Need Help With SERVO VALVE Control

I have been given a project where we need to use a servo valve to control hydraulic pressure, particularly the transient spikes seen during any one full cycle of the valve.

The servo valve will be cycled on/off at 1-2 Hz to deliver "controlled" hydraulic line pressure to a test part (another valve). We need to get rid of our current setup where we are simply using an on/off hydraulic valve in favor of the servo valve so that we can better control the pressure spikes seen on the pressure profile. Both the "ON' and "Off" sides need to follow specific pressure profiles during cycling.

My question is, how do I control and setup a profile to the servo valve using a SLC-500 with RSLogix500? Do I need some sort of motion control card for the SLC?? All I have is analog out and In cards. The Servo Valve is a high end Moog valve. It has its own configuration software but I'm just having a bad feeling that that is not going to be good enough and I want to be covered if I'm forced to bypass their software. The Servo Valve that I am getting is the analog input one (non-fieldbus version) - Moog D638. Any assistance or suggestions would be helpful.

busarider29 said:

I have been given a project where we need to use a servo valve to control hydraulic pressure, particularly the transient spikes seen during any one full cycle of the valve.

The servo valve will be cycled on/off at 1-2 Hz to deliver "controlled" hydraulic line pressure to a test part (another valve).

Click to expand...

Pressure builds as oil is added to the pressurized volume. You need to open the servo valve SSSLLLOOOWWWLLLY to avoid the pressure spikes and there probably isn't a need to open the valve more than about %5 but usually less.

We need to get rid of our current setup where we are simply using an on/off hydraulic valve in favor of the servo valve so that we can better control the pressure spikes seen on the pressure profile. Both the "ON' and "Off" sides need to follow specific pressure profiles during cycling.

Click to expand...

Yes, it is obvious to me that you would see spikes using simple on-off control. Real pressure control is don't by carefully controlling the flow to control the rate of change in pressure. See the equation below

My question is, how do I control and setup a profile to the servo valve using a SLC-500 with RSLogix500? Do I need some sort of motion control card for the SLC?? All I have is analog out and In cards.

Click to expand...

You need a hydraulic motion/pressure controller. PLCs are not fast enough.

dP/dt=ß*Q/V
Where
dP/dt is the rate of pressure change.
ß is the bulk modulus of oil
Q is the net flow into the pressurized cavity
V is the volume of the pressurized cavity

dP/dt is very high unless the flow Q is controlled.

SLCs don't have fast analog inputs nor do they have high resolution inputs

The Servo Valve is a high end Moog valve. It has its own configuration software but I'm just having a bad feeling that that is not going to be good enough and I want to be covered if I'm forced to bypass their software.

Click to expand...

Moog makes good valves. Which one is it? True servo valves don't need configuration software.

The Servo Valve that I am getting is the analog input one (non-fieldbus version) - Moog D638. Any assistance or suggestions would be helpful.

Click to expand...

That is a good valve. It is what Bosch calls a servo solenoid valve in that it uses electrical power to move the spool rather than hydraulic power like a true servo valve.

It is also good you got the analog version. The field bus versions are slow and non-deterministic.

Are you doing any position control?

What you need is this:
http://www.deltamotion.com/products/motion/rmc70/
The RMC75E is powerful enough to be stand alone but you can talk to it using Ethernet if you want to attach a PLC or HMI.

I am the president of Delta Computer Systems. We make hydraulic motion/force controllers to do sophisticated hydraulic control.

Peter Nachtwey said:

Pressure builds as oil is added to the pressurized volume. You need to open the servo valve SSSLLLOOOWWWLLLY to avoid the pressure spikes and there probably isn't a need to open the valve more than about %5 but usually less.



Yes, it is obvious to me that you would see spikes using simple on-off control. Real pressure control is don't by carefully controlling the flow to control the rate of change in pressure. See the equation below


You need a hydraulic motion/pressure controller. PLCs are not fast enough.

dP/dt=ß*Q/V
Where
dP/dt is the rate of pressure change.
ß is the bulk modulus of oil
Q is the net flow into the pressurized cavity
V is the volume of the pressurized cavity

dP/dt is very high unless the flow Q is controlled.

SLCs don't have fast analog inputs nor do they have high resolution inputs


Moog makes good valves. Which one is it? True servo valves don't need configuration software.


That is a good valve. It is what Bosch calls a servo solenoid valve in that it uses electrical power to move the spool rather than hydraulic power like a true servo valve.

It is also good you got the analog version. The field bus versions are slow and non-deterministic.

Are you doing any position control?

What you need is this:
http://www.deltamotion.com/products/motion/rmc70/
The RMC75E is powerful enough to be stand alone but you can talk to it using Ethernet if you want to attach a PLC or HMI.

I am the president of Delta Computer Systems. We make hydraulic motion/force controllers to do sophisticated hydraulic control.

Click to expand...

