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Need Advice on PWM and other stuff
- 05-25-2007
Joe posted on 
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
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
John Nagle posted on
Re: Need Advice on PWM and other stuff
The frequency isn't the problem, if increasing the frequency
reduces heat dissipation. It's the waveform. You're probably driving the
MOSFETS with a waveform that has too long a rise time. If
you're getting overheating like that, you're spending too much
time in the linear range of the device. Put a scope on the inputs
and measure the risetime of the MOSFET's control signal. It should
be well below 1us. Commercial MOSFET gate drivers have rise
times like 35ns. See
http://ww1.microchip.com/downloads/en/DeviceDoc/21933a.pdf
for some commercial drivers.
A 555 ought to have an output rise time of about 125ns, so you
should be seeing rise times like that at the MOSFET gate.
Also note that the output of a 555 is not rail to rail. Are you
getting enough voltage swing at the MOSFET gate?
There are also issues with driving an inductive load like a motor,
but that's less likely to be the problem here. That's a problem
which becomes worse as the switching frequency increases.
The real issues with switching frequency is that if it's too low,
the motor vibrates, and if it's too high, the motor's inductance
limits current into the motor.
John Nagle
Re: Need Advice on PWM and other stuff
Thank you for the response. I should have specified, I am using the
ts555cn, which according to STMicro has a rise time of 25 ns.
Yesterday, I did just what you suggested and measured an actual rise
time of 40 ns. I didn't think too much of the discrepancy at the time.
Is this significant? Although, now that I read the data sheet again,
it says 'trigger propagation delay is 100 ns" Is this what you meant?
It (the ts555) is connected to the 12V power supply the whole time.
Now you gave me an idea, my power mosfet (the IRF 520) has a rise time
of 23ns at 50V, but the number I quoted you above for the ts555 is
into a 30pf load, where the input capacitance of the mosfet is 330
pf.
BTW, the microchip link you gave me above just timed out. I did not
receive the pdf yet. Is there a part number you were referring me to?
I will try the link again, but just in case, if you do not mind,
perhaps I would have better results looking for the part number.
Oh, I am whacking the gate of the mosfet with 12 V from the ts555cn.
I noticed that using 5V resulted in unacceptable heating as well.
Experimenting showed that any voltage above about 7V resulted in
significant less heating of the mosfet. I only wanted to have 2
voltage sources for my electronics, 12 V for the eventual SONAR, and
5V for the micro and other electronics. But that is still flexible.
I'm still stuck on this but I will try the link you gave me once I log
off this forum to see what that's all about. Oh, and while I am at it,
since I did not specify enough info in my first post. I am using the
20L15T Schottky diode b tween the drain and source of the mosfet. Do I
really need something that hefty? They are expensive, but I didn't
want to take any chance on blowing mosfets due to the inductive
kickback.
Thank you again for the info,
Joe
Bob Smith posted on
Re: Need Advice on PWM and other stuff
Since you have an AVR, you might want to replace the 555
with a pin(2) from the AVR. Depending on which AVR you have
you may already have a couple of PWM counters on board.
Even if you don't have the PWM counters you can build one
easily enough with a timer interrupt.
Using an AVR for the PWM output gives you a little more
flexibility in how to set the motor speed.
I'm about to do the same thing (AVR/PWM) for my bot so let
me know if you want more specific help.
Bob
Re: Need Advice on PWM and other stuff
Yes, a very kind offer. Just one pin? I would love to use the avr, but
I don't know how, even after reading the dox. I am going to have one
micro whose sole function will be to control the motors and steering
(and, at first, will have to decode the signals from my RC and timeout
when it stops receiving the once every 18ms pulse that the transmitter
generates when it is on, just a safety precaution in case the thing's
running full tilt and my RC transmitter batteries go dead, or it goes
out of range). I will add micros later for sensors, navigation, etc.
as needed.
I have in my possession the following:
ATTiny 2313 (I know that has 4 PWMs aboard), but limited memory
ATMega 8515, my favorite
ATMega8535, because of the A/D's. I haven't used this one yet.
Bascom full version programming language. 1.1.83
STK 500 programmer.
I cannot seem to wrap my head around how to use the timers for PWM and
get a timeout if needed also. Being more inclined to hardware, I
thought I would use a separate PWM (which probably sounds insane).
Anyway, suggestions on which micro would be the best to use, and, If
possible, I would like to control each motor with its own PWM so I can
slow down instead of stopping one of them to make a turn.
Now, if it so happens that you don't use Bascom, then a flow chart
(antiquated term, I know), or description of which pins I have to
use, details, details, and I can probly figure out how to code it
myself in Bascom.
Thank you for the reply, I look forward to this to enhance my
understanding of all the functions these avrs can perform.
Joe
Re: Need Advice on PWM and other stuff
Sorry, the "(2)" was *supposed* to be "(s)" indicating more
than one pin. You'll probably want, per motor, one pin for
the PWM output and one pin for the direction.
I sit at a Linux desktop and really like the AVR since you can
program it using gcc. The site that got me going on this was:
http://www.linuxfocus.org/English/March2002/article231.shtml
Some AVR books to consider are: John Morton's "AVR, An Introductory
Course", and Dhananjav Gadre's "Programming and Customizing the AVR
Microcontroller".
