Question about counterbalance valves
Question about counterbalance valves
(OP)
My question related to the back pressure produced by the valve. From what I've learned so far on my own is that the Pilot ratio determines how much pressure is going to unseat the pilot and along with the bias spring determines the amount of back pressure that is maintained while the pilot is opened, correct? My application uses a counterbalance valve and the generated back pressure is taking away much needed net force. I have a 6:1 valve, would a 10:1 reduce the back pressure? If so by about how much compared to the 6:1?





RE: Question about counterbalance valves
ISZ
RE: Question about counterbalance valves
If a counterbalance valve is used to support a cylinder, the total pressure at the cylinder side of the valve is a sumation pressure required to overcome the spring force plus any pressure drop caused by the flow of oil through the valve.
Overcentre valves have external pilot ports to assist the opening of the valve. The pilot ratio is the differnece between the annular area of the main stage and the area of the pilot piston. Because the pilot signal is remote it has no effect on the pressure drop across the valve, when the valve is piloted open, it will open fully. When used on a double acting cylinder, the pilot signal pressure has a direct relationship with the spring pressure of the valve that it is opening and for that reason, high ratio valves can cause the cylinder operation to become very unstable. It is always best to use a low pressure ratio wherever possible, although application limitations may mean that a high pilot ratio is unavoidable. In these cases there are valves that have special pilot control features to suppplement the pilot signal and prevent the cylinder shaking that is often seen.
'hope this helps a little.
Adrian
RE: Question about counterbalance valves
Maybe 'Backpressure' was the wrong choice of words. What I meant by that is the resistance created on the opposing side of the stroke to keep the load under control. In my case I need as much Net Force in the piston rod area direction (down stroke, for stripping) as possible and the pressure created by the valve (in doing what it's supposed to do) is reducing that net force significantly.
Ice, differential area seems to make sense... Is this area independent of the pilot ratio? If that is the case then the counterbalance valve may not be the right choice for my application.
RE: Question about counterbalance valves
For me there is a clear difference between the two types of valves and their application. What Ice said is true to a certain point. If a cylinder is pulling a different load than it pushes the differential goes out of the window.
If your application is moving on the down stroke with a constant load then a 10:1 will be OK. The problems tend to occur when the resistive load drops away and the pilot is doing all of the work. The valve becomes over responsive and the cylinder will become unstable.
I am not sure that you have fully grasped the concept of motion control valves. It is the resistive load on the cylinder that is used to open the valve to let oil out. What will provide the greatest resistance to the cylinder down stroke, the stripping process or the motion control valve?
It is possible to get a similar result using a pilot operated check valve, although they do tend to be slightly agressive in closing and opening. They can induce shock to the system.
Adrian
RE: Question about counterbalance valves
You can see an explanation of the three types of Counter Balance Valve pilot operation in my Basic Fluid Power book in Chapter 14 starting on Page 14-3.
Bud Trinkel, Fluid Power Consultant
HYDRA-PNEU CONSULTING
RE: Question about counterbalance valves
http://www.hydraulicspneumatics.com/200/eBooks/
look in the basic book.
Bud Trinkel, Fluid Power Consultant
HYDRA-PNEU CONSULTING
RE: Question about counterbalance valves
From what I can tell I don't need the counterbalance valve, but I still don't know what is causing such a high piston pressure during my downstroke. Things I've considered are the cylinder area ratio, which is nearly 2:1, or the proportional valve maximum flow. For example, during the UP stroke the pressures on both sides of the piston are about the same but during the DOWN stroke the bottom pressure is always half the top pressure no matter how much I increase it.
My conclusion/hypothesis, the line sizes and orifices are the same on both sides of cylinder therefore this behavior is perfectly normal for the design and if I want the bottom pressure to drop more I'd have to either beef up the lines or return straight to the tank, i.e. change the design. Hydromec, Bud, does that sound about right?
