Why can't I adjust the proportional band on a pump?

[PNachtwey]

I can't believe it? You hydraulic people put up with what ever garbage the manufacturers give you.

I want to adjust the proportional band of a pump so I can set the band to be 0% at 1500 psi and 100% flow at 1400 psi or even 1450 psi. If I can I like to know how. If not I would like to know what is wrong with you guys?

 

[hydromech]

Are you refering to a type of PQ control?

You say you want to get a 100% command signal change for 100 or 50 PSI pressure change?

I suspect that you are setting trick questions, you are after all, of electrical origin. You will know what is required.

Has someone put you upto this to increase the traffic in this forum?

Adrian

 

[PNachtwey]

Variable displacement pumps usually are at full stroke or flow when the pressure hard dropped 250-300 psi below the set point. This is too much. The hydraulic pump manufacturers don't seem to think you need to adjust the point where pump is on full stroke. The hydraulic guys just put up with what is provided by the pump manufacturers.

Also, there is no transfer function or formula for these pumps. A transfer function is used to calculate the flow as a function of pressure and time. Anything without a transfer function is a kludge. How can you simulate a hydraulic system with out knowing the transfer functions for all the parts in the system? Doesn't anybody design anything or do you just guess?

No it this isn't a trick question. If I want to design a system that has a constant oil supply what do I do? I can get an electrical power supply that is rated to supply so much current at such a voltage with a rated ripple. What do you do? Why do you put up with it?

Yes I am a EE by schooling and degree but I spend more time with hydraulic circuits. I find it frustrating because one really can't design anything because there are no specifications for how a device responds except for some servo valves. There are just specs for size.

 

[hydromech]

Peter...

With reference to a piston pump...

A pump with 9 pistons working between 40 - 350 BAR. Oil viscoity 5-300 centistrokes. Ambient temp -40 - +60 degrees C. Minimum inlet pressure 650 milibars, maximum inlet pressure 1000 milibars. Pump operating speed 1000-5000 RPM. Medium can be synthetic oil, mineral oil or water glycol.

A single piston will potentially go from -0.350 BAR to +350 BAR in 0.006 seconds, assuming that the piston reaches full pressure in half a rev of the pump.

Quantify the effects of all of that on the pump..?

What do you do with the results, these pumps are being sold into a commercial market. In most cases the end users would not have clue what the result were telling them.

Why not, instead, keep the results for yourself and publish the noise levels, volumetric efficiency and overall efficiency?

Yes, a pump with a standard pressure compensator will go to full flow just below the set pressure. But those pump are really a one size fits all option. They are cheap and readily available.

Bespoke pumps that match flow and pressure to demand within very tight limits are available if you can provide the manufacturer with the operating perameters and heaps of cash.

I am not defending the suppliers, I think they have done too little to advance pump technology for a very long time. Having said that I do know the difficulties involved in designing high pressure pumps for sale into a mass market.

As I have said in other replies, I gave up fighting because the hydraulics business in the UK is in termial decline. No risks, no design inovation.

It is much more fun designing pumps for commercial diesel engines.

Adrian

 

[PNachtwey]

A single piston will potentially go from -0.350 BAR to +350 BAR in 0.006 seconds, assuming that the piston reaches full pressure in half a rev of the pump.

Quantify the effects of all of that on the pump..?

That must be assuming that the outlet of the pump is blocked and the pump is on full stroke.

It takes time move a swash plate. Much more than 6 milliseconds. The pumps I see in the field don't realize they need to be on full stroke until the move is over.

I never heard of Bespoke pumps. I will search for that.

I would solve the problem by using a VFD to drive a fixed displacement pump. The controller can anticipate the load and keep the pump ahead of the game instead of responding to what happened 100 to 200 milliseconds ago.

 

[ScottyUK]

Peter,

'Bespoke' is synonymous with 'custom-built'.


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If we learn from our mistakes I'm getting a great education!

 

[hydromech]

On closed loop hydrostatic marine winches, the pumps have PID controllers, and run off of fixed speed motors.

