Author Topic: Current Sensors to Limit Range of Motion  (Read 585 times)

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Offline MECH_ENGTopic starter

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Current Sensors to Limit Range of Motion
« on: October 24, 2012, 11:42:30 AM »
Hello all I am currently in the process of designing a robotic arm for a ROV (underwater robot). The arm only needs to move up and down and the claw will only need to open and close. I am considering using current senors to detect a spike in current when either the arm hits the frame or the claw closes on an object. The program would be written such that it would not allow the arm or claw to continue in that initial direction without first moving in the other direction once a maximum current is detected.

I'm just looking for feedback on this idea and any other ideas yall may have.

Offline Billy

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Re: Current Sensors to Limit Range of Motion
« Reply #1 on: October 24, 2012, 01:46:31 PM »
Hello all I am currently in the process of designing a robotic arm for a ROV (underwater robot). ....... I am considering using current senors to detect a spike in current when either the arm hits the frame or the claw closes on an object.

Even if you're using the ROV in clean water (poor conductance), you still would have to isolate the frame pieces from each other. So you'll end up with wires running out to contacts that are sitting on plastic. In other words, you'll be using home made switches.

Why not just use sealed magnetic reed switches that will be inherently water resistant, not subject to corrosion, cheap and easy to use, etc.? I think you're way over complicating it.

Offline waltr

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Re: Current Sensors to Limit Range of Motion
« Reply #2 on: October 24, 2012, 01:51:21 PM »
Lots of ways to do this.
Does the motors have a dedicated controller/driver?
Or just a driver controller by the micro-processor?
I assume the driver is an H-bridge. Are there current sensor resistors on the bottom legs of the H-bridge?
This would be the first step to sensor motor current. Typically these are fairly low values, to keep the voltage drop small, so need an op-amp to increase the voltage. Then either a comparator in hardware or ADC processor input to sensor the current limit. The processor code would then stop the motor and not allow movement to the direction the motor was moving until the current limit is not exceeded.

Offline Billy

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Re: Current Sensors to Limit Range of Motion
« Reply #3 on: October 24, 2012, 01:53:29 PM »
Hello all I am currently in the process of designing a robotic arm for a ROV (underwater robot). ....... I am considering using current senors to detect a spike in current when either the arm hits the frame or the claw closes on an object.

Even if you're using the ROV in clean water (poor conductance), you still would have to isolate the frame pieces from each other. So you'll end up with wires running out to contacts that are sitting on plastic. In other words, you'll be using home made switches.

Why not just use sealed magnetic reed switches that will be inherently water resistant, not subject to corrosion, cheap and easy to use, etc.? I think you're way over complicating it.

Oh wait, I just got it. Total brain fart!!
You're talking about current through the motor windings caused by the motor stalling at end of travel!!  Sure that's possible.

I guess you're thinking of using DC brush motors. Not sure that scheme would work with steppers. With steppers you can run them into the end stops and then just wait a certain period of time to be sure you got there.

But I still suggest the switches. You can move them around to set limits without making other mechanical changes.

Offline MECH_ENGTopic starter

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Re: Current Sensors to Limit Range of Motion
« Reply #4 on: October 24, 2012, 03:13:48 PM »
Thanks for all the replies, the micro controller will be an Arduino and the motor driver is the Pololu VNH3SP30 which has a built in current sensor. So the plan is to connect the current sensor on the board to the analog pin on the Arduino. I will look into the sealed magnetic read switches as well. 

 


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