WO1997046834A1

WO1997046834A1 - Fail-safe networked control system for ventilation units

Info

Publication number: WO1997046834A1
Application number: PCT/CA1997/000373
Authority: WO
Inventors: Francois Routhier
Original assignee: Monitrol Inc.
Priority date: 1996-05-31
Filing date: 1997-05-29
Publication date: 1997-12-11
Also published as: AU2884097A; CA2177941A1; EP0906547A1

Abstract

The ventilation system comprises a master controller (10) connected by communication links to slave controllers (12, 14, 16, 18) that regulate the operation of individual fan units (20, 22, 24, 26). The master controller (10) transmits to the slave controllers (12, 14, 16, 18) temperature or rotational speed set points calculated to maintain within the premises desired environmental conditions. At least one slave controller (12, 14, 16, 18) is provided with a temperature sensor (72) allowing the slave controller to modulate the speed of the fan according to the local temperature. This feature is useful in situations when the master controller (10) is no longer capable of sending valid set point data to the slave controller (12, 14, 16, 18), either due to a malfunction of the master controller (10) or because the communication link is interrupted. The local temperature sensor (72) can also be used as backup to the main temperature sensor (34) of the master controller (10). Should the main temperature sensor (34) fail, the master controller reads the temperature remotely, over the communication link and this data is used to continue generating set points data that is transmitted to the slave controllers.

Description

TITLE: FAIL-SAFE NETWORKED CONTROL SYSTEM FOR VENTILATION UNITS.

FIELD OF THE INVENTION

The present invention relates to air conditioning control ana, -nore particularly, to a networked ventilation on rcl system sufficiently rooust to maintain πnimai environmental conditions m an enclosure upon occurrence of a nardware failure or software problem that renders the network inoperative. The ventilation control system _s particularly useful for intensive livestock farms were adequate supply of fresn air is critical to avoiα massive livestock αestruction.

BACKGROUND OF THE INVENTION

Intensive livestock farms designed for the breeding and production of poultry, pork, veal and other animals intended for human consumption have very stringent air quality requirements. The population density in those farms is such that fresh a r must be continuously supplied to evacuate noxious gases such as ammonia and carbon dioxide. The supply cf fresh air also reduces the humidity levels ana lowers the temperature of the enclosure. Failure to maintain adequate environmental conditions, even for snort perioαs, may resu.t in massive livestock destruction. The danger is particularly severe during the summer period, when the amoient temperatures are high. If the ventilation system cf the farm ceases functiomnα the temperature and the concentration of noxious gases increases rapidly to reach fatal concentration levels.

One possible solution to this proolem s to install high capacity ventilation systems that continuously supply fresh air to the livestock. Althougn this approach is satisfactory for the summer period, when temperatures are hign, it creates a proolem αurmς winter time. Indeed, wnen the outside temperature is below the freezing point, the amount of fresh air introduced m the enclosure must be carefully controlled to avoid lowering the internal temperature to the point where the animals may suffer from hypothermia and die. At the same time a minimal amount of fresh air must still be supplied to flush away noxious gases. Excessive cooling of the farm by massive mtroαuction of fresn air can be controlled by heating the enclosure, however, this is undesirable oecause it may increase the production costs. Occasionally, some neating will be required, particularly when the outside temperature is very low, however, it is desirable to maintain the heat supply as low as possible for obvious economic reasons.

Against this backgrounα it appears that the operation cf the ventilation system must be modulated accorαmg to the ambient temperature to satisfy the requirement of adequate air supply, while avoiding overcoolmg of the enclosure. Currently available ventilation systems use individual fan units whose speed can be mαividuaily adjusted by the operator. This approacn, nowever, is not very practical because the typical installation comprises a plurality of fan units, say 10 to 16, and adjusting them individually is a time- consuming task. Considering that such ad ustment may need to be performed several times during the day to compensate for external temperature variations this is evidently not the most efficient approach. - A possible solution to this problem s to provide a central controller connected to the individual fan units by dedicated communication links. An example of such system is described in the European patent application publication No. G545499 filed in the name of INDOLEC B.V. This prior art reference describes a ventilation control system designed for use m agricultural applications. The ventilation control system mcluαes a -aster controller connected v a communication links to slave controllers that regulate the operation of the individual fan units.

