PWM - Heating fan

[RagnarF]

Hi,

I am trying to have my PLC start a heating fan in a linear manner. A certain percentage at a certain received setpoint.

To do this, I use a PWM and Solid State Relay and break up the current. I have been using free software from Crouzet and I can’t figure out how to do this. Any hints are most appreciated.

 

[DaDaDadeo]

Try using Timed Proportional Control. This is basic Proportional control with no "ID". It works well for digital control when the variable reverses when in an opposite state. Ex: Temperature increases when on and decreases when off which can also be reversed for cooling control. Timed Proportional also does not wear out components as much as PWM.

Below is a calculation example:

TimerPreset = Always running repeating timer set to 4 seconds (4000 milliseconds or whatever. A higher resolution is more accurate)
PBand = 1.0
StartPoint = SetPoint - PBand
Variable = Analog Input
Error = SetPoint - Variable

Output% =:
Variable < StartPoint = 100%
Variable > SetPoint = 0%
Variable > StartPoint &< SetPoint = Error/PBand

PropOut = TimerPreset*Output% (Ex: 4000*0.25 = 1000)

Compare = TimerPreset - running timer value

DigitalOut =:
At repeat start of timer & PropOut > 500 = 1
PropOut < Compare = 0

The 500 is a 1/2 second limit for anti-chatter.

An example of Increase Control with a 4 second repeating timer:
If the input value is less than the set point by more than the PBand, the output is always ON.
If the input is above the set point, the output is allways OFF.
If the input is in the PBand at 25%, the output is ON for only 1 second and off for 3.

The best about this type of rudimentary calculation is that all different types of controllers from Allen Bradley to Raspberry Pi will work exactly the same way.

 

[TheWaterboy]

If the PLC has a PWM output use that...
Or use a DO to start your existing ramp circuit ...
Or provide a lot more detail.

 

[DaDaDadeo]

"A certain percentage at a certain received setpoint."

If an analog output is used, remove the timer and use the "Output%" from the last post. This is basic raw Proportional control. Expand the PBand if necessary for a longer ramp.

 

[lfe]

Are you sure that solid state relay is fast enough to follow the frequency of the PWM pulses?

You should use a specific amplifier for PWM

 

[RagnarF]

Try using Timed Proportional Control. This is basic Proportional control with no "ID". It works well for digital control when the variable reverses when in an opposite state. Ex: Temperature increases when on and decreases when off which can also be reversed for cooling control. Timed Proportional also does not wear out components as much as PWM.

This sounds interesting. Being new to this, I have never heard of this. I have read some articles about this now. Some questions arise. 'TPC' will then controle my Solid state relay or Thristor, right? So I can have any current!

I will receive a Setpoint four times per second which I then will "translate" to an Output. Would that frequency be a problem?

 

[RagnarF]

If the PLC has a PWM output use that...
Or use a DO to start your existing ramp circuit ...
Or provide a lot more detail.

Hi TheWaterboy,
I can't stress enough how new I am to this. And eager to learn.

The PLC I am planing to use does have a PWM output. Would I controle it with a timer? Or is there an other "block" / funktion to use?

 

[RagnarF]

Are you sure that solid state relay is fast enough to follow the frequency of the PWM pulses?

You should use a specific amplifier for PWM

Hi lfe,
My ignorance is unlimited I have not yet purschased the SSR nor Thyristor so I will make sure I verify the capability of the SSR. Thank you!

 

[DaDaDadeo]

This sounds interesting. Being new to this, I have never heard of this. I have read some articles about this now. Some questions arise. 'TPC' will then controle my Solid state relay or Thristor, right? So I can have any current!

I will receive a Setpoint four times per second which I then will "translate" to an Output. Would that frequency be a problem?

We use TPC very effectively for controlling a valve for temperatures and pressures. There is a popular article describing it to control a chemical pump. In your case, the PLC output will send a signal to a SSR. The current draw is limited to the SSR rating and not the PLC control.

PWM will turn on and off rhythmically at different speeds whereas TPC may turn on and off at odd intervals according to the reaction of the input in relation to the setpoint.

If "I will receive a Setpoint four times per second" means that the target is moving or bouncing every 1/4 second, I am not sure what the reaction would be for both TPC and PWM. With TPC, the output is only turned on at the timer reset point and then turned off according to the output percentage. If you meant ramping, then that is commonly added by resetting the setpoint limit at every timer interval (4 seconds in my example).