CA2113344A1 - Electromagnetic apparatus for producing linear motion - Google Patents

Abstract

An electromagnetic piston and cylinder device for producing linear motion has a cylinder provided with a plurality of magnetic devices along its length for producing radial magnetic fields of alternative polarity. The piston is also provided with at least one magnetic member producing a further radial field and there is control means for carrying piston radial field or the cylinder radial fields to alternate whereby to cause relative movement between the piston and cylinder. The piston and cylinder device is preferably sealed but connected to a fluid reservoir whereby the device operates as a combined spring and actuator.

Description

WO.g3/Q1~6 - . 211~ 3 4 4 PCT/CB92/01277 !~

ELECTROMAGNETIC APPA~ATUS ~OR PROpUCING LINE~R MOTION

The present invention relates to electromagnetic apparatus for producing linear motion.
The conventional way of producing linear motion is by using a hydraulic or pneumatic piston and cylinder device or by using a rotational device such as an electric motor with a rotary to linear transformation mechanism. Each solution has its strengths and ~
weakne~ses. In recent years a further solution, the dc or ac linear motor, has been used particularly where precise positio~ing of a mova~le load is re~uired as a linear motor avoids the need for a rotary to linear transformation mechanism~ A disadvantage of the linear motor is that it produces an actuator which is asymmetrical with a strong force im~alance ~etween the armature and the stator in that they are mutually attra~ted. - PurtherJ linear motors~have been designPd to be placed in a horizontal position, driving the load : along tracks or ~earings within or alongside of the motsr.
The:present invention is intended to provide anel~ctromagnetic apparatus for~producing linear motion whi~h must be capable of causing linear motion at angles ~etween the horizontal and the vertical and be totally enclQsed;without;;any~signifi~ant stray magnetic~fields.
;. Preferably,~no clamping forces-or special straps ~hould~be requir~d or the:apparatus.
The~preferred~embodiment of the present invention is an~e1e~tromagnetlc device~arranged to operate as~a piston and cylinder device. Conceptually, the s~ator of the device can be considered as being the stator-o~:~a conventional~motor slit along~:a plane -:
passing through the axis of the armature and unrolled.
The un~ut ends of the stator are now ~roug~t together to SU~5TlTUTE SHEET

W~ 93/fl1646 ~ t ~, j.; PCr/~B92iO12i7 21133~ - 2 -produce a tubular stator f orming a cylinder . The piston mem~er is provided with ring shaped magne~ic portions.
This results in a device which has the fol lowing ~eatures: -( a ) Attitude independe~t. Whilst generatingthrust the actuator is capa3: le of swinging through large angles horizontal ly and vertical ly ~.
(b) Totally-enclose~. The ~ctuator is capable of operating in an unfriendly (oily, wet and/or unclean) enviro~ment and there are no significant stray magnetic fields from the actuator in any position of the force rod.
(c) Axially-symmetric. There ~re no clamping force~ or special s~apes of the actuator system which make it dif~icult to operate in any position s~r rotation of the axial rod . Pref erably the thrust ro~ shal 1 }:~e f ree to rotate .
( d ~ Me~ str~ke . Strokes f rom O . 2 metres to 1 metre are easily provided and it is pos~ le to constru::t a ram with a 3 metre stroke.
(e) ~=~1~--~. One of the principal ~dvantages is that the ram; has a high electrical to mechanical efficie~cy ratio~ Mere generation c:~f adequate thrust~without regard~ to power consumption isunsatisfactory.
(f):~SL~ple an~ low cost. The operating conditions~-for the~machines which~we concei~e as the first markets for these:actuators~are such that reIia~ility has a very high~level of-importan~e. The !
actuatox has a ~inimum:number of moving parts and wear components such as~sliding sea1s;and~earings.~
Further~ such:~a device c~n be used in the constru~tion of a com~ined~elec~romagnetic/pneumatic a~tuator. The pneumatic part of~the ~ctuator provides a ~s~atic force whi~h may ~e sustained indefinîtely with little or no power consumption whilst the electromagnetic part, working simultaneously, is capable SUBSrITUTE Sl I~Er W093/01646 2 113 3 ~

of providing extremely fast changes in net actuator force and therefore providing the control and precision which are missing from the pneumatic actuator above.
Depending on the applicationt the pneumatic part may be replaced by a different compressible fluid~
It should be noted that the ~luid within the cylin~er actuator does not itself need to ~e compressible providing that it is connected directly to a reservoir filled with air or other compressible gas whose pxessuxe is modified by the movement of the non-compressi~le liqid, so that the li~uid itself appears to ~e compressible. Alternatively the fluid in the actuator may be constrined by a sprin-loaded piston or other compliant surface of a reservoir.
In order that the present invention be more readily understood, embodiments thereof will now be descri~ed ~y way of example with reference to the accompanying drawings, in which:
Fig. 1 is a diagram of the ~asic electromagnetic forces;
Fig. 2~shows diagrammatically the basic concept of an electromagnetic actua~ox according to the : prese~t invention;
Figs. 3 and 4 show the actuator of Figure 2 in more detail;
: - Fig. ~S is a diagram showin~ the principle of a phase control:led~actuator;
~Fig~.^ 6 shows a modificatîon of the basic c~ncept sh~wn in ~igure 2/
m :~ ig. 7 shows an alternative to the modification shown-~-in Fi~gure 6; ~
Fig.~8~shows a further alternative to the ~ modifications shown::ln Figs. 6 and 7;~
;~ ~ Figs 9 and lO show two implementations of the :: ~asi~ concept shown in Flg. ~;

:

~ ~ SlJBSTITUTE SHEEr 2 1 1 3 ~ k ~ PCT/CB92iO1277 Figs. 11 and lla show two constructions of a piston according to the present invention;
Fig. 12 shows diagrammatically parts of three configurations for a piston according to the present invention;
Fig. 13 shows the magnetic layout of four different arrangements of the basic concept shown in Fig. 2;
Fig. 14 shows a basic arrangement of a brush commutator for a moving magnet unit;
Fig. 15.shows a schematic of m~chanical commutation for a moving coil unit;
Fig. 16 shows a diagram of commutation by a 4-coil array as it moves relative to ~ typical magnet/spacer array;
Figs. 17 and 18 show alternative driYe units;
Figs. 19 and 20 show alternative f~ed~ack arrangement~;
Fig. 21 shows a further embodiment of the present invention;
Fig. 22 shows one way in which to provideelectrical connection to a moving piston;-Fig~ 23 shows diagrammatically how to asse~blea device such as is~sho~n in Fig. 2;
Fig. 24~shows in detail a part of a piston assem~ly accordin~.to the~present invention; and Fig. 25~shows in detail a:part of a stator assem~ly ac~ording to-the~present invention.
~lectromaqnetic_For_ s :: ~Ele~tromagnetic rams differ from electric motors, not just ~ecause the ele~tromagnetic forces are .
:arranged so as to;produce linear motion rather than ratational torque, ~ut also because~the power is :~ generated by a large force-at:à slow speed instead of a small force applied at high speed.
The force generated in a magnetic circuit is a product of the magnetic flux B and the electrical : 5UBSTITUTE SHEET

WO93/01 ~ . 5~ Z ~ ? i current I flowing in a wire at right angles to the magn~tic flux of length L (Fig. 1). For large forces it is therefore necess~ry to think in terms of large currents and large wire lengths~
In MKS units, the force in Newtons is equal to the product af the flux in Tesla, the current in Amperes and the length of the wire in metres. There are a variety of magnetic materials available which have a xemanent flux density which varies between 0.4 Tesla and 1.2 Tesla; that is to say, the achievable flux density in the region of the coil will be bekw~en 0.15 Tesla and 0.6 Tesla depending on the efficiency of the magnetic material used in the permenent magnet system.
Unfortunately, the cost of the magnet increases very much faster than the rate of increase of flux density. For example, using a ferrite magnet it is possi~le to generate a field of a~out 0.2 Tesla in the a~ea of a coil for a cost of perhaps 50p while to incxease that flux to 0.6 Tes~a will cost something like 100. It is:usually possible to increase the total flux ~and therefore the *~tal force which can be produced) ~y a factor of 3 by in~reasing the area of the magnet, instead of inGreasing the flux density in the same area at a very muc~.higher cost. This will increase the weight of the actuatorassem~ly; it will also increase -the inductance::-of the actuator coils by a.factor of 9 and therefore the:time~constant of the control sy~tem ~y an order of magni~ude.
In this do~ment it is the general objective to produce a~varlety of possible a~tuation systems using low:cost magnetic;materials, rather than to produce an especially-~ompac:t actuat~ion assembly. ^Sin~e, in some applications, size and control r~sponse may be at a premium, due reference is made to the use of hiyh flux density materials.

