transducer controlling an vfd through a plc

[Money4Nothing]

The PID control is just a mathematical way (a set of instructions in the PLC program) for the PLC to calculate how to tell the VFD to drive the motor. If you are trying to get the motor to a certain speed so that the pump stays at a constant pressure (based on your transducer input), the motor may have a problem staying at the right speed without a PID control function.

How constant does the pressure need to be on this pump? If the motor is being controlled by a VFD, it probably has a pretty good (fast) response to changes. You probably won't have to worry about overshoot; that is, the VFD driving the motor too far past your desired pressure setpoint. What you might have to worry about is ripple. How much pressure are we talking about here, and how big is this pump? Its quite possible that if the pump runs at a very steady pressure for a given speed setpoint, you might not need a PID at all, and just a simple linear feedback control will suffice.

But if you have a very low tolerance for pressure changes, or the pump pressure output is not very steady, you may need a PID control to dampen out the ripple so the VFD isn't changing the speed back and forth to try to match the setpoint.

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[Clay B.]

To get you started

The presure value you enter from the HMi is the SETPOINT. What the PLC needs to do is ramp the motor up and down to maintain that setpoint. Thats where the PID comes in. In basic terms think of PID as the cruise control on your car. As the car slows down the cruise control adds gas to maintain the speed you selected. As the car speeds up the cruise control lets off the gas to maintain the speed.

Your pump will do the same things thru a PID Loop. Your transducer is your feedback. As your pressure goes down the PID loop speeds up the motor. How fast you accel depends on the loop. Same with decel. That is the tricky part of PID. Finding the values that work for your process. DO some reading on PID and search for examples and you will get it.

 

[stu]

How much pressure are we talking about here, and how big is this pump? Its quite possible that if the pump runs at a very steady pressure for a given speed setpoint, you might not need a PID at all, and just a simple linear feedback control will suffice.
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Hi
the pressure i am trying to maintain is between 40-50 psi,the pump is about 1700 rpm? i am running dough thru the pump which has a open manifold block at the end.

 

[Stationmaster]

Just exactly WHAT does a pressure transducer for DOUGH look like?

 

[stu]

Just exactly WHAT does a pressure transducer for DOUGH look like?

not sure yet still on order, i had a guy come in from ab to tell me what one i will need for the applaction

 

[stu]

hi
can someone help how do you set up a pid instruction,
i am not to sure what it put in where-
pid file?
Process variable?
control variable?
i have an input value from a hmi tag (set point), and i would like a transducer to control the value (at the given set point)?
but where do you enter the output? (vfd)
i hope you can help
all the best stu