Electro-hydraulic pressure control system

[Leandro Fernandes]

My team is designing a pressure vessel testing machine. This testing machine can be simplified and assumed to be a 120ton hydraulic press with a vertically mounted hydraulic cylinder.

Our goal is to use an HMI to set the force the cylinder shall apply to the test piece. The force range is from 10ton to 120ton. We want the accuracy of the applied force to be +/- 5% of the nominal force set in the HMI. The extension and retraction speed of the cylinder is not relevant.

We are designing the electro-hydraulic circuit for this cylinder and have come across 3 possible solutions to control the pressure on the piston side of the cylinder:

1) Servo directional control valve​

2) Proportional pressure regulator valve​

3) Proportional pressure reducing valve​

We believe that solution 1) will require a control loop with pressure sensor data feedback to the controller (which, considering also the higher cost of the valve, would make this the most expensive solution).
We are looking at existing solutions and usually they use a proportional pressure regulator valve. We don't mind to use a proportional pressure regulator valve but I am trying to understand if a proportional pressure reducing valve would have any advantage, and if not in which applications would a proportional pressure reducing valve be chosen over a proportional pressure regulator valve?

So, in summary my question is: which possible solution is more appropriate for our application?

 

[Leandro Fernandes]

Leandro, you need a controller.

Perhaps you are right. But can you lecture me on the differences between an hydraulic controller and a "simple" PLC? What does an hydraulic controller do that I can't program a PLC to do?
I was thinking that I could control the set pressure of PPR3 with a PLC and said PLC would communicate with the HMI the press operator would use.
Can you tell me if an hydraulic controller communicates with a PLC or directly with an HMI? I.e.: can an hydraulic controller replace a PLC and allow us to have I/O for other parts of the system?

We don't know what the cycle time is. Are all pressure vessels being tested so that testing pressure vessels quickly is important?

No. This press is being design to test upto 50 pressure vessels per day. The cycle time of the cylinder is insignificant compared to the testing time of the pressure vessels.

 

[PNachtwey]

Where to start.
1. Your design is flawed. Why haven't the others said this?
It has already been acknowledge that there needs to be an opposing force on the rod side. So how does PPR3 control net force?
I posted Net force = Pa*Aa-Pb*Ab.
What is the pressure or opposing force on the rod side? If you do not know you can't control force unless you use a load cell which may be preferred in a clean, low volume test environment.

2. If you are only testing 50 parts a day why not simply use a high pressure pump and forget the low pressure pump. You don't need the speed. An accumulator may still help. Keep the design simple. Don't make the cylinder any longer than what is necessary to get parts in and out.

3. PLCs are not deterministic. The inputs and outputs are slow. The scan is slow. A pressure or force transient can happen within a PLC scan time so how is the PLC going to keep from overshooting the force and possibly apply too much force on your pressure vessels?
Even the PPR3 valve is slow.

4. Servo valves can be much faster depending on what you want to spend but I mentioned a key trick which seems to be ignored. Put something compliant below the pressure vessel to absorb some of the shock. This is why big presses have cushions.

5. A good hydraulic motion controller is fast and deterministic. A good motion controller can scan at 250 microseconds if necessary. This is something that a PLC can not hope to do. A good motion controller has necessary algorithms built in. A key trick is to make the switch from position to force control smoothly and on-the-fly.

6. PLCs people sell PLCs. They do not specialize in hydraulic control applications like I/we do.
You should do a search for "Peter Nachtwey hydraulic testing"


Peter Nachtwey
Delta Computer Systems

http://www.deltamotion.com

http://forum.deltamotion.com/

 

[Leandro Fernandes]

It has already been acknowledge that there needs to be an opposing force on the rod side. So how does PPR3 control net force?
I posted Net force = Pa*Aa-Pb*Ab.
What is the pressure or opposing force on the rod side? If you do not know you can't control force unless you use a load cell which may be preferred in a clean, low volume test environment.