RE: Question about counterbalance valves
You should have mentioned you are using a counter balance valve with a motion controller in the first post. There can only be one valve that controls the flow. You have found out that counter balance valves interferes with the motion and cause errors that the motion controller tries to correct and often over correct.
I have ranted and raved about designs that include counter balance valves. I normally recommend simple blocking valves that are energized open for safety.
However, you still can have problems with cavitation on the cap end. I prefer having a fill valve in these cases. If you must use a counter balance valve use the lowest ratio possible and an don't size the counter balance valve too big. 3:1 will be better. If you must use a counter balance valve then make sure the design opens the counter balance valve relatively slowly. Don't connect the pilot to the cap side.
I got a call once from a control person trying to tune up one of hydraulic motion controllers. He had been trying to tune a system up for a couple of weeks and finally he called for help. He was tuning a system with a counter balance valve AND regen! The oil on the rod side didn't even go through the proportional valve! The counter balance valve controled the flow on the rod side when going down. There was nothing the control guy could do and there was nothing I could do to help him except to tell him the hydraulic design wouldn't work.
Do you see that Budt? I am not the only one that has seen counter balance valves interfere with motion control. I hope others that monitor this forum pay attention. Designing bang-bang circuits and servo circuits is quite different.
RE: Question about counterbalance valves
You may well be correct about not needing a motion control valve, but only if you can allow the cylinder drop when it is not mechanically supported.
Proportional valves are designed to run with high pressure drops because it makes them more responsive. It may be that the spool in the directional control valve has over/under laps to compensate for the differetial in the cylinder. That may account for why the pressure is always high and why the pressure has not dropped after you removed the motion control valve.
Peter...again I see a very simplistic response. Blocking valves are not a reliable replacement for motion control valves! To control a cylinder the starts with pressure in bore which goes to zero as the cylinder approaches the centre position and then has the pressure increasing in the annulus side, the only safe solution is an overcentre valve.
Hydraulic valves are sensitive to viscosity and will obviously take a longer to accelerate and decelerate than an electronic command signal.
Electronic controls augment the performance of hydraulic systems but there will always be performance limitations in the hydraulic system that cannot be solved using electronics.
Adrian
RE: Question about counterbalance valves
"Do you see that Budt? I am not the only one that has seen counter balance valves interfere with motion control. I hope others that monitor this forum pay attention. Designing bang-bang circuits and servo circuits is quite different."
As you know from experience with a local company, I always recommend you or another moreloal person when I am asked to Design or work on a circuit with Proportional or Servo valves. I learned many years back that those types of Fluid Power circuits were out of my knowledge base and getting into trouble with one circuit was enough for me. I'm not a slow learner and have no problem saying to anyone that I am not proficient at a particular system design or trouble shooting job. No one has ever complained or given me a hard time when I say their request is out of my experience range. Of course, I always have the names of persons who can handle their request as you know from experience.
Bud Trinkel, Fluid Power Consultant
HYDRA-PNEU CONSULTING
RE: Question about counterbalance valves
I don't understand what you are trying to say here. What pressure goes to zero? Why doesn't it depend on the cylinder position? Are you referring to the cap pressure and the possibility the cap side pressure can cavitate?
What you are telling freddyj is that he can't do servo control with hydraulics! An over center valve has dead band. That is not a solution for freddyj. freddyj wants to do servo control on a press. Counter balance and over center valves are not linear or servo quality. So what is your servo solution for freddyj?
It takes a little, but only a little, thought and care to make sure the pressure on both sides of the blocking valves are approximately even before opening the blocking valves otherwise the load can be dropped. This is done by using a small open loop command to raise the load slowly before opening the blocking valves. These interlocks can be hard wired or done within a PLC. Meanwhile the relays that keep the blocking valves open can be de-energized by any fault you care to wire up.
You should know that there are plenty of presses out there using servo control without counter balance valves.
RE: Question about counterbalance valves
Your background is electrical, you are not qualified to make the statement "First I don't consider a blocking valve or a counter balance valve to be a motion control valve."
