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Motor's Load Performance for Robotic Movement

Motor's Load Performance for Robotic Movement

Motor's Load Performance for Robotic Movement

Dear All,
> I am using SENSORED BLDC MOTORS for some robotic movements.
> I am driving my motors using BLDC MOTOR DRIVERs.
> I am using LEADSCREW mechanism for lifting a pedal up and down.
> On the pedal, there will be always a very HIGH Load, placed on it.

Now, my motor can lift up the pedal with my desired speed accurately but while going downward, the pedal just goes down freely overcoming the output speed of the motor. In other words, when the motors are on downward rotation, they can't hold the weight of the load and fail to conform to the desired downward velocity; instead they just let the pedal goes down freely until it reaches the lower threshold.

Please see the attached picture

with Thanks

1. This problem happens to my system in both directions. That's to say, when the pedal is on its upward movement, if I try to pull the pedal up with my hand then also the motor fails to keep up the input speed. I am meaning that, in both directions, if I give any force toward the motor's rotating direction, it completely fails to comply with the desired input speed. However, when the pedal is in the upward move, it's load performance (weight on the pedal) is quite nice.

2. From my microcontroller circuitry, I am feeding the Motor Driver with a PWM signal. The duty cycle of this PWM defines the speed of the motor. If I set the dutycycle of the PWM as 0 it means the motor speed to be zero too (it doesn't mean the STOP of the motor). Now my motor shows the same behaviour as I said above when I set the speed of the motor to 0 too. Say the motor speed is zero (after turning the motor ON) and the direction of rotation is DOWNWARD. In that case also if I press the pedal down, it descends freely. And if the direction is UPWARD, I can pull it up freely. As far as I can understand, the motor shouldn't go down in this case when the speed is zero, as the current input then is also zero.

RE: Motor's Load Performance for Robotic Movement

This is because your BLDC motor controllers are not doing the correct thing.  When you zero the drive the motor essentially goes away.  Hence the rapid movement.

When you assist the pedal in the forward direction, (from the motor's perspective), the controller decides that less power is needed and reduces it. Down to and including to zero.

You should be servo controlling the motors to maintain the same forces (torque) in any direction.  That way pushing, pulling - whatever - the pedal will behave identically.

The motors need to actually reverse direction and start driving backwards during the forced pedal condition even if they continue to move in the same direction.  The torque must reverse even if the rotation direction remains forward.

Keith Cress
kcress - http://www.flaminsystems.com

RE: Motor's Load Performance for Robotic Movement


Thanks a lot for the reply.
Please tell me, Do you think changing only the driver will solve the problem or I have to change the motor too. Can my existing BLDC Motor act with a servo driver and serve my purpose?

Please see the attachment - this picture shows my existing drivers configuration

Thanks again

    * http://lh3.ggpht.com/_PNj0pJFSZyA/Sgu8XpEZwoI/AAAAAAAACGY/oKzdat13pnw/s400/

Beats me. Probably.  Hard to tell with just a fuzzy picture.

Does the drive say anything about being "servo".

Have you looked at the drive manual?  It will talk about those things as they are fundamental operating aspects of drive systems.

What make and model?

There are some others who will come along that do this more than I.

Skogsgurra and PNactchway.

Skoggs will probably show up an a few hours as he's in Europe.

Keith Cress
kcress - http://www.flaminsystems.com

RE: Motor's Load Performance for Robotic Movement

From your description, it seems that your drive is not capable of applying torque in the opposite direction to velocity. That is, it is not a "four-quadrant" drive -- it cannot cover all 4 combinations of signs of torque and velocity. (A servo drive is always four-quadrant.)

Looking at the diagram of the drive you provide, there is a single pulse-train output to indicate velocity, with no indication of direction. This says to me that this drive is not intended to be used in the kind of bi-directional applications you are using them in.

Curt Wilson
Delta Tau Data Systems

RE: Motor's Load Performance for Robotic Movement

I knew you could say it better than me Curt!

Keith Cress
kcress - http://www.flaminsystems.com

RE: Motor's Load Performance for Robotic Movement

Thanks to all of you. Now I have got a clear picture on what I should do next.

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