Controling ceramic heating resistor, pwm cycle?

culebrasx

Hi,
I will control some zones with ceramic heating resistors,
I will use just a digital output, reading the heated area with a PT100, closed loop and PID function,

I want to control the ON/OFF of the resisor with SSR contactor like this:
RGC1A23D42KGU
it has and response to ON of: 0,5 cycles + 500us -> 10ms + 500 us

it has and response to OFF of: 0,5 cycles + 500us -> 10ms + 500 us
I want to change the time ON time OFF during a period, so is not exactly a pwm high frecuency, as it has not a high frecuency carrier, I want to vary the relation time ON/OFF in a period, for example of 1 second, with a resolution of 1/25, so variations of 40 ms (4% resolution)

1.-So, my minimum changeover cycle is 21ms-> the cycle must be less than 47 hz,bigger than 21 ms,right?

2.-Anyonse has used the ratio ON/OFF , to control ceramic resistors?

3.-Which will be a good base period to control it? 1 second is fine?

4.-If a divide de maximum output of my PID output by 25 (27648/25) and I discretize it, I have the output in steps of 1/25 and asign a value to a word ,from a comparation; I can make an cyclic OB with 40 ms and make an inncremental counter and compare it whith the discretized PID output, and then stop the high level of my duty cycle, restart the counter for counter>25

5.-It will work?

Thanks for all, I hope my explation is clear,

1. We need your project to write more
2. Use standard function from PID library to implement your project
3. More design tips can be found HERE

I hope these comments help

Hello culebrasx;

To add to Hamid's good advice, you could have a look at an integrated PID Temperature Control instruction available in Step 7, FB 58 "TCONT_CP". It should provide a solution to your heating resistors control.

See the attachment for a brief description of the pulse output function of FB 58.

Start with this manual:

PID Temperature Control

FB 58 "TCONT_CP"

FB 58 "TCONT_CP" is used to control temperature processes with continuous or pulsed control signals. You can set parameters to enable or disable subfunctions of the PID controller and adapt it to the process. These settings can be made simply with the parameter assignment tool…

Application

The functionality is based on the PID control algorithm with additional functions for temperature processes. The controller supplies analog manipulated values and pulse-duration modulated actuating signals. The controller outputs signals to one actuator; in other words, with one controller, you can either heat or cool but not both.

Look also at these FAQs for guidance:

What should you pay attention to with feedforward control on control block FB58 "TCONT_CP"?

How can I optimize PID Temperature Control using STEP 7 V5.3 SP2?

And possibly this discussion can help also:

Question about FB58 "TCONT_CP"

Hope this helps,

Daniel Chartier