hydraulic motion control question

[agarb]

Hoping our resident Delta expert will weigh in on this one....

Consider a somewhat hypothetical situation. I say hypothetical because the sales engineer has a potential opportunity and he is asking about our capabilities.

As best I can describe it, it would be a 3-1/4" bore cylinder with a integrated SSI position transducer of 5 um (0.0002") resolution. The cylinder rod would be used much like a syringe to inject a particular volume of material into a cavity. The volume of material needs to be controlled very precisely and is determined by the distance the cylinder strokes. Under normal circumstances, what type of position control accuracy could I realistically expect? Could I control the stroke to within 0.001" with little to no overshoot?

 

[TConnolly]

Based on my experiences with the Delta RMC, the RMC is certainly capable of doing that...

provided the hydraulic system was capable and correctly designed.

We are using SSI transducers and RMC controllers to position 500mm pistons to within .02 mm (.0008 in)

Once you get it tuned and get your feed forwards set up right you shouldn't see any overshoot.

 

[Peter Nachtwey]

Should be easy but as always, don't let the hydraulic designers screw it up.

The high resolution SSI rod is a good start. 5 micron is good but 2 micron is better and I don't think it costs any more. You want a good servo or servo solenoid valve with a zero overlap spool. The valve should be mounted on the cylinder to minimize the trapped volume of oil between the valve and the piston. Do not have hose between the valve and the cylinder or all bets are off.

The mass and rate of deceleration are critical. You didn't specify.

Reaction Injection Molding ( RIM ) requires injecting two or more cylinders of material at the same time where two materials need to be 'mixed' or injected at a constant ratio. This is more of synchronizing problem.

How much does the material you are injecting compress? If you want to be that precise then you need to take into account that the material you are injecting will compress so if the cylinder and feedback thinks you have injected 100cc the material may be compressed so the actual injected volume is 99.9cc because it is under pressure.

 

[agarb]

Thanks for the feedback.

I've done about 15-20 axis of closed loop hydraulic motion control with RMCs (and one HACD!) in the past year or so, but this would be the most demanding application and is strictly position control.

In the past I've been more concerned about advancing to a position at a setpoint velocity and then transitioning over to force control. I have never really examined how accurate the positioning can be.

I mentioned a 5 micron rod above because that was the best resolution in the Balluff datasheet that I have. Will definitely keep the 2 micron option in mind.

 

[CharlesM]

MTS Temposonics type R are the higher resolution probes. They are the same price however MTS has delivery problems. Most sensors are 4-6 weeks out or $500 expidite for 7-10 day.

 

[TConnolly]

MTS has rods down to 1 micron.

 

[Peter Nachtwey]

MTS has rods down to 1 micron.

Yes, and we prefer 1 micron rods. It is easier to differentiate 1 micron positions to get velocities and accelerations.

Even so, with 1 millisecond sample time the best resolution one can get is 1mm/sec velocity resolution and 1 m/sec^2 acceleration resolution.

We have tricks that make our velocity and accelerations estimates much more accurate.

They are the same price however MTS has delivery problems. Most sensors are 4-6 weeks out or $500 expidite for 7-10 day.

That is what you get when you transfer your production to Germany. I have heard from just about anybody that will talk that lead times for German hydraulic components is too long.