The slave controllers are individually addressable to enable the master controller to transmit data to each slave unit cf the group. The master controller can thus set eacn fan to rotate at a predetermir.eα speed, which, determines the quantity of fresn a r introduced m the enclosure or extracted from the enclosure. More specifically, the speed setting proceαure consists of impressing on the communication link a variable frequency signal, the frequency being indicative of the speed of the fan. When the addressed slave controller receives that signal, the data is stored in a local register. The slave controller continuously observes the speed of rotation of the fan and compares it with the set point. If a αeviation is noted, the slave controller aαjusts the voltage impresseα across the excitation winαing of the electric motor A tne fan to maintain the actual fan speed as close as possible to the set point.

In situations when the communication links are mterr pteα, ceca se cf hardware failure or a software program m the -aster controller, the mαividual fan units are αesigr.eα to continue operating based on the last speeα set point. Theoretically, this approach donates to the system a fail-safe capability by maintaining the operation of the fan units even when the network crashes. In reality, however, the system may not always be aυle to provide sufficient supply cf fresh air to avoid massive αestruction of livestoo: in such conditions. If the network crash occurs summertime during the night wnen the temperature is relatively low and the fan units are set to low speed to avoid over cooling the enclosure and the failure condition is undetected during the day, the rate of fresh air supply may become insufficient particularly if the temperature rises significantly during daytime. OBJECTS -A D STATEMENT OF THE INVENTION An obj ect of the invention s to provide a ventilation system with averts the drawoao:s c f the prior art devices .

As embodied and broadly described r.erein the invention provides a ventilation control system, comprising: - a master controller; - at least one slave controller, said slave controller including a temperature sensor generating an output signal representative of a temperature in a vicinity of said sensor; a communication link between said master controller and said slave controller allowing said master controller ana said slave controller to exchange data; - said slave controller including control means for varying a rotational speed of a fan unit associated with said slave controller at least partially in relation to said output signal.

In a most preferred embodiment, the ventilation system in accordance with the invention includes a master controller and a plurality of slave controllers connected to one another over the communication link. The master controller generates control parameters that are transmitted to the individual slave controllers so they can regulate the operation of the associated fan units at least in partial dependence upon the control parameters. Each slave controller can adopt several modes of operation. In one specific mode the master controller issues a temperature set point cn the basis of which the slave controller will regulate the speed of the fan. The regulation also takes into account the output signal generated from the local temperature sensor. On the basis of the temperature difference between the set point and the actual temperature value the slave controller will adjust the speed of the fan as to reduce the error between the two values. In a different mode cf operation the master controller issues a rotational speed set point enforced locally by the slave controller. More specifically, the slave controller measures the actual speed of the fan and regulates the voltage across the excitation winding to adjust for errors.

If there is a malfunction, the slave controller can still regulate the operation of the fan based on the output of the local temperature sensor. Th s provides the slave controller with the ability to react to local temperature changes and adjust the speed cf the fan accordingly.

As e boαied and broadly described herein the invention further provides a ventilation control system, comprising: - a master controller; - at least one slave controller, said slave controller including a temperature sensor generataing an output signal representative of a temperature in a vicinity of said sensor; - a communication link between said master controller ana said slave controller allowing said master controller and said slave controller to exchange data; - said master controller including means for generating at least one control parameter that is transmitted to said slave controller over said communication link; - said slave controller including control means for varying a rotational speed of a fan unit associated with said slave controller at least partially m relation to said control parameter; and - said slave controller being capable of transmitting to said master controller data related to said output signal and representative of a temperature in a vicinity of said temperature sensor; and - said master controller including means for processing said αata related to said output signal and for generating a new control parameter for transmission to said slave controller.

Under this embodiment the master controller is preferably designed to read the temperature remotely as measured by the temperature sensor of the slave controller, thus it can continue to issue valid control parameters shoui the temperature sensor cf the master controller fails. The remote reading operation is effected by transmitting a command to the slave controller to put on the communication link the value in the register that contains the current output of the temperature sensor. Such reading can be affected a number of times within a predetermined time period, this allowing the master controller to follow the evolution of the temperature over time aαequately. In a somewhat different embodiment, the master controller can reaα the temperature from several slave controllers. The received data can then ce averaged out or otherwise mathematically processed to provide a more accurate indication of the temperature in the enclosure.

