US3796993A - Analog input device for data transmission systems - Google Patents

Abstract

An analog input device for data transmission systems wherein a digital address message from a central station to a plurality of remote terminals initiates sequential scanning of the analog parameter points at the addressed remote terminal. A sensor has a resistance which varies in accordance with the magnitude of its associated analog parameter point, and a time-shared differential sensing circuit measures the voltage drop across the variable resistance of the sensor while a pulse of a precise reference current passes from a time-shared trunk line through the sensor.

Description

KR 3079699 33 M o M sly I 1x} Umted States q Fox et al.

[ ANALOG INPUT DEVICE FOR DATA TRANSMISSION SYSTEMS R 151 Mar. 12,1974

OTHER PUBLICATIONS IBM Technical Disclosure Bulletin, Two-Amplifier Inventors! Duane Flfllefton; Richard System for Rejecting Common Mode Voltages, E. J.

lchinose, Placentla; Harold Lewis, Peterson, Vol. 6, N0. 7, Dec. 1963, pp. 85, 86, Villa Park, all of Calif. [73] Assignee: American Multiplex Systems Inc., Primary Examine'4 Dfald Anaheim, Calif: littoi'ney, Agent, or Fzrm-Harns, Kern, Wallen &

- ms ey [22] Filed: Oct. 4, 1971 [21] Appl. No.: 186,246 ABSTRACT An analog input device for data transmission systems [52] U S C 340/151 340/182 wherein a digital address message from a central sta- [51] Int Cl iiiiiiiiiiiiiiiiiiiiiii 5/22 & 4 /02 'tion to a plurality of remote terminals initiates sequen- [58] i "340/183 15() 182 tial scanning of the analog parameter points at the addressed remote terminal. A sensor has a resistance [56] References Cited which varies in accordance with the magnitude of its associated analog parameter point, and a time-shared UNITED STATES PATENTS differential sensing circuit measures the voltage drop 3,636,537 1/1972 Terry 340/182 across the variable resistance of the sensor while a 3 223 322 g f pulse of'a precise reference current passes from a rvm 3,479,645 11/1969 Singleton 340/150 shared trunk lme through the sensor 3,268,665 8/1966 Miller et al 340/183 X 7 Claims, 3 Drawing'Figures 12. aTl-lE/G v EEMOTES l PIPEC/S/O/V 2E ANALOG /NPU7' 45 No @EF'ERE/VCE pp c s a DEV/CE 10-. L Paws; ANAL-O6 cmeEE/vr CURRENT I supp;

INPUT +l 5008c: 36 {7 SIG/TW- am; I '70 xm-k 42 i 3515;??? d i l f a/vp (40c) M: 25 ANALOG 4 fl/VGE lA/PUT PEA-6,570 ONTROL -Li] SIG/VAL E w ic CU/PGENT J A REMOTE ANALOG INPUT DEVICE FOR DATA TRANSMISSION SYSTEMS Generally speaking, the invention relates to digital data transmission systems, and more particularly to an analog input device for sensing the value of an analog parameter point at a remote terminal by measuring the voltage drop across a variable resistance carrying a precision reference current.

The general advantages of multiplex digital data transmission systems are discussed in Reprint No. 949 from Control Engineering written by Richard L. Aronson and entitled Line-Sharing Systems for Plant Monitoring and Control and include the capability of monitoring thousands of parameter points through shared lines connecting each of the points with a central station, thereby substantially eliminating wire costs and cable congestion. However, much of the analog input circuitry at the remote terminals of prior art systems of this character are unduly expensive, complicated, and inefficient and are sometimes prone to inaccuracies.

Accordingly, it is a primary object of the present invention to take advantage of the operational benefits inherent in a multiplex system while at the same time providing a simplified, inexpensive, accurate, efficient, and flexible method and apparatus to achieve automated scanning and monitoring of analog inputs in heating and air conditioning systems, security systems, fire detection systems, process control systems, pollution control systems, medical monitoring, and the like.

