GB1589563A

GB1589563A - Comparison techniques

Info

Publication number: GB1589563A
Application number: GB25511/76A
Authority: GB
Original assignee: Planer Products Ltd
Current assignee: Planer Products Ltd
Priority date: 1977-06-20
Filing date: 1977-06-20
Publication date: 1981-05-13

Description

(54) IMPROVEMENTS IN OR R > ELATING TO COMPARISON TECHNIQUES (71) We, PLANER PRODUCTS LIMITED, of Windmill Road, Sunbury-on-Thames, Middlesex, England, a British Company, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates to comparison techniques.

In comparing the actual value of an analogue parameter with its desired value, it is known to apply signals representative of these values to cams operating a potentiometer so as to derive an output representative of any difference there-between. However, such a mechanical arrangement has the disadvantage that it is subject to wear and to backlash, which can lead to inaccuracy in control of the parameter.

It is an object of the present invention to avoid, or at least reduce, inaccuracies in controlling the value of a parameter.

In accordance with the present invention, there is provided apparatus for controlling the value of a parameter, comprising means for converting an analogue representation of the actual value of the parameter into a digital signal, comparator means arranged to effect digital comparison between said digital signal and a further digital signal representative of the desired value of the parameter, the comparator means being arranged to provide an output for means for minimizing any difference between the actual and desired values of the parameter, and control means for selectively varying the desired value of the parameter, said control means being arranged to control a variation of the desired value at a selected predetermined rate between given limits or to maintain the parameter at the desired value.

Also, in accordance with the present invention, there is provided a method of controlling the value of a parameter, wherein an analogue representation of the actual value of the parameter is converted into a digital signal, the digital signal is cornpared with a further digital signal representative of the desired value of the parameter which is varied at a selected predetermined rate between given limits, and a control signal is derived from the comparison for minimizing any difference between the actual and desired values of the parameter.

The parameter being controlled may Ibe temperature or pressure, for example.

A temperature controller and its method of operation, in accordance with the present invention, will now be described, by way of example, with reference to the accompanying drawings, in which : Figure 1 is a view of the control panel of the temperature controller; Figure 2 is a block diagram of the electrical circuit of the controller of Figure 1; and Figure 3 shows an example of the temperature as controlled by the controller.

Referring to the drawings, the temperature controller 10 can be arranged to control heating and/or cooling equipment so as to control the temperature of an associated medium between given limits in accordance with eight predetermined programmes or "ramps". A ramp is a temperature variation at a predetermined rate for a given time or until a predetermined temperature is reached.

For example the controller 10 may be constructed so as to effect control of temperature between -200"C and +2000 C, but is may be desirable that two, or more, controllers be arranged to cover consecutive or overlapping temperature ranges between these values. The rate of change of temperature may be selected from the following ranges: + 1 to 39 "C/hr, +0.1 to 3.90C/min, + 1 to 39 C/min, and' zero, i.e. hold at constant tempera ture.

In addition to these temperature ranges a selection may be made from two "latent heat" settings whereby a predetermined out put from the heater or cooler is available at a selection of rates: +0.1 to 3.9 pulses/1/10 minute, + 1 to 39 pulses/1/10minute, where the pulse length is fixed at 0.1 second.

The limits of a ramp may be chosen from values within the above-mentioned temperature range appropriate to the machine, or from within the following time limits: 0 to 59 mins, 0 to 5.9 hours.

Figure 3 shows an example of eight ramps forming one programme of the controller, indicating how the temperature of the medium is to be controlled over the cycle time.

When the power switch 12 is turned on, the preprogrammed ramp rates appear sequentially on a digital display panel 14.

These can be altered within the limits described above by operation of a selection array 19. A similar procedure is carried out with respect to the ramp limit selection array 22 in association with its display 24. The first ramp thus chosen is indicated by the display 26, as ramp No. 1 and is then stored in a memory 44 (Figure 2) by pressing ENTER button 28. This causes the ramp number display 26 to indicate the next ramp number. The rate and limit settings for ramp No. 2 is similarly selected and stored. This process is then carried out for the eight ramps available to give temperature control such as that shown in Figure 3 for example.

Fewer or more ramps may be available in the controller, and it is not necessary that all the available ramps are used.

