Need Advice on PWM and other stuff
- From: Joe <nuisancewildlife@xxxxxxxxxxxxx>
- Date: 25 May 2007 17:52:57 -0700
Hi Folks,
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.
TIA,
Joe
.
- Follow-Ups:
- Re: Need Advice on PWM and other stuff
- From: Bob Smith
- Re: Need Advice on PWM and other stuff
- From: John Nagle
- Re: Need Advice on PWM and other stuff
- Prev by Date: Re: Microcontroller options
- Next by Date: Another humanoid robot - about 5 feet tall
- Previous by thread: A tad too scary for bats!
- Next by thread: Re: Need Advice on PWM and other stuff
- Index(es):
Relevant Pages
|