The most recent prior art multiplex data transmission system employing analog input signals with which we are familiar is disclosed in patent application Ser. No. 176228, filed on Aug. 30, 1971, now US. Pat. No. 3,747,067, entitled METHOD AND APPARATUS FOR DATA TRANSMISSION, which application was assigned to the assignee of the present application. Accordingly, it is a primary object of the present invention to provide an analog input device adaptable for. use with the data transmission system of said patent application without having to modify the basic circuit design of remote terminals therein. A related aspect is to provide an analog input device which operates on the same voltage supplies used in the remote terminals.

Another object is to provide an analog input device which incorporates sensors having resistances which vary linearly with the change in magnitude of an associated parameter point, such as resistance temperature devices, potentiometers, and the like. A related object is to eliminate the disadvantages of a Wheatstone Bridge circuit which is traditionally used to measure variations in a resistor, and provide an improved differential sensing circuit for directly measuring without calibration a voltage drop across the variable resistance of the sensor.

A specific object of the invention is to provide an analog input device of the foregoing character wherein a precision current is carried through a single timeshared trunk line to the sensors, and the voltage drop across the sensors is measured by a single time-shared differential amplifier circuit, and wherein switching means simultaneously connects both the trunk line and differential amplifier circuit to a single sensor at a time.

Another specific object is to provide an analog input device of the foregoing character wherein a relatively large precision current is pulsed through each sensor during a relatively short duty cycle, thus virtually eliminating self-heating in the sensors.

A further specific object is to provide an analog input device of the foregoing character which includes in each sensor a two-wire line for carrying the precision current to the sensors and a separate two-wire line for communicating the voltage drop across the sensor to the differential amplifier circuit, and wherein both twowire lines on each sensor are bundled together in adjacent parallel positions within a shield, thereby making the measurement of voltage across the sensor substantially independent of common mode noise and voltage drops in the precision current lines.

An additional specific object is to provide a precision current source within an analog input device of the foregoing character, wherein a pair of matched resistors having their bases connected are activated by a common voltage to produce identical precision currents, one of which is monitored by comparison with a stable reference voltage circuit such that any variation of current magnitude in the monitored current produces a feedback voltage on the bases to compensate for and eliminate the variation.

Further purposes, objects, features, and advantages of the invention will be evident to those skilled in the art from the following description of the preferred embodiment of the invention.

In the drawings:

FIG. 1 is a block diagram of a presently preferred embodiment of an analog input device in combination with a remote terminal of a data transmission system;

FIG. 2 is a schematic circuit diagram for the analog input device portion of FIG. 1; and

FIG. 3 is a schematic circuit diagram for the precision current source of FIG. 1.

Generally speaking, the invention provides an analog input device 10 for sequentially sensing a plurality of analog parameter points at a remote terminal 12 of a data transmission system. Each parameter point has a sensor, such as 14 or 16, having a variable resistance which, under stable environmental conditions, changes in a direction and magnitude proportional to the parameter being monitored.

When a digital address message carried from a central station 18 to a plurality of remote terminals 12 identifies a specific remote terminal and certain parameter point(s) therein, control logic 20 initiates the scanning of the parameter point(s) by passing a pulse of current having a known precise value through a timeshared trunk line 22 to the sensors 14,16. A timeshared differential sensing circuit 24 measures the voltage drop across the variable resistance of the sensor as the precision current pulse passes therethrough. Thus, one, or some, or all of the analog parameter points can be sequentially scanned in any order at a remote terminal 12 by using only one trunk line 22 for carrying the precision reference current and only one differential sensing circuit 24 for measuring the voltage drop across the sensors l4, 16. Since each variable resistor 14, 16 is connected in parallel to the trunk line 22 and the differential sensing circuit 2 4, parameter points can be added or deleted without changing the basic circuit design of the analog input device 10. Moreover, the usual inaccuracies due to self-heating and cable voltage drop are virtually eliminated, and fluctuations in the value of the precision reference current are prevented, all as described in more detail hereinafter.

