Well, school's out for the summer, and I am ready to build my new bot. If you recall, last winter break, I built "bang-bang", so named by the people on this group because of my use of dpdt toggles connected to RC servos to bang him off and on. Well, bang bang's motors died after about 3 weeks of use.
Since then, I have read Gordon's book, along with a few others, finished my course in electronics, and built a SONAR range finder device for my final project, along with some optical gadgets for my LASER course.
I have a circuit for PWM using a 555 and a potentiometer to vary the duty cycle. Now, at first I thought of driving the potentiometer with a small telescope motor, but then I started thinking about how to let the micro know (I am using Atmel AVRs) when the pot was against the stops so it wouldn't keep trying to turn the motor in that direction.
Just some background I am building my own H-Bridges, using power mosfets, and my PWM circuit (actually Horowitz and Hill's circuit). Also, I am using 2 windshield wiper motors that use 12V at 2.5 Amperes at 60 RPM (unloaded), With a load, I have found the stall current to be 4.5 amperes and the stall torque is 13.5 Newton-meter each. This is to be a 2 front wheel drive (differential system) vehicle using a sprocket and chain system to transfer the power from the motors to the wheels. Oh, this is an outdoor bot.
My question is, what frequency PWM?. All the above tests were conducted with the motor connected directly to the battery. I have heard that anywhere from 60 Hz to 100Khz is acceptable, but I tried first (on the bench, ie no load) 1Khz and the power mosfet was dissipating too much heat (like finger burning heat). At 10Khz it is much better (ie, much less heat dissipation). I have also tried using power darlingtons (TIP102's) and they reach 100C within about 5 seconds of power on. These tests are being conducted without heat sinks as I was able to rig up a temperature alarm to sound when the metal portion of the mosfet reaches 100 degrees C. Of course, I know I will be using heat sinks in the real deal. And probably active cooling (ie a fan).
I get absolutely no rotation until my duty cycle reaches about 60%. That did not change from 1Khz to 10 Khz.
I am using separate power supplies for motors and electronics with just the grounds commoned.
I decided that using the motor driven potentiometer was folly, and I will probably use mosfet switches to change the resistor values (and, hence, the duty cycle) in my PWM control module. Unless folks have a better idea.
I guess I wanted to bounce my ideas and design thinking off the group, as I know some of you have been building bots for years.
If anyone has ideas on any of what I have mentioned above, your comments are welcomed. As this is only the second bot I have built, I need all the help I can get. It will be radio controlled at first, as I have been able to decode the pulses from my futaba using the AVR. Ultimately, I hope to make it at least semi autonomous. I have many, many sensor ideas for it that I would like to experiment with.