Using PWM to control electric resistance heater

[PlasticsDude]

I want to use pulse width modulation to control an electric resistance heater. I've got "real" thermocouple data being fed into memory. How does one generally go about feeding this data into a loop to run a pulsed output? Thanks! (S7-200 CPU 224XP)

[P Daniil]

PWM is generated by having a variable time interval (´pulse width´) triggered at constant frequency. So you need a constantly repeating event (say at T secs). Within this period you generate a pulse (with any suitable timer arrangement) which is proportional to the required power output (say t secs). The output power equals the ratio t/T times the full power of the heater. If the heater power is large and the system has large thermal inertia you should also consider integral cycle control, ie driving the heater with a number of whole line cycles (synchronized at the line zero crossings, at twice the line frequency 100/120 Hz) every fixed number of cycles. In this way minimum noise is generated in the line. Edit: To derive the required heater ouput you must first process the thermocouple measurement to generate an error value which must then be further processed with PID functions. You can find many and very good posts on this subject in MrPlc. Edited March 2, 2007 by P Daniil

[beegee]

you will probably have a slow system where changes are not so fast. In heating solutions you should dimension your resistor to be able to keep the temperature at the typical nominal temperature at 50% power. You don't have to switch it on and off so fast (this cycle is called the control period) if you don't have a fast system. we have had very precise temperature control with 1 to 2 seconds control period. You will then use a solid state relay to switch off and on the resitors. The time you switch it on and off will be the result of the percentage of output calculated by your PID function. Depending on the brand of PLC you will have special functions or dedicated TC cards. An other option is to use a low cost TC (48x48 ...) from for instance JUMO, Hero or Omron. (Simple and cheap)