SUE~STITUT SHEET

WO 93/01~46 ~ . ~ PCI~/CB92~12i7 2 1 1 3~ 4 ~ ~

C. PERMANENT MAGN13T 5YSTEMS.
In per~anent magnet systems the magnetic field with which a current in the coil interacts is produced by a series of permanent magnets whose shape and arrangement in conjunction with the steel of the actuator ram determine the key parameters of the actuator design.
Permanent magnet systems ha~e the following advantages:-1. High efficiency. No electrical power needs to besupplied for the generation of magnetic fields by coils with iron pole pieces.
~v 2 . Rapid response. The energization of field coils often requires the build up of current in a high inductance coil which takes a signi~icant time and slows the response of the system. In contrast the armature coil i~ usually of a low inductance design and, if the magnetic field already exists as a result of a permanent magnet circuit, the thrust can be changed much more ~quickly by ~arying the:current in the "armature" .

3. Light weight. Permanent magnet materials axe less dense than iron.and much less dense~than copper, ~oth o~
which would have to be used in q~lantity to repla~e them.
It is an advantage in the applications which we co~sider -: ,,t~at~the actuator unit shall bç relatively-light;-~weight.

4.~!~Dynamic braking.~ The~existence of a permanent magnetic field system~:allows, in:prlnciple, the armature to be.arranged as a passive element, short-circuited by a chosen value of l~ad resistor so that movement of the ~armature coil through:the permanent magnet fields causes a current to flow which brakes the movement of that cs~il. By sy~etry, dynamic braking is also possible for a movlng-magnet piston system, the stator windings then SUBSTITUTE SHE~ET

WO93/01~6- s ~ ~ PCT/CB92/01~77 7 3~; 1 . 5 ~ L. _ ~, being short ci.rcuited to cause power loss. Such an arrangement is clearly power-efficientJ especially so if the dissipat~d energy can be fed back into the original power supply system.

D. ELECTROMAGNETIC SYST~MS.

Whilst we concentrate ~he bulk of our design activity at the present time in relation to actuators in the form of piston and cylinder devices which incorporate permanent magnets with the advantages listed in the preceding section, we can see that under some circumstances it may be preferable to replace the permanent magnets by an electromagnet syst~m . The chief advantage of such a configuration is that ~oth the stator and the ar~ature (or ~'piston" ) may be supplied from sources of alternating current with a known and contxollable phase relationship.

This arrangement ha~ the potential advantage that a~connection to a moving:part may be made by^~!
inductive-coupling, removing the requirPment f~r slip rings or commutators in some circumstances. The design may have similar ad~a~tages when the actuator is placed within a sealed contSainer or ~in a corrosive atmosphere where stronsg:electrolyti~ action is common . DC currents : : and vol~age~ within such a system would enhans~é the ' corrosion wherea~ the effect~ of AC voltag0s of onSe-~polarity are cance~lled a few milliseconds`llatér by` th~
use of:the opposite polarity .
~ universal actuator design is shown in~Figure 3, in which a cyllnder 30 of a magnetic material sussch s steel houses a plurality: of annular co_l~ 31 whi~h are separated from ~each other ~y pole piece rirlgs 32. A
piston member 3S slides in the central bore of the cylinder 30 and comprises a cylindrical steel sleeve 336 on the exterior of which are mounted a plurality of bUBS~lTUTE SHEET

WO93/0l~6~ PCT/GB92/01277 21133~ ~ - 8 ~

segmented windings 37. Cuxrents in the stator windings produce radial magnetic fields which interact with currents in the piston coils, whose phase is controlled according to the position of the piston and the required thrust direction~ The mirror image of this design is one in which currents fl~wing in the piston produce radial magnetic fields which interact with currents in the seg~ented windings of the stator. The piston of the mirror image design is shown in Fig. 4 where the piston comprises a steel core 40 provided with annular polepieces 41 and coils 42.
E. DC SYSTEMS.
The inclusion of a permanent magnet within the actuator does not necessarily imply that the coil el~ment of the actuator must ~e energized with a DC
~that is to say a no~-zero m~an current per cycle) ~urxent system. For example, it is possible to conceive of an actuator in which the piston element of the ram carries permanent magnet(s) which interact with a field of variable fre~uency which is made to travel up or down the axis of~the ram and with~which the speed of the permanent ma~net:system is brought into synchronis~ .
Alternatively , the actuator may be designed to move the pistoD in steps~between magnetic detent positions.
~ ~nless the application of the:actuator is very sensit~ive to hum and/or is resonant::under some :: : condi~ions~3to~a harmonic or sub-harmo~ic o~:the mains frequency,:-it is clearly possible for the actuator to ~e energized with raw rectified AC ( 100 Half c~cles per !
:second) rather than ~y a steady DC curreni with : ~ ~negligible ripple. -- ,~ ,.
; :The a~ailability of fast switching power : ransistors also allows us to consider the mea~ing of "DC" to include a supply of current from a source which consists of a train of unipolar high frequency pulses supplied from a switching transistor~

SU~STI~UTE SHEET

W0 93/Ol~K 2113 3 4 ~ PCT/CB92iO1277 We take a "DC system" in the context of our designs to mean one in which the magnetic field is unidirectional or time invariant.
Alternatively, the windings o~ the stator of the design shown in Figure 3 or Figure 4 may be ~
energized with DC (steady, pulsed or raw-rectified) to react with currents in the piston, similarly derived and controlled~
F. AC SYSTEMS.
We have earlier referred to the potential advantages in difficult circumstances of an AC-powered electromagnetic actuator. The control of the actuator force may now ~e effe~ted either ~y variance of the relati~e phases of the currents flowing in the stator and the.piston or by the variation of at least one of the current amplitudes. (~f both currents are varied together the force v~ries approximately as the square of the amplitude. ) It will be clear that when the method of force control is chosen to be that of phase variation, the minimum force ( theoreti~ally zero ~ will be generated when the phases of the two currents differ by 90o and will attain a maximum when the currents are precisely in phase. It is, however, clearly wasteful in terms of en~rgy to supply-current to the system when no force i~
produced.
,,. ;There lS, ho~wever, an-:-interesting variation on this technology which:allows the design of an a~tuator which is-position-sensitive. That is to say at any:-chosen position the phase of the current in the~piston may be adjus~ed to be~orthogo~al to the current in the stator:but this will not be so if the piston should-move a small distance. The arrangement conceives of a:wave travelling down the stator at the same speed as a similar wave propagatad i~the piston. This does of course imply that the energizing current to both the stator and the piston shall be multi-phase, consisting 5UBSTITUTE SHEF~

WO93/01646 ~ ` PCT/CBg2i~1277 2ll33a~ - 10 ~

of at least two phases applied to spatially-separated coil systems. Such a travelling wave can be generated from a ~ingle phase supply ~y a capacitively-tuned coil system in which the current flows 90~ out of phase with the current in the directly energized coils.
It will be seen that, with the piston stationary with re~pect to the stator winding the net effect of the two travelling waves will be to produce steady force on the piston in one direction or another, the amplitude of the force ~eing a function of the amplitude and th relative phase of the currents in the stator and the piston .
Now consider that, the piston being fixed, the phase of the energization of the piston coils shall ~e adjusted so that the wave travelling across the piston is at all times 90o out of phase with the wave travelling do~l the stat:or. There will therefore be no net force between the two systems and, at that position of the piston, no force is generated. This situation is illustrated in Figure ~. ~
- ; ,Nevertheles~, if the piston should be displaced to the rlght or the left then the electromagne~ic fields will no longer be orthogonal and a force will be produced on the piston , which for small displacPments will tend to return the piston to the original zero force cond:iti~n . If the piston is ~displaced by more:than 90-(spatial) degrees away from one null force position then the~direction of-the net force on the pi~ton,will be reversed1and it,will be driven even further away into the next null state.
The charactérîstics of this~form of actuator system provide for an intrinsically-stable po~ition control and a means of generating variable-speed displacements of the piston in either direction without changing the energizing ~requency of the actuator itself. (Clearly, a slow phase advance applied to the SUBSTITUTE SHEET

