GB2043268A - Analogue indicator - Google Patents

Abstract

An analogue indicator having high linearity and accuracy over a wide range comprises a moving coil meter 1 (Figures 1 and 2) having a pointer 2 movable over a scale 3. The meter 1 is movable as a whole relative to the scale 3 by a servomechanism including motor 4 and position feedback potentiometer 6. The meter 1 indicates the difference between position signal 9 and a measured variable 8 and when this difference is large a hysteresis circuit 11 allows movement of the meter in the appropriate direction. The motor 4 may be driven by discharging pulses into the motor from a capacitor 12. A set point indicator may be mounted adjacent pointer 2 and connected to a potentiometer matching potentiometer 6 for controlling a process and indicating the difference between the measured variable and the set point. <IMAGE>

Description

SPECIFICATION Analogue indicator This invention relates to analogue indicators.

Analogue indicators are used in many applications in industry and have the advantage over digital indicators that they may be read quickly and easily and in particular the difference between a measured value and desired value may be easily seen from an analogue indicator. For example, in the control of a process it may be necessary for an operatorto control a variable, such as temperature, so as to maintain it at a desired value. An analogue indicator may be provided with a set point marker on the same scale as the indicator and the operator will then be able to see easily when the indicated value deviates from the set point value.

It is commonly desirable for an analogue indicator to be able to display a wide range of values but yet be able to indicate values around a set point for most of the time with high accuracy. Presently available analogue indicators capable of displaying a wide range of values include the moving coil meter having a scale of 240 , for example, and the servodriven indicator in which a pointer is moved over a scale by a servo mechanism in which a comparator compares the value to be indicated with the position of a pointer as sensed by a position feedback potentiometer. The moving coil meter has the disadvantage that its accuracy is low, typically one percent, because of mechanical and metallurgical inaccuracies of the pole pieces.Further, the full accuracy is generally only maintained over a limited part of the total range of movement and known meters are generally of poor linearity over the whole of the scale. The servo-driven indicator is capable of good accuracy and linearity (both determined by the position feedback potentiometer) but these devices have the disadvantage that they are relatively expensive. Furthermore, in a system in which the measured variable is maintained for most of the time very close to a fixed value, heavy localised wear of the indicator takes place around the fixed value.

Even a small variation in the measured variable will cause the servo mechanism to drive the indicator a small amount causing wear around one point on the position feedback potentiometer slide wire, for example. A precision gear box with a high reduction ratio is required between the motor and the indicator in this kind of device.

According to the invention there is provided an analogue indicator comprising a meter mounted so as to be movable relative to a scale, which scale is also traversed by the indicating element of the meter, and a servo mechanism arranged to move the meter relative to the scale, the servo mechanism including an electrical circuit arranged to compare an input signal to be indicated with a position feedback signal indicative of the position of the meter relative to the scale and to supply a signal representative of the difference between said input and position feedback signals to the meter and to drive the motor of the servo mechanism in accordance with said difference signal via a circuit providing hysteresis.When the difference signal is large, the hysteresis circuit operates to drive the motor and move the meter relative to the scale until the difference signal falls within a small range. Within this range the hysteresis circuit operates to prevent driving of the servo motor and any difference signal is indicated by the meter. The meter may itself have a total range of only a fraction of the total scale and may effectively be moved to different parts of the scale by the servo mechanism. The advantage of this is that a meter having high accuracy and linearity over a small scale may be used to indicate values over a wide range. Furthermore, when the measured value varies slightly around a fixed point, the servo mechanism is not energised and the meter follows the variations.

The meter may be one of many types, and may have a linear or circular scale. Preferably, however, the meter is a moving coil meter operating over a circular arcuate scale, for example 90 .

Advantageously, a set point marker can be mounted so as to be moveable relative to the scale in the same way as the indicating element of the meter.

If the indicator is to be used to supply an electrical set point or control value, the set point marker can be connected to a set point potentiometer mounted in a similar manner to the position feedback potentiometer. By this means the set point potentiometer and position feedback potentiometer may be closely matched.

