Control.com - Nerds in Control

I need to replace the control hardware for a friction welding machine. A stationay part is welded to a rotating moving part.

1)The axial movement of the rotary part is controlled by a servo valve. A Vdc signal is wired to a servo valve which controls the hydraulic pressure of the rotary chuck and therefore its axial movement
2)The position of the chuck is measured by an incremental encoder with signals A and B
3)There is a pressure transmitter that sends a Vdc signal corresponding to the current pressure that is being applied to the rotary chuck

My problem is:
1)The current control applies an open control loop of the pressure. The required pressure is applied with a certain position of the servo valve, but is doesn’t use a correction method according to the real pressure
2)This pressure variation produces an undesired variation in the length of the final welded piece

Should I try to control position or pressure?
PLC motion controllers are designed to control servo motors. Can they be used to control servo valves?

Help will be highly appreciatted.

I would try to control position rather than pressure - though this is not necessarily an either/or case. I have used our controllers in a number of applications to control position using servo hydraulic valves. But if you also wish to control pressure you can run nested PID loops in our controller, maintaining a constant pressure after achieving a desired position (this assumes that position with then become subordinate to pressure and may vary) or vary pressure to maintain position. This is a fairly common application using our stuff, and I have machines from several companies in the field using this technique. The applications doing this range from semiconductor to sawmills.

Davis Gentry
Applications Engineer
Delta Tau Data Systems
dgentry@deltatau.com
804.795.4288

Davis,

I appreciate your comment it really helps.

I took a look at Deltatau products and they are dedicated motion controllers. I will later review the details of your products.

Dou you think that the same control philosophy that you recommend may be implemented with a PLC using standard analog cards and high-speed counters? Are they fast and accurate enough for a one-axis application? (from a technical point of view)

It’s a difficult question to answer but any comment will be important to me.

Damian Matsumura

Damian -

That will depend greatly on the resolution and
response of your system. If you have low resolution requirements and if your system can deal with relatively slow updates, then maybe you can make it work using standard PLC I/O and processors. Usually even the PLC manufacturers acknowledge that their main processors cannot do this well and instead use dedicated motion control cards on their backplanes. To give you a reference point, all of our controllers have a default servo update rate of 442 microseconds
(usec). I have run our cards at rates up to 50 usec and in one particularly demanding application am working to achieve 30 usec for three axes. Compare this to the usual loop update times in your PLC. We actually have a VME card which can be used in some PLC backplanes (GE 90-70, one of the Siemens racks, etc). So the answer is, if you have fast I/O and loop update
times on your PLC, and low resolution requirements, you MAY be able to get that to work.

Davis Gentry

All I can say about servo valve control is that it requires a good system control software. Also, it depends upon the type of controller being used in a certain process variable such as pressure. But in this case, the only way to control the valve is to make some adjustments in the length of the welded piece that requires some human intercention in the control system. Lastly, it is so impractical to adjust the pressure from the valve without automation.

I have several applications where I control hydraulic cylinders using proportional valves with spool position feedback and an A/B SLC5-03. I can control position, velocity, or pressure. The thing to recognize is that you are primarily controlling the hydraulic valve spool. Depending on the mechanical design, the driven load may be rather loosely coupled.

Position or pressure control? The answer depends on what you are trying to accomplish. With many presses, the only time you want pressure control is when the press is closed and you are forming. The rest of the time velocity control (position) is more important. Keep in mind that under pressure control, if resistance is not met, the cylinder will move as fast as it can and as far as it can in the attempt to exert the pressure called for.