W~ 93!01~ 2 1 13 3 41~4 i~ PCT/C~9i/~12i7~;

piston system will cause the piston to drift in the direction of the advancing phase. ) G. IN~CTION ACTUATORS.
In the previous section concerned with AC
actuator systems we describe the effect of the interaction between a wave travelling down the stator and a wave travelling down the piston in the same direction, showing that, by varying the phase relationship ~etween them the force on the piston can be modulated in a positive or negative sense.
It is also possible as shown in Fig. 6 to conceive of a piston coil which is, in effect, a number o~ sleeves of copper 60 in which currents are induced which tend to drag the piston in the direction of the:
travelling wave in ~he stator which is similar to that shown in Fig. 3. The force on the piston may now be varied in strength and in direction by phase and amplitude modulation af one of ~he stator phases causing it to lead or lag the primary energizing phase and thereby to vary the streng~h and ~direc~îon of the -induced currents in the piston sleeve coils. -:
On a reciprocal con~iguration as shown i~ Fi~.
7, the stator consists of a steel~ylinder 70 haYing a of a number of copper rings 71 in whîch current is induced by a travelling wave field:produced by 2 or 3 phase coils 72 on t~e~piston 73. Such a ~onfiguration :
is cheaper to make and con~umes less electrical power.
H.~ SER~O INDUCTION COILS. -;
A variation on the arrangement descri~ed in Fig. 7 is~:shown-in Fig.i8-and one in whic~ the currents ~l~wing inithe pisto~ ~oils 72 are the ~utputs from power amplifiers whose inputs-are~derived~from sensing coils 7~ which pick up the signals from the stator~.
trave}ling wa~e system.
: - The advantage of such a design improvement is that strong interaction ~etween the piston and the stator fields can be maintained when the slip speed is SUBSTITVTE SHEEl W~93/~1~6 ~< .
2113 3 4 ~ - PCT/CB92~01277 very small ~ecause the amplifier can be used to drive the piston current at maximum level when only a small voltage is generated in the pick up coil. (The voltage in the pick up coil is a function of the relative velocities of the pis~on and the travelling wave in~the stator. ) (A reciprocal arrangement is possible, using pick-up coils and amplifier-driven coils in the stator , the piston being enPrgized . ) J. MOVING MAGNET SYSTEMS.
By analogy with the loudsp~aker system from which the pre~ent range of electromagnetic a~tua~ors has been derived , the moving element îs assumed to be ~he piston of the combined pneumatic and electromagnetic ram system and the " stationary " elemcnt is assumed to be that relat~d to the outside cylinder surfa~e , whether of circular or rectangular cross-section. However, the opportunity is taken here to point out that it may sometimes be convenient to couple the force elements in the reverse sense,~so that the piston elem@nt is .anchored to a~stationary objec~ and the moving object is cunnected to the sleeve:.:We also point out that the stationary element is not necessarily an external sleeve ~ut may al50 ~e.a central rod, over which the "pist~n"
element of the actuator is arranged;to:travel. (Such a design has:recently ~een disclosed by a British ~compan~
In ~he context of:this~discussion the element which we.refer to~as::the~"piston" has.a dimension in the - direction along which the thrust:is applied which is shorter than the dimension of:the "stator" in the same -dire~tion. By a "moving magnet-systeml' we therefore :~
imply that the:dimensions of the element which ~arries : the magnetic field generating parts ( whether~permanent magnet or elec~romagnet ) is smaller than the element which carries the length of wire on which ~he magnetic flux is concei~d to act so as to generate the required S~B~ TE S~~T

wog3/0!646 ~- 21133~ P~/~B92jol277 ~

force. ~igure 9 shows such an arrarlgement where a piston 90 slides in a cylinder 91 which carries annular segmented stator coils 92. The piston 90 carries radially magnetised ring magnets 93.
The essential feature of a moving ma~net system is that there must be in the stationary element a means wh2reby the magnetic flux emanating from the mo~ing magnet may be returned to it by a low reluctance magnetic path. This is sometimes difficult to arrange.
It should also be noted that as the moving magnet elem~nt traverses its path of action the flux linkage through components of the stator will change as a result of that movement and the ch~nge of flux may be ~ound to generate eddy currents in metalwork associated therewith. Precautions may therefore ~e necessary to eliminate this ed~y ~urrent effect, which would cause power to ~e dissipated in the resistance of the metalwork. Such precautions mig~t include, for example, the interruption ~f a continuous metallic path ~y the incision of a slot at right angles to the path in which the eddy current would otherwise flowO
. By analogy with-the moving coil loudspeaker system the magnetic field is usually conceived to ~e radlal (in a c~ircular magnetic system~) or at right angles to the path of~motion ~in a rectilinear electromag~etic actuator~system)~
K.~ MOVING COIL~-SYSTEMS. . :: :
- As we:ha~e-;explained:in ~he preceding ~ection, ~the d~mensio~s of the~'~moYing"~parts of the system arel ;
assumed *o be significantly less than those of the "stator" part: of the:~syst~m. For a long-stroke actuator the choi~e between a moving coil:and a moving magnet system often redu~s to a consideration of cost of the magnetic elements:versus the cost of the copper:coil sy~tem. When the magnetic material i5 expensive it is clearly an advantage to use a small n~mber of magnetic elements as the "piston" and to construct a l~ng SUBSTITUTE SHEET

WO93/016q6 . f.
~11'33~ ~ 14 -a~sembly of coils as the stator. In contrast, when magnetic material is cheap it may be prefer~ble to produce ~ long assembly of magnets and to use a small coil assembly on the piston. Figure lO shows such an arrangement with the steel stator cylinder lOl carrying a series of ring magnets 102 which are radially magnetised. The piston lO3 carries segmented ~oils 104.
There is also a consideration of the cost ~f co~mutation~ Whexe commutation is not directly mechanical - achieved by means of a brush assembly mounted on the moving piston and making connection to a fixed commutator on the stator - a moving c~il system is to be preferred. This is because the number ~f commutated ~lements in a moving coil assembly is less than the number of elemen~s in a stator / so that the number of solid state switches is also reduced, together with the complexity of their wiring to a co~troller.
Nevertheless, a moving coil system does împly an electrical connection to a moving part p~rhaps by a cable which will be subject to wear under continuous flexure. (A c~mmutating ~rush system will also be subject to wear.) L . SINGL~ SIDED SYST~MS~
The force generat~d by an electroma~netic actuator is proportional:to B.I.L. where B is the flux, I is the current and L is the length of wire. It may therefore be~thought to be an advantage to intercept the flux from the magnet~:twice,:once a~ the north pole-and once at the~south pole., so as to make availa~le the maximum ~urfaee~area over which the length-of coil may be arranged to interact with the flux and the magnet.
However, it is also nece~sary to redu~e to a minimum the reluctance in the path of the magnetic flux because the effect of such reluctance is to reduce the flux density and therefore the force which is generated by a cuxrent carrying coil which the flux interceptsO An 5U8STITUTE SHEE~T

