Hello,

I need help choosing a motor/method to push an object against a wall.
This holding mechanism is needed in a machine that performs an operation on an object that can't move.

Here is a sketch to show the idea:
https://plus.google.com/u/0/1059972...6142049766195760418&oid=105997285739033039869

Ideally I would use an pneumatic cylinder, but since that is quite expensive I am looking for other solutions.
I would like to be able to control this holding mechanism with an Arduino Uno since the entire machine is controlled by one. I am also using Adafruit motor shields and stepper motors.

The torque motor described on Wikipedia looks like a solution:
A torque motor is a specialized form of electric motor which can operate indefinitely while stalled, that is, with the rotor blocked from turning, without incurring damage.
But I can't find a torque motor on the internet suitable this application.

I was also looking at a normal DC motor. I thought about using the stall torque when the holder pushes against the object and can't move any further. But I don't know if I can use this because of the stall current...

If you can think of a good, affordable solution, please let me know!

 

Most quality motors have a rating for continuous stall torque and peak torque, in the continuous mode, the motor can be taken to this level of current and held there indefinitely, the peak torque range should only be entered for a very brief period, for a long term will destroy the motor.
Any motor can usually be used this way if the drive allows torque limit setting.
I would look at picking up one of the servo drives off of ebay for the relative motor size, one make come up often is Advanced Motion Controls for analogue control or one of the hobby units that uses step direction like a stepper drive does.
One make is Gecko.
You would then set the torque limit, but I am not sure if the stand alone stepper type can be used without encoder as an A-M-C could.
The majority of my designs uses ±10vdc control.
Max.

 

How about adding a spring?
The motor runs once, the spring compresses, you sense that the current required to compress the spring has happened.
Turn off the motor.

 

That's what popped into my head, too. Cheap motor or solenoid plus a spring. The pressure applied could be fine tuned by a screw on the spring holder.

I'd stop the motor just based on position. In other words, it would be stronger than needed and would slam into position.

 

How about adding a spring?
The motor runs once, the spring compresses, you sense that the current required to compress the spring has happened.
Turn off the motor.

Brilliant. If employing this method it would probably be best (and easier and cheaper too) to use a lead screw instead of a rack/pinion. A rack/pinion could be pushed back by the spring if no brake is used. A threaded rod is not suceptible to the pushback.

 

Don't know how much travel you need or want or what you call inexpensive but you may want to look at "electric Trailer Jack" (Google it). They use various drive systems including ball screw (ball bearing) and have travel of arond 18". Most also have limit switches and controlling one with a uC should not be a problem. They are less costly than expensive linear motors and you may find one that will do as you want. Husky makes a pretty good line of them as well as other manufacturers. Just something to consider.

Ron

 

"performs an operation on an object that can't move" is really quite nebulous. You might be drilling a 1/64" hole in a thin-wall plastic pipe; then again you might be performing a 1/2" keyway broach operation on a 3" stainless steel bar; these two operations would require vastly differing clamping forces to ensure that the part being machined is immobilized.

Is this going to be for a machine to be used in a production line, or something for casual/intermittent use?

You might find that a pneumatically powered toggle clamp will be less expensive and much more compact than a large pneumatic cylinder alone.
Just as an example:
http://www.mjvail.com/destaco/destacoTOC.html
Have a look at some of the straight-line action clamps, like the model 803.

 

I got a vague feeling that this might be a robot which needs to capture an object as part of a competition.

Would you like to give us a clue, @RenzovS ?