# How to make speed of DC motor match speed of another?

Discussion in 'Electronic Basics' started by Eric R Snow, Jun 11, 2006.

1. ### Eric R SnowGuest

I have a situation where a dc motor drives a wheel which pushes an
aluminum wire at variable speed through a 15 foot tube. I would like
to be able to pull this wire also at the tube exit at the same time to
avoid jams. When the speed of the pushing roller changes the speed of
the pulling motor must change with it. I can think of several ways to
sense the push motor speed but not how to use this to make the pull
motor spin at the same rate. Is there a simple and cheap way to do
this?
Thanks,
Eric

2. ### purple_starsGuest

this is a non-trivial problem. i used to work with a printer
manufacturer and some of their printer models had two motors, one at
either side of the printer, to move the mechanisms across the page.
they used stepper motors to do it, and spent a lot of time and energy
tuning them to work together, working out their step tables, etc.
yours is a slightly different problem because you just need your "front
end motor" to pull up the slack, so probably it is easier than their
problem.

3. ### Greg NeillGuest

It might be easier to just have the motor provide a constant
torque (within reason!) so that it maintains a constant force
on the wire. This should be proportional to the power being
drawn by the motor.

4. ### John Jardine.Guest

Watched on in amusement while a similar project was delayed by 2 months
while they tried to run 2 motors in synchronism. Told 'em it's not that
simple and wouldn't be happening but they'd a bit between their teeth, 2
months already down the tubes and "hey! we're precision engineers, we
understand gearboxes and we've face to save". Eventually solved by a single
motor, long PU toothed belt and timing pulleys.
It's not straight forward.
If you've just got to have the 2 motors, then maybe continuously overspeed
the pull unit while feeding the power through a variable magnetic clutch. If
wire is thin, could result in secondary stretch problems and extra work of
needing to monitor tension and forming a local servo loop with the pull
motor and mag clutch. Although I suspect what I know as "wire", is lots
john

5. ### ehsjrGuest

The forces don't have to be equal. Have the pull motor
run faster, and allow the pull mechanism to slip.
Ed

6. ### JamieGuest

use ABS (absolute position using gray code) encoders on both motors
of the same type and same ratio of swing etc..
2 vector or alike drives the monitors each others "wait/syncro" signal.
etc..
vector drives also work very nice for this.
but all in all, if you want to hack it, a couple of
gray code encoders properly aligned going into a
PLC 8 bit data input cards you can force both motors
to stay in perfect track of each other.

7. ### Jasen BettsGuest

Gee, that sounds a whole lot like a MIG welder
I won't suggest oil as that's no be real useful in a mig welder,

can you get a teflon coated liners for your tube?
one way would be use a stepper motor with the coils switched by an encoder
on the pushing motor.

8. ### DeefooGuest

Didn't they invent synchro's for that? Search for synchro/resolver/LVDT.

Example: in old marine radars they used to have two motors, one rotating the
antenna and a second one rotating the deflection coil on the CRT to follow
the antenna.

--DF

9. ### Guest

How about sensing torque on the puller motor, and using that to control
the pusher motor proportionally? The higher the puller torque, the
more push is needed at the other end. This might require some sort of
PID control to keep from "hunting" due to the intertia of the system,
but it seems a safer bet than trying to match the speeds.

Just a thought...

Bob Masta

10. ### kellGuest

Just get rid of the push motor.

11. ### Eric R SnowGuest

Greetings Greg,
Thanks for the suggestion. I had thought about that and it helps when
others come up with suggestions that tend to validate my ideas. There
are available adjustable slip clutches meant for constant duty. So
maybe that's what I'll need to do. I'm trying to keep the drive as
compact and as light as possible though.
Eric

12. ### Eric R SnowGuest

Gee Jasen, it seems like you may be reading my mind. It's kind silly
maybe but it's a small welder and the weld shop says that even with
the teflon liner the wire tends to tangle. With larger MIG welders
there are pull systems, like the Cobra, that work great. I could even
adapt one to my small unit. But they are expensive and heavy. So I'm
trying to think of ways to make a cheap one. Mostly just for my own
pleasure. Stepper motors could work. US Digital sells a chip that
accepts signals from an encoder and outputs step and direction. I have
one of these, an encoder, and a chip that accepts step and direction
and will drive steppers up to a couple amps at 24 volts. But stepper
drives are sensitive to electrical noise. And the encoder is slated
for a different project. It looks like a slip clutch just may be the
way to go.
Eric

13. ### bgGuest

Eric R Snow wrote in message ...
Think in terms of a reel to reel tape deck. A capstan and pinch roller can
spool out tape at a constant speed with much less than 1 percent variation,
while a torque motor takes up the slack on the output. The input to your
tube could be set up like a capstan and pinch roller. The motor could be
running at the desired speed and the pinch roller engages only when wire is
needed. The output of the tube would again be a capstan and pinch roller but
continuously energized to pull the wire. Depending on how much tension you
put on the wire, a brake may be needed at the input.
A simple AC motor running at less than full power should work at the output
side.

14. ### feeboGuest

similar problem to cassette tapes. In those, the speed of the tape is
controlled by the capstan and pinch roller but the wind-on spool runs
much fatser, the speed is maintained by tension on the tape and a
slip-clutch on the wind-on spool drive. you can see this by "playing"
the cassette mech with no tape in - the wind-on spool spins rapidly.

I would go down a similar route with the pull wheel running faster but
with a slip-clutch. This way it applies as much tension as you need
but you have no sync problems.

The slip-clutch is fairly easily constructed by using a belt drive
onto the wheel axle (best onto another wheel mounted so as to get the
gearing right). The amount of tension can then be easily adjusted by
tensioning the drive belt, but if the wheel is stopped (i.e. by your
hand) the belt slips on the axle.

Get an old cassette mech and play - it'll give you a load of clues for
this

15. ### Eric R SnowGuest

The slip clutch is the way to go. I'm gonna build one using a wave
washer for a spring and aluminum bronze against hardened steel as the
friction surfaces.
Eric