Servo Control using High Speed Counter and Analog output

cottagewood

Member
Join Date
Oct 2005
Location
Kent
Posts
177
S7-315
I have inherited a job that someone else designed and started. It has Hydraulic Servos 0-10v from an analog OP card and Encoders back to an FM350 High speed counter module.

Now I've done this same application successfully in a 1500 CPU using the technology motion control functions, but I have never attempted to do it with code.
The code has been written using continuous PID controllers, I've started commissioning and the servos drive but the motors are sloppy and not accurate enough.

My question is:
Is this actually possible or should I throw the CPU away and start again with an S7-1500
 
That's the route I want to go but I have to convince the customer, he has no knowledge of PLC's but I don't want to tell him it's not possible if it actually is, albeit difficult.
 
The motors only turn one direction?
Is there hose between the servo valves and the motor?
What is being moved?

You are too worried about the PLC and not worried enough about what the PLC will be controlling.

One direction in automatic , reverse is only for maintenance.
no hose, the valve is mounted directly on the motor
6mm wire rod moving 2.5 metres +/- 1mm
 
One direction in automatic , reverse is only for maintenance.
no hose, the valve is mounted directly on the motor
6mm wire rod moving 2.5 metres +/- 1mm
That should be easy if the valve is mounted on the motor but
can you post a link to the valve? The valve may not be good for servo control.
A good test is to output a small voltage like 0.1 volts and see if the valve move at 1% of the expected velocity. Some valves have a dead band of 2 volts so any fine adjustment the controller makes for position is ignored.
In short do a linearity check
 
The smaller valves are good enough. On page 6 you can see the flow vs signal curves are linear EXCEPT for the progressive spools.
So I doubt the problem is with the valve.
Is there an accumulator to maintain a constant pressure? Most HPUs have a very poor response to changes in the pressure so an accumulator helps to keep the pressure relatively constant.
I would still do the linearity check. This will check the motor's linearity too. Usually they are very linear but they suffer from a lot of static friction.

Eventually we will get to the PLC itself. Is it fast enough? S7-315s are very fast and a 10ms time interrupt is probably not fast enough. I have a few old S7-315s too. The PIDs in the PLC are usually not good enough by themselves. A velocity and acceleration feed forward term is required.
 
The linearity was fine but I did find a couple of other problems. The encoders that have been fitted don't have enough resolution for the application, only 0.75 pls per mm.
The valves should have a +/- 15v power supply but there's only +12 /-11
They may not solve my issues but need fixing no matter what.
 

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