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Old March 13th, 2008, 08:28 PM   #1
mcafone
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Pressure rate control PID loop

Hi,
I'm working on a hydraulic rupture disk testing system. I have a variable speed DC motor/gearbox driving a hydraulic generator (a big stainless steel syringe) which is then attached to a valving system, water reservior and a fixture to hold the rupture disk. A pressure sensor provides 0-10 volt feedback to an analog input on an AD DL06 PLC which then controls the DC motor/hydraulic generator speed.

The process starts off with the motor driving at a fixed speed until an initial pressure is reached (around 5000 psi). This pressure is held for a little while to check for leaks and then the pressure needs to ramp up again while controlling the rate of pressure increase at around 100psi/sec.

The question I'm getting around to asking is: what is the best method to control this ramp rate? I was thinking of using a timer to trigger a bit of logic to calculate the current rate of pressure increase and feed that to a PID loop to control the motor speed but, If I remember correctly these PLCs only have 100ms timers and I don't think this will give me the response time I need. Somebody mentioned that a cascaded PID loop might be applicable here but, I don't have any experience with that. The fastest update rate you can set on the PID loop is 50ms so any pressure rate calculation would have to take less time than this. Any suggestions?
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Old March 13th, 2008, 11:07 PM   #2
TConnolly
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1) What is your hydraulic fluid? Is it water?
2) What is your PLC?
3) What pressure are you ramping to?
4) Whats the overall volume?

Chances are you can't achieve the desired results with a PLC, especially if the fluid is water, the bulk modulus is just way too high. You're going to need a screaming fast motion controller.

Do you need to actually control pressure or will controlling the position of your hydraulic generator and measuring pressure be sufficient? Are you proof testing or are you rupture testing?
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Old March 13th, 2008, 11:36 PM   #3
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In addition to Alaric's questions, what is the feed constant of your generator or the volumetric feedrate at max motor speed.

As Alaric said, unless you have alot of entrapped air or something is mechanically absorbing volume things are likely to happen faster than a PLC can keep up with. But some of that depends on total volume versus max feedrate.

Is there something special about the rate of increase or do you just want something slow enough that you get an accurate reading of where rupture occurs? Assuming the test conditions are reasonably consistent you should be able to calculate a drive speed command that gets you pretty close to a desired pressure change rate without cllosed loop feedback.

Keith
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Old March 13th, 2008, 11:43 PM   #4
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Thanks for the answer

I am using water and I am rupture testing. I'm not quite sure that I understand what you mean about the bulk modulus of water (and my backround is in physics). However, I do know that water is essentially incompressible. The burst pressures I'm shooting for are around 10,000 PSI. I have seen other systems that do this with a single board computer (which of course is much faster) but they do it very slowly. The output shaft of the gearbox (70:1) that drives the hydraulic generator runs around 24rpm at max speed and around 3 rpm when performing the final ramp. I figured that since I'm only looking to acheive a 100PSI/sec ramp rate, that a 50ms response time (20/sec) would be good enough. If think otherwise, please help me to understand this.
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Old March 13th, 2008, 11:52 PM   #5
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Kamenges - I was posting at the same time as you, so I missed your questions. I'm using a part #50-5-75-30 hydraulic generator with an 18cc capacity from High Pressure Equipment Inc. I don't know the thread pitch on the driving mechanism nor the length of traverse right now so I can't calculate the volumetric feedrate at the moment.
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Old March 14th, 2008, 12:00 AM   #6
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You said it yourself; water is incompressible. Taken to the extreme that means that, once the vessel is full, any displacement of your pump will cause an instantaneous infinite increase in pressure. We know that water is not truly incompressible. This is where bulk modulus comes in. For the purposes of our discussion it is effectively a spring rate for liquids. Liquids with high bulk modulus are very stiff. Small changes in volume equate to large changes in pressure in a fixed vessel. Water has a relatively high bulk modulus. So small changes in volume cause large changes in pressure.

Basically your system needs somewhere for the liquid to go if it is to be controllable. The slower your control loop the less pressure change a given change in volume can cause without creating instability.

That is the basis of my question. In your response you gave me the speed of the generator driveing shaft. But you didn't tell me how much volume change occurs at those speeds or whatthe volume change is for a given shaft revolution. You also didn't tell us the volume of your test chanmber.

You obviously feel that 3 RPM on the output shaft will give you something in the neighborhood of 100 PSI/sec. Why not just run it at 3 RPM? If the rate of pressure change is 80 PSI/sec or 120 PSI/sec will it have a significantly detrimental effect on the test?