BRIEF DESCRIPTION OF THE DRAWINGS - Figure 1 s a block diagram of a ventilation system in accorαance with the present invention; - Figure Z s block diagram of a master controller of the ventilation system snown in Figure 1; - Figure 3 is a block diagram of a slave controller of the ventilation system shown m Figure 1; - Figure - is a diagram illustrating the methcα of fan speeα control impiementeα by the slave controller; -Figure 5 is a diagram showing the strategy enforced by the slave controller for regulating the speed of the fan on the basis of local temperature if there is a malfunction of the master controller; and - Figure 6 is diagram showing the strategy enforced by the slave controller for regulating the speed of the fan under normal operating conditions. DESCRIPTION OF A PROFFERED EMBODIMENT Figure I of the annexed drawings illustrates a fail- safe ventilation system particularly well suited for use in intensive livestock f rms. The ventilation system features the capability of maintaining an adequate supply of fresh air when a malfunction of one or more of its components occurs, such as an interruption of the communication Iιnκ, a harαware failure of the master controller or a software problem in the master controller.

The ventilation system includes a control section comprising a master controller 10 connected to a plurality cf slave controllers 12, 14, 16 and 18 by a communication link 19. In the example shown, four slave controllers are useα. This is only a question of design as the memoer of slave controllers may vary without departing from the spirit of the invention. Each slave controller regulates the operation of a fan unit that includes an electric motor driving a mechanical device for producing currents in order to circulate, exhaust, or deliver large volumes of air. Such mechanical device may be in the form of a rotating paddle wheel, an air screw, with or without a casing or a positive displacement pump including meshing lobes rotating inside a casing to transport volumes of air from the inlet to the outlet. The rate at which a fan unit displaces air is dependent upon the speed of rotation of the electric motor. The slave controller can thus regulate the operation of the fan unit by varying the speed at which tne electric motor turns. This 10 achieveα by controlling tne voltage impresseα across the excitation winding of the electric motor. Usually, the higher the voltage the faster the motor will rotate.

The function of the master controller 10 is to enforce a global ventilation strategy cy transmitting to the individual slave controllers operational set points, either m terms cf rotational speeα or local temperature. In turn, the slave controllers enforce those set points in order to implement the global strategy.

The master controller 10 includes a CPU 28 connected to a memory 30 through a bus 32. The memory 30 contains the instructions of the program executed by the CPU. It also has a reserved portion for storing data such as variables or parameters that the program calculates during its execution. Most preferably the memory 30 is non volatile to avoid loss of data when the power is turned off. This avoids the requirement of reloading the program every time the unit is energized and also allows to retain the current status of all variables and parameters m the event of a sudden power loss. Thus, when the master oontrciier is re-energized it can continue operating m the same state at which the power loss occurred.

The CPU 23 can read the outside temperature by observing the output of a temperature sensor 34. The sensor 34 generates an analog output supplied to an analog/digital converter 36 that, n turn, feeds the digitized information to a register 33. At predetermined points in time, established by the program executed by the CPU 28, the latter reads the information stored in the register 38. On the basis of this information, the CPU 28 calculates temperature or rotational speed set points that are successively transmitted to the individual slave controllers. The set points data is written into a register 40 before being sent in serial 1 form over the communicaticn link 19. That same register

2 s also used for storing bytes of information sent from

3 the slave controllers so they can be read by the CPU 28.

4 A master controller based on a micro controller

5 manufactured by MICROCHIP under the product number

6 PIC16C73 has been found satisfactory. 7

8 The structure of the slave controller 20 is

9 illustrated in figure 3. The controller comprises a CPU 0 42 connected to a non volatile memory 44 through a bus 1 46. A number cf interfaces are connected to the bus 46 to 2 allow the CPU 42 to exchange data with external devices. 3 External block 48 designates an AC current power supply 4 that, typically, would be the main power transmission

15 line in the enclosure that also provides electrical

16 energy to the motor of the associated ran unit. The

17 sinusoidal waveform is directed to an analog/digital

18 converter 50 that digitizes the incoming data and loads

19 a register 52 that is periodically observed by the CPU 42

20 to allow the CPU to follow the evolution of the line

21 voltage with respect to time. This feature is used to

22 enable the CPU to detect the zero crossing points. This 23. information gives the CPU a reference point to calculate the firing angle of a bilateral gate controlled rectifier (Triac) used for regulating the voltage impressed across the excitation winding of the electric motor. This characteristic will be described in greater detail later. The line voltage is also applied to a power supply unit 54 that steps the voltage down and rectifies it to provide a stable source of DC power sufficient to energize the electronic components of the slave controller.

