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pressure PID control issue

pressure PID control issue

(OP)
Hello,

I am newbie to hydraulic system and working on a project which is pressure control.
the hydraulic system has a motor driven pump which gives a constant pressure (~3,000psi). I have attached a schematic to show the control schematic. the output pressure of the intensifier is the controlled variable and the pressure is controlled through a proportional valve.
The issue I am encountering is that there is undershoot when the pressure setpoint changing from high to low and the undershoot seems happens randomly. Please see the attachment. Green line is setpoint and red line is actually pressure output. I am guessing that the undershoot is caused by the intensifier since I can hear click sound anytime when there is undershoot. I understand that the intensifier automatically correct itself when it runs out of stroke.

Anyone has experience in how to control intensifier pressure?
Any tips are very appreciated.

Best regards,

RE: pressure PID control issue

What is the intensifier? What is being pressurized? I suspect the undershoot may have to do with the valves in the intensifier not holding while the piston takes in a new charge of fluid.

Ted

RE: pressure PID control issue

(OP)
Hi Ted,

Thank you very much for your reply.
Regarding the intensifier, it is minibooster and ratio is 1:5, the max pressure we use in our application is 12,000psi. Do you think PID control is the suitable controller in the application?

Please let me know if you need more information.

Your help is greatly appreciated.
Best regards,

RE: pressure PID control issue

All the intensifiers that I'm familiar with are reciprocating motor/pump units which I do not see working well after your servo valve. The intensifier should be your pressure source with an accumulator feeding a servo valve.

RE: pressure PID control issue

(OP)
Hello Ted,

Do you mean the intensifier should be put before the proportional valve and after the pump, as well as an accumulator? The system has been built and I am developing control software. I can ask vendor to change it if that is the only solution.

Thank you very much.

RE: pressure PID control issue

I think you have done a pretty good job of tuning your system. I doubt the problem is with the controller. I would look at how constant the 3000 psi supply really is. I would look into adding an accumulator on the output of the pump. Pumps can't respond to flow changes that quickly and it looks like your pressure transients are pretty sharp.

Peter Nachtwey
Delta Computer Systems
http://www.deltamotion.com

RE: pressure PID control issue

Hello,
It does not seem to be any problem with the pressure source because the problem occurs when you send the oil to the tank.
I am convinced that your phenomenon does not occur randomly, but it depends on the volume of oil or elasticity in the part of the circuit where you measure the pressure. The gain to adjust the PID depends on the elasticity of the system. A you to vary the gain with info taken on your machine.

RE: pressure PID control issue

The intensifier is designed to work in one direction and that is to increase pressure. The pressure on the output side will respond to increase in input pressure to create higher pressure, but to decreasing output pressure the output must drop below the desired pressure before the intensifier will 'reset' and deliver the lower pressure. The intensifier will not continuously vary output pressure as the input pressure is varied from high to low.
The response to decreasing pressure steps appears to be characteristic of the intensifier. The check valves in the intensifier prevent flow backwards when the output pressure is higher than the lower desired output pressure. There must be some means other to reduce output pressure other than back through the intensifier.

Ted

RE: pressure PID control issue

It is also possible that your intensifier is a single acting with controlled internal pilote operated check valve system (see documentation PIN Enerpac PID models and models). If it matches the PID model: it is obvious that the pressure drops suddenly when you open the pilote operated check valve. Take a PIN model and add to it instead of the pilot valve item 6 a counterbalance valve that keeps your high pressure 12000PSI. Or to the rise in pressure by the proportional valve that you have on a single coil and put a proportional relief valve to drain without passing through the intensifier.

RE: pressure PID control issue

(OP)
Thanks everyone for the reply.
I am sure I joined to the right forum, a lot helpful info.

I don't know the hydraulic system much and try my best to understand and explain it to you guys.

@Ted, the intensifier is HC2-5.0-G

the system doesn't have flow or very little, at least no flow in the UUT.

Best Regards,

RE: pressure PID control issue

Thank you for giving the reference. It is sure that if I had the hydraulic diagramm and nomenclature I can find better options to solve the problem.

Several ideas:

We delivered a HC2-5.0-B model and you have connected you A proportional valve to the IN and B to R: in this case it does not work well and your DV controlled valve opens abruptly and falling pressure too fast.

It has given you the right model, namely HC2 -5.0 - G and you have made the same connection as described above. Not good because the pressure in R when you want to lower the pressure disrupts the operation of the pressure intensifier.

You have the right model and the R is permanently connected to the tank. This should work. The first thing to check is the absence of abnormal internal leakage by maintaining a constant pressure to the IN port, and measuring with a bucket to drain the hole R. Ask the manufacturer if this value is correct. The solution to the electronics and check that the DV valve works well: Vary the pressure on the IN port on the rise and the descent. Record the pressure at the IN port and port H. You will see that we have an offset between ascending and descending pressure. For example the rise to 1000psi in IN at the rise was 5000psi H and for these to 5000psi descent found in 900PSI IN. So after he would get to set your electronique to account for this offset.

A test that could be done (if the mechanical elasticity to the cylinder and the volume of oil in the pipe is very low) is to add a larger dead volume of oil: for example a long flexible purged well.

You can also try putting a needle valve with check valve on the outlet H for limiting the flow of the actuator to the multiplier.

If you've always random phenomenon there must be pollution which disturbs the operation of the RV valve: remove and clean or put a new component

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