Let's take the last circuit (even though it's missing a few check valves, but please bare with me) with a counterbalance valve as suggested by HPost.
Once the rod touches the workpiece why can't I assume that Pb is 0? The counterbalance valve is fully open and the rod side is connected to tank.
As such can't I calculate the net force the press exerts on the workpiece as Pa*Aa?

If you are only testing 50 parts a day why not simply use a high pressure pump and forget the low pressure pump. You don't need the speed. An accumulator may still help. Keep the design simple. Don't make the cylinder any longer than what is necessary to get parts in and out.

Your suggestion was duly noted.

3. PLCs are not deterministic. The inputs and outputs are slow. The scan is slow. A pressure or force transient can happen within a PLC scan time so how is the PLC going to keep from overshooting the force and possibly apply too much force on your pressure vessels?
Even the PPR3 valve is slow.

Conceptually I was thinking that PPR3 pressure would be set by discrete increments, i.e.: the operator loads a new piece; sets the type of workpiece on the HMI; presses start; the cylinder extends until the rod touches the workpiece and the pressure could never go above the set pressure of PPR3 (unless the control circuit is badly tuned and allows for overshoot). Then the PLC would increase the set pressure of PPR3 by, for instance, another 10bar, and repeat the previous steps until the clamping force is equal to the value set on the PLC memory for the type of workpiece under test.

4. Servo valves can be much faster depending on what you want to spend but I mentioned a key trick which seems to be ignored. Put something compliant below the pressure vessel to absorb some of the shock. This is why big presses have cushions.

The workpiece is "sandwiched" between flanges that either: are made of Teflon or have o-rings.

5. A good hydraulic motion controller is fast and deterministic. A good motion controller can scan at 250 microseconds if necessary. This is something that a PLC can not hope to do. A good motion controller has necessary algorithms built in. A key trick is to make the switch from position to force control smoothly and on-the-fly.

6. PLCs people sell PLCs. They do not specialize in hydraulic control applications like I/we do.
You should do a search for "Peter Nachtwey hydraulic testing"

I understand your frustration. I have visited your YT channel but the thing is, I need to understand the basics before I delve deeper into hydraulic controllers. Perhaps they are the right solution for the application I am describing, but imagine I am a possible client, how are you going to explain to this possible client the technical benefits of an hydraulic controller over a PLC, when this possible client has told you that cycle times of the cylinder extension are not important? How are you going to explain to him what are the benefits of controlling the position of the cylinder, when his pressure vessels, for the same dimension and pressure class, can have a length tolerance of +/-5mm?

I am not trying to make you leave the discussion, quite the contrary. I just need very good arguments to tell my boss: "Well, the hydraulic circuit is going cost 10 times more than initially planned". I would prefer to defend that argument with the technical advantages of an hydraulic controller and a servovalve over my proposal instead of having to say "Because I'm a dumbf***".

 

[PNachtwey]

Let's take the last circuit (even though it's missing a few check valves, but please bare with me) with a counterbalance valve as suggested by HPost.

Counter balance valve turn on and off outside the control of the motion controller.
Servo control is like the movie "Highlander" there can be only one control valve.

Once the rod touches the workpiece why can't I assume that Pb is 0?

It won't be. It better not be or the press will come down fast due to the force on top and gravity.

As such can't I calculate the net force the press exerts on the workpiece as Pa*Aa?

You can assume that if you use a directional valve since they will fully open and close. A servo quality valve still needs pressure to force the oil out the rod side. If you open the valves long enough the force can be roughly determined by Pa*Aa but it is what happens in between that will determine if your simple idea will work. Usually servo control systems with rods pointed down chatter on the way down unless the CBV is self piloting. If the CBV is self piloting then there will always be pressure on the rod side. Do you understand why the system will chatter on the way down?