See here...
http://www.oilcontrol.com/website/
To clarify...
Imagine a cylinder pushing an arm that pivots at the bottom. As the cylinder begins to push the arm the load in the full bore is at its maximum. As the arm moves towards the vertical/centre position the load decreases and eventually reaches zero when the arm is vertical and the load is acting vertically on the pivot at the bottom of the arm. As the arm continues to move it will tend to drag the cylinder and the pressure begins to rise in the cylinder annulus.
I am absolutely not telling freddy that he cannot do servo control with hydraulics. I am simply stating that to control a vertical load safely some sort of motion control valve is required.
All spool valves leak and to rely on them to support a load is simply foolish. If a blocking valve is used to lock the pressure the in the cylinder the line pressure between the blocking valve and the spool valve will decay. When the blocking is opened the line pressure increases rapidly and produces a damaging shock wave to the system.
If I were in freddy's shoes, I would leave the motion control valves off and let the directional control valve do the dynamic control and then lock the press mechanically when it is idle.
Adrian
RE: Question about counterbalance valves
What exactly are you controlling with what you call a motion control valve? Counter balance valves don't control position, velocity or acceleration. If you aren't controlling position, velocity, acceleration then you aren't doing motion control, you are just slamming cylinders around.
http://www.oilcontrol.com/website/
I don't see how that is relevant. Are you a distributor?
We have a difference in opinion about what a motion control valve is. At best you can say a counter balance valve is a flow control device.
I was hoping you would come up with a suggestion for making the system safer that doesn't include counter balance valves. I have provided mine.
I agree
I mention above how I would solve the problem above. freddyj is an electrical person that can probably program a PLC so the blocking valves aren't opened until the pressure are equalized.
Direction control valves are not servo valves. freddyj said he has a motion control card. He should be able to get his actuator to smoothly follow a target position within a few thousandths of an inch with a servo valve or a proportional valve with a servo quality spool. That can't be done with a directional control valve.
At least we can agree here.
Peter Nachtwey
http://www.deltamotion.com
RE: Question about counterbalance valves
This will be my last comment on the subject as I feel that we have hijacked freddy's thread.
My comments about your background were tongue in cheek and I am sorry if caused offence.
I pointed you to the oilcontrol website to highlight the fact that the people who make the valves call them motion control valves for that is what the valves actually do.
All hydraulic valves are mechanical and their function is reliant on many factors that electronic controls are immune to. For that reasons precise electro-hydraulic control is not always possible when using motion control valves and servo/proportional control valve.
I would not fit a counterbalance valve to a servo cylinder on a flight simulator, the same as I would not fit a moog servo valve to backhoe loader. That does not mean that hydraulic valve are troublesome pains in the backside, which most of your comments seem to infer.
It's all about application!
Merry Christmas!!!
Adrian
RE: Question about counterbalance valves
The tolerances are tight as hell, the tooling load is about 1100lbs and it holds with no assistance from any valves whatsoever, I noticed that I wasn't getting a pressure reading with just the load so I decided to remove the lines to see what would happen and it just held itself up with friction. Left it there overnight and it barely moved at all. The bore is 11in. in diameter with a 9.5in rod, hollow in the middle so the areas are only 44.89in^2 and 24.3in^2 respectively. At 1500psi this produces a tremendous amount of force on either side but the downstroke has the tooling weight on top of the 1500psi driving the fluid out of the 1/2" line. Flow should be significantly higher during downstroke, which is why I get the unwanted pressure reading. I spoke to a hydraulics specialists that suggested changing the proportional valve for a directional valve. I did that and got the same pressure response. So I concluded that whatever valve I use is going to do the same thing because I'm driving the fluid out too fast either for the valves flow response or for the tube size I'm using. I was advised that the tube size is adequate so I came up with another potential solution.