The clock 56 represents the bilateral gate controlleα rectifier used for regulating the excitation voltage, hence the speed of the fan unit. To achieve the highest possible speed the maximum voltage is applied to the excitation winding. This is accomplished by sending to the gate controlled rectifier a pulse that coincides with the zero crossing of the line voltage. The CPU 42 issues a pulse by loading a register 58 with a certain value that is then passed to a digital/analog converter, transforming the digital value into an analog pulse directed to the bilateral gate controlled rectifier. The excitation voltage can be reduced by delaying the actuation puise by a predetermined angle. Thus, instead of issuing a pulse that almost coincides with the zero crossing, the pulse is delayed by an angle that depends upon the desired fan speed. In general, the higher the angle the lower the excitation voltage would be which m turn commands a lower speed. This feature is best shown at figure 4. The value of the delay angle is adjusted continuously cy the CPU on the basis data supplied by sensor 60 that s indicative cf the rotational speed of the electric motor.

Most preferably, the speed sensor 60 is a Hall- effect device including one or more magnets secured to the rotor of the electric motor. A Hail-effect sensor is mounted to the casing cf the electric motor and it is periodically swept by the magnetic field generated by the magnets. In response to each curst of magnetic energy observed by the Hall-effect sensor, the latter issues a pulse that is received by a counter 62. The counter 62 totalises the number pulses over a predetermined period of time say 500 milliseconds. At the expiration of this period the total number of pulses is loaded into a register 64 where it can be read by the CPU 42. On the basis of the rotational speed data, the CPU 42 can correct the αelay angle of the firing pulse applied to the bilateral gate controlled rectifier m order to adjust for any difference oetween the actual speed and the programmeα speed. This procedure could be implemented by comparing the actual speed of rotation with the speed set point to αerive an error value. The magnitude of the error value rnen determines how much correction to be made cn the f πnα angle delay. This feeαbac loop is particularly iseful as it allows to maintain the speed constant, even when strong winds are present that generate currents Having the effect of either accelerating or decelerating the fan. Thus, m the absence of sucn feedback loop the actual speeα of the fan may vary significantly with relation to the desired speed, αependmg upon the prevailing wind conditions.

A temperature measurement device, such as a thermistor, represented by the blocκ 66 is mounted to the casing of the electric motor to sense any overneatmg of the motor. The output of the thermistor 66 is digitized by the analog/digital converter 68 and the resulting value is loaded in register 70 wnere it can be read by the CPU 42. If the electric motor overheats, the CPU 42 is programmed to take action primary to avoid permanent damage to the electric motor. Such action depends upon the particular strategy to be enforced. For instance the operation of the motor could be interrupted until it has cooled down sufficiently so its operation can be resumed. .Another possibility is to reduce the speed of the motor so ventilation is maintained by the fan unit while overneatmg of the motor is controlled. In all instances, once the CPU 42 has senseα a condition of overheating, information is dispatched to the master controller 10 over the communication link allowing the master controller to take any appropriate action. Such action may be the implementation of a compensating strategy that increases the speed of the remaining fan units to maintain the overall supply of fresh air constant.

Temperature sensor 2 is provided to measure the ambient temperature in the vicinity of the fan unit. The output signal generated by the sensor 72 is digitized by analog/digital converter 74 and the resulting value written in the register 76 so that it can be accessed by the CPU 42. The temperature sensor 72 is an important input device as it enables the fan unit to modulate its speeα with relation to temperature even when the communication with the master controller 10 is lost. This feature will be discussed in more detail later.

Finally, the slave controller is provided with a register 78 m which is stored data transmitted over the communication link 19, so it can be accessed by the CPU 42. In addition, the register ^8 is used to receive αata from tne CFU 42 that s to be sent over the link 19.

A slave controller based on a micro controller manufactured by MICROCHIP under the product number PIC16C622 has been found satisfactory.

