Hydraulic control loop

[John_Hoekstra]

Good evening everybody

I like to increase my knowledge about using control systems with hydraulic systems. And I hope that somebody can help me out. I made a system with two hydraulic cylinders. The request is that the two cylinders must move synchronize with each other. I have something figured out, but I'm not sure if this works. Maybe there are better alternatives because my background is not a control system engineer. ( SEE VRAAG4-1)

The proportional valves that have been used are no expensive valves with a position feedback from the valve position. Unfortunately, I was not allowed to use these because of the low budget.Both hydraulic cylinders are equipped with a position feedback. When the master-slave is activated cylinder 1 will move. The signal from the moving cylinder 1 is feedback to the slave system as a reference signal. I limited the maximum stroke of the master proportional valve 1. ( Otherwise it is not possible to catch up with master cylinder 1)
The reference signal from the master is compared with the position feedback from the second slave cylinder 2. The error is given to the controller to adjust the valve. When the position difference is more than 2% the system will stop. I know that there always be a difference between the master and the salve system because the system is in series.

My question is :

• Does my first control system block works and is there a simpler solution?

Thank you in advance

 

[John_Hoekstra]

Hi

At my previous company, they didn’t use a master and slave option. They compared the speed of each cylinder. The cylinder with the highest speed will be reduced so that the other cylinders can catch up. I tried to understand the control loop but I was missing integrators and knowledge to completely understand the loop. Is this a feedforward loop see attachment??

 

[PNachtwey]

First, you should say what the application is. Believe it or not some of us have exeripence with many thousands of of hydraulic servo applications and I/we have probably done it before.
The mass is important.
It doesn't look like you are expecting much.
From the diagram it looks like the flow in the pipe to and from the proportional valve is very high. At this speed the pressure drop in the lines starts to make a transition from proportional to the flow speed to proportional to the flow speed ^2.
There is no accumulator to keep the pressure constant so quick accelerations is out of the question.

Does my first control system block works and is there a simpler solution?

Yes, you realize that if you gear the slave to the master the slave will always be behind. This is good. So why not gear both actuators to a virtual master that accelerates slowly to a constant speed and then ramps down. Try using two gains. One gain is multiplied by the error between the virtual master and the slave actuator positions. If the two actuators are similar they will both lag behind the master the same amount. The second gain is multiplied by the difference between the faster and slower actuator positions and the result added to the slower actuators output. You could also subtract this term from the faster actuators output but the system will slow down.

This solution is crude but will work if you don't expect perfection. The main thing is that this can be implemented in a PLC but programming the virtual master will require some math.

I have seen even simpler systems used on dam gates that didn't even use proportional or servo valves. Pumps feed both cylinders at each end of the gate. If one cylinder got higher than the other then oil was let out of the higher cylinder. This is super simple.

I have customers synchronizing sliders or platens on four corner presses with errors less than 0.1mm even when the load is uneven. The system must be designed correctly to do this and a real hydraulic servo controller is required.

Warning, we see many people try to go cheap and fail.











Peter Nachtwey
Delta Computer Systems

http://www.deltamotion.com

http://forum.deltamotion.com/