I don't need all that net force all the time so I'm going insert a solenoid bypass circuit that dumps capside fluid directly in the the tank when I need to maximize downforce. I still expect some restriction but hopefully it will make some difference. I know there is some safety and stability issues that could arise by doing this, what should I look out for?
By the way the counterbalance valve is outta here, the proportional valve works so much better without it.
RE: Question about counterbalance valves
Ted
RE: Question about counterbalance valves
Run through Jack Johnson's VCCM equations to understand the concept. VCCM is for steady state motions and force, not dynamic accel/decel that peter talks about, but the concept is there.
Hollow rod doesn't matter, the area at end of the hollow spot still adds to the piston (closed) side area.
RE: Question about counterbalance valves
Kcj, I mean that I have higher back pressure on the piston cap end during down stroke. I need net stripping force, down force. In my case case rod retracting force. Back pressure on the piston side during the retract takes away net force. For example, during a retract, if I have a reading of 1500psi on the rod side, I get 750psi on the piston cap. Using the area figures I gave earlier you can see what I end up with compared to what it could output. Ideally I'd like the full 18Ton output but naturally like you said backpressure is needed for control. If I cut this backpressure in half I go from 1.4Tons to 10Tons net force.
RE: Question about counterbalance valves
There will be back pressure on the cap side while retracting because there is flow and a pressure drop across the valve. The areas are about 2/1 so you can expect twice the flow through the cap side as the rod side. Since the pressure drop is proportional to the flow squared you can see the pressure drop across cap side land of the spool is going to be 4 times higher. but once contact is made there will be less back pressure as the material will
There are valves with 2:1 spools that may keep the pressure drop across the spool lands closer to 1 to 1.
What motion controller are you using? Doesn't it provide graphs of the positions, control outputs and pressures? I know ours would.
RE: Question about counterbalance valves
Ted
RE: Question about counterbalance valves
The only 'active' valve in the system should be the servo valve. Other valves interfere with the PID control.
RE: Question about counterbalance valves
Ted
RE: Question about counterbalance valves
A 2:1 spool will allow double the flow to or from the cap end of the cylinder for the same pressure drop. Ideally a servo system works best when the valve spool is cut in proportion to the surface areas on the piston.
I would follow kcj's advice about working through the VCCM equations to calculate the steady state speed when retracting. There is a term in the VCCM equation that takes into account valves with spools cut to provide 2:1 flow. If the calculations show the system can't attain the desired speed then a bigger valve is required. Other key points that haven't been brought up. Is there an accumulator? The VCCM equation assumes the supply pressure is relatively constant which will only happen if there an accumulator big enough to smooth out the pressure ripples. This is like having a big capacitor on a power supply.
This should be a reasonably easy press to control as the surface areas are about 2:1 and not 10:1.
RE: Question about counterbalance valves
Ed Danzer
www.danzcoinc.com
www.dehyds.com
RE: Question about counterbalance valves
RE: Question about counterbalance valves
Ed Danzer
www.danzcoinc.com
www.dehyds.com
RE: Question about counterbalance valves
http://www.hydraforce.com/Proport/Pvd4w.htm
I didn't see anything suitable for closed loop control. Hydraforce makes no claims as to the linearity or response.
The valves aren't even that big.
So now that we have agreed that a proportional valve can be used somewhat interchangeably how does that help?
I didn't follow what you are trying to say except that I saw another key word and that is pilot. Counter balance valves have pilots too. When pilots open or close valves they change the way the system works during operation. This makes it hard for the controller to do a good job of controlling position or force. The person tuning the system will go nuts trying to figure out what is happening. This doesn't sound like a good servo system to me.
I would stick to the 2:1 asymmetrical spool idea. When designing hydraulic servo system remember that there can be only one active valve and that is the servo valve. As soon as freddyj said he was doing closed loop control the design rules change.