The communication protocol between the master controller 10 and the individual slave controllers is baseα on the current loop standard that is implemented by having the link consist of a loop that carries a current with a magnitude when it is in a mark (1) state i.e., logic level 1 is being signalled and does not carry current when it is in a space (0) state i.e., logic level 0 is being signalled. Data is exchanged between the master controller and the slave controllers as

i^«9 asynchronous bit packets. Each packet would typically include a start bit, 8 data bits followeα by one, one and a half, or two stop bits.

The slave controller can be installed either outside of tne electric motor or inside the casing thereof. An internal installation is preferred.

Before initiating the operation cf a particular fan unit the master controller transmits to the associateα slave controller a configuration data set that determines the mode of operation of the slave controller and also loaαs certain register with default values. Generally speaκιng, the proceαure for writing αata m a register of the slave controller is a two pass operation effected by transmitting twice a configuration data set. The first pass begins by senαmg the following data set:

TABLE 1

The start byte is a header that signals the addressed device the beginning of a data set. The following identi cation oyte is essentially an address that uniquely identifies the slave controller. An identification byte of 3 bits allows 256 different address combinations. The command byte signals the slave controller that data will be written into one of its registers. The address of the register to be loaded is provided by the address byte. The next byte is the data byte that is the actual information stored in the register. The configuration data set ends t; a checksum byte provided for data validation.

The followinσ registers of the slave controller can be configured through a configuration data set. Those registers are essentially memory locations in the physical memory 44 and they are different from the registers 52, 58, 64, 70, 76 ana ^~3 descnoea earlier.

TABLE 2

Register PO contains the speed set point of the fan. Typically, the number loaded by the master controller is expressed in revolutions per minute in steps of 20 rpm. For example a speed setting of 50 would mean 1000 rpm.

4 Register PI contains the temperature set point expressed 5 either m terms of degrees Celsius or Fahrenheit, 6 depending upon the value of a bit in the configuration

-7/ byte P3. The structure of the configuration byte F3 is as

8 follows : 9 TABLE 3

0 1 2 3

4

5 6 7 Bit No. 7 of the configuration byte P3 designates 8 the units of temperature in which the temperature set point is expressed, either Fahrenneit or Celsius. Bit 5 indicates whether the fan is allowed to attain its maximum speed determined by the structural capability of the electric motor, f this bit is 0, the speed is limited to the value stored in the register P7, otherwise full voltage can be applied across the excitation winding. Bit No. 3 and No. 2 determine the mode of operation of the slave controller m the event a malfunction _s determined to exist with the master controller or with the communication link 19. Such determination is made upon occurrence of either one of the following events:

A) current flowing is interrupted in the communication link 19 for a period exceeding the delay storeα in register ? 11. Th s means that no data packets are being sent by the master controller which can be the result of a hardware failure, such as the wiring m the communication link 19 being interrupted, or a software problem causing the master controller to hang up;

B) no valid communication has been received from

2.4 the master controller for a time perioα exceeding the delay stored in the register P 12. Each communication originating from the master controller s validated on the basis of the checksum πyte. If the checksum s incorrect, the slave controller will not accept the communication as a valid one. If this event continues over the period or time stored ir. P 12, the s_^ave oontroller will assume that communication is lost. Communication time out is likely to occur when the master controller hangs up due to a software problem.

In the default mode 00 the slave controller will rely cn the output of temperature sensor 72 to modulate the speed of the fan. This s accompi snec by caκιnσ into account the default temperature stored in register P6, the temperature αifferential stored in the register P 13 and the minimum fan speed stored in register P 14. Figure 5 of the annexed drawings illustrates this feature. The values stored in registers P6 and P6 + P13 define a range of temperature over which the speed of the fan is varied on the basis of the signal received from temperature sensor 72. Thus, wnen the local temperature corresponds to the value P6 + P13 the fan is rotating at the maximum speed. The relationship speeα/temperature over the range ? 13 is linear. It should be noted that the speed of the fan never goes below the value stored m register P14 even when the temperature reported by the sensor 72 drops below the value stored in P6.

If mode A is set then the fan will immeαiately stop when a conαition of malfunction is detected. In mode 10 the fan will continuously rotate at the αefault speed setting stored m register P 5. Mode 11 is a continuation of the normal operation under mode 00, described below. The slave controller will simply continue to enforce the last temperature setting received from the master controller.

