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BLDC motor load performance

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faysal_002299

New Member
Hi,
I am using Sensored BLDC motors for some robotic movements. I am driving my motors using motor drivers. I am using ball screw 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 summery I would like to say, 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.

If you have any questions please let me know

thanks in advance
 

Mr RB

Well-Known Member
Cool project. :)

You need proper "servo drivers" for your DC servo motors. These can decelerate your motors and stop at the correct position, holding the load weight stationary.

Try a google search for "Gecko servo driver" I think Mariss is still making these sensibly priced servo drivers.
 

faysal_002299

New Member
Thanks for your reply.

Could you please make your answer clearer? Actually before going for any further step I have to justify my attempts before my professor and colleagues.

Please write me in detail on it at your convenience.

Thanks again & again
 

Sceadwian

Banned
I don't really see that there is a problem here. Just weld the ram to the pedal, if you don't want to weld it then come up with a hard locking mechanism that prevents the load pedal from pulling away from the ram that can be detached, a simple bolted on clip would work.
 
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Mr RB

Well-Known Member
Sceadwian, I think his problem is not mechanical, its the motors and leadscrews freewheeling because of the downward load after they move downwards.

Faysal, a "proper" closed-loop servo driver will move the servos and leadscrews to the correct position. If the downward load tries to turn the leadscrews and motors the drive will apply the correct amount of current to keep the motor at that same position and the load will not move.

You said your motors are "sensored" meaning they have optical position encoders, so just buy a proper servo driver (See google for "Gecko servo drive") and it will do the whole job for you.
 

faysal_002299

New Member
Mr. RB thanks for all your supports. I completely agree wity you- here the problem is not with the mechanical design. The problem is entirely related to the motor and driver.

Ok, I will look for the driver you have suggested. Before that, for your information, I want to say you one more thing- 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.

Now the thing is, I have already bought 6 drivers (made in Korea) for my system. They were quite expensive for me and it would be quite difficult for me to change them now. I am attaching the driver's manual. Though it's written in all-Korean, I am sure you can get a clear idea from the schematics. Please take a look at it and say me if I have any way to solve my problem with these drivers.

Please let me know if you have any further query

Infinite Thanks for all your helps :)
 

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Mr RB

Well-Known Member
Ok I looked at your schematic, they dont look like DC servomotors at all, they seem to be 3-phase brushless motors??

I would think these are totally wrong type of motor and driver for your application, you need low speeds and very high torques with closed loop control, ie DC servos with optical encoders.

The motors you have might be able to make the torque required but it seems your drivers won't produce a negative torque. To hold a heave weight on a vertical leadscrew you will need a different type of motor, or maybe a custom built driver.
 

faysal_002299

New Member
Dear Mr RB,
I have one more thing to tell you. 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 descend 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. Why this is happening? If the motor acts like this then do you think the servo control driver you are suggesting can serve my purpose with this motor?

Thanks a lot
 
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