According to a preferred feature of the invention, the servo motor may be driven from a pulse power supply in which short pulses of electrical energy are periodically supplied to the servo motor under the control of the hysteresis circuit. For example, a capacitor may be charged to several times the rated continuous voltage of the motor and then discharged into the motor when the hysteresis circuit indicates that the difference signal exceeds a predetermined amount. The circuit will of course be bi-directional. The use of such a power supply for the motor is possible because the motor is only required to move from time to time in discrete steps which are not necessarily of accurately controlled size since any difference between the input signal and position feedback signal is indicated accurately by the meter.

Considerable advantages arise through this power supply method, namely that a very low power supply may be used since the total energy required is small; an inexpensive low power motor may be used and a simple, not necessarily accurate, mechanical drive may be employed; and the pulsing mode of operation overcomes minor mechanical faults and stiffness and gives immunity from poor contact resistance between the brushes and commutator of the motor.

The pulse method of driving a motor has applications in other fields such as in driving motors used for operating fluid valves. Thus according to another aspect of the invention there is provided a method of driving a d.c. motor from a power supply, comprising charging a capacitor from the power supply to a voltage several times the rated continuous voltage of the motor and repeatedly discharging the capacitor into the windings of the motor. For example, the capacitor may be charged to four or-five times the rated voltage of the motor.

Certain embodiments of the invention will now be described by way of example and with reference to the accompanying drawings, in which: Figure 1 is a schematic view of an analogue indicator in accordance with the invention; Figure 2 is a schematic circuit diagram of the electrical portion of the servo mechanism of the indicator of Figure 1; Figure 3 is a schematic circuit diagram similar to Figure 2 but showing means for providing an electrical control signal; Figure 4 is an exploded perspective view of a preferred analogue indicator in accordance with the invention; Figure 5 is an electrical circuit diagram of the indicator of Figure 4; and Figure 6 is a diagram of a modified electrical circuit.

Referring to the drawings, Figure 1 shows an analogue indicator comprising a meter 1 having an indicating element in the form of a pointer 2 movable over a scale 3. The meter is mounted so as to be itself movable relative to the scale 3. The indicator also comprises a servo mechanism including a servo motor 4 arranged to move the meter via a drive 5. A position feedback potentiometer 6 is arranged to supply an electrical signal representative of the position of the meter relative to the scale.

An electrical circuit for driving the servo motor includes a comparator 7 arranged to receive an input signal on line 8 representative of a variable to be indicated and a signal on line 9 from the position feedback potentiometer representative of the position of the meter relative to the scale 3. The output of the comparator on line 10 represents the difference between the two input signals and is supplied to the meter 1 and to a hysteresis circuit 11. The hysteresis circuit 11 controls the discharge of a capacitor 12 into the windings of motor 4 via a bidirectional switch arrangement 13, 14.

Figure 3 shows 3 circuit arranged to produce an electrical control signal for controlling a process, for example. The measured variable on line 15 is supplied via a buffer amplifier 16 to the comparator 7 and to a comparator 17 which is also arranged to receive a signal from a set point potentiometer 18.

Comparator 17 produces a signal representative of the difference between the measured variable and the set point and this is supplied via a control amplifier 19 to line 20 to control the process.

Briefly, the operation of the apparatus is as follows. The meter 1 is an accurate moving coil meter having a small range of movement, such as 90 , and when any small variation in the input signal on line 8 occurs the meter 1 indicates the change on the scale 3. When a larger change of the measured variable occurs, the output from comparator 7 on line 10 becoms sufficient to cause hysteresis circuit 11 to operate switching arrangement 13, 14to discharge capacitor 12 into the windings of the motor 4 to drive the motor 4 in the appropriate direction. Initiallythe pointer 2 moves to full scale deflection on the meter but as the meter is rotated the pointer comes nearer the center of the meter movement.The total extent of the pointer movement relative to the meter is made equal to or greater than the deadband of the motor drive system provided by the hysteresis circuit. The position feedback potentiometer 6 supplies a signal to the comparator 7 indicative of the position of the meter 1 relative to the scale 3.