RE: Question about counterbalance valves
Freddyj has not stated the maximum speed, acceleration velocity, pressure or accuracy required of the system. Many presses can have a high speed low tolerance primary movement that is controlled by large flow on-off valves and slower speed higher accuracy finish movements. Many of the proportional valves are more linear as the flow decreases so the tuning of the control is easier if a high response speed is not required. If the system does require high flow (40+ GMP) the response of a large servo valve may not be any better than a small proportional valve at low flows.
Ed Danzer
www.danzcoinc.com
www.dehyds.com
RE: Question about counterbalance valves
Typically no servo motor accelerates linearly so why does a hydraulic valve have to be linear?
[quote]
The is a big difference between have a linear system and the non-linear ramps!
What a control person wants to see is that the actor moves at 10% of maximum speed with a 10% control output, 20% of maximum speed with a 20% control output. 30% of maximum speed with a 30% output etc.
Hydraulic controllers should never use linear ramps! They need to use s-curves or s-ramps. A linear ramp assumes the acceleration can instantly change which it can't. It takes flow to change the force and that takes time so s-curves are the way to go.
That is possible but not with a simple PID controller. You are expecting a control person to figure out the gains at high speed and then at low speed. Most control people have a hard enough time just trying to figure out how to tune a PID. If you make life difficult for the control guy you will not make any friends.
Response times and linearity are two different issues. Freddyj is controlling a press. High response times are always required on a press. Fortunately most valves dither well enough to control pressure. The ability to accelerate and decelerate is limited by the response of the valve, the diameter of the cylinder and the mass of the load.
As you said, freddyj didn't state the flow or speed required but if he wants to close the press at 10 inches per second he will need a very big valve. Moog has some valves that can control this easily. The issue is whether there are 2:1 valves for big valves.
Why do you resist? What is wrong with my solution?
I have explained what is wrong with other non-linear solutions. If you want to remain friends with the control guys you will heed my advice.
I have many different press customers. Some are controlling four corner presses. In these cases the ability to synchronize the actuators within a few thousands of an inch is critical. In the past there were Metal Form shows. 1/3 to 1/2 of the hydraulic press manufactures there are our customers.
Hydromech wants to know what qualifies me to post on this forum. I would say that our company has been involved with more hydraulic presses than all of you combined. As soon as the words PID, closed loop or servo control are mentioned the design rules change. If you don't design the press to be controllable you will not be asked to design another press. Is that simple enough? Why do you resist?
RE: Question about counterbalance valves
I guess I wasn't absolutely clear on what type of press I'm using. Hydraulics are only used to fill the die cavity and remove the pressed part, not to press the part. We all know that there's more than on way to skin a cat or in my case a powdered metal slug.
PNatchwey, you're all over the place man, almost every hydraulics forum I come across. Question for you is, how do I tell if a valve has a 2:1 spool, namely proportional valves. In most of my findings this is not something that is explicitly stated under any specifications. Definitely not on the specs for the parker valve I'm using. It does mention that Qa=Qb and that valves are available with Qa>Qb and Qa<Qb where Q is flow, no specific ratio but I presume this is related to spool ratio. Just curious because I may have to head in that direction.
http://www.utronkinetics.com/technology.html
RE: Question about counterbalance valves
at least for me to navigate.
Maytag
RE: Question about counterbalance valves
I can't tell by looking at the valve. I must see the specifications. The last valves specifications I saw for a 2:1 valve is on the Eaton/Vickers site. I just looked a few days ago at the start of the thread and couldn't find anything about 2:1 spools otherwise I would have posted a link to an example. I get frustrated trying to find things on the hydraulic web sites like many others.
A good distributor can help find the right valves.
Freddyj, I figured you had your solution. I just wasn't going to let the thread die without first making the point that the design rules change when doing servo controls. I hope the hydraulic designers now know that counter balance valves are active devices that ruin servo control and make life harder for the control guy. At one time I thought is was a plot by the hydraulic designers to make life miserable for the control guys but I soon realized it was just ignorance of what it takes to make a good servo hydraulic design. Some bigger companies have had me give training sessions. At first the hydraulic designer don't like my ideas because they are more expensive but they found that in the long run they saved the money because the systems tuned up more quickly. You can use auto tuning on a well designed servo hydraulic system but you can't auto tune an actuator that has a valves with dead band, dual gains or has a counter balance or other piloted valves that may open or close by them selves with out the knowledge of the controller.
