digitalPIDcontroller.pdfVIP

GENERAL INTEREST digital PID controller closing the loop with a PIC The term ‘PID controller’ normally brings to mind an analogue controller circuit with a couple of opamps. This article shows how such a controller can be implemented in digital form. In this universal controller circuit, based on a PIC and a D/A converter, all control para- meters can be entered via a keyboard to adapt the controller to the task at hand. The digital PID controller described in this article, based on a PIC microcon- troller, is on the one hand very well suited to studying how Specifications: the controller operates with various parameter ? controller type: PID values, using simple con- ? parameter entry: hexadecimal keypad ? parameter display: alphanumeric LCD ? adjustable parameters: - set value - initial control quantity value - upper limit - lower limit trol loops. On the other hand, it can of course also be used for specific control tasks. A complete control loop requires an appropriate actuator and a suitable sensor, in addi- - sampling interval (100 ms to 25 s) tion to the controller ? parameter storage: EEPROM itself. ? A/D and D/A conversion: 8 bit The core of the circuit (measured value, control quantity) is an inexpensive PIC ? microcontroller: PIC 16C71 16C71 microcontroller, whose built-in A/D con- verter digitizes the mea- sured value (actual value) and pro- Design By D. Kohtz vides the result to the controller pro- 14 gram. The program determines the control error relative to the set value (target value) for each actual value and, based on this, calculates the control value that is output to the external D/A converter. The control parameters, which are entered by a keypad, are stored in a non-volatile memory (EEPROM). In order to manage with the program memory of the PIC, which is only 1 kB, the parameters are entered as hexadecimal codes. They are however displayed on the LCD as text, which is easier to understand. BASIC THEORY Control circuits are encountered very oft