4 rpm measurement and 4 pwm control

chelitog555 · Joined: 25 Feb 2008 Posts: 5

Hello, I'm going to make a sumo robot for a competition, and I want to connect my 4 motors directly to the wheels without gearhead. I need to measure the rpm in each motor to control independently their pwm, because even the motors are of the same model and brand they can have small diferences on speed.

but I need some help because I don't know it all this work can be made by only one PIC or DsPic(especially for the timers), for example the DsPic 30f4012 or maybe using 4 Pics?? Shocked

Ttelmah · Guest

Hint. We are not telepathic.
What are the motors?. stepper?. DC motors?.
How are they actually driven?.

Best Wishes

chelitog555 · Joined: 25 Feb 2008 Posts: 5

Sorry, there are 4 mabushi dc motors, I want to use drivers like l298

RLScott · Joined: 10 Jul 2007 Posts: 465

If the motor has an attached quadrature encoder, then that can be used to measure the RPM. If not, then you are faced with the less accurate means of estimating RPM by means of the back EMF generated by the motor. So which is it?
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chelitog555 · Joined: 25 Feb 2008 Posts: 5

or maybe I can use any LM331, AD650, VFC32, XR4151 to make a frequency to voltage converter and use one on each motor??

SherpaDoug · Joined: 07 Sep 2003 Posts: 1640 Location: Cape Cod Mass USA

I would tend to use a PIC for each motor. Have a master processor that commands four virtual motors. Each virtual motor is a slave PIC controlling its own real motor.
With only one motor to worry about the slave code should be easy to write, and then just duplicate. PICs cost less than the time it takes to write more complex code.
Plus there is inherent fault tolerance. If one motor gets damaged, the others won't be affected.
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RLScott · Joined: 10 Jul 2007 Posts: 465

chelitog555 · Joined: 25 Feb 2008 Posts: 5

the hardware is an optocoupler using an encoder on each motor

FvM · Joined: 27 Aug 2008 Posts: 2337 Location: Germany

I guess, you mean a single channel photointerrupter, not a two channel quadrature encoder? In this case e. g. a 30f4011, that has 4 ICx pins and sufficient PWM outputs could be used to control four motors. The said 4012 unfortunately is missing just the ICx pins.

Ttelmah · Guest

There would also appear to be a very large problem with the timing involved. If this is an interrupter, giving just one pulse per rev, at 4RPM, you are only going to see one pulse every 15 seconds. A frequency to voltage converter, itself adds a time delay to detecting any speed change. Even with multiple teeth on the interrupter, the data the processor receives is going to be so far 'out of touch', that the code is going to be very hard to generate, without massive overshooting, or very slow responses...
Better to measure _time_ between the interrupter pulses. If you measure how long the gap is, using the clock of the processor, you have a measure that reflects the speed during the last interrupter gap, and which is therefore much more recent. Look at the example code for tacho applications.
I agree with Sherpa Dog's comment. It really is going to be easier to use a relatively 'basic' PIC for each motor. Or going just a little more upmarket, something like the PIC18F1220, which gives the ECCP, which is ideal for controlling a motor through the L298.
I would worry that the motor is going to have enough torque without reduction. Sit down and do some estimates of how much torque you need (there are formulae on the web for this), and then look at the motor data sheets. You may be suprised at how large the motors have to be, for direct drive. Generally, smaller motors, spinning faster, with a reduction gearbox, will be able to deliver much better performance, than direct drive units. Typical 'short cut' motor assemblies, are the units in modern cheap rechargeable electric drills. These have reduction gearboxes built in, and the basic units, are often cheaper than you can buy the motors for!...

Best Wishes

chelitog555 · Joined: 25 Feb 2008 Posts: 5

Thanks so much, well I think my motors have torque enough, there are four mabushi fs-578va power window motors, and now I can sense the speed of each motor using only one timer, but when I try to use the dspic 30f4012 to drive the 4 motors I found that this dspic have only 3 pwm generators, what can I do now? I need to use 2 Pic?
thanks