  DPRG List

 [DPRG] PWM Frequency and torque Message index sorted by: [ date ] [ thread ] [ subject ] [ author ] Previous message: [DPRG] PWM Frequency and torque Next message: [DPRG] PWM Frequency and torque Subject: [DPRG] PWM Frequency and torque From: Chuck McManis cmcmanis at mcmanis.com Date: Tue Feb 27 00:19:36 CST 2007 ```Its not that complicated. You need to just remember some basic facts: 1) Torgue is proportional to current, more current = more torque. 2) Current is proportional to voltage, more voltage = more current = more torque 3) Voltage is a function of the driver circuit's frequency bandwidth. So you're pushing a square wave into a motor, the circuit looks something like: +-------\/\/\/---+----+ | | | +-+-+ | ( |V~ | --- ) Effective Motor +-+-+ C--- ( Inductance | | ) | | | +----------------+----+ And what you're looking at is a "low pass" filter, where the frequency determines what the output voltage is across the inductor (aka the motor). What you get is a resonant circuit where the voltage across the coil is a function of the duty cycle and frequency of the input and that changes the motor's torque. The simplest way to "tune" this circuit is to up the voltage. Basically the motor's inductance is going limit the voltage ramp and if you can vary the voltage (or build a chopper but more on that in a second) you can precisely control the current through the motor and hence its torque. I'd suggest for home lab users, put your Ammeter (multimeter set to amps) in series with your motor. Vary the parameters of the circuit (input voltage, frequency, and duty cycle) and watch how that affects the current through the motor. As you've noticed these circuits are not linear in their response to duty cycle so if you want a motor controller where 10 is 10% and 50 is 50% and 100 is 100% of the motors torque, you need to compute the voltage/current curve for the resonant circuit and fill in your values from there. what you'll find is that 1% torque is like 33% duty cycle and the difference between 75% torque and 100% torque is like 95% and 100% duty cycle. So the simple answer is, there is no simple answer. You can finesse the problem by using active feedback. The simplest form of active feedback is called a chopper circuit. A chopper is fairly simple in concept. Basically you apply a voltage across the motor and measure the current through the motor using something like a shunt. When the current reaches the desired set point you turn off the voltage, when it drops below your minimum set point you turn on the voltage. What this does is drive the motor with a PWM voltage wave form that is modulated by the chopper feed back to provide the desired current. In this way you can precisely control motor current, and precisely control motor torque. Further, even as the mechanical load on the motor changes, the torque output of the motor will remain constant because the chopper circuit will compensate. I have to believe that there is probably a 50 line program you could put into one of the 8 bit PICs or AVRs to do exactly this. --Chuck ``` Previous message: [DPRG] PWM Frequency and torque Next message: [DPRG] PWM Frequency and torque Message index sorted by: [ date ] [ thread ] [ subject ] [ author ] More information about the DPRG mailing list