In the normal conditions of operation, four different modes can be selected. In mode 00 the speed of the fan is a linear function of the temperature measured by the local sensor 72 over a range determined by the temperature set point stored in register P 1 and the temperature differential value stored m register P 13, taking also nto account the minimum fan speed m register P 14. This relationship is best shown in figure 6. It will be noted that the graph s similar to figure 5 with the exception that the base temperature value (P 1} is not fixed and fluctuates in accordance with the control strategy implemented by the master controller. Thus, the line segment that defines the relationship speeα/temperature moves norizontally along the temperature axis as the temperature set point m the register P 6 is changed by the master controller. Also note that the speed of the fan can never drop under the value stored in the register P 14.

Mode 01 is identical to mode 0C described above with the exception that when the temperature drops below the temperature set point m register P 1 the speed cf the fan will be controlled on the basis of the values stored in the registers P 8 and P 9. P 8 determines the duration of the operational cycle of the fan that includes an on segment during which the fan operates and an off segment during the fan is idle. The αuration of the off segment is determined by the value stored in P 9. Thus, if P 8 specifies a time period of say 5 minutes and P 9 (duty cycle) is of 50f a fan will operate over two and a half minutes and remain dle for two and a half minutes. The operating cycle then repeats itself indefinitely.

Mode 10 does not take m consideration the output of the temperature sensor 72 and operates the fan on the basis of the speed set point stored m register PO. The value storeα in the register P 0 ±s an initial speeα; that speeα is then progressively reduced on the basis of the reduction rate stored in the register P10. finally, during mode 11 the speed of the fan is held constant at the set point m register PO.

The specific procedure for setting up anyone of the registers P 0 to P 14 is to generate over the communication link 19 the con iguration αata set of Table 1, where the register address byte is the address of the register to be loaded and the data byte contains the actual data to be stored m that register. When the configuration data set has been correctly received by the slave controller, the latter responds to the master controller by sending back a number of parameters allowing the master controller to compare the response with the configuration data set to ensure that a valid transmission has taken place. Table 4 describes the response generated by the slave controller.

TABLE 4

Once the master controller has received the data set of Table 4, the first pass of the configuration of the particular register is considered complete. The second pass is essentially a duplication of the first pass with the exception that the header byte of the data set transmitted by the master controller is different to allow the slave controller to determine that a second writing pass is taking place. When the data has been validated by the slave controller, the value loaded in the register may be copied in a non-volatile portion of the memory so the data will not be lost m the event of a sudden power disruption. Thus, when the master controller is re-energized following a power disruption it will be able to resume its operation precisely at the point when the power loss occurred. The second pass writing operation is completed by the transition from the slave controller to the master controller of the data set in figure 4. If the data set is correctly received by the master controller, the latter assumes that the register configuration operation has been correctly implemented.

The slave controller contains a numoer of status registers in which is stored information relating to the current operative condition of the controller and also data generated by external sensors. The master controller can interrogate the slave controller to read the information in anyone of those registers. The information thus obtained can be used to compensate for malfunctions of the slave controller (for instance the master controller can increase the speeα cf the remaining fan units), or remotely reaα temperature, among others. Table 5 below describes the various status registers of the slave controller. Some of those registers correspond to the registers 52, 58, 64, 70, 76, or 78, while others are memory locations in the pnysical memory 44 which can be individually addressed by the CPU 42.

TABLE 5

Register VO corresponds to register 64 that contains the value of the actual speed of rotation of the fan. Register V I, on the other hanα contains the speed setting that has been generated by the master controller. For all practical purposes, register V 1 could be the same memory location has register PI. Register V2, corresponαs to register 66 ana contains a thermal flag that is raised when the temperature of the motor exceeds a preset value. This enables the srave controller to stop the operation of the motor so it cools αown. A number of possible strategies can be implemented, depending upon the degree of overheating. If the overheating is severe, the motor may be stopped altogether. In mild overneatiπσ conditions, the speed of the motor may oe reαuceα. The register V2 also allows the master controller to be aware of the overheating condition. This enables the master controller to implement a compensating strategy m order to maintain the supply cf fresh air m the enclosure substantially constant. This strategy may consist, for example, of increasing the speeα cf the remaining fan units. If desireα, the task of controlling the overneating condition may be left entirely with the master controller in which case the master controller could either command the fan to stop or reduce its speed by loading the appropriate speed setting in the register PO. It is preferred, however to provide the slave controller with a software capability to respond to an overheating event so the response is available even when the communication with the master controller is lost.