A running switch 30 of the controller 10 is operated to start the sequence of temperature control, and electrical output from the controller is then arranged to contol a heater, e.g. a resistance element, or a cooler, e.g. a supply of liquid nitrogen.

A sensor, e.g. a platinum resistance thermometer, is placed in contact with the medium to be controlled, and the output from this is fed back to the controller 10.

A panel 34 indicates temperature, and according to the position of the three-way switch 36 indicates either the actual or the required temperature of the medium. An alarm is arranged to indicate a discrepancy of more than a predetermined amount between these values.

Referring to Figure 2 of the drawings, the individual ramp rate and limits set by the arrays 19 and 22 respectively are fed along lines 40, 42 into the memory 44, these values being displaved on the respective displays 14 and 24. Operation of the running switch 30 is effective to feed the rate data via a divider 46 into an up/down counter 48. An output from the counter 48 is fed via a selector 50 into a comparator 52, where its instantaneous value is compared with the limit value received directly from the limit data in the memory 44. When the comparator 52 detects equality between its inputs, indicating that the rate data has brought the temperature of the controlled medium to the limit set by that control ramp, the comparator 52 delivers an output to an address counter 54 which operates via a select switch 56 to select the next ramp.

The displays 14 and 24 show the new conditions, and the up / down counter 48 operates at the rate demanded of the next ramp.

A platinum resistance thermomenter 60 is arranged to sense the temperature of the medium being controlled, and has its analogue output converted to digital notation by a logic circuit 62 the outputs of which are fed to input terminals 48a, b, c of the counter 48, and also to respective input terminals 64a, b, c of a further up/down counter 64.

A comparator 66 receives input from the up/down counter 48, corresponding to the required temperature, and also receives input from the up/down counter 64, representative of the actual temperature. The comparator 66 provides an output along one of three channels, depending upon whether the inputs from the counter 48 and 64 are equal, or the input from one is greater, or less than, the input from the other. When the comparator 66 registers a difference between its inputs, it enables the counter 64, in the corresponding direction of counting, via a delay counter 70 at one of fifteen predetermined sampling frequencies.The up/ down counter 64 acts as a control circuit for a power circuit 68, which is arranged to switch on a heating circuit so as to supply heat to the medium sensed by the thermometer 60, when the actual temperature is less than the desired temperature, or to operate a solenoid valve so as to supply liquid nitrogen to the sensed medium to cool it down when the actual temperature is higher than the required temperature.

The sampling frequency of the delay counter 70 may be chosen in dependence on the time constant of the medium being controlled. Thus, if the medium has a large time constant, then the sampling frequency would be low, so that the temperature is given sufficient time to adapt to its new value in response to operation of the power circuit 68 before the comparator 66 receives an output from the counter 64 for making its next comparison. The control counter 64 is further regulated by a power control counter 72 which regulates, in accordance with one of fifteen selected rates, the counting rate of the counter 64. This control affects the gain or sensitivity of the temperature control exercised by the power circuit 68.

The operation of the comparator 66 in controlling the power circuit 68, and thus the cooling or heating of the sensed medium, will now be described. The sampling time required for the delay counter 70 is set, and the sensitivity required of the system is set for the control counter 72. At the end of the predetermined delay interval, the counter 70 enables the up / down counter 64 via a monostable circuit 73 to receive that output of the converter circuit 62 which indicates the actual temperature of the medium.

The output signal of comparator 66 is passed to the power circuit 68 and counter 64 for appropriate action, as mentioned above.

The counter 64 is clocked in the appropriate direction until the comparator 66 detects equality between the inputs from the counters 48 and 64, at which stage the comparator 66 passes a stop signal to the delay counter 70. Control of the power circuit 68 by the counter 64 is then stopped, and the delay counter 70 initiates its sampling delay time.

Thus, the power circuit 68 is enabled for a time proportional to the difference between the required and actual temperatures.

It is envisaged that an anticipatory control circuit, for example a 3-term circuit, may also be employed, for improving the response of the control apparatus. This circuitry would be particularly useful in controlling the temperature of a system having high thermal inertia, for example a muffle furnace, where a small change in temperature can be used to initiate a disproportionately large change in the controlled output of the heater or cooler.

