Motor Speed Problem

Hi Folks!.. i've got a problem with motor speed i am building an Atutonomous Ground Vehicle(AGV). And controlling its two wheel with diffrential movement...
The thing is the motors i have bought have diffrent RPM (diffrence of 8 RPM); basically they are gear motors 24 Volt DC .Can you plz tell me how to make their RPM Equal.
I have two Sugesstion ...
1. Operate both motors at diffrent voltage to make their RPM Equal. 2. Attach some High Value Resistors to one motor and apply same voltage to both motor to make their RPM equal.
Folks plz tell me which appraoch is better ? or u have some new sugesstion. One thing more if i use one of above mentioned approach what will be the affects when i apply load on the motors..
Reagards,Abbas..
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M Abbas wrote:

Without using a magnetic clutch between the two motors, you can't get them to go at the same speed without some sort of closed loop motor control system.
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Hi ! "mlw" can you tell me about megnetic clutch,what it is how it operated also is it possible to use when u are operation motors in diffrentional movement ? bye.
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M Abbas wrote:

The first time I saw the technique used was in the late 1970's on a Mattel "BigTrak" toy. Presumably, behind the gear box, the torque load on the motors is pretty low. The two motors are mounted back to back. A magnet is attached to the back of each shaft in such a way that they attract one another. When the motors are goin forward at the same speed, the magnet from one motor can help pull the magnet of the other when the load is uneven. When you are going in different directions, the magnetic clutch breaks.
It is a subtle effect, in that it doesn't stand up against directly opposing power to the motor, but when driven at the same power, has enough influence on the drive of each motor to keep them in sync.
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#1 is possible, but I suspect that the motors will respond differently to different loads, so that if it goes uphill your required voltage differential might be different than when its on a flat surface or going downhill. Also, the question of how to operate them at two different voltages is unanswered.
#2 Is an answer to #1, but unless these are very small motors you will a) have a hard time finding resistors large enough to dissipate that much power, and b) even if you do you'll still be wasting a lot of power through those resistors.
The best solution here -- if not the easiest -- is to implement a closed loop controller, e.g. PID controller.
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mehaase(at)sas(dot)upenn(dot)edu
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---snip---
Consider using a voltage control for each motor and use that combination to steer the robot. You most likely are sending a voltage signal now, and it may not be too difficult to change your design by adding one more to your joystick or to your servo controller. You may be lucky enough to get a balance of speed with a circuit divider and resistors, but the instant you load one motor more than the other you will be back to square one.
Wayne
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