Peter,

Thank you for the detailed reply. Looks like you and your company are the ones to talk to on this. I'll give you guys a call later this afternoon my time (EST). As for your questions:

Like stated, the valve is the Moog D638/639. I tried to upload the PDF documentation on it with my original post but it didn't take. I'll try again with this post. It's on Moog's website. It's the analog input version that I'm looking at getting. I'm supposed to get a quote back today from our vendor. I'll mention some of the things you brought up as well even though he's a pretty knowledgeable guy on hydraulics. Although he doesn't know much about the Moog D638 other than its a really good servo valve. He may recommend something else for our needs once I talk to him today. I don't know, we'll see.

As for the Moog D638, assuming that is the valve that we go with, it has analog input for the trigger. It also has it's own configuration software for setting up specific profiles. Great......it if works for my needs. From my understanding, one doesn't need to use the configuration software but can if need or wants to. I'm thinking that I'm going to need/want to use it since I have no experience setting up a pressure profile on a servo valve and this is a hot project that we need up and running ASAP.

We are not doing any position control. This project is simply an off/on cycling of the servo valve where the "controlled" line pressure will be delivered to the test part, which is a simple hydraulically actuated valve. The test is required to run 1.8 million cycles where 1 cycle = on and off of the hydraulically actuated valve. Although not a test requirement, we would like to run the test as fast of a frequency as possible without jeopardizing the integrity of the test. So say 2-3 Hz??

With the SLC and its analog's being slow, wouldn't it still be doable if all we are doing is triggering the valve, since the valve has it's own configuration software and the closed loop control is internal to the electronics of the valve.

analog is fast enough, however PWM control is about 10 Hz so not good.

Use an analog output on the PLC to control the valve.
have the power to the valve only on after the analog output card is initialized, as 0 out means hardleft.
Do all control in the PLC, and have the valve just follow a simple pattern 2.5 Volt is hardleft and 7.5 Volt is hardright. It is standard.

busarider29 said:

Peter,

Thank you for the detailed reply. Looks like you and your company are the ones to talk to on this. I'll give you guys a call later this afternoon my time (EST). As for your questions:

Like stated, the valve is the Moog D638/639.

Click to expand...

OK, I downloaded the pdf. This valve is a smart valve. It has its own PID built into it. You may be able to get buy with just the on board PID controlling around a analog pressure reference from the PLC.

This may work for you if the pressure set point doesn't and actual pressure control is not that critical. The on board PID will simply try to follow the pressure reference from the PLC.

With the SLC and its analog's being slow, wouldn't it still be doable if all we are doing is triggering the valve, since the valve has it's own configuration software and the closed loop control is internal to the electronics of the valve.

Click to expand...

The SLC's analog is too slow to control the spool directly but in this case the SLC's analog is just a pressure reference. It should be fast enough for that. If you are just going change the pressure set point every 500 millisecond between two voltages.

Our controller would not be needed in this case unless you need tighter control, more flexible SP profiles, better data acquisition, graphics, interface to labview, tech support etc.

It now depends on what your needs are. With our controller the PID in the valve would not be necessary so you could buy a cheaper valve without the PID.

Peter Nachtwey said:

OK, I downloaded the pdf. This valve is a smart valve. It has its own PID built into it. You may be able to get buy with just the on board PID controlling around a analog pressure reference from the PLC.

This may work for you if the pressure set point doesn't and actual pressure control is not that critical. The on board PID will simply try to follow the pressure reference from the PLC.


The SLC's analog is too slow to control the spool directly but in this case the SLC's analog is just a pressure reference. It should be fast enough for that. If you are just going change the pressure set point every 500 millisecond between two voltages.

Our controller would not be needed in this case unless you need tighter control, more flexible SP profiles, better data acquisition, graphics, interface to labview, tech support etc.

It now depends on what your needs are. With our controller the PID in the valve would not be necessary so you could buy a cheaper valve without the PID.

Click to expand...

Yeah, was able to get some feedback on the D638/639 from Moog's tech support. It is not able to store a pressure profile but simply follows a set pressure referenced from the PLC (like you stated). It has a built in pressure transducer and PID. So......I WILL need a motion controller since I need to be able to follow a tight pressure profile on both the "ON" and "OFF" cycle of the valve and not just cycling between two different set pressures. And like you stated, I can go with a less expensive valve now. We are still looking at valve options and are in touch with a vendor in the Detroit area who we will also go through for the Delta motion controller. I should get some more feedback from them sometime today.

I've used the Delta RMC controller before. It really does work quite well. I've used it on press controls where the pressure on the die was absolutely critical (aerospace industry). The Delta was able to do the job and do it well. It could keep the pressure within 1 PSI of the setpoint (and these are setpoints in the 4,000 PSI neighborhood, so around 0.03% deviation to be conservative). To send it commands I was using a Micrologix 1400, so in your case you'll need a SLC 5/05 with Ethernet if you're going to stick with that platform. The documentation is quite thorough and you can control it with MSG commands over Ethernet.