Stop Servo Move on Torque Limit

[plccontrols]

Hello,
I am working on an application that inserts a splined shaft into a 'gearbox'. The shaft is rotating slightly to allow engagement of the splines and is moved forward on a ballscrew controlled slide. It uses a Powerflex 755 drive set up for CIP motion at a reduced torque. This prevents the servo driving forward if the splines don't engage and causing damage to the shaft or the part.

I am simulating a blocked condition and I have tried using both position and velocity instructions with the same result. Using some sample code from the customer (they are very particular on code used), I either move (MAM) or jog (MAJ) the shaft to a position (currently a block of wood) at low torque (10%). Monitoring the position error and current feedback, I issue a stop (MAS), followed by a disable (MSF). It is somewhat inconsistent in that sometimes I get a smooth transition, and others get a quick reverse movement of the slide. It only retracts a few millimetres, usually, but it is quite sudden and usually causes a bang, due to the quick movement of the servo. Sometimes the axis faults, usually not.

I have monitored the torque reference, position error, and even set the current output to 0% prior to disabling the drive, but it doesn't appear to make a difference. Also, I don't seem to have access to the velocity error tolerance parameter in the PF755, like the Kinetix products do. The customer uses this parameter in conjunction with their MAJ command, which they set to a high number for this move alone (SSV), and then set back after it's completed.

Should the MSF not disable the drive completely, position error and all? How is the drive moving when the current level is set to zero? Why would it move on a disable command? What am I missing?

Thanks in advance for any assistance.

 

[robertmee]

MSF removes any holding power. At that point, any windup (drive system, compression, etc) in the system will move the axis in the opposite direction. Once an axis is turned on with an MSO, I rarely turn it off. MAS to stop motion, but leave the axis enabled to hold position. Issuing the MSF, the drive isn't moving at that point, the mechanical system is moving the drive upon release of power.

 

[plccontrols]

Thanks for the feedback Robert, and I agree, once the MSO is on, I typically don't turn it off either. I thought it was just mechanical at first as well, but I noticed that when the drive has been stopped with the MAS, the torque reference would continue to buildup to maximum, while the output was limited. Also, the longer the drive was only stopped and not disabled, it would travel much further in the opposite direction when disabling, more than 100 mm. This must mean the servo is activated as it is on a ballscrew. I figured this was due to the position error.

Initiating a jog in the opposite direction would bring the torque reference to zero, where it would then stop or fault when the torque reference hit zero. Very odd.

 

[robertmee]

Thanks for the feedback Robert, and I agree, once the MSO is on, I typically don't turn it off either. I thought it was just mechanical at first as well, but I noticed that when the drive has been stopped with the MAS, the torque reference would continue to buildup to maximum, while the output was limited. Also, the longer the drive was only stopped and not disabled, it would travel much further in the opposite direction when disabling, more than 100 mm. This must mean the servo is activated as it is on a ballscrew. I figured this was due to the position error.

Initiating a jog in the opposite direction would bring the torque reference to zero, where it would then stop or fault when the torque reference hit zero. Very odd.

Are you doing an MAS ALL? Are you sure there isn't something else issuing a motion command to that axis? I do Torque Homes all the time, and never seen this behavior. MAS should stop the motion, regardless of position error. As a test, do a Motion Axis Shutdown instead (MASD). See if you get the same movement.

Also how is your MAS configured in regards to Change Decel and Decel Rate? The torque is building as your system is under mechanical compression and the axis is trying to hold position against whatever mechanical push back is occurring. The servo loop is still closed after an MAS so it's trying to hold last position. Do you need to stay at this position? In my Torque Homing routines, after I set the new position, I back up to relieve any stress.

Also, how is the motor coupled to the ball screw? Direct shaft, chain/belt?

 

[plccontrols]

Yes, I'm using an MAS all. No other motion commands as I'm doing this manually at the moment using test logic. I will try your suggestion of the MASD when I go back later today.

The motor is directly connected to the ballscrew. My next step was to set up another instruction to back off a mm or so, as I saw the torque level drop when I issued a manual negative jog. What you describe is likely what is happening. Thank you so much for your assistance.

 

[plccontrols]

To follow up, I did try the MASD instruction and got the same results as an MAS followed by an MSF. It stops with a bang from a rough retraction.

