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# DC motor current limiter

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#### Vikky

##### New Member
Hi all,

I am trying to build a small robot operated wirelessly. Almost all the things are been completed. The only problem i am facing is the draging of the robot.
It is not going perfectly straight. If i allow it move 15 - 18 cms it tilts from it path for 10 - 20 degree.

I searched for the cause and found that the two DC motors are not been running at same speed. One is consuming 90 mA and other one 65 mA. Hence i am in search for a current limiting circuit which would supply max 65 mA of current.

I searched for it on the web but found only circuit which allowed the current flow in one direction. Since the robot can move in either direction i need a circuit which will limit the current in both the directions.

Thanks

I guess using the same current for both motors won't make them run synchronously. Even identical motors draw different currents at the same rpm.

Better add a circuit (hall sensor, back EMF measuring to determine motor rpm) and make one motor the master while the other is the slave.

Boncuk

What kind of circuit are you using to drive the motors?
Have you attempted PWM?
If anything you can use an lm317 to control the voltage with a pot until the two motors have an equal speed, this is probably the easiest fix without the need of a feedback loop.

Hi all,

I am trying to build a small robot operated wirelessly. Almost all the things are been completed. The only problem i am facing is the draging of the robot.
It is not going perfectly straight. If i allow it move 15 - 18 cms it tilts from it path for 10 - 20 degree.

To make it go straight you need some kind of feedback, otherwise (even with identical currents) it won't go straight.

I thought that almost by definition, even the simplest mobile robot utilizes two motors on two different wheels just so it can be steered differentially! This requires a controller that regulates the current to the two motors in real time to accomplish the stated task:
"Follow the white line" or "Go straight" (requires a compass or GPS)!

Otherwise, what is the point?

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The problem is glaringly obvious to me! It's analogy is a blind two-legged animal that takes longer strides with one leg and runs around in a circle.

If instead the animal is allowed to see where it's going, it'll change the stride lengths accordingly and keep on the line it wants to go.

Speed of the motors is largely dependent on the voltage applied to the motor. The resulting current taken by the motor relates to mechanical loading applied to the motor. Limiting the current will only reduce the maximum available torque, not the RPM of the motor. Current limited motor is how the Reel Motors are operated in a recording studio's tape machine, the current limit of the motor dictates the tension of the tape wound onto the spool, and the RPM of the motor falls as the spool fills up with tape, the same reel torque. So you can see, reducing the current in your robot drive application is a folly!

I say, if you're trying to arrange the motors to run at matched speeds, then you might as well make it a fixed axle with same sized wheels instead, or add rails to run on.

Normally robots have independent motor speed controls for each wheel (doesn't have to be matched, just independently variable). Then control these motors according to the robot's (or commander's) perception of running straight ahead (or the lack of).

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Use 2 voltage regulators, LM317 adjustable type will do nicely. Control the voltage of each with a trimpot (variable resistor).

You will be able to adjust the speed of the 2 motors easily within the need for "straight" seeing that there is always some minor wheel slippage etc in differential robots and they always drift a bit after a couple of feet travelled no matter what system you use.

Thanks all of you for the advice.

I first made up mind to use the LM317 to regulate the voltage suplied to motor. I even tested it and got the motors running at the same speed. But i stuck for reversing the robot. For this purpose i have to use 2 more regulators which will add up the BOM cost.

Is their any other approach which is less costly??/

You don't need to add 2 more 317's, have a motor reversing relay. (4PCO)

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