In the temperature controller of the present invention, the two quantities being compared are both in digital form, which allows more extensive use of solid-state circuitry than is the case if the comparison of the required value with the actual value were made in analogue form. Furthermore, the sensitivity of the control is enhanced since this is exercised digitally and is thus not subiect to noise, as is the case in analogue comparison and adjustment.

The temperature controller 10 also has a re-cycle facility, which is operable when the final temperature of a cycle comprising one ramp, or of a sequence of ramps, is equal to its initial temperature. A reset button 80 (Figure 1) can be pressed so as to return the programme to ramp No. 1 after the eighth, or any other, ramp has been completed. A further button 82 of the controller 10 feeds into the control programme an END signal, and when this is reached the system is stopped and arranged to maintain the existing temperature until the controller 10 is switched off manually.

The BCD output from the digital thermometer may be fed internally of the controller 10 to a socket on a rear panel thereof, to allow for connection to a data logging system so that the profile of the actual temperature of the medium can ge reproduced at any time.

Such a temperature controller finds application, for example, in the field of biochemistry, where controlled freezing of a sample may be required. In this respect the "latent heat" provision of the controller is particularly useful, since heat may be added to or removed from the sample at a constant rate, either for a given time, or until a temperature a few degrees to one side of the critical temperature is reached. Thus, controlled freezing of a number of constituents in a sample can be achieved, without damage.

WHAT WE CLAIM IS:- 1. Apparatus for controlling the value of a parameter, comprising means for converting an analogue representation of the actual value of the parameter into a digital signal, comparator means arranged to effect digital comparison between said digital signal and a further digital signal representative of the desired value of the parameter, the compartor means being arranged to provide an output for means for minimizing any diference between the actual and desired values of the parameter, and control means for selectively varying the desired value of the parameter, said control means being arranged to control a variation of the desired value at a selected predetermined rate between given limits or to maintain the parameter at the desired value, 2.Apparatus for controlling the ternperature of a medium, comprising means arranged to receive a signal in analogue form representative of the actual value of the temperature, means for converting the analogue signal to a digital signal, means for deriving a further digital signal representative of the desired value of the temperature, comparator means arranged to effect digital comparison between said digital signal and said further digital signal, the comparator means being arranged to provided an output for temperature controlling means for minimizing any difference between the actual and desired values of the temperature, and control means for selectively varying the desired value of the temperature, said control means being arranged to control a variation of the desired value at a selected predetermined rate between given limits or to maintain the temperature at the desired value.

3. Apparatus according to claim 2, comprising means arranged to sense the temperature of the medium and to derive said analogue signal in accordance therewith.

4. Apparatus according to claim 3, wherein the temperature sensing means comprises a platinum resistance thermometer.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. controlling the power circuit 68, and thus the cooling or heating of the sensed medium, will now be described. The sampling time required for the delay counter 70 is set, and the sensitivity required of the system is set for the control counter 72. At the end of the predetermined delay interval, the counter 70 enables the up / down counter 64 via a monostable circuit 73 to receive that output of the converter circuit 62 which indicates the actual temperature of the medium. The output signal of comparator 66 is passed to the power circuit 68 and counter 64 for appropriate action, as mentioned above. The counter 64 is clocked in the appropriate direction until the comparator 66 detects equality between the inputs from the counters 48 and 64, at which stage the comparator 66 passes a stop signal to the delay counter 70. Control of the power circuit 68 by the counter 64 is then stopped, and the delay counter 70 initiates its sampling delay time. Thus, the power circuit 68 is enabled for a time proportional to the difference between the required and actual temperatures. It is envisaged that an anticipatory control circuit, for example a 3-term circuit, may also be employed, for improving the response of the control apparatus. This circuitry would be particularly useful in controlling the temperature of a system having high thermal inertia, for example a muffle furnace, where a small change in temperature can be used to initiate a disproportionately large change in the controlled output of the heater or cooler. In the temperature controller of the present invention, the two quantities being compared are both in digital form, which allows more extensive use of solid-state circuitry than is the case if the comparison of the required value with the actual value were made in analogue form. Furthermore, the sensitivity of the control is enhanced since this is exercised digitally and is thus not subiect to noise, as is the case in analogue comparison and adjustment. The temperature controller 10 also has a re-cycle facility, which is operable when the final temperature of a cycle comprising one ramp, or of a sequence of ramps, is equal to its initial temperature. A reset button 80 (Figure 1) can be pressed so as to return the programme to ramp No. 1 after the eighth, or any other, ramp has been completed. A further button 82 of the controller 10 feeds into the control programme an END signal, and when this is reached the system is stopped and arranged to maintain the existing temperature until the controller 10 is switched off manually. The BCD output from the digital thermometer may be fed internally of the controller 10 to a socket on a rear panel thereof, to allow for connection to a data logging system so that the profile of the actual temperature of the medium can ge reproduced at any time. Such a temperature controller finds application, for example, in the field of biochemistry, where controlled freezing of a sample may be required. In this respect the "latent heat" provision of the controller is particularly useful, since heat may be added to or removed from the sample at a constant rate, either for a given time, or until a temperature a few degrees to one side of the critical temperature is reached. Thus, controlled freezing of a number of constituents in a sample can be achieved, without damage. WHAT WE CLAIM IS:-