WO93/01 ~ PCT/~B92/01277 `~ 21133~

optimum is reached when th~ magnet is operating at i~s "BH max't point on thP magnetization curve of the magnetic material. If the air gap represented ~y khe thi~kness of the coil is reduced, the flux will increase but the product of current and length will be reduc^ed so that there is no net gain. Con~ersely, if the air gap is increased so as to include a greater volume of copper i~
the magnetic flux path, the flux leaving the magnet is reduced and no net gain results.
Because it is difficult to devise a mechanical cons*ruction which takes foxce from a coil system which is arranged on both si~es of a magnet system (or vice-versa) it is preferable that one side of the magnet be arranged to have a short high permeability (low reluctance) path and for the other side to incorporate the coil in an air gap system coupled to the force element.
M. CIRCULAR CROSS SECTION ACTUATORS.
By ~nalogy with the moving ~oil loudspeaker system and also with the conventional forms of pneu~atic actuator, the general~purpose elec~romagnetic/pneumatic ram may be thought to be of:circular symmetry~
As explained in pre~ious sections, the essentlal dif~erence~between~the designs of electromagnetic actuator which~are discussed in this document and~those~whlch haye previously been described in the literature~is::~that~although previous designs ~have~conceived of the:~sli~ing and unrolling of a conventional electric mot~r systèm,ithe~second:stage ~f the topologi~al transformation has~not been conceived;
namely tha~ of taking::the sides--~of~:the u~rolled motor strip and:curving them together to-form a long cyl:inder. .~
.: : The cylindrical cross section is an advantage when the coil: system:is wound on the sur ace of a cylindrical piston or on a cylindrical sleeve which is SU~STITUTE SHEET

WO93/01~6 ~ PCT/GB92/0i277 2113~4 l 16 '~) then inserted in the out~r cylinder tu~e to form a statcr.
When the electromagnetic actuator is combi~ed with a pneumatic actuator it is clear that the design of an air sealing system to work with the electromagne~ic piston is simplified in a device with a circular cross section ; sliding seals of circular cross section are well known and r~adily available.
Finally, an actuator of circu~ar cross section has the advantage that the actuator shaft may be orientated in any direction relative to the mounting brackets of the external sleeve, subject to the arrangement of elec~ric connections to a moving coil piston if employed.
The di~advantage of an electromagnetic actuator of circular ~ross s~ction is that the permanent magnets must be arranged in a radial format; that is to say, the shape of the external periphery of the magnet must be circular . Whilst this is not a problem for ferrite magnets having a flux density in the order of 0.2 tesla, it is o~ten a problem for high performance magnets having a 1ux density in the order of l tesla.
N. ~ECTANGULAR ~ROSS SECTION ACTUATORS.
Previpus designs~by others have descrihed flat electromagnetic actuator systems in which the moving element travels on~a ~earing a~ove a stator or surface to which~it is:strongly attra~ted , producing a strong bearing:rea~tion:force .~ Alternative designs show twa plates~ between which the moving element passes ~o that!
the strong downwards~force is counter~alanced, ~ut we have not seen the e~tension-of this concept to include left and right hand:~stators or plates. It will ~e understood tAat such a balanced system might be constructed w~thin:.a teel ~ylinder of square cross section and using a square " pistnn " arrangeme~t approximately ~quivalent to the cylindrical conformation previously discussed. This is shown in Fig. ll where SUBST~T~ SHEET

W~93/01~6 2 1 1 3 3 4 4 P~T/CBg2/~277 ~

-- 17 -- , four rectangular magnets 111 are mounted on a steel core 112 to form a first "ring" whose external periphery has a North pole. A further "ring" is cr~ated by four further magnets 113 but the external periphery is now a South pole and the two "rings" are separated by a ~on~
magentic spacer 115.
The advantage of a rectangular cross section system is that magnets of all types ar~ available as rectangular plates, allowing a high performance piston to ~e designedt intrinsically capable of a higher thrust than that of a comparable ferrite unit. See Figure 11~
The disadvantage of such systems , as alluded to in the preceding section, is that s~uare cross section pneumatic seals are a problem at the corners of the rectangulsr piston and it is ne~essary to ensure that a mini~um radius is employed. There is also a difficulty in the control of coil winding machines for rectangular cross section devices in that the ~angential speed varies rapidly during the rotation.
Pole pieces may be added to magnets of rectangular cross ~eetion so that a~cir~ular cross~
section is achieYed in the complete assembly.
O. DISC/PLATE MAG~ETS.
~ In:oxder to produce a radial magneti~ field, disc:or plate magnet:s may be arranged back to back between steel pole piec~s and form the piston of the d2Yi~ s ~
All magnetic materials can be produced in the form of:plates of circular or rectangular ~ross section, so that--the-technique may be universally adopted.
:However, ~ecause the~:complementary magnet generates a high opposing magnetomotive~ orce, the flux ~: -output from the magneti material~is ~riven well down its characteristic.~ Since there is a danger of irreversi~le de-ma~netisation with some magnetic materials, the technique cannot always be employed.

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WO 93/01646 ~ s -~ P{~/GB92/01277 211334~ - 18 - '~~

The advantage of the technique is that it is possible to stack a series of magnets in a compact space so as to produce a magnetic field of high density, enabling significant mechanical force to be produced from a coil system of compact dimensions. ^
Plate magnets can be used in an alternative configuration i~ a rectangular cross section cylinder unit. In this case the magnets are arranged so that the four plates generate a radial magnetic field on the four faces of the rectangular cylinder as in Figure 11. The return patn of the magnetic flux is via steel pole pieces or via magnets of opposing polarity and of half the width of the main magnet as illustrated in Figures 12a, ~ and c.
In Fig. 12a, the stator cylinder 125 which is preferably of steel, is provided;with the segmented stator windings 126. The piston comprises a steel core 128 on which i5 mounted two magnets 129 separated by a non-magnetic spacer 130 e.g. of aluminium. This produces high thrust with an unbalanced flux path and requires the stator cylinder 125 an~ the core 128 to be t~llC~
An alternative~arrangement is shown in Fig }2~b) where the.piston arrangement~ls altered to use a magnet lZ9 disposed~between two half magnets 129 and 129b~and sepa~ated therefrom~by:two;~spacers 130. This provides all the:- attrlbutes of the :arrangement in~Fig.
12a but needs thinner~steel. ;
Fig 12:(;c? shows~a:.further alternative where the:half magnets of Fig.~:12(b) are replaced ~y steel pole pieces. This~results in~-further thru~t.
Again, because~flat plate magnets can be.
suppli d in: any magnetic material, magnets of the la~est and most efficient~:~type:may-be employed to produce .
actuators ~f Yery high~ef~iciency.~

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P. SEGMENTED MAGNETS.
In that the "natural" shape of a piston is conceived to be circular in cross section and the most efficient field configuration is expected to be radial, the i~eal form of a magnet in such an actuator system might be thoug~t to be that of a radially-magnetized cylinder. Magnets of this foxm are theoretically possible in a ferrite such as " Ferroxdure" but there has ~een no demand for such magnets in other applications~ . -Nevertheless, magnets which are segments of acylinder are extremely common and used in electric motors over a wide range of dimensions. A ~uppliers catal~gue for such materials includes many e~amples of segments having arcs of 160, 120, 60 and 50 etc.
These magnets may be supplied ~ith the field magnetized north pole inwards or south pole inwards and in a variety of diameters and thickn~sses. The magnets are relatiYely cheap, owing to their large volume u~e in other applications, so that it is conceiva~le that the technique can be used in a movlng magnet or a mo~ing ~oil configuration as shown in Figures 9 and 10.
The system is shown in the preferred arrangement in which the magnetic fields are wholly contai~ed within the outer steel sleeve of the long stator. Figure 13a-d illustrate alternatiYe designs.
Q~ :iSINGLE-M~GNET-SYSTEM5~
We have earlier described systems such as that ~-in Figure-12c;(or its mirror-images)-in which the magnet : is :arranged to have~two return paths arranged symmetrically on ei~her side of the magnet, thus making it independent of position in a long stroke actuator.
The total air~ gap distance from the pole of the magnet out: through the coil to the bounding ~ylinder and then back:~hrough the coil ~o the flanking~pole piece must not be significantly greater than the magnetic length of the energizing magnet . The design has the principal SUBST~TUTE SHEEI