Looking now in detail at the preferred embodiment of the apparatus, Figure 4 shows an analogue indicator comprising a frame 21 in which is mounted the scale 3. A board 22 is mounted to the frame 21 by means of brackets 23. The board 22 carries a circular slidewire on each face, the slidewire 24 forming a set point potentiometer and the slidewire 25 forming the position feedback potentiometer. Servo motor 4, such as a small 10 volt 5 pole permanent magnet d.c.

motor, is fixedly mounted behind board 22. The motor is arranged to drive a pulley 26 via a neoprene rubber belt 27 and the pulley 26 is connected to a shaft 28 on which is mounted the meter 1. The shaft 28 passes through a bush in the board 22. The bush is connected to the drum 30 described below and is mounted in the board with relatively high friction; the shaft 28 passes through the bush with relatively low friction. This allows the motorto rotate the meter 1 easily and also allows the position of the drum 30 to be manually adjusted for adjusting a set point indicator 31 and for holding it inthe set position. The pulley 26 carries a spring wiper contact 29 arranged to contact the slidewire 25, which may be of 50 mm diameter, for example.

Atransparent plastic drum 30 surrounds the meter 1 and is mounted via the bush on the meter shaft 28.

The drum carries a set point indicator 31 arranged so as to be movable relative to the scale 3 in the same way as the meter pointer 2. A short shaft 32 fixed to the drum extends through a hole in the scale and is provided with a knob 33. The drum 30 carries on its rear another spring wiper contact which is arranged to contact the set point slide wire 24. It may be seen that the drum 30, with its attached set point indicator 31, shaft 32 and knob 33 may be moved relative to the scale 3 independently of the position of the meter shaft 28 and the meter 1.

A plug 32 is mounted on the board 22 giving access to the terminals of the set point and position feedback potentiometers. These potentiometers may be substantially identical and thus closely matched.

In operation the motor 4 is driven from a pulse power supply and belt 27 operates to partially smooth the pulsed high torque. The moving coil meter 1 is rotated in use about the same axis as its own pivot and although the meter may have a total movement of about 90', the total scale 3 may be nearly 360 .

Referring now to Figure 5, the electrical control circuit for the analogue indicator is shown in detail.

The circuit includes a capacitor C2 (e.g. 330 uF) which is charged to almost 50 V via a constant current charging circuit 41. The capacitor C2 is periodically dischargeable via thyristors SCR1-4 into the windings of motor 4 via resistor R12 (e.g. 3.3 ohms) under the control of a pulse generator 42. The circuit includes diode clamps 43 which only allow triggering pulses to pass from the pulse generator 42 to the-motor drive circuit 44 when the difference between the measured variable supplied on line 8 and the position feedback signal on line 9 exceeds the hysteresis provided by deadband and clamp drive circuits 45 and 46.

The measured variable and position feedback signals are supplied to a comparator IC1 arranged to produce a difference signal which is supplied to the meter 1 and to the deadband and clamp drive circuits 45 and 46. A trimming resistor VR1 is connected in series with meter 1 for calibration purposes and the meter operates to indicate the difference between the measured variable and position feedback signals.

The deadband and clamp drive circuits 45 and 46 each include a transistor (TR4 and TR7 respectively) which may be turned on to operate one of the clamps 43. When the output of comparator IC1 exceeds +0.6 V,transistor TR7 is turned on and transistorTR4 is off. When the difference signal is less than -0.6 V, transistor TR4 is turned on and transistorTR7 is turned off. For valuers of the error signal between +0.6 V and -0.6 V both transistors TR4 and TR7 are turned on.

Transistor TR7 is controlled by transistor TR5 and when the base-emitter voltage of TR5 is more negative than -0.6 V transistor TR5 turns on; turning on transistor TR6 and turning off transistor TR7. This releases the clamp comprising diodes D1 and D2 and allows pulses to pass from the pulse generator 42 to thyristors SCR1 and SCR4. When the difference signal is above -0.6 V transistor TR7 is turned on to prevent pulses from the pulse generator reaching thyristors SCR1 and SCR4.