Keith

Edit: Just saw your reply. Will wait for feedrate and volume.
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Old March 14th, 2008, 12:04 AM   #7
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The rate of increase is part of a process specification I need to meet. I am trying to capture the peak burst pressure. At the end of each burst run, some water is allowed to pop out through the rupture disk which is not replaced so there is some air entrapped. I might be able to set a fixed drive speed command but, I'd rather not.
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Old March 14th, 2008, 12:13 AM   #8
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I'll check the specs on the generator tomorrow - got to crash now...
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Old March 14th, 2008, 12:26 AM   #9
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This page lists the generator you are using:

http://www.highpressure.com/pumping....=hp&section=10

It is the 5-5.75-30; 1/2" shaft diameter, 30,000 PSI max pressure, 5.75" max stroke, 18mL per stroke, 14 turns per inch.

So the feed constant is 0.2236 mL/rev.

This is a quick blurb on bulk modulus:
http://www.engineeringtoolbox.com/bu...ity-d_585.html

As you can see the total volume of your test assembly has a significan effect on how quickly the pressure will change with generator displacement. You can use this information, the bulk modulus of water and the pump information to determine an absolute worst case pressure rise. You will never reach thins since your test equipment will bend and you have an unknown amount of air to deal with. Ultimately this is your saving grace as it gives the fluid you are adding somewhere to go. However, it is also your biggest unknown.

Back to your initial post, this doesn't look like a good fit for cascade control, at least in the PLC. In effect you already will have cascade control in a sense if you develop a drive speed command based on pressure feedback and you allow the drive to take care of its own velocity. Your easiest bet may be to stay away from the PID instruction and do a simple proportional control of pressure. Ramp the pressure command at whatever rate you want. Develop the speed command to the drive based soley on the gained difference between the ramped pressure command and the pressure feedback. You will never be at the exact pressure but the system will settle in to increasing at the pressure ramp rate with some constant offset between commanded pressure and actual pressure. You don't need to try and calculate pressure rate this way; just let it naturally follow the command ramp rate. And this can run as fast as you can get the processor to run as it is just basic instructions.

Keith
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Old March 14th, 2008, 12:55 AM   #10
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If all that you want to do is to get the burst pressure, then what do you need the PID loop for? Just read off of your pressure sensor, and select your motor speed based on how close you are to your expected burst pressure (basically ramp down to slow once you pass 9,000 PSI or so).

If you really need the PID, then load up different gains based on where you are in your pressure range.

Either way, you should be able to tune it to be within your rate increase spec.
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Old March 14th, 2008, 02:53 AM   #11
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I think the BIG question is how accurately do you want to read the burst pressure? Do you want 10K +/- 1000 or 10K +/- 1 psi.

The second question is how many of these tests do you intend to do?
IF 10 then forget all the automation and do it manually. IF 1000 then the automation has a good point.

The prime rule of any hydrostatic testing is to ENSURE there is no air in the system. Water of course is used because it is for all practical field purposes non compressible, non flammable, and easily cleaned up.

I do not see how you are going to use a pump with that volumetric output with some kind of powered drive and be able to control it AND be able to "see" the burst pressure - yes I know you are using a transmitter but it has a response time also. Looked at web page above
http://www.highpressure.com/pumping...e=hp&section=10

and saw manual screw type pumps that will give you the resolution you need and the pressure output.

Dan Bentler
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Old March 14th, 2008, 10:30 AM   #12
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You could do this with the RMC70 motion controller without any problem. They have dual loops so you can do a position/pressure control. I would just tell the motor to move in one direction with a pressure limit of your setpoint. I bet you could do all the logic in the motion control and not need the plc. If you did want to keep the plc they have examples on how to talk back and forth.


http://www.deltacompsys.com/

http://forum.deltamotion.com/index.p...0cdf507c32d73d
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Old March 14th, 2008, 03:38 PM   #13
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CharlesM - This Delta Computer Systems company - Are they related in any way to Delta Tau motion controls? If so, I will be running in another direction at top speed. I worked on a system with delta tau hardware over last summer and had nothing but trouble. Their hardware is awsome but, the development environment, setup software and manuals are ****. Very agravating to work with, you practically have to be married to the thing to understand it (boy do I hate being an "un-intentional" beta test site).
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Old March 14th, 2008, 03:41 PM   #14
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Production rate is an issue here. My company is foreseeing a large increase in the number of tests they will need to perform. They expect to need two more machines by July this year.
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Old March 14th, 2008, 03:48 PM   #15
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As far as the response time of the sensor is concerned I don't think this will be the limiting factor. I am reverse engineering an existing system (that uses this sensor) because the exisiting unit(s) we own are continually breaking down. The problem seemd to lie in a custom designed valve actuator they are using. I will be replacing these with pneumatically driven valves with a high MTBF. The ony issue I can see at this point is the response time which may not be so bad considering that I have some air intentionally trapped in the system to provide some limitation on the rate of pressure rise. I think I may hedge my bets on this and use a motion controller as previously mentioned. Any other suggestions on motion controllers?
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