I tried multiple decel configurations with the MAS, using the existing customer logic, custom set values and the max value (setting change decel to zero). I thought this might reduce the torque buildup, shutting down sooner.

The results I observe are essentially the same, but somewhat inconsistent. It has improved from my first round in that it's not banging on retraction (I'm not disabling the servo). However, on the reverse move, it seems to be very briefly out of control, springing back, but then regaining control and moving gently to the set position.

When stopping on torque, the torque reference builds up and I'm assuming that because the ballscrew binds, the error increases as it tries to move back, hence the initial violent retraction. I am going to set the retraction torque higher and see if that helps.

When you are doing your torque to home movements, are you moving to home with a MAM or jogging with a MAJ? Similarly when you back off, I'm assuming a MAM? What torque level (low, med, high) do you typically use? Do you have a smooth transition or do you get a jump initially? I'm just trying to understand your settings so I can improve the performance on this application.

 

[robertmee]

To follow up, I did try the MASD instruction and got the same results as an MAS followed by an MSF. It stops with a bang from a rough retraction.

I tried multiple decel configurations with the MAS, using the existing customer logic, custom set values and the max value (setting change decel to zero). I thought this might reduce the torque buildup, shutting down sooner.

The results I observe are essentially the same, but somewhat inconsistent. It has improved from my first round in that it's not banging on retraction (I'm not disabling the servo). However, on the reverse move, it seems to be very briefly out of control, springing back, but then regaining control and moving gently to the set position.

When stopping on torque, the torque reference builds up and I'm assuming that because the ballscrew binds, the error increases as it tries to move back, hence the initial violent retraction. I am going to set the retraction torque higher and see if that helps.

When you are doing your torque to home movements, are you moving to home with a MAM or jogging with a MAJ? Similarly when you back off, I'm assuming a MAM? What torque level (low, med, high) do you typically use? Do you have a smooth transition or do you get a jump initially? I'm just trying to understand your settings so I can improve the performance on this application.

I torque to home at about 70% for my application. Normal operating range is 200%. Using an SSV, I set the torque to +/- 70%, then do an MAJ at slow rate and monitor position error. Once position error exceeds a limit, I do an MAS and when the MAS sets PC, I do a MAM to final position and Home that to 0. I do the same on several applications whether its a pulley/polychain setup or a linear actuator. The sequence is pretty much the standard Torque to Home sequence that Rockwell published with some of my own tweaks. But the core function is the same. I don't get any jump. In the case of a polychain setup, I can see the belt tension to one side at the end (indicating no more movement), and then it smoothly relaxes and sets position.

If you are doing an MASD and you still see movement, that is absolutely mechanical system issues, not the drive. MASD turns all output power off to the axis, so the drive is incapable of doing anything. Are you sure you are setting your torque limit to 10% during this procedure? Since it's a 755 (I mostly work with Kinetix), I'm not certain what the equivalent parameter for torque limit (current limit in the PF) would be. Maybe the shear pin limit parameter? I vaguely recall having issues setting the right parameter in a 755, because the motion parameters get translated to 755 drive parameters, and it wasn't clear the actual parameter in the drive being changed.

 

[plccontrols]

With the MASD, I only got a slight reverse travel but still got the bang.

I am able to change the torque limit and saw the results of the different settings so I know that's working. There are some other parameters that aren't available in the PF755 though. Unfortunately, the lead times were (are?) really long for a lot of AB items, including Kinetix. That's why they made the PF substitute. It's OK, but I would prefer the Kinetix.

Thanks for the detailed description of your applications. I am going to write some test logic first to incorporate a dedicated jog function for the initial move and see how that works. The customer is working on other tests today, and I'm supporting those tests, but when I have the machine back I'll give it a try. I'll update on how it goes, success or not.

 

[plccontrols]

Hi Robert. After some experimentation, I was successful in getting smooth torque stops. It didn't seem to matter too much whether I used MAJ or MAM for the initial move. I did reduce the velocity a little further (it was already slow), as it's weight is quite substantial (~9000 lbs). I set the MAS decel to maximum for the axis, and performed an immediate move back with no disabling of the servo. Seems to be working quite well and is repeatable. Thanks again for your help.