DE2322694A1 - PROCESS FOR CREATING A MOVING SET POINT IN ELECTRONIC TEMPERATURE CONTROLLERS - Google Patents

Description

Gebr. Böhler & Co, Aktiengesellschaft, Vienna

Process for creating a sliding setpoint in electronic temperature controllers

In practice, the heating or cooling of ovens, workpieces etc. is usually not done with linear heating or Cooling rates, but usually follows an exponential time-temperature law. But it is also common to run through only certain temperature ranges with a linear heating and cooling rate and the heating or cooling in the adjacent temperature ranges according to an exponential time-temperature law to undertake. In FIG. 1, the linear and exponential time-temperature curves are compared with one another, with the abscissa represents the time axis and the ordinate represents the temperature, thermal voltage, etc.

While the linear time-temperature curve e.g. through the

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Specification ° C / min is sufficiently defined, the exponential time-temperature curve can be described by specifying the time constant X. The value of the time constant τ in seconds means that after this time t has elapsed, the initial value of the temperature at time t has in the specific case decreased by a factor of 0.368. After a period of time of 2 τ has elapsed, the starting temperature is the

Time t _{Q has} decreased by a factor of 0.368, etc.

During the regulation of heating or cooling processes

■ "- - Practice with a linear time-temperature curve in the / mostly relative can be carried out easily, the regulation of such processes prepares an exponential time-temperature law should follow, usually considerable difficulties.

The heating or cooling processes are generally regulated in such _{a way} that, for example, the actual workpiece or furnace temperature measured with a thermocouple is continuously compared with the specified temperature setpoint and the difference between the actual value and the setpoint is adjusted Minimum is regulated.

The actual temperature value is usually measured using a thermocouple or a comparable Meßwertauf carried out takers, with an electrical measured variable Voltage is generally in the order of mV. Accordingly, an electrical setpoint generator is also used Voltage source is used, which supplies a voltage of the same order of magnitude for comparison with the actual value. These Tension is usually regulated by adjusting a

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adjustable voltage divider (potentiometer) in the nominal voltage circuit. The voltage provided by the setpoint generator is then fed to the actual controller, which continuously compares the actual value with the setpoint and, if there are differences, the heating output of the furnace in the direction that the difference reaches a minimum.

The regulation of heating or cooling processes with a linear time-temperature curve is generally used in the Way carried out that the setpoint potentiometer the desired Time-temperature curve accordingly with the help of a simple electromechanical drive, e.g. in form a gear motor with stepwise speed adjustment.

In contrast, the regulation of heating and cooling processes requires an exponential time-temperature law should follow, for the adjustment of the setpoint device a comparatively much greater effort, e.g. special functional gears or complicated cam disks.

As is well known, however, follow an exponential law of time not only for heating or cooling processes but also, for example, charging and discharging of electrical capacitors. During the time-temperature profile with exponential heating or cooling due to the heat capacity of the system and determines the heat supply or removal to or from the system depends on the time-voltage curve during charging or dis-

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charge an electrical capacitor from its capacity and the size of the resistance through which the Charge or discharge current flows. Accordingly, the time constant of a charging or discharging process after the formula

T = R.C (equation (1)

where X is the time constant in seconds, R is the discharge resistance in megohms and C is the capacitance of the capacitor in microfarads.

The registration proposal is based on the fact that Heating and cooling processes as well as charging and discharging processes of capacitors follow the same exponential law of time follow. To regulate such heating and cooling processes, the charging or discharging process can therefore be a suitable combination of capacitor and resistor (RC element) be used in such a way that the voltage supplied by this RC element during charging or discharging Is used as a setpoint. Fig. 2 shows those discussed below and characterizing the proposed invention Assemblies in the form of a block diagram.

The voltage required for charging the RC element 3 is fed to the RC element via a switch Z from a voltage source 1. The switch can be used to initiate and terminate the charging process. Charging corresponds to heating up, discharging corresponds to cooling down.

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In order to be able to precisely regulate very slow heating and cooling processes in the manner described, it is necessary that the time constant of the RC element (3) is as large as possible «, According to equation (1) this means that for such an RC element very high resistances and Very large capacitors must be used.

If such an RC element is now connected to the actual control loop (6), it must be ensured that the The internal resistance of the control circuit (6) is significantly greater than the resistance of the RC element (3), since the opposite occurs In the case of the capacitor of the RC element not exclusively via its resistance but more or less would also be discharged via the internal resistance of the control loop, causing a change in the time constant of the RC element and thus to a change in the characteristics of the setpoint generator.

In order to eliminate this source of error, there is another suggestion that between the setpoint generator and the control circuit a buffer amplifier (4) with a very high resistance of the Input circuit (dofcu with very high input resistance or with a very high input impedance) «, This amplifier (4) is to be designed so that its input resistance is greater than the discharge resistance of the RC element (3) used, preferably by at least the cell.

The values from the RC element (3) and the downstream buffer circuit

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stronger (4) voltage output is of the order of magnitude of a few volts. In order to be able to carry out a setpoint / actual value comparison in the control circuit (6), the output voltage must of the buffer amplifier (4) to the order of magnitude of the temperature sensor (7) (thermocouple, resistance thermometer) output measuring voltage can be adjusted. For this purpose, between the buffer amplifier (4) and the Control circuit (6) nor a suitable voltage divider (5) ge ^ be switched.

The subject of the present application is therefore a method for forming a sliding setpoint in electronic temperature controllers for exponential cooling or heating processes using a transducer for the actual temperature, preferably a thermocouple, the voltage of which is compared with the voltage of a setpoint potentiometer and the invention consists in that for the formation of said sliding setpoint that voltage is used which results in each case when charging or discharging a capacitor, the time constant χ determining the course of the exponential function being set as the product of the capacitance and resistance of the capacitor.

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Claims (4)

-7- Ma 1461 -Patent claims: ί1. ) Procedure for creating a sliding setpoint in electronic temperature controllers for exponential cooling or heating processes using a Measured value transmitter for the actual temperature, preferably of a thermocouple, whose voltage corresponds to the voltage of a setpoint potentiometer is compared, characterized by g e k e η η that for the formation of said sliding Setpoint the voltage is used that results from charging or discharging a fine capacitor, where the would determine the course of the exponential function Time constant is set as the product of the capacitance and resistance of the capacitor. 2. The method according to claim 1, characterized in that a partial discharge of the condensate via the internal resistance of the control circuit (6) is prevented by this that between setpoint generator (3) and control circuit (6) a buffer amplifier (4) is switched, its input resistance compared to the discharge resistance of the RC element used (3) is very large, and preferably at least ten times. 3. The method according to claims 1 and 2, characterized in that to adapt the temperature sensor (7) output voltage to the output voltage of the buffer 309848/0841 ■ -β " Dimensions Ί 461 Amplifier (4) between these and the control circuit (6) a suitable voltage divider (5) is connected. 4. Arrangement for performing the method according to claims 1-3 »characterized in that the transducer (7) for the actual temperature of a system to be heated up or cooled down and the associated control circuit (6) is electrically conductively connected to the following elements connected in series: a voltage source (1), a RC element (3), a buffer amplifier (4) with high resistance of the input circuit and a voltage divider (5), between the voltage source (1) and the RC element (3) a switch (2) is located. Gebr. Bohle r & Co Public company patent office 309848/0 841