Linear motion setup

[Ether]

Originally Posted by 1075guy

The goal is to be able to stop and change direction at any arbitrary point in the travel.

Will your controller be told ahead of time what the arbitrary point is, so that it can plan for that and start to slow down as it approaches? Does the 0.1" precision (accuracy?) apply to this arbitrary point?

Or will the controller need to respond to a "change now!" command given at an arbitrary point in time? In which case, how much overshoot is acceptable?

You can provide a lot more detail without revealing the specific application. The more detail you provide, the more likely you will get useful help.

[Racer26]

The controller will know its operation profile ahead of time. It will have a command list along the lines of

Go 8.5" forwards at a speed of 1"/sec
Go 4.5" backwards at a speed of 10"/sec
Go 1" forwards at a speed of 100"/sec
Go 5" backwards at a speed of 50"/sec

Repeat ad infinitum.

The profiles its requested to move could be completely arbitrary, within its mechanical limits of stroke and speed, but they will be known before it starts, so yes, it will be able to plan for, and begin to decelerate as it approaches the arbitrary points.

Conversely, the profile could be as simple as:

Go 12" forwards at 1000"/sec
Go 12" backwards at 1000"/sec

Repeat.

(Obviously, 1000"/sec is way faster than it would be able to go, its an example of the simplest sort of command string. It will always return to the starting point before repeating the cycle.)

I have considered a pneumatic setup, with the sort of fancy valves we're not allowed to use in FRC, and it would probably work and meet the needs, but pneumatics are much noisier than electric motors (quiet operation is preferred for this application), and speed control would be difficult.

[PAR_WIG1350]

I'm thinking something like a tapered cam on a stepper attached to a lead screw attached to a second stepper might work for adjusting the stroke in one direction, while allowing for some very high speeds. Varying the stroke in the other direction would require more complex motion and would slow it down a bit, but it would still be fast.

EDIT, It would probably be faster to use a standard cam and drop the lead screw and second stepper. The tapered cam would be better only if you had a set return point and only one end of the stroke varied.

[DonRotolo]

Originally Posted by 1075guy

Go 8.5" forwards at a speed of 1"/sec
Go 4.5" backwards at a speed of 10"/sec
Go 1" forwards at a speed of 100"/sec
Go 5" backwards at a speed of 50"/sec

Forget pneumatics, they would not work as you expect for this kind of application.

An alternative if the load is light enough - say a magic marker - is to use a stepper with some Gates toothed belt and sprockets. That could be made to work pretty darned fast with a decent stepper. 0.1-0.5" is not a problem with either a lead screw or belt drive.

[Racer26]

Don, with all due respect, given my past experience in FRC with using pneumatics in non-standard configurations to achieve an innovative solution to a problem, I'm not convinced that they couldn't be used like this, however, I do agree that it would be much more complex than using a stepper motor with leadscrew or belted drive or similar.

[Racer26]

What about something like the 23L2 motor from this page: http://www.anaheimautomation.com/pro...75&pt=t&cID=19

connected to a leadscrew setup, driven by this driver chip: http://www.sparkfun.com/products/10735

Assuming the stepping + leadscrew combination could create fast enough motion with enough torque/thrust, it seems to put the price point in the right ballpark.

[DonRotolo]

Originally Posted by 1075guy

Don, with all due respect, given my past experience in FRC with using pneumatics in non-standard configurations to achieve an innovative solution to a problem, I'm not convinced that they couldn't be used like this, however, I do agree that it would be much more complex than using a stepper motor with leadscrew or belted drive or similar.

I think we're saying the same thing. Yes, of course it's possible. In fact, if the endpoints were fixed, pneumatics make it trivial and would be the better choice. But variable endpoints in both directions, with variable speed, make it far too complex to consider (IMHO).

...And if #23 steppers aren't strong enough, they come in several larger sizes.

[Racer26]

New question for everybody (I think I've decided to go with a NEMA 34 stepper as the powerplant for this operation).

Does anybody know of a stepper driver chip capable of handling 5-10A of current? Something that I feed enable/step/direction lines to, and hook up the 4 (or 8) wires of my stepper motor to? A higher current version of this chip: http://www.allegromicro.com/en/Produ..._Numbers/3977/

Bonus points if it can handle more than 35V too (as high as 70 or even 90 VDC would be good)