Although the analog input device is useful in any system as a separate element for sensing analog parameter variations by measuring the voltage drop across a variable resistance, the' invention was specifically developed for and is presently being used in combination with the previously identified digital multiplex data transmission system, as shown in FIGS. 1 and 2. Binary address messages are carried over one transmission line 26 from the central station 28 to the remote terminals 12. When a receiver'28-in the remote terminal 12 identifies its own address, a scanning signal is transmitted over a line 30 to a'multiplexer 32 which initiates scanning of one or more of the sixteen exemplary analog parameter points. When each point is scanned, the resulting analog sample passes through the differential sensing circuit 24 including a differential amplifier 34, and a range or scaling amplifier 36 which amplifies the analog signal over the broadest range possible for that particular parameter point, as determined by the range control 38 coupled between line 30 and the range amplifier 36. The output signal from the range amplifier 36 is converted to a digital word by an analog to digital converter 40 for transmission by remote transmitter 42 along line 44 back to the central station 28. A power supply 46 in the remote terminal 12 provides voltages such as volts and ilS volts along line 48 to the differential sensing circuit 24, the multiplexer 32, and also to a precision current source 50 which generates the precision reference current through the trunk line '22. Thus, all necessary power for the analog input device is provided by the existing power supply 46 of the remote terminal 12.

Referring now to FIG, 2, exemplary sensors Nos. 1, 2 and n, identified as 14, are platinum resistance temperature devices having an electrical resistance which varies linearly with the: change in temperature. Thus,.a decrease or increase in the temperatureof the surrounding medium being sensed causes a proportional decrease or increase in the resistance of the sensor. Ex-

. emplary sensor No. x, identified as 16, is a potentiometer which can be coupled to certain analog parameters being sensed such that the variation of the resistance of the sensor is proportional to and accurately reflects the variation in the parameter. Of course, the invention is not limited to any particular sensor or parameter, but is applicable to any analog parameter whose variations can be reflected as proportional changes of resistance in a sensor. The use of the symbols No. n and No. x

- indicate the expandability of thenumber of analog parameter points that can be sensed without changing'the basic circuit of the analog input device 10.

Each sensor 14, 16 is connected in series both'with a first two-wire line 52 attached at one end through a switch 54 to trunk line 22 and to ground at the other end, and with a second two-wire line 56 which is connected at each end through switches 58, 60 to the differential amplifier 34. Thus, since the differential amplifier 34 measures only the voltage drop across the sensors 14, 16 and is unaffected by the voltage drop in.

the first two-wire line 52, the sensors can be placed hundreds of feet away from the differential sensing circuit 24 and from each other without hampering the accuracy of the scanning. In this regard,'the differential amplifier 34 draws no substantial current through the two-wire line 56, thus avoiding any inaccuracy due to I a possible voltage drop in that line when it is also extended hundreds of feet. Moreover, by aligning all four wires of lines 52 and 56 parallel to each other within a shield, such as 61, most common noise is rejected and does not interfere with the accuracy of the scanning operation.

The multiplexer 32 through proper operation of its switches 54, 58, 60 by its control logic 20 causes only a single sensor ata time to simultaneously conduct current and have its voltage drop measured. In the preferred embodiment, a relatively high precision reference current of 10 milliamps is sufficient to obtain an accurate measurement of voltage drop across the sensor, while the duty cycle during which a pulse of current passes through the sensor is short enough to prevent any significant increase in the variable resistance due to self-heating of the sensor. Also, since the switches 54, 58, 60 remain open for, all sensors not currently being scanned, the precision current source 50, trunk line 22 and differential sensing circuit 24 can be time-shared, thereby reducing significantly the number of wires and circuit elements needed in a remote terminal 12 having a number of analog parameter points.

In addition to the differential amplifier 34, range amplifier 36, range control 38 and analog to digital converter 40, the differential sensing circuit 24 includes a reference voltage source 62 connected through line 64 to the precision current source 50 and also connected through line 66 to an offset bias generator 68 which is coupled to the range amplifier36. When the analog parameter point is at the bottom of its range, the offset bias generator 68 assures that there is no response signal produced by the range amplifier 36.

In order for the voltage measurement across the sensors 14, 16 to be accurate and thereforecorrectly reflect the instantaneous resistance ofthe sensors, the precision current source 50 has built-in safeguards to assure a constant current output on the trunk line 22. Referring now to the illustrated embodiment of the precision current source 50 in FIG. 3, a supply voltage 70 of 15 volts is applied across matching pnp'transistors 72, 74 to produce a precision reference current of 10 milliamps on the trunk line 22 and 'a monitored refer ence current of 10 milliamps .on a line 76 through a resistance 78 to ground; One ofthe matching resistors 80, 82in the respective transistor input lines is variable to assure identical current outputs in the initial condition.