The winch can be switched into and out of constant tension mode. Constant tension is where the winch can slow, stop and then reverse unidirectionally in fractions of a second(100s of miliseconds) and under full load.

The contollers look at various pressure and load signals from around the system and signals the pumps servo piston to respond. The pump anticipates the next step, but can also respond to unexpected load changes. Any losses in speed due to viscocity or internal leaks are made up by a boost pump or an accumulator if a faster response is required.

You are correct, a pump swash plate cannot move in 6 milliseconds but I think that considering the load and acceleration forces at work, they do pretty well to respond as quickly as they do.

No matter what we do we cannot get hydraulic oil replicate the behavouir of electrons.

 

[PNachtwey]

hydromech, what you suggest is fine for winches. In fact our controllers are sometimes used to control speed and tension on winches. Ocean applications are not precision applications and their challenge is that a winch controller must respond to what the water is doing. A flying shear is a different matter. The traverse part must quickly accelerate to match speed so the shear can cut. I have customers that want to be cut to within 0.01 inches. This requires that the hydraulic gain be kept constant which in turn requires the supply pressure be kept constant. The motion profile is known and happens at a rate proportional to the production line. I run across all the time. The acceleration part of the motion is the most critical part and yet most pumps don't respond have enough for hundreds of milliseconds. The pumps don't supply the rated flow until the pressure drops 250-300 psi.

You guys know how to calculate the hydraulic gain don't you? You know meters per second per volt? How does the pressure affect the gain?

Keeping the oil supply pressure constant requires a pumps that can respond quickly to deviations in pressure and big accumulators. The proportional band must be adjustable so one can adjust the pump so it supplies the rated flow with only 50 or even a 20 psi drop. Can you imagine the power supply in your TV set varying 20%. You expect electrical power supplies to be close to perfect. What about hydraulic oil supplies? What if the electric company varied your AC voltage by that amount?
I bet those electric servo guys have excellent power supplies ( drives ).

 

[hydromech]

Yes...yes...yes!

There are so many variables, if a pump was setup to respond to such a small delta P, the inherent mechanical hystersis would cause it to start hunting with the slightest of changes within the system. It would never be stable.

You must see that oil does not behave like electrons and that a variable flow pump is subject to mechanical forces that make it awkward to control with a high degree of accuracy.

There will always be applications where even the most advanced hydraulic pump systems cannot be used.

 

[IceStationZebra]

Quote Hydromech: A single piston will potentially go from -0.350 BAR to +350 BAR in 0.006 seconds, assuming that the piston reaches full pressure in half a rev of the pump.

Quote PNachtwey: That must be assuming that the outlet of the pump is blocked and the pump is on full stroke.

Basically yes - why design a system for 350 bar if you aren't going to use it?

And I have to agree with Hydromech. Are you going to tell me that the system using the output of this "perfect" pump is never going to vary its demand by more than 25psi? It will probably vary more than that just due to differences in oil temperature!! And setting up a mechanical system this tight but not expect it to resonate?!?

And don't even think that me complaining about a pump control will cause any fixes by the manufacturer. Unless the loss of my business will hurt him it ain't happening!

ISZ

 

[PNachtwey]

There are so many variables, if a pump was setup to respond to such a small delta P, the inherent mechanical hystersis would cause it to start hunting with the slightest of changes within the system. It would never be stable.

Yes, you are one of the few that understand that. I know that decreasing the proportional band down to 20 psi is similar to increasing the gain on a proportional only controller. Once the gain gets too high the system while hunt and be unstable.

But what is too high? The pump pressure gain is the flow/PSI_of_error. The hydraulic system( not counting the pump ) gain is rate of change in PSI /flow. It is obvious that the pump gain can't be high as well as the hydraulic system gain being high because the system will oscillate. However, if I put accumulators on the system that reduces the hydraulic systems gain. The rate of PSI change / flow will be reduced. If I stick a big enough accumulator on the system the down stream pressures will change slow enough so the pump can respond without oscillating.

