Help - Gear Motor Controller

[dteshome]

Does someone know someone who can help me write
or buy and implement the proper controller - circuite
to control a gear motor?

The gear motor has an arm (attached to to it horizontaly)
that rotates in a circular manner and stops at 12
positions around the circumfrance.

I am in the Los ANgeles area and would appreciate someone
who can help be design the necessary components, of cource
for a few.

Thanks.
Dan

[Caleb Fulton]

Are you referring to a stepper motor?

[dteshome]

No, I am referring to a gear motor, an electric motor.

[Caleb Fulton]

If you have a part number, you might be able to find a spec sheet/pinout online, from which you could devise some sort of control circuit. My guess is that it would need to have--at the most--a rather elaborated PWM output.

[Adam Y.]

No, I am referring to a gear motor, an electric motor.

Im really confused.
a)How many leads are going into and out the motor?
b)Can you post a picture?
c)Where did you get the motor?

[dteshome]

I am sorry if I caused confusion. I am very new at this.
I do not have the motor yet, I am trying to find someone
to help me pick the right motor (considering the torque,
etc. necessary to move the arm which has a waight of
about 25 lb. attached to it's end) and write the controller
for it.

Most of the technical ?s asked are going over my head. What
I need is someone who has done this type of work to
provide me with consulting service to implement this
project.

Thanks.

[Caleb Fulton]

How long is the arm? I ask because torque is force at the end of the arm (weight) times distance, not just the force at the end...

[Jnadke]

I think he's referring to a Stepper Motor. MicroMo is one company that sells them along with stepper motor controllers. It's basically a motor that has 2 (+) inputs and 1 (-). As you alternate between the two (+) inputs the motor completes a fraction of a revolution. You use it like a regular motor, but the speed is determined by an external controller and how fast it switches between the inputs.

For example, if you get a stepper motor with a 30 degree resolution, every time you alternate to the other input, the output armature moves 30 degrees. Coupled with a gear reduction, these motors can be made to be very accurate. However, with these motors you need a potentiometer for positioning to determine electronically to when the motor has reached its next step. If you switch early it will go back to the previous step.

If you haven't purchased it yet, then I think it might be easier to use a servo motor for arm positioning. Although they cost a lot more, it's a lot easier to use. The position is determined by how much power you send it.


Basically:
If cost isn't a factor and time is, go with a servo motor.
If cost is a factor and time isn't, go with the stepper motor.

Whatever you get, you'll need a motor that can hold ~112N if your mass is 25lb.

[Caleb Fulton]

No...he IS referring to a gear motor...they are much larger and have more torque. They seem to be mostly AC devices, though...


Personally, I would use one of the window motors with a potentiometer connected to the rotation shaft to monitor the position. I would use a PID system to make the whole thing work.

[dteshome]

The arm that the electric motor has to move is 3 feet in radius.
The weight at the end is about 25 lb.

If you think you can go the job (i.e., write the circuit/controller)
I will send you the picture and call you.

Please let me know.

[Eric Bareiss]

How fast does it need to go?

[dteshome]

It does not have to move too fast, 3 to 5 seconds
to move from one station to the next is enough.
That is, it can take upto a minute to make a complete
revolution. The arm, however, has to stop at 10 or
12 stations fixed around the perimiter of the table.

[Jared Russell]

A stepper motor would be ideal for that application.

[Jnadke]

Originally posted by Abwehr
A stepper motor would be ideal for that application.

The bad thing about stepper motors is that they are designed for very low torque applications where maximum precision is key. The holding torque of a stepper motor is the maximum torque it can hold at the current step. The maximum torque the stepper motor can maintain while rotating to the next step is:
Max Torque = (Holding Torque) / sqrt(2)

The speed of a stepper motor is determined by the speed of the controller, so you'll need a very fast controller if you want a decent speed.

[Al Skierkiewicz]

dteshome,
What you are asking to do here is a rather sophisticated setup. With a large arm and the weight you have specified you would need a drive system with low RPM, lot's of torque and some form of brake. Getting the precision you are looking for (about 30 degrees/ move) with the rest of the specifications would require rotation/position sensing and the ability to slow the motor and brake at the location you want. You can of course use some form of indexing so that the motor turns the arm until it hits a mechanical stop. Then you can remove the stop when you want to move to the next position. If you are not concerned with precision you can put a microswitch at each position you want to stop at. Use the switches to interrupt the motor current. Then use a start switch which bypasses the cutoff switches for a time after the motor moves. The start switch is wired so that the stop switches are not part of the circuit until the arm has passed the current position and has moved towards the next position. This would require 12 switches in series, wired for normally closed, and those in series with the motor. The start switch would bypass the series string while pressed and then put them back into the circuit after the start time is over. American Science Center is a good place to get surplus motors, switches, etc. They have a website and are located here in Chicago. I am sure there are stores out by you as well.