A monitoring and feedback circuit is incorporated between the line 76 and a line 84 connecting the bases of the two transistors 72, 74 and includes a high gain comparator amplifier 86 which compares the .two voltages at 88, 90-and generates any necessary compensating output through a resistor 82 to the line 84. The voltage at point 88 is the same as Online 76 between the transistor 72 and the resistor 78 since point 88 is connected to line 76 through a large resistor 92 whichnormally conducts no current. Point 90 lies on'a line connected in one direction through resistor 94 to ground and in the other through resistor 96 to a stable reference voltage 98 of about 8 volts. In the exemplary embodiment, a current of 10 milliamps through a resistor 78 of 300'ohms causes point 88 to be 3 volts, and an.

8 volt reference voltage at 98 applied to a resistor 96 of 5K ohms and a resistor 94 of 3K ohmscauses point 90 to also be 3 volts. The comparative amplifier 86 is. activated by :15 volts'at 100, 102. It will therefore be 22, and also provides a feedback output to the base of transistor 74 to eliminate immediately and compensate for such variations.

In the illustrated form, a voltage of 5 volts at 104 is applied through two antilatch diodes 106,108 in series to line 84 to keep the feedback under control. Also, a clamp circuit of three diodes 110, 112, 114 in series connected from trunk line 22 to ground and side couplecl through resistor 116 to a vbltage 1 18 of 5 v olt s 7 serves to draw current from the transistor 74 when no current is being drawn through trunk line 22. The clamp circuit thus prevents the transistor 74 from becoming saturated and possibly developing characteristics dissimilar to its identical companion 72.

Although an exemplary embodiment of the invention has been disclosed herein for purposes of illustration, it will be understood that various changes, modification and substitutions may be incorporated in such embodiment without departing from the spirit of the invention as defined by the claims which follow.

We claim as our invention: 1. A method of obtaining analog input signals from a plurality of independent sensors each displaced an appreciable distance from each other and each having a resistance which varies according to the magnitude of its own associated analog parameter point including the steps of:

generating a precision direct current; and monitoring repeatedly the resistance value of each sensor over a substantial period of time to detect any variation of the magnitude of its associated parameter, said monitoring step including conducting a pulse of said precision current through said sensor during a first time period while simultaneously measuring the voltage differential between the beginning and end points of the sensor, and terminating and ceasing said conduction and measuring steps during a second time period subsequent to and substantially longer than said first time period to prevent the magnitude of the resistance from increasing due to self heating caused by the current passing through the sensor, and wherein said conducting and measuring steps are sequentially performed on each of the other sensors during the second time period without performing said conducting and measuring steps on more than one sensor at any given time. 2. The method of claim 1 wherein said monitoring step is performed on a plurality of separate temperature sensors each having a resistance which varies in direct proportion to its change of temperature.

3. A method of obtaining analog input signals from a plurality of sensors each having a resistance which varies according to the magnitude of an associated analog parameter point including the steps of: generating a precision current including applying a supply voltage to a pair of matched transistors having a common base voltage to produce two identical current outputs, monitoring one of the current outputs to detect any variation in current output, and changing the base voltage to compensate for and eliminate any variation in current output detected by said monitoring step; 7

conducting a pulse of the precision current through a sensor; and

measuring the voltage drop across the variable resistance of said sensor during said conducting step.

4. The method of claim 1 wherein said conducting step includes conducting a pulse of the precision direct current to and from each sensor through its own twowire transmission line;

5 said measuring step includes measuring the voltage drop across the variable resistance of each sensor through its own two-wire monitoring line separate from said two wire transmission lines and connected thereto only at the opposite ends respectively of such sensor;

and said monitoring step further including timesharing a single trunk line to carry the precision current sequentially to the two-wire transmission line of each sensor, and time-sharing a single differential amplifier circuit connected to the two-wire monitoring line of each sensor.

