Thursday, April 21, 2011

PLC for Custom-design of a PID/PWM Program

The use of a DSP (Digital Signal Processor) to evaluate the actual value of an analogue process output and then compute a correction signal has many benefits. Digital Signal Processor does not suffer from the long-term drift effect that analogue circuits do. Changes to constants can be made easily without the actual physical change to the circuitry and simply modifying the loaded program or loading a new one can alter radically the mode of control. Realizing techniques of PWM and other advanced functions is some of the vast power points of digital controllers. In most cases DSPs are created to replace the ON-line analogue ones. This provides the continuous approaches to implement digitally the traditional analogue control modes such as PID actions.

Because of the advantages of the PLCs, a PLC type S7-200 was utilized as a DSP. Currently most modern sequential control systems are based on PLCs, which are in fact specialized industrial computers. Thus, the goal of this project is to design a PLC program for PID control algorithm and to develop it to get at the PLC output a PWM signal proportional to the value at the output of PID controller. In this case there is no require for a DAC IC nor for a specific power amplifier stage. Here the PWM signal that has the power static switch emulates the function of a D-type power amplifier. From another side, with the designed interface board, one can exclude the implementation of a high cost proprietary power interface module, this simplifies the circuit and reduces its cost.

PLC manufacturers are very often to provide the option of analogue I/O and support instant PID functions for extra cost. Such functions can be used immediately by entering the control parameters and constants. This feature is able to use if the analogue I/O module is installed. Therefore PID algorithm was created instead of using a ready one. This, also, offers more flexibility to use the program with those PLCs, which do not support ready PID loops. Designed I/O board satisfies our demands and costs one third of the I/O module's cost approximately.

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