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advantage that the piston is rela~ively compact.
However, the density of windings is high for similar rea~ons, and this increases the complexity of the coil switching system and the cost of the coil arrangement.
R. MULTIPLE MAGNET SYSTEMS.
The simplest and most compact piston arrangement is to use two plate ~disc) magnets side by side in oppo~ition but separated by steel pole pieces.
The advantage of the paired magnet sys~em is that other disc magnet pairs may be a~sembled to increase the thrust to any desired figure at the expense of piston length.
In the case of the segmented cylindrical magnets which are made in quantity for electric motors and are therefore available at low cost, it is possible to design a long cylindrical sleeve which incorporates alternating rings of such magnets producing a design of moving coil actuator with excellent performance.
S. MOULDED MAGNET SYSTEMS~
~ Certain manufacturers now have the ability to produce a mouldable magnet material with properties similar to those of a Ferrite. This makes it possible to assemble an array of magnets for ei~her the stator or the piston of an actuator from two half ~ylinder mouldings of the màgnetic material, with a significant reduction in actuator assemb1y:time-.
(See also the section on Ass~mbly-~echniques) .
:T.- ~ : SQUARE-WOUND:COILS.~
By "square-wo~nd coils"-we mean an arrangement of the windin~s of the coils which presents the boundary of each coil section at right angles to the axis of thrust so that the movement of a-piston carrying a magnetic syste~ through one of these ~oil segments results in an a~rupt transition from zero flux linkage to high flux linkage and visa versa at a later instant.
It might be thought that the control of square-wound coils: is difficult in that there is no SUBSTITUTE SHEET

WO 93/01~6 ~ 2113 3 4 4 PCT/GB92/Ot277 tolerance of coil switching at an instant which is slightly e~rlier or slightly later than that in which the transition actually occurs. Such mistiming might be expected to create a perceptible force transient.
~ owever~ a study of the mechanism of commutation in, for ex~mple, a four-coil fixed magnet design , shows that commutation tak2s place at a position of near zero coil flux, so that transient foxces are acceptable.
U. SK~W WOUND COILS~ -The tolerance to switching is improved byarranging for the coils to be wound at a~ angle to the perpendicular to the force axis. Since this would of itself tend to generate a rotational tor~ue on the piston about an axis at right angle~ to its line of travel, this is compensated by an opposite skewing of the orientation of the magnets. (~or a cylindrical actuator concept, a skewing of the magn~ts on the piston will require an elliptical cylinder cross section. ) V~ WA~E WOUND COILS. -: ` f In a wave wound coil the torque whi~h`would be generated~y a sk~w wound coil configuration is `:~
counterbal~nced~b~ an alternation in the angle of skew during each revolution~of the coil , so that in one part of~the rotation the skew is to the left and in another part to ~he.,right. Thls effectively spreads the interface~of~the coil system~with:.the magnets: ~o:that the transition;becomes smoother and less a~rupt and : :therefore easier to control by means which will be described in later sections of this document.
The~disadvantage~of this:coil arrangement,~as indeed for-~skew:wound coils, is~that the spreading of the coil effetively reduces the mean flux density in the coil and therefore the thrust per Ampere of ::-current. -SUBSTITUTE SHEET
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W. SLOTT~D COIL ASSEMBLIES.
In the construction oE motor armatureassemblies, it is common practice to design the steel core of the rotor so as to fill almost the whole of the internal volume of the motor cyli.nder, lea~ing only a small air gap between the periphery of the rotor and the internal surface of the stator iron. This reduces the magnetic reluctance of the motor system to a minimum.
The armature coils are then insulated and fitted into slots which are generally arranged t~ lie parallel to the axis of the rotor. Although it might be thought that the iron on either side of the slots would effectively shunt the flux away from the armature windings and render them ineffective, this is found not to be the case. The interaction ~etween the magnetic fields due to the st~tor and those caused by current flowing in the rotor coils then occurs in the iron of the slotted rotor.
The technique has the added advantage that the forces on the wires of the coils ar~ immediately ~-transferred to the steel of-the rotor. A similar practice o~ slotted coil assem~ly is also applied to the stator windings of elec~ric motors, with equally-~eneficial results.
~ ~ : In the diagrams in~this do~ument we have, as a general practice, simply shown the coil~sections to lie : on the surface of:~he iron core of-:the pis~on (for exampIe);~they could,:however, lie in-slots and in Figure~24 we show an illustration bf a piston having a~
num~er of iron or steel rings or peripheral ribs 240, r, forming slots whlch~are~perpendi~ular to the thrust axis of the assembly. Th~se ribs or rings ~educe the reluctance of:the magnetic~:circuit-and transfer the thrust of the coil winding s 241 to the actuator shaft in the same way as the slots of the motor armatures descri~ed earlier.

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WO93/01646 2113 3 ~ 4 pcT/rJBs2io1277 - 23 - j 1 The rings (and the steel of the piston cylinder itself) are arranged to include an axial slot forming a ~xeak in their electrical conductance, which would otherwis~ form a short~d turn coupled to the actuator coils.
It should be noted that the copper coils cannot be pres~ed into slots in a preassem~led armature in the same way as it is possible to do in a rotary motor. They can, however, be preformed as rings 241 which can be stacked alternately with steel rib rings 240 to form an assem~ly which is then integrated electrically as sections or individually-commutated elements and terminated hy clamping end rings 242.
~rictionless hearing sleeve 244 e.g. of Teflon can be provided. ~igure 2~ illustrates the method for stator construction.
Here, stator coils 250 are assembled on to abn inner non-magnetic sleeve 2 51 with magnetic rib rings 252 which form slots receiving the coils 250. The assembly can then either be slid into the outer ~ylinder 255 of magnetic material or cl~mped between halvesiof the inter cylinder. ~ -X. MECHANICAL COMMUTATION.
It is possible to imagine a moving-magnet system as shown in~Figure 14 which carries three brushes 149a, b and c~beaxing on commutator segments l~0 which select, connections.~to:~a sîngle layer of copper~
conductor. It is~conceived that along one strip of-this conduc~or.the insulation has been removed and the wir~
h~s been plated with~a hard material to withstand the weari~g effects-of commutation. Alternatively a technique of coil:construction might be~devised which welds coil se~ments~150 at frequent intervals to a printed commutation strip l~l laid into the coil former.
As thé magnet assembly moves backwards and forwards down the length of the stator coil, current SUE~STITUTE SHIEET

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2 1 13 3 4q ~ - 24 - 7 flows into the central brush 149a and flows in parallel to the left and to the right, leaving the system via brushes 149b and 149c which are toget~er connected to the opposite terminal of the electrical supply.
Clearly, current will flow (say) clockwise from a t-o ~
and anti-clockwise from b to c, over the face of maqnets having an opposite polarity~ In this way the thrusts generated in the two coil areas will be equal and additive. Clearly, if more than two magnets are used in the piston assembly, it is possible to conceive ~f an arrangement of coil segments and commutator ~rushes which will optimize the thrust generated for any particular configuration.
This is a mechanical commutation arrangement similar to that employed in many motor systems. Its disadvantage is that the brush contacts are not perfect, resulting in power loss and wear. The ~rush wear generates dust which is not likely to be carried away in the air system of an electro-pneumatic actuator and there is danger of electrical interference which will :
need~to ~e suppressed. The piston cannot~be a~lowed to rotate within the cylinder, or the brushes: will leave the commutator strlps.~
Nevertheless, the technique is well proven and is suitable for~lo~w-cost~applications.
A mirror-image~system,~providing mechanical commutation~for;~a~movi~g-coil~piston having eight coil segments is;shown in Fig.-15. -~
'~ r "' ~ Clearly~,~as~thelcoils move across~the faces of~the magnets, the direction of~current flow must be reYersed one section~at a time in a manner which repeats oyer~a distance~equal to the cycle in magnet configuration - ie over twice (l ~ y)~where 1 is *he axial length of the~magnet~and y is the width of the spacer.
This effect can be produced by the use of two printed circuit~commutator strips, set flush with the ~UBSTITUTE SHE~T
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surface of the thrus~ rod as shown in figs~ l~b and l~c.
The arrangement has the additional advantage that the power brush~s 151 are external to the ram cylinder 152 and that it removes the requirement for flexible wire current co~nections to the moving piston. The brushes 151 co-operate with commutator strips 153 inset in the thrust rod which in turn is shrouded ~y a telescopic gaiter (not shown).
Y. CHOICE OF COIL WIDTH.
We have described alternative designs of actuator in which an array of magnets of alternating polarity moves relative to a number of coil sections -whose current direction must be chosen in accordance with the instantan~ous position of the magnets in relation to t~e coil sections. The magnets may be fixed and the coils moving or vice-versa.
Assume that the magnets are of axial length 1 separated by non-magnetic spacers of axial length y.
Assume that there are n coils of axial length x. We need to consider the choice of n and x, given 1 and y.
-Clearly,:n cannot ~e 1, because when x lies symmetrically over any number of alkernating msgnetic poles, the net flux can ~e zero ~ the system will be "dead" . The same is true for any coil individually in an array ~f coils.
- - Thus, if n ls 2, thrust will fall to (less than) ~O~ of maximum for-at least one positio~ ; for n=3j less than:67%, and for n=4 less than 75%, by:
similar considerations. Clearly the variation will - .
continue to decrease as the number of coils increases.
~ However, as the number of coil sections ~-increases the cost and complexity of the system will also ~e increased~by:the num~er of switching transistors and their control elements. We suggest that for an actuator with solid state commutation a reasonable minimum number of coil sections is 4 and that a maxim~m is 10.