5. Apparatus for obtaining analog input signals from a plurality of independent sensors displaced apart from each other and each having a resistance which varies according to the magnitude of its own associated analog parameter point, including:

current source means for generating a precision direct current;

a trunk line connected to said current source means;

a plurality of separate current transmission lines each connected in series with a sensor and in parallel through its own respective current switch to said trunk line to carry a pulse of direct current through a particular sensor only when said current switch is closed;

differential amplifier means for measuring the voltage drop across each of said sensors;

a separate pair of detection lines for each sensor connected at one end through their own respective pair of detection switches to said differential amplifier means, and connected at the other end to said current transmission line immediately adjacent the ends of the sensor to measure only the voltage drop across the sensor itself while said pulse of direct current is passing therethrough;

shield means surrounding the transmission and detection lines for each sensor to prevent errors caused by extraneous voltages;

switching means connected to all of said transmission and detection switches for repeatedly monitoring the resistance value of each sensor over a substantial period of time by sequentially operating said switches to simultaneously connect said generating means and said differential amplifier means to only a single sensor at a time to allow a pulse of said precision current to pass through said sensor during a first short time interval while said differential 'amplifier means measures the voltage drop across the variable resistance of said sensor, and for disconnecting said sensor during a subsequent longer second time interval to prevent self-heating of the sensor due to current flow therethrough;

6. Apparatus for obtaining analog input signals from 0 a plurality of sensors each having a resistance which varies according to the magnitude of an associated analog parameter point, indludin g:

means for generating a precision current including a pair of matchedtransistors having a common base, monitoring circuit means connected to one of said transistors for detecting variations in the magnitude of precision current being generated, and feed-back circuit means coupling said monitoring circuit means to said common base of said matched transistors for eliminating any detected variations of said precision current;

separate transmission circuit means associated with each of said sensors, respectively, for connecting said generating means with each of said sensors;

differential amplifier means for measuring the voltage drop across said sensors;

separate detection circuit means associated with each of said sensors, respectively, for connecting said differential amplifier means with each of said sensors; and

switching means coupled to each of said transmission and detection circuit means for simultaneously connecting said generating means and said differential amplifiermeans to only a single sensor to allow a pulse of said precision current to pass through said sensor during a short time interval while said differential amplifier means measures the voltage drop across the variable resistance of said sensor. 7. A data transmission system for transmitting binary address messages from a central station to a plurality of includes:

a plurality of sensors each having a resistance which varies according to the magnitude of an associated analog parameter point;

first circuit means connected in parallel with each of said plurality of sensors for carrying a current through said sensors;

a precision current supply circuit connected to said first circuit means;

second circuit means connected in parallel with each of said plurality of sensors and aligned with and adjacent to said first circuit means for measuring the variable resistance of said sensors while said precision current is passing therethrough;

a shield member surrounding the aligned portions of said first and second circuit means;

switching means connected with said first and second circuit means for allowing a pulse of said precision current to pass through only one sensor at a time while limiting the measurement of said voltage drop to said particular sensor carrying said precision current pulse; and wherein said single low-voltage power supply is connected to said precision current supply circuit, said second circuit means and said switching means.

Claims (7)

1. A method of obtaining analog input signals from a plurality of independent sensors each displaced an appreciable distance from each other and each having a resistance which varies according to the magnitude of its own associated analog parameter point including the steps of: generating a precision direct current; and monitoring repeatedly the resistance value of each sensor over a substantial period of time to detect any variation of the magnitude of its associated parameter, said monitoring step including conducting a pulse of said precision current through said sensor during a first time period while simultaneously measuring the voltage differential between the beginning and end points of the sensor, and terminating and ceasing said conduction and measuring steps during a second time period subsequent to and substantially longer than said first time period to prevent the magnitude of the resistance from increasing due to self heating caused by the current passing through the sensor, and wherein said conducting and measuring steps are sequentially performed on each of the other sensors during the second time period without performing said conducting and measuring steps on more than one sensor at any given time.

2. The method of claim 1 wherein said monitoring step is performed on a plurality of separate temperature sensors each having a resistance which varies in direct proportion to its change of temperature.