Transistor TR4 is controlled by transistor TR3 and the base biassing resistors R22 and R23 of transistor TR3 are selected such that when the difference signal from comparator IC1 is less than +0.6 V the base potential of TR3 is more negative than -0.6 V causing transistors TR3 and TR4to turn on, clamping diodes D6 and D7. When the output of comparator IC1 is greater than +0.6 V diodes D6 and D7 are unclamped allowing thyristors SCR2 and SCR3 to operate. Diodes D9 and D10 are provided to protect transistors TR3 and TR5 from excessive reverse base-emitter voltages.

The pulse generator 42 includes a programmable unijunction transistor TR1 connected as a blocking oscillator. The gate is connected to 0 V and in operation capacitor C1 charges via resistor R1 until it reaches 0 V whereupon the unijunction transistor fires to discharge the capacitor and produce a short pulse rising from -25 V to O V and falling again. The repetition frequency-is typically a few seconds. The short positive-going pulses are shorted out by diodes D1 and D2, for example, when they are clamped, i.e. their centre point is connected to -25 V by transistor TR7. If the diodes D1 and D2 are not biassed into conduction in this manner the short positive-going firing pulses are supplied by diodes D3 and D4 to thyristors SCR1 and SCR4 as mentioned above.

Conduction of thyristors SCR1 and SCR4 causes capacitor C2 to be discharged into motor 4 via resistor R12 in one direction, and, alternatively, conduction of thyristors SCR2 and SCR3 causes capacitor C2 to be discharged into the motor windings in the opposite direction.

Capacitor C2 is charged by constant current charging circuit 41 comprising transistor TR2 whose base voltage is defined by resistors R15 and R16. Zener diode Z1 with its associated feed resistor R13 and noise decoupling capacitor C3 provides a small negative bias (e.g. 5 V) for the thyristor gates to ensure that they switch off as the voltage across capacitor C2 approaches zero.

The motor 4 is typically rated at 10 V 250 mA and the capacitor C2 is charged to about 45 V at 40 mA and discharged into the motor when necessary every few seconds.

In operation the pulse generator 42 produces firing pulses continuously but the clamps 43 and deadband and clamp drive circuits 45 and 46 operate to allow triggering pulses to reach the thyristors only when there is a large difference between the mea suredvariable and position feedback signals on lines 8 and 9. If, for example, the measured variable increases suddenly by a large amount, the output of the comparator IC1 on line 10 will be large and the pointer of the meter 1 will move towards its maximum position. Transistors TR3 and TR5 will be turned off, transistor TR4 will turn off and transistor TR7 will turn on. Diodes D1 and D2 will clamp the anodes close to -25 V but diodes D6 and D7 will allow triggering pulses to pass through diodes D5 and D8 to thyristors SCR2 and SCR3.This will cause pulses of energy to be discharged from capacitor C2 into motor 4 to move the meter in a direction towards the higher end of the scale. After one or more such pulse movements the pointer 2 will again be close to the centre of the total range of movement of the meter 1 and both transistors TR4 and TR7 will be turned on so as to prevent triggering pulses from reaching any of the thyristors. The meter will then remain fixed in position until a further large change in the measured variable occurs. Small changes in the measured variable are indicated by corresponding small- movements of the pointer relative to the meter.

The width of the deadband provided by the hysteresis circuits 45 and 46 corresponds to a pointer movement equal to the range of movement of the pointer relative to the meter. If desired, the deadband can be made somewhat smaller than this meter range, for example, 30" on a 90 meter, so as to use a portion of the meter movement providing the greatest accuracy and linearity.

Figure 6 shows some modifications of the circuit of Figure 5. The constant current charging circuit 41 includes transistor TR8 biassed by diode D11 and controlled via resistor R29 by. a 20V a.c. source.