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The lPngth of the coil sections must be chosen to avoid harmonics of the length between centres of the spacers ~etween the magnets , otherwise more than one coil section can be "dead" at the same time.
The total length of the coil assembly ~ho~ld therefore be arranged to cover the distance between the extreme ends of two magnets having a spacer between them i~e. that n.x = y + 2l.
Z. HALL-EFFECT COMMUT~TION.
The disadvantage of the arrangement of magnets as in Figures 12a, ~ or c i~ that the dire~tion of current in the coils must be reversed from time to time in correspo~dence with the changing position of the magnet assembly in relation to the coils. For example, in the moving magnet system illustrat~d in Figure 9 the coil~ must be divided into segments a~d a commutation arrangement m~st ~e devised so tha~ as each coil passes from a north pole to a south pole the electrical connections to the coil are reversed..
It is therefore possible to place in the centre of each~coil a Hall-effect sensor so that, when the sen~or is adjacent to a north pole the current in the coil is arr~nged to ~e switched from left to right (for example) and when the coil is adjacent to a south pole the current i~arranged to ~e rom right to leftO
The transition occurs~as the ~all-ef~ect sensor passes acros~;the mid-p~int ~etween th~ magnets. In a ~imilar manner,.:the~moving coil illustrated in FiguFe 10 requires~that the coil be ~u~divided into a number of segment in ~hich the-;direction of current flow is revexs~d in a~manner which~depends upon the relative position of the~coil segment and the polarity-of the nearest magneti~ segment.
: (It:should b~ noted that the Hall-effect sensors do not in themselves determine the direction of current flow because the actuator is re~uired to produce SUBSTITUTE SHET

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accelerations in either direction, independent of precise position and velocity. That is to say, if the actuator is moving to the left and requires to be accelerated further ~o the left then the current may be positive; but i~ is also possible that the actuator~may be moving to the left and is now required to accelerate to the =right: in this case the direction of current in the coil will be negative.) POSITION TRANSDUCER COMMUTATION.
It will be recognized that the technique of sub-dividing the actuator coil into a finite number of segments and of commutating these segments at appropriate tim~s may also ~e achieved under microprocessor control if the computer is always aware of the relative positions of the piston and the stator.
Microprocessor-controlled switching has ~-he addi ~ional advantage that the approach to the point of current reversal ca~ also be modified to avoid ~witching transients. Further, it is possible for the microprocessor system to vary the magnitude of the currents in the coil segments so as to smoothe out the force ripple which would otherwise result~ . -~or example, Figure 16 shows a diagram of commutatio~ as a 4-coil array (A~CDi moves relative to a typical magne~/spacer array. In:Figure 16(i) maximum thrust is achieved with peak currents in all coils. Howeverl in Figure 16(ii), if ull lthrust~:is~ no lonyer achieved }:~ecause the intercepted areas o f -the: coil segments are now different; and so on -for ~igures l~(iii? through 16 (Vi ?
If r.`o correcting mechanism was employed and the current was allowed to remain constant at its peak value in all coils then the force generated would vary ~y 3~ during the thrusting movement illustrated in this example. This would not of course be acceptable in a simulator application although it might be so in an industrial application with no extreme sensitivity to SUBSTITUTE SHFET

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force changes. In later sections of this document we describe alternative or combinational means of smoothing the force output.
TRA~SISTOR COIL SWITCHING.
Figure 17 shows a complementary ~.F. coil^
drive arrangement whereby a coil 170 having a common return to zero potential may be connected to a positi~e or negative ~C power supply via transistors x or y. In operation, transistor x or transistor y is used as a high frequency switch, being either saturated ON or totally switched OFF and ~eing modulated such that the mark: space ratio of the ON time to the OFF time determines the mean current in the coil.
The protective diodes dl and d2 across the switchin~ transistors are ne~essary to prevent voltage overswings of the coil ç-onne~tion voltage. For exampl~, if txansistor x is turned hard ON for a fraction of a s~cond, current begins to build up in the coil~ When transistor x is su~denly switched OFF the potential at q will:swing hard negative until clipped by the diode d2 at negative rail voltage. The curre~t will then begin to decay in the coil until transistor x is turned hard ON
again , causing the current to increase in a positive direction . Similar considerations apply to negative coil currents and to the positive supply protectibn diode.
The;drive:;wave form:to the ~witching transistors may~e:pre-determined by the microprocessor ~controlling the pos~ition~aatuator and~will take ints account the following paramete~s : ~
- Relative;positions of magnet:and coil assemblies.
.

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Desired position of the actuator.
Desired velocity of the actuator.
Desired acceleration of the actuator.
Approach to limiting positions (eg. end stops).
Emergency conditions (eg. function upon power failure).
Current mo~ulation to smooth force ripple.
Compensation for variable coil efficiency.
Inductive time constant of the actuator coil section.
Force or accelerati~n feedback may be superimposed on the programmed variation. ( See sections AG and AH.) AC. TRIAC SWITCHING.
Figure 18 illustrates an arrangement of coil control in which a single low-cost triac element replaces the two switching power transistors and two diodes illustrated in the previous section.
One end of the aoil is now permanently connected to an alternating power supply which is switched between~plus V~and~minus V~;at a high frequency (in excess of l KHz ) by'~an inverter sy~tem operating from~a mains-power supply~bridge rectifier system used to charge two ac~umulators.~
It will~,~be cl~ear;that,if the trigger voltage f is applied when the~alternat mg~voltage is at v plus, then current wil~l~flow through~the~coil in one direction,~ whilst~if~the ~trigger Yoltage f is present when the supply~-is~'`'at~v minus-~then current will'flow hr~ugh the coil in thè opposite dirè~tion. (The triac is capable of ~onducting, with thë~oltàge in either sense')~-. '`'""~;~ ''~' ~ ''`' ~ '-' ' ' The~mean'current'~through~'the~coil can be varied by controlling~the insta~t~'during the power--supply cycle~at~which~the trigger~voltage is applied to the~triac. The current-~having built up during one half cycle, the triac turns off as the current decays to zero during the next half cycle under the influence of the ; SUBSTITUTE SHEEl~

W~93/01646 PCT/~B92/01277 21133~4 - 30 -voltage of opp~site polarity applied to the coil element.
It should b~ not~d that, if the actuator has been accelerated up to a speed of 5~cms per second and the width of a coil is in the order of lcm (so thak-switching must occur over a distance in the order of l~m) then the switch transition time is required to be l/~OOth of a second or 2msec.
These parameters set an upper limit on the inductance of the coil seg~lent. F~r instance, if a current of lOamps is required to build up in a time of 0. ~m seconds from a power supply of 50 volts then the inductance of the coil must be less than 2.SmH.
AD. DC POWER SUPPLIES.
It will be clear that the system is dependent on the exi.~tence of DC power supplies fxom which a current in the order of 30 amps ( 10 amps per actuator) may be drawn for a period in the order of 0.1 seconds.
It is anticipated that such a power ~upply ~an be constructed from a bridge-rectified main~ p~wer supply source which supports a re~hargeable battery acting ~s an ~nergy reser~oir. : ;
The advantage o~ this techni~ue is that a power supply reserve is~ retalned in the accumulators whi~h will allow the predetermined emergency procedures ( for example~ levelling and lowering the capsule) to be carried out with ade~uate power reserve~
AE. AC~PO~E~ SUPPLI~5~(MAINS)~
.~ Triac-con~rolled coil systems are a simple an~
low-cost means of controlling high currents in actuator coil assemblies.~The disadvantage of a: 50Hz or 60Hæ
mains supply is~the relative slowness of action in turning of~ an energized coil, of whatever inductance~
. .
This can be compen~ated-for by applying power in the opposite sense to an adjacent coil but clearly this is rather wasteful of power.
.

