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cantcliff

Webinar Request: PID with digital Output

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Thank you for the webinar video about PID, was immensely helpful for a project I'm working on.

 

 

Would it be possible to have a short webinar on PID with digital output only instead of analog out?  I know there are some key differences and it would be helpful to see some examples.

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Hello,

 

Currently, we are working on a PID webinar for UniLogic, which should incorporate a digital output for the control value.

 

However, if you are working with a normal transistor output, we recommend using a PWM function block and linking the control value to the duty cycle.  This example can be seen in the example project files of VisiLogic under "PID" and is call "V350_PID simple demo.vlp". 

 

If you are using the controller's HSO, you would also link the control value to the duty cycle, you just would not need to use a PWM FB since the HW configuration handles that for you.

 

Remember, the duty cycles have a range of 0-1000 (0% - 100.0%), so this would have linked to the control value output limits.

 

Let me know if this helps.

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Hello,

 

Could you elaborate a little more on how you wish to control the relay output?  With PID, you are usually either controlling a digital output via PWM or controlling an analog output.

 

We normally do not advise using a relay output with PID since it is a mechanical switch and will degrade much quicker than a transistor output, and the switching frequency is rated much lower as well.

 

If the relay will be on/off for long periods of time, you could manipulate the frequency at which you run PWM to that output.  However, you would need to have a constant duty cycle for that output.

 

Please let me know if this helps.

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You can link the pid output to an MI and run a self resetting timer with your desired preset time.

Then use "less or equal" compare function so that your output will be energized when the timer's current value is smaller than the PID output.

 

For example if you set the timers preset value to 10 seconds, your min PID output should be 0 and the max should be 1000 (10mS resolution).

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A great example is a home project I'm working on using an SSR to run a fridge compressor for temperature control in a home brewing setup.

 

The compressor can't work on a PWM power cycle as cycling it on and off frequently will destroy the compressor motor, and it won't allow the system to get into a cool state where it would function correctly.

 

Essentially it requires a long On-Off cycle time to protect the hardware components, but still needs the advantages of PID control (yeast can get very particular about large temperature fluctuations, +-1 degree can drastically alter the flavor of the final beer.)

 

PWM is useful if the equipment can function analogous to an analog signal but doesn't work well if there are distinct power cycle limitations that can ruin the hardware with the extremely short power cycles.

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Hello,

 

What you are describing above is still essentially PWM.  In order to produce a longer cycle time, simply increase the frequency to a very large time frame.

 

Let me know if this helps.

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