Mechanical program controller
The mechanical program controller is a temperature controller that uses the Wheatstone measuring bridge as the basis for its control part. The program part is based on a timer which is connected to a "contour disk". As a concrete example, a program controller from Jumo GmbH & Co. KG from the 1970s is used as the basis for this article.
The program part
The program part has mechanical scanning that is coupled with a setpoint display ( scale ). By changing the "program" (ie changing the radius of the contour disk) the stylus moves at a certain angle . This angle is passed on to the controller part as a change in resistance.
The drive of the program part consists of a synchronous motor with a triple change-over gear. The cycle time is 24 hours, so a program can be run that runs through different temperatures within 24 hours; after this time it repeats itself or switches itself off.
The rule part
The control part is constructed like a Wheatstone measuring bridge , i. H. two voltage dividers are connected in parallel to each other, as shown in the figure, with two unchangeable resistors ( R1 and R2 ), a potentiometer ( Rv ) and a temperature-dependent resistor ( Rx ), more precisely a PT100 (= a temperature-dependent resistor with a positive temperature coefficient, whose resistance increases linearly with temperature). The potentiometer is connected to the program part and is changed depending on the angular position of the spacer pin. Rv is therefore the setpoint that is sent from the contour disk to the measuring bridge via the stylus.
A PT100, which is installed in the area to be tempered, outputs the actual value .
The program controller now registers the control difference and depending on the value of the bridge voltage , the command is given to heat or cool.
(Note: Article is under construction, pictures and further elaboration will follow)