Electronics > Electronics

Effect of PWM on Motor Torque

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Billy:

--- Quote from: oscyes on February 25, 2013, 08:52:43 AM ---well i used the motor for raising my robot's arm and when i put low pulse width it cant raise it as it does with fully pulse width and i need to raise my robot's arm at a very slow speed but i cant still do it

--- End quote ---

To control the speed of the arm using a non-stepper motor, you need to implement a servo loop using a motor sensor and PID (or PI or P) loop in a controller.
The sensor can be an encoder, a tachometer, a resolver, etc. Something that gives indication of motor speed.
The PID in the controller can then modify the PWM signal to keep the arm moving, but keep it from moving too fast.

Do you have a sensor on the motor? 

oscyes:
Hi billy, thanks for your reply
The real question is How can I obtain the max motor torque at low speeds using PWM for controling this speed through an h-bridge like in this tutorial:

http://www.instructables.com/pdf/Control-your-motors-with-L293D-and-Arduino/Control-your-motors-with-L293D-and-Arduino.pdf

I have tried this but changing each digital input A of L293 by PWM pulses, but when I send max pulse width for moving at high speed the robot arm can raise itself but when I send the pulse width for moving at low speed it can't. I hope you understand me.

Pdta. I haven't put the capacitors in my circuit I dont know what does it do.

jwatte:
You need to add an encoder, so that you know how fast the motor is moving. Once you have an encoder, you can drive the motor using PWM at a high voltage, and modulate the duty cycle so that the motor doesn't move faster than you need it to. As long as the load is within motor limits, the actual current flowing will not be too much, and the motor won't burn out.

In addition to an encoder (so you know how fast the motor is moving) you also need a H-bridge/motor controller that can run at PWM frequencies, and a microcontroller to read the encoder and figure out what PWM duty cycle to use. You are basically building your own servo at this point.

oscyes:
yes you are right is like a servo but im not sure if the servo can have slow speed with high torque

i mean  i need to lift a charge with the same motor (wich have some geared transmision) at different speeds, i am using pwm for varying the speed but when i try to move the motor at a very slow speed it has not enogh torque for lifting the charge

the deal is i need to decrease the speed of my motor without losing the torque so the charge can be lifted

thank you so much for your help =)

waltr:
Do you ever get to run the motor at its fastest speed? If not then you need a greater gear reduction to increase torque while running the motor at a higher speed.
If the motor does run at its fastest speed then you may need a larger motor.

Also, the greatest torque output from a motor is when it is staled and the maximum current is applied.
Study these tutorials to understand motor and motor selection.
http://www.micromo.com/dc-motor-tutorials.aspx

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