Application Advise

I have been asked to propose a control system for a four axis hydraulic lift system. There will be four, 10" bore x 42" stroke cylinders required to perform synchronized lift/lower moves. There is a 100Ton offset load that will not be shared equally between all cylinders. Each cylinder will be supplied with an SSI transducer and counter balance valve, and be powered by independent gear pumps, driven by variable speed ac motors. The motors will have closed loop speed control using motor mounted encoder and variable frequency drives. The drives will accept an analog speed command signal. Speed will be approx. 0.5 to 1.5"/sec. Direction will be controlled by basic soft-shift directional valves.
I am thinking about using an RMC150E-S3-DI/O, to do the control. I have used an RMC75E once before, to control two hydraulic cylinders via servo valves. I have not done a variable speed motor/gear-pump system before. Details about positioning requirements are scarce, I am told that as long as each cylinder is within 0.250" of each other, that is enough. I would prefer to keep them much closer together, say 0.050".
On my previous application, I was not able to “play” with the system before start-up and relied on the simulation capabilities of the RMC Tools to come up with initial tuning values. This time, I will at least be able to perform no load testing with all four cylinders.
Any input would be very much appreciated.


I would like to know why someone came up with this design. Four VFDs can’t be that cheap so money can’t be the issue. Gear pumps are used so that efficiency can’t be the issue and the design will be a real pain to setup and never perform as well as the traditional methods.

I recommend against using a counter balance valve. Counter balance valves are semi active devices that are not controlled by the motion controller and they will interfere with the motion control. From reading the rest of your post it looks like the gear pump is going to push the actuator down and the counter balance valve is going to do what? It will oppose flow until the pressure exceeds some set point and then the valve will open and the actuator will drop way to quickly. The motion control will see this and reduce flow. The pressure will drop and the CBV will close. This chattering will occur all the way down. You will not get smooth motion.

Gear pumps are not the most efficient. Why not use fixed displacement pump(s)? The leakage and control at low speeds would be much better.
There may be some noticeable pulses from the pistons though but your oil volume is large so that should dampen out. It doesn’t look like the pulses would be of concern in this application.

Each cylinder would require about 120 in^3 of oil per inch per second or 31 gpm. How often does the lift move?

Controlling the direction using directional valves is a pain and will cause problems holding position if you must hold position accurately. With the directional valve you will need to have a small dead band so the directional valve doesn’t get beat to death as the controller tries to maintain position.

The S3 can control 6 actuators but you mention only 4.
The DI/O will be necessary. You will need to write a user program that monitors the target velocity when moving and the error when stopped switch the directional valve accordingly. If the DI/O card is only being used to control the directional valves then getting rid of the directional valve will remove the need for the DI/O card.

I don’t know about this combination. The variable speed motor with fixed displacement pump has been used before.

OK, at least you are being realistic except for when coming down because of the CBV.
I would use blocking valves that must be kept energized open instead of CBVs but then you still need a resistance to flow when going down.
With 100 tons on the platform little if any hydraulic force will be required to make the actuators go down. It may be that the gear pumps only need to supply the bottom side of the cylinder and push oil in or pull oil out of the bottom side. The top side can be filled with oil at atmospheric pressure. Is the platform heavy enough without load so that it will come down due to gravity?

Since you are willing to try a VFD I would use 1 VFD to drive 1 fixed displacement pump for all four actuators. The VFD would charge a small accumulator and maintain a constant pressure. The RMC can control the VFD too and stop it when the actuators are not moving and the pressure in the accumulator is high enough. The flow between the accumulator and the actuators would be regulated with servo grade proportional valves. If possible I would get valves with asymmetrical spools so the flow on the rod side pushing down is less than the flow on the cap side pushing up. They wouldn’t need to be of high performance since the speeds and accelerations are so slow. I would use energized open blocking valves for safety. I would have a series of relay or switches like the emergency stop, the fault line from the RMC, and safety curtain relays and any other things you think of all wired in series so that if any of the relays open the blocking valve will de-energize to the safe shut position.
You would be able to get much better than 0.050" with this setup.

The servo grade proportional valves would allow the oil to be metered in and out the bottom of the actuators.
I do not like the idea of directional valves. They work but are a pain forever. Like there CBV they are yet another active device and reduce performance.

Edit, more thoughts. While looking at gear pumps I notice that some have a minimum speed. I also know that some VFDs do not perform well at low speeds and some have a minimum speed. The minimum speed requirements may limit your choices.

Thanks for the quick and in-depth response. Exactly what I was hoping for.

me too…I will try to find out. I’m not a hydraulics specialist by any means, but It seemed a little strange to me.

average 1 once every 10 minutes

fluid is water glycol

I used the online configurator…Looks like I mistakenly added an extra module, I thought I had selected only two, dual axis SSI modules.

I was not using proper nomenclature…It was an RMC75E-MA2 and they were servo grade proportional valves.

A quick search revealed that there are fixed displacement pumps that will work with water/glycol.
Now the question is whether one uses a simple AC motor or are VFD to drive the pump. Both motors would be off for much of the time between the moves. The VFD would be more energy efficient because it can be programmed to pump exactly what is needed where as the fixed speed pump motor would pump excessive fluid at low speeds and need to relieve over a relief valve. This isn’t very energy efficient but then the duty cycle is so low. Heat probably won’t be a problem but that must be checked. One gear pump with a VFD supplying oil to an accumulator and 4 valves would work too. Gear pumps aren’t that efficient at high pressures.