1. Apparatus for controlling the value of a parameter, comprising means for converting an analogue representation of the actual value of the parameter into a digital signal, comparator means arranged to effect digital comparison between said digital signal and a further digital signal representative of the desired value of the parameter, the compartor means being arranged to provide an output for means for minimizing any diference between the actual and desired values of the parameter, and control means for selectively varying the desired value of the parameter, said control means being arranged to control a variation of the desired value at a selected predetermined rate between given limits or to maintain the parameter at the desired value,

2.Apparatus for controlling the ternperature of a medium, comprising means arranged to receive a signal in analogue form representative of the actual value of the temperature, means for converting the analogue signal to a digital signal, means for deriving a further digital signal representative of the desired value of the temperature, comparator means arranged to effect digital comparison between said digital signal and said further digital signal, the comparator means being arranged to provided an output for temperature controlling means for minimizing any difference between the actual and desired values of the temperature, and control means for selectively varying the desired value of the temperature, said control means being arranged to control a variation of the desired value at a selected predetermined rate between given limits or to maintain the temperature at the desired value.

5. Apparatus according to any of claims

2 to 4, comprising said temperature controlling means.

6. Apparatus according to any preceding claim, wherein the control means is arranged to control the variation at the selected rate during a control period which is ended when either a predetermined value is reached or a predetermined time interval has elapsed.

7. Apparatus according to any of claims 2 to 5, comprising control means for controlling variation of the temperature at a selected predetermined rate during a control period which is ended when either a predetermined temperature is reached or a predetermined time interval has elapsed, display means for successively displaying said predetermined rates of change of temperature, said predetermined temperatures and said predetermined time intervals, and selection means enabling selection of a rate of change and a temperature or time interval when displayed.

8. Apparatus according to claim 6 or 7, wherein the control means is arranged to perform a plurality of different control periods in a predetermined sequence.

9. Apparatus according to any preceding claim, comprising first and second UP/ DOWN counters arranged to receive inforrnation representative of the actual and desired parameter or temperature values respectively and to derive therefrom the said digital input signal and the said further digital signal.

10. Apparatus according to claim 9, comprising means for selectively varying the rate at which information is passed from the first UP/DOWN counter to the comparator means.

11. Apparatus according to claim 9 or 10, comprising means for selectively varying the counting rate of at least one of the UP/ DOWN counters so as to vary the gain or sensitivity of the control apparatus.

12. Control apparatus substantially as hereinbefore described with reference to the drawings.

13. A method of controlling the value of a parameter wherein an analogue representation of the actual value of the parameter is converted into a digital signal, the digital signal is compared with a further digital signal representative of the desired value of the parameter which is varied at a selected predetermined rate between given limits, and a control signal is derived from the com parison for minimizing any difference between the actual and desired values of the parameter.

14. A method according to claim 13, wherein the parameter is temperature.

15. A method according to claim 13 or 14, wherein signals representative of the actual and desired values of the parameter are passed into respective UP/DOWiN counters before being passed to a comparator to effect said comparison.

16. A method according to claim 15 when dependent on claim 14, wherein the rate at which signals are passed from the UP/DOWN counters to the comparator is selected in dependence on the thermal time constant of the medium whose temperature is to be sensed.

17. A method according to claim 15 or 16, wherein the counting rate of at least one of the counters is varied to vary the gain or sensitivity of the control.

18. A method of controlling the value of a parameter substantially as hereinbefore described with reference to the drawings.