Hotplate heat Control

[sparks32]

Hi, I have a large sealed cooking pot and I need to keep it at between 75 - 85 degrees Celsius for 90 mins at a time.
I have an electric hot plate hooked directly into a 240V 50 HZ AC supply Via a Solid State relay controlled by input 1 of a  Fx2424 and a sealed lm34 hooked into ADC 1 reading the temperature.
So I was thinking of trying to work out the PID function. But because it is a 50 HZ ac supply i don’t think using PWM would work well. I was thinking of a low switch rate duty cycle control to regulate the temperature similar to what the simmer stat would do. Should i be use the pid function or try to use 2 timers and try to use a custom function to alter the set values? Thanks for the advice in advance.  

[sparks32]

OOPS I meant the SSR was controlled by output 1 not input 1 thanks

[garysdickinson]

I think that you are correct in your assessment of the PWM capabilities of the PLC.  The lowest PWM frequency for the current PLCs is 50Hz.  To get a 1% resolution on PWM you'd need a frequency closer to 0.5Hz.  The PLC's PWM hardware/firmware doesn't go slow enough!

You need a slow PWM system for your SSR. I have sample code from an older project that does slow motion PWM.  I used COUNTERS to get a PWM output with 1% resolution.  At 1% the output will be one for 20mS. out of every 2S At 99% the output will be on for 1.98S out of every 2S.

I think that PID is an excellent approach for computing the next value for your hot plate.

The following is what I have done to get slow PWM

There are 2 custom functions:

[*]SetPWM - Function to adjust PWM duty cycle

'SetPWM - Custom function to determine PWM duty cycle
'
Call ComputePWM      ' Compute next PWM value in A. A must be in the range of
               ' 0..SV of the TotalPeriod COUNTER
               '
               ' This function is a dummy, the value in A changed via the
               ' on-line debug interface for the purpose to test/debug.



' The SV (Set Value) of the COUNTER, OnPeriod, determines the overall duty cycle of the PWM output.
' The range for the SV is bewteen 1 and the SV of the TotalPeriod COUNTER.
'
' If the SV of the OnPeriod COUNTER is set to 1, this is the minimum duty cycle.
' If the SV of the OnPeriod COUNTER is >= the SV of the TotalPeriod Counter, then the PWM output
'   is always on. 100% duty cycle.
' If the SV of the OnPeriod COUNTER is set to 0, then the contact associated with this COUNTER
'    is undefined, but treated by the ladder logic as being "FALSE".  Setting the SV to the
'   OnPeriod COUNTER does not get a 0% PWM output, but rather a 100% PWM output. The "fix" to
'   this problem is to clear the EnablePWM RELAY when a 0% output is desired.
'   EnablePWM is cleared to disable the PWM output when a 0% duty cycle is required.
'
IF (A = 0)
   ClrIO EnablePWM         ' 0% PWM is special, so disable the hardware
ELSE
   SetCtrSV OnPeriod, A   ' Non-zero PWM, so let the hardware run...
   SetIO EnablePWM
ENDIF


[*]ComputePWD - Function to compute next PWM duty cycle value
This function, is currently a placeholder (dummy).

' ComputePWD Function to compute new PWM value, Value is returned in A
'
' This is a dummy function and is to be replaced by working code
'
RETURN

[/list]

Gary D

[sparks32]

Thanks for the advice Gary D I'll have to digest that. Much appreciated. I have a bit of time to read up on those functions and tinker with them a bit I should have it running properly some time next week. i'll let everyone know how it all paned out for anyone else who wants to do something like this.