A counter balance valve is kind of like a zener diode. You wouldn't put one in a linear circuit.
BTW, I really should have (control) after my name. I have EE degree but don't do any electrical design. I concentrate on hydraulic controls.
RE: Question about counterbalance valves
fairly often I run across significant momentum or overruning loads where the valve must be remote from the load, i.e hose in between. No positon feedback, no real tolerances or accel or decel requireements. In the past we used soft shift or bang bang bangs with cylinder mounted CB just fine.
Now, I have several applications I'd like to go to prop valves, but need some sort of load blocking valve for hose break protection, and the valve cnnot be at the actuator.
A solenoid blocking valve won't know when hose is broken.
I have other servos with PO checks activated by the main P line before the valve. That is to lock the load upon shutdown when pump pressure is gone. Also won't work for hose break protection.
POCB would work for now as the applicaiton is not closed loop. Hwoever I can see adding position feedback later (+ or - a half inch, not in the thousandths of an inch range)
POCB I already new not the preferred choice, but what other ideas does anyone have? will never be a servo valve application.
What do boom trucks or aerials, etc applications use for protection?
kcj
RE: Question about counterbalance valves
If you read what I wrote about the blocking valves above you can see that blocking valves must be energized open and opening any one of a number of relays will cause the blocking valve to close. The motion controller can detect errors. It wouldn't take long ( milliseconds ) for a motion controller to detect that it can't control the actuator because a hose broke and pressure is gone. The motion controller can thengenerate a fault which causes the block valve to de-energize. There may be other methods that don't require a motion controller too like a simple pressure switch.
However, with a motion controller you should have smooth moves and shouldn't need to worry about the hose breaking. At least the hose will be stressed less. As I pointed out above, a good motion controller uses s-curves for the acceleration and deceleration ramps. S-curves can be achieved with smooth pressure changes. Linear ramps require and instantaneous step change in acceleration ( infinite jerk ). This can only be achieved if there is an instantaneous step change in force which requires an instantaneous step change in pressure which we know can't happen because it takes flow to change pressure and that occurs over time even if it is milliseconds.
RE: Question about counterbalance valves
Maytag
RE: Question about counterbalance valves
Not every system can justify the expense and maintenance of a servo. Meter out position control will provide a way to use PO checks to stop over running loads, broken hoses, fittings or pipes and will reduce spring if the oil has air entrapment. If a load sensing pump is used it can reduce energy consumption and cooling requirements. With some work regeneration can be implemented to further reduce energy requirements.
Even though energy prices have gone down they will go up and then system efficiency will become a major priority. Servo valves are not the most energy efficient way to position hydraulics.
Making the motion control supplier or the person tuning the system job easy by increasing the cost of operations is poor business practice for any manufacture or distributer.
Ed Danzer
www.danzcoinc.com
www.dehyds.com
RE: Question about counterbalance valves
BTW, do you know that regeneration reduces the natural frequency of the system.
I have noted that that use servo valves isn't very 'green' but sometimes speed and precision are more important.
RE: Question about counterbalance valves
Even with control, granted less stress on hoses, but still need some sort of safety backup.
The PO checks from the supply pressure work great, but those are for sensing pump shutdown (another application that does use a servo). If the pump is on, even with hose break,the load would move. In that case it doesn't matter so the load holding checks work great for shutdown time.
Granted it is not the recommended choice, but we may try 10:1 pilot motor POCB valves. I will get data acq on the cylinder pressures first to find load pressures and servo/prop pressures.
maytag, hows the steel mill business down there? pm me.
k
RE: Question about counterbalance valves
Kevin you have a pm on the IFPS board.
Maytag