Cheap Motor controll

Hi again,

Just a few words, - in the morning haze before I am fully awake and
starts to work. ( Lots of coffee and smoke ;-)

I am not going to answer a lot of questions about what I said about
a motor control for a few dollars - as I bet a lot of you would ask
me about, so here is:

First PMW versus analog:

A PWM bridge is what you would like for a large motor, - it is far
more efficient. An analog would have to take up the same wattage as
the motor itself - and then have a good heatsink.

Let say you just wanted a small motor, - say a "dremel" tool. If
you feed the +-10V to a power opamp, - or a transistor buffered
opamp, then you wire this up as a closed loop voltage fashion. A dc
motor will have an rpm according to the voltage you supply it. The
rpm may drop a bit because of load. A bit more electronics, and you
can compensate for this rpm drop. So a opamp and a power
transistor, you have a simple servo controller for a few dollars. (
Yes, it is a servo ! The one used in a tape recorder or a tune
table - but the often use a velocity feedback. Small tape recorders
often use a motor with a tachometer in the same housing )

Okay, - this is an analog type of motor control, - for a just a few
dollars. You can also use a PWM bridge, - adding a few dollars.
Now it is a little harder to have a voltage feedback. Add a few
dollars - and you can have that too, without any tacho generator.
But the best performance would be with a dc-tacho generator for
feedback.

All I have just said, is for a small motor - like a "dremel" tool.
But add larger transistor output stage, and it can turn big motors.
This would add more dollars to it - until you reach designs in Jon
Elson's category.

But back to the very simple design - you could use it to control a
"dremel" tool, or small motor. If you add an encoder feedback, -
then you will close the loop for position. The EMC will handle
that, so this very simple design could be used as a simple
interface to EMC. It will stop in position, and crank up as others,
- and if the load don't vary much, it could run on several axis.

The dc feedback tacho - is used to keep a constant velocity. Say
0-10V is equal to 0-100% on your rpm scale. Output 8 volt, and the
motor should output 80% rpm. The EMC will home it in to the correct
position, so way bother if the RPM drops some % ? Why use a dc
feedback ?

Let say you would like a 45 degree motion on your X & Y. If you
output 50% rpm to both motors, then it will move in a straight
line, but say if your X axis drops to just 30% of the commanded rpm
- what then ? You will not be moving in a straight line, - the Y
motor will reach it's position before the X motor catches up - but
it will zero out to it's commanded position.

A lot of commercial motor-amps, have compensation for velocity drops
and for a lot of uses, - you don't need any dc - tacho. A large XY
table or different loads, - may not work so good.

But for a little test - we could make a "stupid simple" motor
control for less than $10, and it could work okay too. If we added
a simple counter, then the RPM on this could use the stepper
interface too. This could make the interface you would need for
controlling the RPM on a router or something too, without any STG
board. This little setup could interface to a 3phase motor
controller, - from the simple par-port.

A "real good" motor-amp, can be made for less than what Jon Elson
has designed. I don't think he would mind to adjust his design a
bit either. If you wanted something cheaper, he could drop the bar
led display and maybe use smaller output transistors if you don't
have the power need. This would save you some dollars. If he use
the same boards, you could add the bar graph LED later on - if you
think you would need it.

So, there is endless of possible solutions, - the main thing is to
get answers as to what your needs is. What motor size, what use,
and so on.

I have not studied the EMC code, - but Fred may have put in some
velocity control loop. In other words, it might work well with a
less than perfect motor-amp. That is, you may not need any high
quality dc-tachometer, and Jon Elson states that he has use a small
dc motor instead.

I hope that this could be of some help to you, - and if we made
list of what most would need, - then we could just present this to
several parties, and they could tell you what you need. There is
also electronic groups that would make a design for us cheap.

//ARNE