All of the above are good and have enough PWM channels that you
don't need to use a timer interrupt. I use the ATMega8 in the
28 pin DIP. It is easy work with since I wire-wrap most of my
projects. (See my hobby web site at: http://www.linuxtoys.org )
The basic idea is the same for both a hardware and a software
implementation. You have a free running counter with the number
of bits of your desired resolution. For 8 bit resolution you
need an 8 bit counter. Set up a timer interrupt and increment
the counter on each interrupt. If the new count is zero, set
the PWM output high. The terminal count is the PWM width times
the resolution. So if you wanted a one-quarter width PWM output,
the terminal count would be 64. If the new count is equal the
terminal count, set the PWM output pin low. There are other
considerations but this is the basic idea.
hope this helps
Bob
Re: Need Advice on PWM and other stuff
was:http://www.linuxfocus.org/English/March2002/article231.shtml
I did some more reading in both the bascom user manual and the atmel
documentation for the 8515. They don't talk about rise times, which is
crucial for using power mosfets.
The bascom user manual has a sample program to output 2 PWM signals
(according to the 8515 dox, "phase and frequency correct outputs are
the preferred modes for electric motors), using timer 1, the 16 bit
timer. It divides it into 2 8 bit pwm signals. I don't know if they
are "phase and frequency correct", nor do I even know what that means,
but I am going to give it a go today.
You said above:
So if you wanted a one-quarter width PWM output,
I don't know what you mean by terminal count. Do you mean clock
scaling?.
I guess I will begin at the beginning. Program the chip using the
sample program in the user manual, and measure the OC1A and OC1B pins
and see what I get on the scope. Then, I will try and vary the width
of the pulses after I make sure the rise time is on the order of
about 35-40 nanoseconds (or less), although I doubt it. If not, maybe
there is some logic (LS series maybe ? that can take a slow rising
edge and make it fast.) Is there a such thing as a Schottky comparator
with the rise times I have mentioned?
Anyway, I am off to the races now, so to speak. I will repost here if
I have any questions, and I will check out your web site first. Maybe
I will get some ideas from there.
Thank you again,
Joe
Gordon McComb posted on
Re: Need Advice on PWM and other stuff
Hmmm, I'm surprised. I thought I recalled seeing these diagrams for the
AVR. The Bascom manual is chip-agnostic, so it wouldn't have anything
specific about any of the chips.
The 8515 is a rather old (and now obsolete chip) and maybe they didn't
use to document this. Or, you have the "summary" or errata
documentation, and not the full documentation. On most of their chips,
the full docs are >100 pages.
-- Gordon
Re: Need Advice on PWM and other stuff
Good to hear from you again. I've been indisposed for the past few
months with studies and finals and all.
Sheeutt, the bascom manual is 750 pages long ! And, the ATMega8515
"data sheet" (ahem) is 250 pages long. Lots of stuff to wade thru
there. I have to laugh when it says "data sheet". A data sheet is
maybe what 20 pages at most? This thing is like a book.
What do you mean the 8515 is obsolete? maybe the AT90S8515 is what you
are referring to. I am the one who got a mega8515 that said it was the
AT90S8515 and it drove me batshit for 2 days till someone on the mcs
forum asked me to send them the chip ID, and then she told me that it
was the ID for the ATMega 8515. Everyone said it was a one in a
million shot. The Atmel app engineer said he'd heard of it, but that
it was extremely rare. Maybe I should have played the lottery that
night !
And, yes, you are right, no mention of rise times. Maybe they don't
even know, but now I do.
I did some experiments today. Bob, You Rock !
I just entered the sample program from the bascavr user manual, but
using the pins from the (ahem) 8515 data sheet and looked at the
waveforms on the scope.
I'll be dipped in shit, it works! Just by changing the pwm1a= and
pwm1b= values, you actually get what looks like a linear relationship
between those numbers and duty cycle. (I haven't plotted them yet,but
it is predictable and repeatable).
Also, when you configure timer1=pwm,pwm=8 compareApwm=clear up,
compareBpwm=cleardown results in a pi shift in phase between the 2
signals, but if both compares are up or down, ie, equal, then both
wave forms are in phase.
So, I have duty cycle and phase control. The signal is clean, and I
didn't see any glitches.
Now for the best part. I measured the rise time of the pulses at 50
nanoseconds. Not quite what I wanted, but I was very pleasantly
surprised.
OK, so now I need to speed up the rise time. I have to run these nice
neat clean waveforms thru something that will give them a gain of,
say, 3, and will speed up the rise time. I have to whack the gate of
these mosfets with at least 7 volts, 14V is their limit before the
fireworks, but 5 volts will just cause a meltdown, so I am thinking 12
volts, since that is my electronics power source (not the same 12V the
motors will be using). All the so called high speed comparators out
there have rise times on the order of 100 ns.
Any ideas?
Joe
Re: Need Advice on PWM and other stuff
One of the few times I am answering my own post. I think I have solved
the above problem I outlined in the last paragraph above. At least on
paper anyway. I'm going to try my solution out today.
Well, thank you gentlemen, for all your advice and information.
I will be visiting the group more often now that I have a little more
time.
Joe
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