Transistor TR8 conducts only during positive halfcycles of the source and so transistor TR2 is non-conductive during the negative half-cycles. This ensures a reliable turnoff of the thyristors SCRl - -4 and enables the firing pulse power to be increased by the omission of resistor R3 of Figure 5, i.e.

transistor TR1 directly supplies the diode clamps 43. Capacitors C4 and C5 have been added to the deadband and clamp drive circuits 45 and 46 to reduce the effect of interference spikes caused when the thyristors fire and resistor R12 has been repositioned to iimit the thyristor current should a circuit fault exist causing two in-line thyristors, e.g. SCR1 and SCR3, to fire at the same time.

The overall accuracy of the indicator depends on the accuracy and linearity of the slidewire and scale and of the meter in the range over which it is used. It is easily possible to obtain an accuracy of 0.5% over a 270 scale. The indicator has the considerable advantage that accuracy does not depend at all on the drive components which means that an inexpensive motor, belt and pulley can be used. As described above, any error between the position of the meter and the measured variable is indicated by the meter movement and of course any small variations in the measured variable are followed by the meter movement. From the point of view of wear, a meter movement is of course better adapted to withstand small movements over a long period than a potentiometer slide-wire of a servo mechanism.

When used as an indicating controller, the apparatus has the advantage that a set point signal can be provided which is accurately matched to the indicated value and so an accurate control signal representative of the difference between measured and set point values can be provided.

The use of a small d.c. motor in a high current pulsed mode of operation in conjunction with an elastic belt drive means that high torque is generated in pulses which overcomes problems of stiffness in the moving parts. The shape of the current pulses supplied to the motor affects the performance of the motor and the shape may be adjusted by adjusting the resistor connected in series between the capacitor and motor.

Although the invention has been described in connection with an indicator having a pointer moving over a scale, the principles of the invention may also be applied for other purposes, such as controlling the position of a fluid control valve. Again the use of a pulse power supply results in advantages in overcoming any stiffness in the system.

Claims (11)

1. An analogue indicator comprising a meter mounted so as to be movable relative to a scale, which scale is also traversed by the indicating element of the meter, and a servo mechanism arranged to move the meter relative to the scale, the servo mechanism including an electrical circuit arranged to compare an input signal to be indicated with a position feedback signal indicative of the position of the meter relative to the scale and to supply a signal representative of the difference between said input and position feedback signals to the meter and to drive the motor of the servo mechanism in accordance with said difference signal via a circuit providing hysteresis.

2. An indicator as claimed in claim 1 wherein the meter is a moving coil- meter operating over a circular arcuate scale.

3. An indicator as claimed in claim 1 or 2 including a set point marker mounted so as to be moveable relative to the scale in the same way as the indicating elementofthe meterwherebythe position of the indicating element may readily be compared with the position of the set point marker.

4. An indicator as claimed in claim 3 wherein the set point marker is connected to a set point potentiometer.

5. An indicator as claimed in any preceding claim wherein the servo motor is arranged to be driven from a pulse power supply whereby short pulses of electrical energy are periodically supplied to the servo motor under the control of the hysteresis circuit.

6. An indicator as claimed in claim 5 in which a capacitor is charged to several times the rated continuous voltage of the motor and then discharged into the motor when the hysteresis circuit indicates that the difference signal exceeds a predetermined amount.

7. An indicator as claimed in any preceding claim including means for continuously producing pulses for controlling the operation of the motor and wherein the hysteresis circuit is arranged to block the application of the pulses to the motor when the difference signal is below a predetermined minimum.

8. An indicator as claimed in claims 6 and 7 including a plurality of thyristors arranged to control the discharge of said capacitor through the motor, said pulses being gate firing pulses for the thyristors.

9. An analogue including substantially as hereinbefore described with reference to the accompanying drawings.

10. A method of driving a d.c. motor from a power supply, comprising charging a capacitorfrom the power supply to a voltage several times the rated continuous voltage of the motor and repeatedly dicharging the capacitor into the windings of the motor.

11. A method of driving a d.c. motor from a power supply, substantially as hereinbefore described with reference to the accompanying drawings.