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W093/0t~6 , 2113 3 ~ 4 PCTtGB92t0127i ~ 31 ~ t t The disadvantages of a mains frequency alternating current supply are probably acceptable for all ~ut the most exacting applications~
AF. AC POWER SUPPLIES (INVERTER).
An Ac mains supply of any frequency can bê
generated by a technique which interposes a high capacity reserve of ~attery power between the mains and the machine. It is this reserve of battery power which can ~e used to control the actuators under mains failure conditions for the periQd of time which is ~ecessary to carry out the safety procedures.
The use of an inverter frequency which is ~u~stantially greater than that of the mains frequency allows tighter control of the coil currents, dependent up~n the design of the ~oil system such that the se~ments possess a low in~ucta~ce under all circuit ~on~itions.
AG. FORCE FEE~BACK.
: We have discussed the effect of finite sized coil segme~ts in producing a thrust whi~h, were the ~urrent~in the~coils not~to be varied, would contain a ~ignificant force ripple.
~ hilst a pre-calculated variation of current can be controlled:by the~microprocessor element having an ac~urate~position feedback from~a position transducer element~within the:system, there~will al50 be variations hetween one magnet~:seg~ent;and-another and minor variations:of flux path~sy~metry:which:will produce small variations in the ~or~e generated by the piston as it travels down th~ cylinder.- ; ;
: Finally~:as has been explained in earlier d~cuments,-the thermodynamic ~ariations in the gas ~spring:beneath~the plston-(when-~present),~1pon which the electromagnetic forces are superimposed, will also cause transient variations in the net thrust from the actuator.

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tThe temperature of the gas will change a~ a result of the adiabatic volume change caused by a sudden movement o~ the piston. There will ~e a short time delay before the heat flow restores the original gas temperature, which will vary the portion of the thrust due to the air pressure on the piston. This will re~uire a continuous change in electromagnetic thrust to compensate)~
A force feedback element l90,is therefore interposed between the piston and the piston shaft and the signals from this force sensing element are fed into the control system as a final variance parameter, as shown in Figure 19.
The force sensing element might be a pressure sensitive resistor, a systém of strain gauges or any other element which produces an output which varies with th~ ~orce applied in a positive ox negative sense.
AH. ACCELERATION FEEDBACK.
In that the purpo~e of the act~ator is to induce accelerations in a load, the precise ~alue of that load not-being pre-determinedt it is necessary to incorporate within a co~trol loop an acceleration:~feed back transducer 192. The transducer(s) may be mounted as a part of the load (for example,:in the case of a simula$or mechanism,: they may ~e mounted within the capsule~. Or, in the-~alternative, they may be incorpor?te~withIn:~the actuator itself. Fox example, the accelerometer may be mounted in~the pisto~ element in the~same~-way as we have previously described for the for e sensing *ransducer. See Fi~ure 20.
- W~en.the accelerometer transducer is used it is no~}onger necessary to incorporate a force sensing resistor ~ecausé,~acceleration being proportional to the fo~ce, the sig~al~from~the~accelerometer also contains the necessary information on instankaneous force. This may be, used in a feed back system to control the current in the actuator coils to take account of all SUBSTITUTE SHEET

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those factors which cause force variation and which we have discussed in earlier sections.
THRUSTRODS.
We have earlier explained our reasons for placing the air gap contaîning the current carrying coils on one side only of the magnetic circuit. ~hat is to say, in a moving coil system the magnets may be arranged on the inside of the outer steel cylinder and the piston may move along the central line of the cylinder inside it. For an e~uivalent moving magnet system the electrical coils are arranged along the inside of the outer steel cylinder and th~ magnet assembly ( "piston" ) moves along the axis of the cylinder. In either of these two arrangements it is clear that the thrust rod is fixed to the mov.ing element or piston ~nd protrude~ through the end of the cylinder via a slide bearing in exactly the same way as a conventional pneumatic or hydraulic ram.
But it is possi~le to conceive of a moving-coil system which is carried on a sleeve over a fixed central rod within a steel ~ylinder lined with radial-field magnets. Such an arrangement results in a piston of minimum weight. In this case the central rod may be constructed to incorporate magnetic elements matched to the magnetic elements of the outex cylin8er so that the coil moves in the air gap between two permanent magnetic materials.. In the:alternati~e, the inner rod may simply be of iron and the coil then moves as a cylinder sliding o~ that rod within an outer cylinder. ~
The problem, of course, is that if the central steel~rod is asswmed to be fixed to ~losing discs at both end~ of the eylinder it is di~ficult to bring-the thrust out of the system.
~ We conceive of:the moving sleeve being connected to the sutside world through a number (say 3) of rods 210 which pierce the disc closure 212 at one end of the cylinder through separate gland seals for each SU8STITlJTE SHÇ:ET

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rod, as shown in Figure 21. A similar axrangement would be necessary for a moving magnet sleeve which would move between coils on the inside of the outer steel case and on the outside of the inner steel rod~
AK. ELECTR:l:CAI. CONNECTIONS.
Electrical connections to the coil segments of a moving magnet system are relatively straightforward, since the coils form part of the stator, which is the extérnal element. If solid state switching is employed, a set of switching transistors and a triac element may be mounted adjacent to each coil section or as a bank in a separate controller to which coil connections are car.ried a~ twisted pairs.
~ igure 14 illustrates an equivalent system in which the commutation is mechanîcal, using brushes bearing on a commutat~r strip. In this case although the coils are stationary, the power to the brushes must be fed to a moving piston.
For a moving coil syst~m, the control elements of Figuxe 17 may ~e mount~d on the mo~ing piston asse~bly, t~ which the power supply is c~rried as a 3-wire system in the same way as in Figure 14. In the alternative, the control elements are mounted external to the actuator.assembly where they can ~e maintained in a ~ooler environment and the coil connections are brought out as twisted pairs.
~ The problem~:which is:~ommon to the~e systems is that the cable must ~e very flexible, and it must be controlled to retract into à small space at one end of the piston cylinder. : :
The cable m~y be of the ribbon type, as used in printers and~plotter units, spring loaded so as to retract into a folded~assembly. The assembly of folded ca~l~ may be housed within a cavity on the face of the piston unit or in a cavity at the base end of the actuator cylinder, as shown in Figure 2~. To facilitate the use of such ribbon cable the design of the coil SUBSrITUTE SHE~ET

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windings should ~e arranged to have a current rating of less than one amp. and a peak voltage requirement of less than l~O volts.
ASSEMBLY. AL.
We have described a moving-magnet system in which the stator compxises an outer steel cylinder lined with a series of copper wire coil sections, which must, of course, ~e wound on an inner liner a~d then slipped into the steel cylinder~ (During the winding process the thin liner is fitted over a rigid cylinder or shaft to resist the compressive stress s of the necessary winding tension).
The movi~g (piston) assembly of magnets and thru~t rod may then be inserted into the liner and the end pie~e or pieces added to complete the unit.
However, in the case of a moving-coil system the stator comprises an outer steel cylinder lined with a series of rings of magnetic ~aterial, magnetized in opposite directions alternately~ It will.be clear that, were the magnets to be assembled on the outsid~ of a ~non-ma~n~tic liner (temporarily fitted over a steel cylinder or:rod to retain the magnets:i~ position) it would not actually be possible to insert the magnet array into~the outer cylinder because of the clamping e~fect of the magnets.: - ~
~ One practical method ~f as:sembly is shown in Fi~ure:~3~ ere;the~outer cylinder:is split into two hal~es and the~magnet segments are~placed as "tiles" on the-iexposed inner surface~bf the cy;linder, with suitable aluminium or plastic spacing elements. The moving-coil piston unit is then inserted into~:a ~thln non-magnetic sleeve which;is placed in:one asse~bled half of the statDr whilst the other hal~ is aligned and ~rought toget~er:to complete the as~embly.
~ t should be noted that there will be a force betrJeen the two stator pieces in an axial direction until the assembly is completed, owing to the tendency SUBSTITUTE SHEET