The problem is that the pump manufacturers think we are all idiots and can't design a system properly for pumps with adjustable proportional bands. We must put up with 250-300 psi pressure sags.

You must see that oil does not behave like electrons and that a variable flow pump is subject to mechanical forces that make it awkward to control with a high degree of accuracy.

You would be surprised. That is what I do.

There will always be applications where even the most advanced hydraulic pump systems cannot be used.

The way things are going there will be more and more of them because the 'advanced' hydraulic pumps are so crude.

Basically yes - why design a system for 350 bar if you aren't going to use it?

I don't understand the point you are trying to make. I am not talking about the maximum pump output pressure. I am talking about the ability to maintain a control pressure like an electric power supply can maintain a constant voltage. I think ( know ) it is possible.

And I have to agree with Hydromech. Are you going to tell me that the system using the output of this "perfect" pump is never going to vary its demand by more than 25psi?

If there is a big enough accumulator then the pump will keep up with the pressure changes. If one adds a smarter pump controller then one can probably get by with a much smaller accumulator. I don't know of any pump controllers that try to predict loads or use a derivative gain.

The perfect pump can already control pressure much finer than 25 psi. It is closer to a PSI or two. It uses a VFD instead of a swash plate though.

It will probably vary more than that just due to differences in oil temperature!!

I don't see why pressure feedback would depend on viscosity.

And setting up a mechanical system this tight but not expect it to resonate?!?

I explained that above.

And don't even think that me complaining about a pump control will cause any fixes by the manufacturer. Unless the loss of my business will hurt him it ain't happening!

I think that the more you complain the more they will respond. I have seen it happen.

 

[EdDanzer]

Peter,

Your comparing response of a hydraulic system to grid power is not taking into account the grid is a large capacitor, but if you are on an overloaded system in rural Thurston County in Washington State, a 20% voltage variation is common. Electronic devices have capacitors in their power supplies to make up for incoming voltage variations.

A hydraulic pumps stroke response time is partially market driven. If your volume is enough to justify the cost of redesign or you are willing pay to have a special design built I’m sure it can be done.

Why do you need such precise supply pressure control? Flow is what moves fluid power, pressure drop creates heat and can make movements nonlinear but pressure by it's self cannot do work.

Ed Danzer

http://www.danzcoinc.com

http://www.dehyds.com

 

[PNachtwey]

The actuator gain varies as the hydraulic pressure varies. The controller can compensate for some variation. A 300 PSI variation at 3000 PSI is not that bad but a 300 PSI variation at 1500 PSI is significant.


Aren't most of those swash plates control by pressure on a small piston pushing against a spring to port oil and move the swash plate? I would think that getting different springs with different spring constants would allow the piston that pistom that port the oil to the swash plate at different pressures. I understand that changing out springs may be tricky and not something you would want to do. A more sophisticated approach would be to use electronic controllers.

 

[budt]

Ed Danzer wrote:
"Flow is what moves fluid power, pressure drop creates heat and can make movements nonlinear but pressure by it's self cannot do work."

You let that statement pass Peter. Now I'm wondering if you have seen the light?????


Bud Trinkel, Fluid Power Consultant
HYDRA-PNEU CONSULTING

 

[budt]

Peter wrote way back at the beginning:
"I can't believe it? You hydraulic people put up with what ever garbage the manufacturers give you.

I want to adjust the proportional band of a pump so I can set the band to be 0% at 1500 psi and 100% flow at 1400 psi or even 1450 psi. If I can I like to know how. If not I would like to know what is wrong with you guys?"

Since there are very few TRAINED/DEDICATED persons in Fluid Power, Why would you expect anyone to give Feedback, When a they try to make observations about a discipline they have a one week Seminar in given by a Fluid Power Manufacturuer and attended 3 years ago?????? On top of that they have worked on everything except a Hydraulic or Pneumatic system during those 3 years. Try that type of training and work situation for Mechanical and Electrical types!!!!!

When a manufacturer hears only 5 complaints about a component and they have sold and applied 40,000 of them, and on top of that the complaint is "This valve stops working when I don't change Filters" or some other feedback as bad. why would they take it to heart?????? And I heard a lot worse feedback than that in my sales days.