3. A method of obtaining analog input signals from a plurality of sensors each having a resistance which varies according to the magnitude of an associated analog parameter point including the steps of: generating a precision current including applying a supply voltage to a pair of matched transistors having a common base voltage to produce two identical current outputs, monitoring one of the current outputs to detect any variation in current output, and changing the base voltage to compensate for and eliminate any variation in current output detected by said monitoring step; conducting a pulse of the precision current through a sensor; and measuring the voltage drop across the variable resistance of said sensor during said conducting step.

4. The method of claim 1 wherein said conducting step includes conducting a pulse of the precision direct current to and from each sensor through its own two-wire transmission line; said measuring step includes measuring the voltage drop across the variable resistance of each sensor through its own two-wire monitoring line separate from said two wire transmission lines and connected thereto only at the opposite ends respectively of such sensor; and said monitoring step further including time-sharing a single trunk line to carry the precision current sequentially to the two-wire transmission line of each sensor, and time-sharing a single differential amplifier circuit connected to the two-wire monitoring line of each sensor.

5. Apparatus for obtaining analog input signals from a plurality of independent sensors displaced apart from each other and each having a resistance which varies according to the magnitude of its own associated analog parameter point, including: current source means for generating a precision direct current; a trunk line connected to said current source means; a plurality of separate current transmission lines each connected in series with a sensor and in parallel through its own respective current switch to said trunk line to carry a pulse of direct current through a particular sensor only when said current switch is closed; differential amplifier means for measuring the voltage drop across each of said sensors; a separate pair of detection lines for each sensor connected at one end through their own respective pair of detection switches to said differential amplifier means, and connected at the other end to said current transmission line immediately adjacent the ends of the sensor to measure only the voltage drop across the sensor itself while said pulse of direct current is passing therethrough; shield means surrounding the transmission and detection lines for each sensor to prevent errors caused by extraneous voltages; switching means connected to all of said transmission and detection switches for repeatedly monitoring the resistance value of each sensor over a substantial period of time by sequentially operating said switches to simultaneously connect said generating means and said differential amplifier means to only a single sensor at a time to allow a pulse of said precision current to pass through said sensor during a first short time interval while said differential amplifier means measures the voltage drop across the variable resistance of said sensor, and for disconnecting said sensor during a subsequent longer second time interval to prevent self-heating of the sensor due to current flow therethrough.

6. Apparatus for obtaining analog input signals from a plurality of sensors each having a resistance which varies according to the magnitude of an associated analog parameter point, indluding: means for generating a precision current including a pair of matched transistors having a common base, monitoring circuit means connected to one of said transistors for detecting variations in the magnitude of precision current being generated, and feed-back circuit means coupling said monitoring circuit means to said common base of said matched transistors for eliminating any detected variations of said precision current; separate transmission circuit means associated with each of said sensors, respectively, for connecting said generating means with each of said sensors; differential amplifier means for measuring the voltage drop across said sensors; separate detection circuit means associated with each of said sensors, respectively, for connecting said differential amplifier means with each of said sensors; and switching means coupled to each of said transmission and detection circuit means for simultaneously connecting said generating means and said differential amplifier means to only a single sensor to allow a pulse of said precision current to pass through said sensor during a short time interval while said differential amplifier means measures the voltage drop across the variable resistance of said sensor.

7. A data transmission system for transmitting binary address messages from a central station to a plurality of remote terminals each having a single low-voltage power supply, and for transmitting binary response messages from the addressed remote terminal to the central station, wherein one of said remote terminals includes: a plurality of sensors each having a resistance which varies according to the magnitude of an associated analog parameter point; first circuit means connected in parallel with each of said plurality of sensors for carrying a current through said sensors; a precision current supply circuit connected to said first circuit means; second circuit means connected in parallel with each of said plurality of sensors and aligned with and adjacent to said first circuit means for measuring the variable resistance of said sensors while said precision current is passing therethrough; a shield member surrounding the aligNed portions of said first and second circuit means; switching means connected with said first and second circuit means for allowing a pulse of said precision current to pass through only one sensor at a time while limiting the measurement of said voltage drop to said particular sensor carrying said precision current pulse; and wherein said single low-voltage power supply is connected to said precision current supply circuit, said second circuit means and said switching means.

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