Linear Motion

All,

I have a question about linear motion. I have a couple of pneumatic cylinders that I currently use to move objects of random size up against a hard stop. I would consider this "rough" motion. What I mean by that is I that the cylinder does not care how large the object is, the cylinder will push it against the hard stop and then stop(I don't have enough bore or pressure to break the object).

I have some experience with a electric actuator with pot feedback and have learned how to program this actuator to index certain distances.

My question is this: Is it possible to program a ball screw(or any electric actuator for the matter) so that it behaves like the pneumatic cylinder in my first paragraph? I understand I can have over-loads on the power supply but is there a device or way to program it so it does not "fault" out and is basically a part of the machine/system? I am just looking for a high level answer here with maybe a little bit of information to get me started in thinking about this. I would not mind tinkering with the above scenario, I could see some good real world examples to apply this if possible.

Thanks for the input,

Niese

A short answer to your question Yes it can be done
But having said that, it will get real expensive real fast so give it a lot of thought before you jump into it.

First you will need a motor drive system that can deliver 100% torque at zero speed continuously.
They do exist. For most applications you will need to oversize the motor – drive to produce the continuous torque. You will also have to supply external cooling on the motor to keep the temperature at safe levels.
If I were to be considering it I would not use a ball screw, my first choice would be a jack screw.
If they are sized correctly they can hold the load with very little motor torque. I have seen them hold the load with no power to the motor at all. This would help with motor cooling.

It would be interesting to program. Setup a variable torque drive system. You would have to select the desired torque for each job. I would also set up a encoder position feedback and monitor both motor amps (Torque) and position to make sure everything is working
It would be interesting to do but doable.

Others will have different ideas to do it but then there are many way to get the same result.

How brittle or compressible are the objects?

Yes, it can be done. This sounds like an even ending application. The trick is to not use 100% torque. If the linear motor has no load it will take very little torque to accelerate. It will take even less torque to move at a constant velocity. Use a torque limit. This should be easy to do. Set the torque limit to a little more than the amount necessary to move a constant velocity, but do it after accelerating. In short, change the torque limit depending on whether the linear motor is accelerating or decelerating. Now when you hit the obj you will not be applying much torque. Now there is a second issue. Is there enough torque to move the object to even end it. Again, you will need to know how much torque is required to move the biggest boxes and move the limit up enough so the torque will move the object but have a timer on this so the higher torque is not applied for too long. The higher torque should only be on long enough to even end the object then drop down to some low value.

I am not saying anything magic. Just be smart about it. There is never any need to apply 100% torque unless that is what it takes to even end the object and even then the 100% torque should only be applied for the time it takes to even end the object. You can tell when the object stops moving.