Shane_Gordon
Member
Hi All,
I am a new member to this site but have been reading posts on this site for years. I now have to ask a few questions for the PID Gurus out there.
I have a small pressure vessel that I have been controlling with a SLC505 using a PID loop for some time. The system works reasonably well but I'm controlling a device bought from another company. The device is a pressure controller from proportion air. I opened the unit and found that it was simply two $30 clippard valves - one to control the air supply and one to control the exhaust. These are either open or closed valves so they are being controlled via PWM control. I basically send a 4-20 mA signal to the black box and it adjusts the pressure from 0-15 psi via these two valves and a circuit board that contains an "on board" transducer. I'm not sure what calculations the circuit board is doing and how the transducer is adjusting the output. I also have a transducer on my pressure vessel going to my SLC505 as the PV.
I've but this device to test on a quite testing bench and found that it makes adjustments even when my CV has not changed. This is obviously the circuit board and onboard transducer within the device making the changes. I hate this.
I currently found that clippard makes a proportional control and have ordered and installed these in my process. So know I have two proportional valves (0-5vdc), one for air supply (flow control) and one for air exhaust (air flow out).
I am controlling the pressure on a piloted regulator acting as a booster due to the limited airflow that these clippard valves have.
Saying all of this, I have tried a few control methods. Variable feed and Variable bleed - Constant feed with variable bleed - Variable feed and constant bleed.
My setpoint ramp up to 4.5 psi in about 3 seconds then remains at 4.5 for a given length of time then jumps to 6 psi.
I have tried having both the valves controlled by one PID loop but haven't got the best results (0-50% air supply & 51-100% air exhaust). If a leave one or the other constant it creates a lag at the beginning of the SP ramp. So now I have two independent PID instructions controlling two valves.
Finally my questions:
1) Does this make sense?
2) How do I tune two PID loops?
3) Is there a way to auto tune on the fly in a SLC505 ie. Does someone have ladder logic code to do this?
I am a new member to this site but have been reading posts on this site for years. I now have to ask a few questions for the PID Gurus out there.
I have a small pressure vessel that I have been controlling with a SLC505 using a PID loop for some time. The system works reasonably well but I'm controlling a device bought from another company. The device is a pressure controller from proportion air. I opened the unit and found that it was simply two $30 clippard valves - one to control the air supply and one to control the exhaust. These are either open or closed valves so they are being controlled via PWM control. I basically send a 4-20 mA signal to the black box and it adjusts the pressure from 0-15 psi via these two valves and a circuit board that contains an "on board" transducer. I'm not sure what calculations the circuit board is doing and how the transducer is adjusting the output. I also have a transducer on my pressure vessel going to my SLC505 as the PV.
I've but this device to test on a quite testing bench and found that it makes adjustments even when my CV has not changed. This is obviously the circuit board and onboard transducer within the device making the changes. I hate this.
I currently found that clippard makes a proportional control and have ordered and installed these in my process. So know I have two proportional valves (0-5vdc), one for air supply (flow control) and one for air exhaust (air flow out).
I am controlling the pressure on a piloted regulator acting as a booster due to the limited airflow that these clippard valves have.
Saying all of this, I have tried a few control methods. Variable feed and Variable bleed - Constant feed with variable bleed - Variable feed and constant bleed.
My setpoint ramp up to 4.5 psi in about 3 seconds then remains at 4.5 for a given length of time then jumps to 6 psi.
I have tried having both the valves controlled by one PID loop but haven't got the best results (0-50% air supply & 51-100% air exhaust). If a leave one or the other constant it creates a lag at the beginning of the SP ramp. So now I have two independent PID instructions controlling two valves.
Finally my questions:
1) Does this make sense?
2) How do I tune two PID loops?
3) Is there a way to auto tune on the fly in a SLC505 ie. Does someone have ladder logic code to do this?
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