Hi,

I am getting axis halt ( Direct output halt (Force Follow Err.) while trying to hold force force of 300lb. I am working on a project where i had to hold the force on the axis while my test part is retracting or extending. Currently the setup is a linear motion with ball screw and the Emerson drive in Torque mode and configured my axis as Position force control (Single input).I have tuned my force and position axis. I am first entering the force control and ramping my force in S curve to 300lb. Post that i am issuing the hold pressure force command and as soon as i start my actuator moving i get above fault. The tuning parameter for force as P=0.05928 ;I=0.1805.

LAT TUNING.rmcproj (12.2 KB)

# Axis is halting during force control

**Himanshu**#1

A graph would have been nice but I think I know what is going on. I bet when your part is retracting the force drops. This happens because the P and I gains must be small but they are too small to respond to disturbances ( your part retracting and extending ) very quickly. The key here is to know how fast your part is retracting and have a user program add or subtract a bias depending on how fast the part extends or retracts.

There is a key formula for this situation:

\frac { dP }{ dt } =\frac { \beta \cdot \left( Q(t)+v(t)\cdot A \right) }{ V(t) }

where:

- \frac { dP }{ dt } is the rate of change in pressure. You want to keep this 0 to keep 300 lb of force constant
- \beta is the bulk modulus of oil. This value determines the relationship between changes in the volume of oil and the changes in the pressure
- Q(t) is the flow into the volume of compressed oil.
- v(t) is the velocity. I am assuming that retracting is negative so the pressure will decrease
- A is the area of the piston.
- V(t) is the volume of oil under compression. This changes as the piston retracts.

The key is to keep Q(t)=v(t)\cdot A to keep the rate of change in the pressure 0.

If you know how fast the actuator is retracting you can multiply the velocity of the retracting cylinder by the feed forward of the cylinder holding force. This will be a voltage that can be used as a bias for the cylinder holding force. Now the cylinder holding force will respond immediately to the motion of the retracting cylinder instead of waiting for the force integrator to respond which is too slow.

**Himanshu**#3

Hi ,

Thanks for your input. My Test part are all electrical motors .In current scenario when i extend my test part i was expecting my delta motion controller should hold the current force of 300lb while moving back on the axis. I have captures the graph of my test part in extending and retracting while the delta motion control as not active.

Plot3.rmcplots (1.02 MB)

Plot2.rmcplots (779 KB)

Plot1.rmcplots (1.26 MB)

More explanation is needed.

I all cases it looks like the control output is at zero and not doing anything. If so how is anything moving.

How are you monitoring force?

In plot 1 it looks like the actual force is the force necessary to move what ever you are moving. When not moving it looks like there is 115 lbs of friction. It doesn’t look like force control is activated yet.

Plot 2 is similar. The target force is still 0. The actual force is just the force necessary to move the object. When stopped there is only frictional force.

Plot 3 is the same. I can’t see where the control output is trying to correct for anything and the actual force is simply the force required to move the load and friction.

It doesn’t look like you are activating force control or limit.

**Himanshu**#5

Hi,

As part of my project i have to control the Force of xlb while my test part extends or retracts. I have defined my axis a position force control but i m not doing any position control.I am trying to control the force only. I was expecting that when my test parts extends my delta motion should retract the drive while maintaining x lbs force on it.The Test part can be moved independently by applying voltage to it.The graph i uploaded was by applying the voltage to the test part to extend and capturing the graph of it on delta motion plot manager as the feedback from the load cell is being read by the Delta motion.

You need to call.

I don’t see where the RMC is control output is doing anything at all.

The green line in your plots is always reading 0.

Also, you can’t apply 300 lbs of force without having 300 lbs of resistance.

If there is an imbalance then the load will accelerate or decelerate.

**Jacob_Paso**#7

Himanshu,

In your plots, the Status Bits show that the axis is in Direct Output or Open Loop. In order to control force, it needs to be in Force Control. To enter force control, use the Hold Current Pressure/Force command or one of the Enter Pressure/Force commands, then the axis will try to control force, and yes, if your test part moves, the axis will move back or forth as required to hold the commanded force.