# DC motor control. auto reversing

Discussion in 'Electronic Basics' started by Nick Zentena, Dec 30, 2005.

1. ### Nick ZentenaGuest

I'm interested in building something that can control a small DC
motor. Basically it needs to turn a wheel 1.5 revolutions. Stop. Then turn
back 1.5 revolutions. The speed needs to be constant. One obvious problem is
I'm not sure of the speed and won't be until I've tested the whole thing.

I've found plenty of websites talking about motor controllers. Some even
have the ability to reverse. But none have the ability to reverse
themselves. Can anybody point me to a website or book that might explain
this sort of thing?

A little help on motor sizing would be great to. I figure the load will
vary from about 500 grams to say a max of 1000 grams.

Thanks
Nick

2. ### Jasen BettsGuest

yeah, but how many watts (or (fdractional)horsepower etc) do you need
from the motor. (you need to describe the task in more detail)

are you lifting the 1Kg (safest to design for the worst case) weight up
a great height, or just moving it a few milimetres to the left?

if the motor has to stop and start for some of the time it's not moving at a
constant speed.

how constant do you need the speed to be, cheap tape players typically
manage 0.5% variation (IIRC) would that be sufficient?

what sort of time frame is involved. do you want it to turn at a rate of 45
degrees per month for half a year and then reverse and turn 45 degrees per
month in the other direction for the other half of the year? or is this
something that must be done in less than one second?

Bye.
Jasen

3. ### Nick ZentenaGuest

Hi,

I need to spin this:

http://www.jobo.com/jobolab/produkte/images/2523.jpg

It's 5" or maybe 6".

My current setup can't handle something that short. The current roller is
rated at 120V 0.15 amps and can easily handle a larger load then the drum in
that picture. That's what 18 watts. The difference is the bigger setup turns
two rubber wheels that the big drum sits on. The gap between the wheels is
too big for a small drum. I'm likely going to try and drive the short drum
using the magnet fitted to it's base.

Just spinning it then reversing direction.
I'd bet even a 10% variation wouldn't be a deal breaker. It might actually
help by introducing some random action into the process. OTOH I'd like
something that wasn't too random -)

Worse case is 30 minutes of steady turning. It won't be used much just
when I need the smaller drum.

Thanks.
Nick

5. ### ehsjrGuest

Conceptually:

-------- --------- ----------
| Sensor |===>| Counter |=====>| Reverser |=====>Motor
-------- --------- ----------
^ ^ ^
2 magnets 4017 DPDT
+ reed counter, relay,
switch debouncer & driver,
optional delay

Glue magnets to the drum, 180 degrees apart.
Each time a magnet rotates to the reed, the reed
triggers the counter to increment by 1. Condition
the signal from the reed with a debounce.
Wire up the counter to count to 6, then reset.
Invert the outputs of counts of the last 3 counts
to cause the DPDT to de-energize; outputs from
the first three counts cause it to energize.

You mention that the motor must stop after 1.5
revolutions, then reverse. Did you mean there
must be a pause before it starts going in the
opposite direction? If so, a delay function
can be triggered each time the DPDT relay
coil voltage changes state.

Ed

6. ### Jasen BettsGuest

hmm a black cylindrical thing with a red band and a stepped end.

looks a little like a land mine but probably isnt.
it appears to be reasonably well balanced so there's not going to a lot of
torque needed.
most DC motors will meet that 10% figure quite easily if not loaded heavily.
I'll assume that's typical... is that slowest or fastest?

my first idea is to make some sort of adaptor to let you use your existing
machine (or add another roller to it etc...)

Otherwise it seems to be a matter of taking a small DC motor and gearing it
down from the 10000+RPM they typically run at to about 0.04 RPM for your
worst case.

I'd suggest getting the largest motor you can power easily, the bigger the
motor the less its speed will fluctuate if there's a rough spot on one of
the gears etc.

small gear sets boxes are available from hobby model making places.
I've seen some that can be "stacked" to give any degree of speed reduction.

I expect the motor will run smoothly enough so as to not need any regulation
of its speed.

For permanent magnet motors with brushes the speed can be reduced quite
effectively by reducing the voltage to the motor.

the next problem is the 520 degree turn and then the reversing at the end.
and coming back (and then stopping?)

It seems to me that a disk with two features that activate a sensor and a
system to count the passing of these features would be the way to go.

start at one of the features, count three new features on the way out,
and three new features on the way back.

a setup like that can easily get 5 degree accuracy, and with a bit of care
could exceed 1 degree accuracy.

a small circuit board with a few dollars worth of parts.

another idea is that a small three phase AC motor could be used they give
an lmost perfect constant speed (but only one speed) and are about as easy
to reverse as DC motors. (they ca be driven off a single phae AC supply
with a simple circuit)

If you need an "industrial strength" solution you could spend a little
more money and do it "properly" with a PLC (programmable logic controller
- basically a small computer thingy designed for process controll I expect
that even the smallest (cheapest) PLC would be capable of tasks many times
more comple than this one) one of those motor controllers you saw earlier
and sensors from the same (or a similar) supplier... this has the advantage
that you don't need to solder it together (and once setup it'll probably be
more reliable) and the disadvantage that you'll need to program the PLC to
get the behavior you want. (this is done by hooking it up to a PC and
running the manufacurers software and entering the program aftwer which
the device is disconected and runs autonomously)

everything for yourself, consult real experts etc...

Bye.
Jasen

7. ### Nick ZentenaGuest

No-) Small film tank. Will hold a max of two rolls of film or various
other combinations.
I think I didn't explain this right. When I said 30 minutes I meant the
setup would need to be on and running for 30 minutes at worst. I looked up
the commerical product that spins the drum. It claims 75rpm. But they also
require a range of 65rpm to 75rpm for the process. I'm guessing a fully
loaded large drum slows down thier motor.

No repeating until the external timer shuts off the power to the motor.

Thanks.

Nick

8. ### Nick ZentenaGuest

No it just needs to change direction. I'm not even sure if it needs to be
exactly 1.5 turns but it sounds like a reasonable number.

Thanks

Nick

9. ### Jasen BettsGuest

At 70rpm the half-cycle of 1.5 turns would take approx 1.3 seconds
so 1.5 turns and 1.5 back would be 2.6 seconds
ahh... I'm no mechanical engineer (watch the flames fly) but,
I'd be looking to do an all-mechanical solution

reversing every 1.3 seconds will be hard on most motors, and/or on any
mechanisms light enough to be reversed in that way.

or could you fit the tank inside a tube that would fit on your larger machine?

it sounds like a old washing machine agitator may have the function you want
but on the wrong axis, the speed van be varied if needed by using different
sized pulleys on the motor iirc 1.5 turns is fairly typical and is a little
over 1 second before reversing...

Bye.
Jasen