Pressure-Position

Hi all,

I have been working a little bit with DELTA COMP. SYS motion controllers. When going through the specs I noticed that their controllers can be switched from "live" position control to "live" pressure control.
So, under what conditions would you sacrifice your position for given pressure??? To me, you can't have both a known position and a known pressure at the same time just as you can not control both speed and position at the same time.

(Peter N.) looking forward to your reply.

Thanks,
Orn

You are right!

You don't know how many people can't figure that one out.

The motion controller has two position/pressure modes:

1. Position or Pressure control.
2. Position and Pressure limit.

In 'position or pressure' mode the axis moves like a normal position controller until the pressure build up above a pressure threshold ( pressure set a ). At this point the controller switch over to pressure mode. The pressure rate is used to compute a pressure trajectory to the pressure set point ( pressure command ) so that entry into to pressure mode is relatively seamless depending on how fast the closing speed is. In most cases the breaking to prevent pressure overshoot starts as soon as pressure exceed the pressure threshold. Once in pressure mode the controller ignores position unless the actuator reaches the position set point ( command position ). Pressure mode can be exited if the pressure drops below another pressure threshold ( pressure set b ), the pressure bit in the mode field is cleared, or one of the above positions are reached.

In 'position and pressure limit' the controller is actually solving two PIDs at the same time and then using a minimum or maximum function to chose the output. This mode is much more intuitive and easier to set up but it is also 'fuzzier'. Some time the controller can find situations where it rapidly switches between the two PID but the minimum of two function is continious so no step jumps in output occur. For a simple example using only the proportional terms

output = min ( position error * position gain, pressure error * pressure gain)

Where:
position error = command position - actual position
pressure error = command pressure - actual pressure

When the pressure term is the smallest we call that being pressure limited.
When the position term is the smallest we call that being position limited.

The actual control algorithm is much more complex.

The same two pressure modes can also be combined with open loop control position control instead of closed loop position control. These modes also work with pneumatics.
These modes also work in a bi-polar mode where the actuator can push and pull which is handy in test systems.

I will post plots of real system using position and pressure mode tomorrow when I am at work.

So, under what conditions would you sacrifice your position for given pressure???

Click to expand...

If you were wondering where this might be applied...

• Closing the platen on a particle-board press
• Closing the clamp on a plastic injection moulding machine

In both cases the desire is rapid movement to the closed position, then significantly large pressure to set the glue or maintain position.

Might even be used in a pick & place system handling eggs (or pickles?)

With electric servo motors there is feed to positive stop. If you need precision that a ball screw can't give you, you move rapidly to a point near the stop in position control, then you switch to torque control and press up against the stop. Quite common. This I am sure is the same for servo-hydraulic systems.

Rick Densing said:

With electric servo motors there is feed to positive stop. If you need precision that a ball screw can't give you, you move rapidly to a point near the stop in position control, then you switch to torque control and press up against the stop. Quite common. This I am sure is the same for servo-hydraulic systems.

Click to expand...

Yes, actually we can control servos too. About 1/3 of the application we do require press/force/torque control or limiting.

Other applications

Many types of presses. Powdered metal and four corner presses are the most popular.

Hydrofroming. This is becoming rapidly more popular.

Some forms of blowmolding.

Rolling metal as is steel and alumimum plants.

Hydraulic test systems for fatigue testing.

Material handling. BIG pick and place machines.

Link to position/pressure files

Link to position pressure files

h500inj.zip is an injection-molding machine. This was done on a HPM 500 ton injection molding machine. Notice the control during the pressure transients. This was using a Rexroth injection-molding valve.

lawton.zip is a silcon rubber press that is used to make power line insolators. The these look like bug spark plugs on top of the power lines. This press had 4 20 inch diameter cylinders with 2 - 8 inch jack rams. This press was made by CA Lawton for NGK, the Japanese spark plug maker.

Both machines where nicely built mechanically and hdyraulically

Each zip file has a .bmp and a .plt file. The .plt file can be imported into Excel to view more closely.