The register V3 contains a number of flags designed to signal malfunctions of external sensors. Bit number 7, indicates that the slave controller is operating in the default mode due to a communication time out or to an absence of current in the communication loop. Bit No. 5 signals malfunctions of the speed sensor. The occurrence of a malfunction is determined when the speeα varies more than 50% over 1/8 revolution. Finally, bit No. 4 signals malfunctions of the temperature sensor 72. A malfunction is determined wnen the temperature abruptly rises or drops by more than 10%.

Register 24 contains the temperature output signal from the sensor A.

To read the data stored m a given status register the master controller issues the following command that is represented by the data set of figure 6:

TABLE 6

To speed up the reading operation, the slave controller will place on the communication link 19 the data contained in the four consecutive status registers beginning at the αesignated address at the fourth byte of the communication data set. The response of the slave controller is provided in the Table 7 below:

TABLE 7

Allowing the master controller to consult the status registers of the slave controller is important because it provides certain security benefits such as when the temperature sensor associated with the master controller is malfunctioning. In those instances, the master controller can effect a remote temperature reading operation and continue to manage the entire system based on the temperature backup data supplied from the sensor 72. Typically, the remote temperature reading would be affected at regular intervals in order to follow the temperature evolution. In an installation where several slave controllers are provided with a local temperature sensor it could be envisaged to remotely read the temperature from several sources and average out the measurements to provide a more accurate indication of the temperature in the enclosure.

The above description of a preferred embodiment should not be interpreted in any limiting manner as many refinements and variations are possible without departing from the spirit of the invention. The scope of the invention is defined in the appenαeα claims and their equivalents.

30

Claims

I CLAIM: 1. A ventilation control system, comprising:

- a master controller;

at least one slave controller, said slave controller including a temperature sensor generataing an output signal representative of a temperature in a vicinity of said sensor;

- a communication link between said master controller and said slave controller allowing said master controller and said slave controller to exchange data;

- said slave controller including control means for varying a rotational speed of a fan unit associated with said slave controller at least partially in relation to said output signal. 2. A ventilation control system as defined in claim 1, comprising memory means capable of storing temperature set point data transmitted from said master controller. 3. A ventilation control system as defined in claim 2, wherein said control means is capable of varying the rotational speed of the fan unit associated with said slave controller at least partially in relation to said output signal and to said temperature set point. 4. A ventilation control system as defined in claim 3, wherein said memory means is capable of storing minimum rotational speed data transmitted from said master controller. 5. A ventilation control system as defined in claim 4, wherein said control means varies the rotational speed of the fan unit associated with said slave controller in a range beginning at said minimum rotational speed, wherein the speed of the fan unit is always above the minimum rotational speed. 6. A ventilation control system as defined in claim 1, wherein said slave controller includes memory means for storing rotational speed set point data and temperature set point data, said slave controller being capable of acquiring either one of a mode of operation in which said control means varies a rotational speed of the fan unit associated with said slave controller at least partially in relation to said output signal, and a mode of operation in which said control means varies a rotational speed of the fan unit associated with said slave controller at least partially in relation to said rotational speed set point. 7. A ventilation control system as defined in claim 1, wherein said slave controller includes memory means for storing rotational speed set point data and temperature set point data, said slave controller being capable of acquiring either one of a mode of operation in which said control means varies a rotational speed of the fan unit associated with said slave controller at least partially in relation to said output signal and said temperature set point data, and a mode of operation in which said control means varies a rotational speed of the fan unit associated with said slave controller at least partially in relation to said rotational speed set point. 8. A ventilation control system, comprising:

- a master controller;

- at least one slave controller, said slave controller including a temperature sensor generataing an output signal representative of a temperature in a vicinity of said sensor;

- said master controller including means for generating at least one control parameter that is transmitted to said slave controller over said communication link;

- said slave controller including control means for varying a rotational speed of a fan unit associated with said slave controller at least partially in relation to said control parameter; and

- said slave controller being capable of transmitting to said master controller data related to said output signal and representative of a temperature in a vicinity of said temperature sensor; and

- said master controller including means for processing said data related to said output signal and for generating a new control parameter for transmission to said slave controller. 9. A ventilation control system as defined in claim 8, wherein said control parameter is a temperature set point. 10. A ventilation control system as defined in claim 8, wherein said control parameter is a rotational speed set point.