Conceptually I was thinking that PPR3 pressure would be set by discrete increments, i.e.: the operator loads a new piece; sets the type of workpiece on the HMI; presses start; the cylinder extends until the rod touches the workpiece and the pressure could never go above the set pressure of PPR3 (unless the control circuit is badly tuned and allows for overshoot). Then the PLC would increase the set pressure of PPR3 by, for instance, another 10bar, and repeat the previous steps until the clamping force is equal to the value set on the PLC memory for the type of workpiece under test.

You forget about kinetic energy. When the press makes contact with the work piece it is the kinetic energy that matters if there isn't enough kinetic energy then the press's force matters. Since your press doesn't need to be fast I advice a slow closing speed. It be easier to control the force. Still you will not be able to tell what the peak contact force is.

If you do it your way you should monitor the pressure on the rod side too and take that into consideration when stepping up PPR3.

I understand your frustration. I have visited your YT channel but the thing is, I need to understand the basics before I delve deeper into hydraulic controllers. Perhaps they are the right solution for the application I am describing, but imagine I am a possible client, how are you going to explain to this possible client the technical benefits of an hydraulic controller over a PLC, when this possible client has told you that cycle times of the cylinder extension are not important?

Accuracy and repeatability are important. Since your cycle times are long you can move very slowly and build pressure slowly. If you do decide to use a PLC, put a pressure sensor on the rod side. To do otherwise is to close your eyes to potential inaccuracies.

You are doing the thing by asking the right questions. So many don't know what they do not know. They screw up the design. Then we get the call when it is too late. Since your testing system is not fast you have options that productions presses don't have.

Don't think you are the first one to ask these questions. I had a customer with a vacuum hot press that thought all he needed was a pressure sensor on the cap side. His cycle time was very slow. I had to show them that even when applying force there is still pressure on the rod side of the cylinder. It took four hours of demonstrating and meetings before they agreed that they had to change their procedures to using net force control instead of pressure control.
Their defect rate dropped significantly due to the changes.

How are you going to explain to him what are the benefits of controlling the position of the cylinder, when his pressure vessels, for the same dimension and pressure class, can have a length tolerance of +/-5mm?

As I said before, the motion controller can detect the rate of change in force so action can be taken before the force reaches the desired set point.
I am very limited here because of the advertising restrictions. I have recorded plots and videos showing what can be done.

My Peter Ponder's PID YouTube channel is not about hydraulic control. It is about control theory. It is also intended for college seniors, graduate students and control theory professionals. Most people look for 3 minutes and wonder what I am talking about and leave. They expect a simple explanation like the P gain does this and the I gain does that.


Peter Nachtwey
Delta Computer Systems

http://www.deltamotion.com

http://forum.deltamotion.com/

 

[73lafuite]

for me i make these schema:
1: gear pump 12l/mn - 0.75kW
2: without leakage on port 1 znd 3 - adjusting 50bar - ratio>4 - with drain
3: piston pump - 0.37kW - 0.6l/mn that mean 60 secondes for go to 250bar with cylinder in middle position
4: electronic pressure switch - cut pump 3 when the pressure is OK - give autorisation to go back when pressure < 3bar
5: adjusting 15bar
6: adjusting 270bar
7: port A up

 

[PNachtwey]

According to the feature of electro-hydraulic system and the requirements of closed-loop pressure control, a new type of pressure controller which adopts digital control and fuzzy self-turning PED control algorithm is presented in this paper

What paper? Most academic articles comparing this or that control algorithm are garbage. They usually compare the algorithm du jour against a poorly tuned PID. I would fail the students and the professors that accepted the paper for not knowing the basics.

I can show you Fuzzy Logic pdf documents and rip them apart for not knowing the basics.

I believe I started a thread on this topic in the control systems engineering section.

What I mean is that spool valves leak. The flow control valve will stop it falling under gravity, but it will creep down as oil leaks over the spool. This may not be a problem for you, but don’t expect the press to stay up, it won’t.

It will! If you have closed loop control for position too.


Peter Nachtwey
Delta Computer Systems

http://www.deltamotion.com

http://forum.deltamotion.com/