People are not Dumb, Just not TRAINED and may not have enough opportunuty to apply the training to keep up with a technical field like Fluid Power anyway.

Oh well, Someday, Maybe????


Bud Trinkel, Fluid Power Consultant
HYDRA-PNEU CONSULTING

 

[stroupaloop]

I agree with Bud here on this one.

Your question is of obvious merit and concern for various working applications. One that comes to mind is the inherentely growing need to have precision robotic control in unique processes. The problem, as Bud stated, is that the manufacturers aren't required or better put, do not see the demand, to make such alterations.

The solution to your answer has been grazed a few times already by you, which is quite expensive. A VFD would be the most direct route in solving your control issues, but to get such fine control, an implementation of many other measuring devices would be required to check constant parameters of the system to allow the VFD to make accurate and timely changes.

Your question is absurd, but very unique for the average "hydraulic" engineer who has not had a large amount of experience with such types of control. Its also important to note that this issue is going to be harder and harder to press as the hydraulic industry is always looking to reduce in size, which means higher and higher pressure, allowing for a greater amount of "slop" in their pressure bands.

Sidenote, although the manufacturer doesn't always provide it, you can calculate the magnitude ripple of the flow supply based on various design parameters of the pump, allowing for a more refined model.

 

[PNachtwey]

So why is a VFD attached to a fixed displacement pump so expensive? The fixed displacement pump must be far cheaper. That should offset the extra cost of the VFD ( motor and power unit ). A lot of the drive units have enough smarts to replace the PLC on small projects. I know our controller does.

I have customers doing this already but just a few. At the IFPE show there was a small company there that specialized in driving actuators directly. This would be expensive on a multi axis system where each axis has its own VFD and controller. I was thinking that one VFD and fixed displacement pump would supply and accumulator just as the pressure compensate pump would. The difference is that the VFD can be controlled much tighter and intelligently. For instance on a flying shear the VFD speed/flow can be geared to the line speed. This would be very efficient. Better yet the VFD could be ramped up and down like an electron cam system that is still geared to the line speed.

Now when the line speed increases the VFDs speed increases instead of responding only to a pressure drop. When the line speed slows down so does the VFD. That is efficient.

 

[stroupaloop]

I suppose it would depend on the sizing requirements of the pump to determine cost efficiency. For some applications, fixed displacement pumps can be 200-300 dollars compared to a pressure compensated pump at 2000 dollars, but a VFD can run up to $4k. You also have to consider the use of a torque and speed sensor depending on the type of motor and pump setup to verify the output of the VFD matches the actual delivery of the pump.

 

[budt]

Interesting exchange Peter and stroupaloup.

BTW, stroupaloup, Peter has not called you Idiot or worse even once. It is good to see him have someone who can discuss matters on his level.

On the use of VFD drives an fixed volume pumps, In my opinion, Cheaper is not always Less Expensive. You must look at the operating cost, downtime for repair and longevity of components.

I have earned many sales by explaining why the added cost, of say Extra Filtration, would payback fast because all the above would be reduced and often eliminated. As I mentioned in another post, adding filtration to one system increased pump life from 2 times a year change out to most pumps now running 5-7 years 24/7/5. And that was just one situation.

Also, can you imagine the extra hours of operation a pump would have if it did not turn 1,200-1,800 RPM anytime the machine was operating.

And, how about heat generation, how much do Heat exchangers cost and how much do you spend on cooling energy to remove HEAT that should not be there and even cost extra to produce????

As I said CHEAPER IS NOT ALWAYS LESS EXPENSIVE!!!!!!!


Bud Trinkel, Fluid Power Consultant
HYDRA-PNEU CONSULTING

 

[budt]

The old saying that says:

"PAY ME NOW OR PAY ME LATER" or at "leas PAY SOMEONE LATER"

Never fails when it comes to doing it right the first time.


Bud Trinkel, Fluid Power Consultant
HYDRA-PNEU CONSULTING