WO93/01646 ; ~ ,; PCT/GB92/01277 2113344 - 36 - ' of ~he magnet half rings to seek alignment with half rings of opposite pol~rity.) This method of assem~ly is particularly convenient for the use of moulded ferrite sheet material which may be formed as flat sheet "striped" with magnets of alternate polarity. The sheets are simply laid into the steel half cylinders, into one of which the lining tube and piston assembly is placed before the two halves are brought together.
An alternative method of assembly for long stator tubes is to construct them as modular assemblies of length equal to two magnet rings and two spacers, perhaps with a temporary inner steel ring "ke~per ".
Since the modules have no unbalanced magnetic fields they can be stored, transported and brought together as necessary to form stator assemblies into which the airtight liner and plston assem~ly may then be inserted.
The innovati~e features which we have described in this document axe:-The two stage topologi~al transformation ofelectric motor systems to cylindrical actuators~
The design of electromagnetic actuators having no external fields.
The use of disc or plate magnets to produce pist~n assemblies. ~ -The use of radially-magnetized segments to produce piston assemblies.
The use ~f radially-magne~ized segments to produce stator sleeves in which a moving coil assembly moves as a pistonO -~-The design of rectangular tube thrustassemblies using plate magnets.
The design of solid state commutated coils of finite width, such ~hat nx=y+21.

SU~STITUTE SHEET

W093/01~6 2113 3 4 4 PCT/GBg2/01277 ,, ~ .
-- 37 -- i :

The use of current control in finite width commutated segments to linearize actuator thrust.
The use of force sensing elements to compensate ~or unpredictable and/or fast acting variabl~
transient changes in actuator thrust.
The use of accelerometer feedback to control the current in actuator coils, 50 as to smoothe thrust variations and to compensate for actuator attitude.
The design of a universal (AC or DC) electromagnetic actuator system.
The design of linear actuator~ having travelling waves in both stator and piston, the phase xelationship between the waves determining the force acting on the piston.
The use of a phase control to lock the piston at a node position in such a travelling wave system.
The~design of inductively-coupled cylindrical piston travelling wave actuator systems.
The use of servo-assisted inductively-coupled piston actuators for increas~d thrust at low relative speeds.
The use of phase control to modify th~ force ana direction of the thrust in an alternating current bi- or tri-phase~actuator system.
The comhination of Hall-effect commutator switching and force-feedback thrust control.
The use~ of a~microprocessor and a position transducer to produce a smooth actuator output thrust by predict~d current variation.
The ~o~ination of position transducer pre~etermined curreDt variation and dynamic feedback from a force-sensing or acceleration-sensing transducer.
The use of triac switching elements to control the amplltude and direction of current in commutated coi 1 elements .

SUBSTITUTE SHE T

WO93/01~6 ~ , PCT/GB92~01Z77 2113344 - 38 - ' A technique of assembly usiny a cylindrical outer housing split along its length into two or more pieces to provide easy access to internal areas.
A technique of assembly using a cylindrical outer housing cut into shorter sections ox modules and abutted after partial assemhly.
The use of moulded Ferrite material to produce a single-piece a~sembly of magnets which may be laid in (each half of) a split steel cylinder skator assem~ly.
The use of moulded Ferrite material to produce a single-piece assembly of magnets which may be laid upon (each half of) a steel~cylinder piston asse~bly.
The construction of actuator coils, directly or as preformed and stacked assemblies, on ribbed (ringed) steel cylinders to reduce the magnetic reluctance and to couple the reaction forces to the thxust components.

,, .`, : .

SUBSrlTUTF SHEE~T

Claims (22)

CLAIMS:

1. An electromagnetic piston and cylinder device comprising a cylinder assembly including an elongate cylinder of magnetic material and means for producing a cylinder magnetic field pattern comprising a series of alternate radially directed magnetic poles along the length of the cylinder, a piston member mounted within the cylinder assembly and having means for creating a radial magnetic field pattern for cooperating with the cylinder magnetic field pattern such that the magnetic fields are contained within the cylinder, a means for modulating the strength and polarity of at least one of the cylinder or piston magnetic field patterns whereby to cause relative linear thrust between the cylinder assembly and piston member, characterised in that the means for producing the cylinder magnetic field pattern comprises means for generating radial magnetic fields within the interior of the cylinder, in that the piston member is shorter in axial length than the cylinder, in that the cylinder is closed at both ends by end members with the piston member being provided with a rod extending through one of the end members, and in that the interior of the cylinder communicates with a fluid reservoir.

2. A piston and cylinder device according to claim 1 wherein the cylinder radial fields are produced by permanent magnets.

3. A piston and cylinder device according to claim 1, wherein the cylinder radial fields are provided by current carrying coils.

4. A piston and cylinder device according to claim 3, wherein the piston radial field is provided by one or more permanent magnets.

5. A piston and cylinder device according to claim 2 or 3, wherein the piston radial field is provided by a plurality of current carrying coils.

6. A piston and cylinder device according to claim 5, wherein the cylinder is of circular cross-sectional shape.

7. A piston and cylinder device according to claim 6, wherein the cylinder is of non-circular cross-sectional shape.

8. A piston and cylinder device according to claim 7, wherein the fluid is a gas.

9. A piston and cylinder device according to any one of the preceding claims, wherein the piston is provided with one or more transducers for influencing the condition of the control means according to the movement of the piston.

10. A piston and cylinder device according to claim 9, wherein one of the transducers is an accelerometer.

11. A piston and cylinder device according to claim 10, wherein one of the transducers is a force transducer located between the piston and actuator rod.

12. A piston and cylinder device according to claim 11 wherein one of the transducers is a position sensing device.

13. An electromagnetic piston and cylinder device comprising a cylinder assembly including an elongate cylinder of magnetic material and means for producing a cylinder magnetic field pattern comprising a series of alternate radially directed magnetic poles along the length of the cylinder, a piston member mounted within the cylinder assembly and having means for creating a radial magnetic field pattern for cooperating with the cylinder magnetic field pattern such that the magnetic field are contained within the cylinder, a means for modulating the strength and polarity of at least one of the cylinder or piston magnetic field patterns whereby to cause relative linear thrust between the cylinder assembly and piston member, characterised in that the means for producing the cylinder magnetic field pattern comprises means for generating radial magnetic fields within the interior of the cylinder, and in that the means for producing the cylinder magnetic field pattern comprises means for generating radial magnetic fields within the interior of the cylinder, in that the piston member is shorter in the axial length that the cylinder, and in that the piston field pattern creating means comprise radially magnetised permanent magnets axially spaced from each other to create a central pole of one direction of magnetisation and two further poles spaced apart on either side of the central pole and having the opposite direction of magnetisation.

14. A piston and cylinder device according to claim 13, wherein the cylinder radial fields are provided by current carrying coils.

15. A piston and cylinder device according to claim 13, wherein the cylinder is of circular cross-sectional shape.

16. A piston and cylinder device according to claim 13, wherein the cylinder is of non-circular cross-sectional shape.

17. A piston and cylinder device according to any one of the preceding claims, wherein the cylinder is sealed and the interior of the cylinder communicates with a fluid reservoir, (whereby to operate a combined spring and actuator).

18. A piston and cylinder device according claim 17, wherein the fluid is a gas.

19. A piston and cylinder device according to any one of the preceding claims, wherein the piston is provided with one or more transducers for influencing the condition of the control means according to the piston.

20. A piston and cylinder device according to claim 19, wherein one of the transducers is an accelerometer.

21. A piston and cylinder device according to claim 19, wherein one of the transducers is a force transducer located between the piston and actuator rod.

22. A piston and cylinder device according to claim 19 wherein one of the transducers is a position.
sensing device.