Chris Loiacono wrote:
> First of all this is a personal project that I took on partly because
> I don't have much experience controlling motors and want to broaden my
> horizons a bit. With that said.......
>
> I'm driving 12V unipolar steppers with a 12V supply and am a little
> disappointed at the performance - namely speed wise. I can get the
> torque needed but only with low step rates. After putting together
> and playijng with my own FET driver, I tried the Allegro 5804 chips,
> which is what I am using now. They made the cotroller code and
> hardware simple for pennies. An Allegro engineer then talked me into
> checking out their bipolar microstepping chopper chips, which I will
> do - but not before I get more out of the unipolars & 5804's. (or is
> this a waste of time?)
>
> I understand that the voltage across the motor can be increased up to
> a published limit as long as I add ballast to limit the current. I
> did a simple test with some resistors and a supply I had on hand, but
> it seemed that with the current at like levels, the performance was
> approximately the same.
>
> Thomson/Airpax recommends L/4R - This sounds like 4X the voltage to
> me. What about a 2R setup?
>
> Allegro suggests using zeners at a V a bit less than the supply level
> to help dump the BEMF quickly. That would mean the diodes would need
> to be rated for the full motor current, I would think....
>
> I guess my question is if I should bother with any/all of this, or
> scrap the uipolar design and move on to the bipolar chopper chips? If
> I only knew how much improvement to expect by raising the voltage and
> adding the zeners......Also, I don't think I can beat the under $10
> cost of the new closed loop chips with my own chopper design.....
> Anyone feel like sharing any clues to help keep me from bad
> decisions? - I really do not enjoy working with motors (yet) and
> obviously have much to learn. Perhaps with more experience they will
> grow on me.
>
> TIA,
> Chris


Hi Chris, it is a common belief that stepper motors work easily as a lamp.
The fact that applying current to the phases make the motor move, doesn't
mean everything is done.
A stepper motor has a complex drunk and emotional personality.
It requires care, study, learning and above all, understanding.

You can read all night long about stepper motors, and find out less than =
5%
of what is possible, and ending up knowing less than you started.

This is why there are so many different kinds, models and type os stepper
motors, everyone was produced or developped trying to solve one or anothe=
r
problem or necessity in the market.

Few things I can resume about them;

Forget about voltage, pay attention in the current on the coils.  It WILL
require to use a power supply at least with double voltage then the state=
d
to the motor.  You can use current limiting resistors, or better, current
limiting electronics.

As anything else, each stepper motor is designed for a particular task,
some have more torque, others speed, other both.

It is common to see stepper motors getting warm or very hot under low
speed, since the coils will drain pure DC current.  Under medium speed,
good torque, the motor can run not so hot, as a matter of fact, they coul=
d
run without even getting warm.  At high speed, the motor start to fail, t=
he
mechanics simply CAN'T react to the fast change in the magnetic field, it
start to loose steps and make crazy things.

A 1.8=B0 step, 200 steps per revolution motor, can't give you speed highe=
r
than 80 to 100 RPM without starting to fail, or with a torque so low that
you can't drive anything with.

We should always remember that a stepper motor, is a precision mechanic
motor, something developped to produce safe and accurate positioning in l=
ab
and industrial equipment or machinery.  They are not intended to drive a
fan blade or a drill.  For those, there are other kind of motors.

What kind or motor are you using?  brand? model? voltage? current?  what
speed you got actually?

Also, it is good to remember, that low voltage and high current motor giv=
es
you more torque and faster response. High voltage and low current motors
are to be used where low torque and low speed is required, so low current
drivers can be used, small transistors, etc.  It will be difficult to dra=
in
high torque and speed from a 12V motor, you can start to hope this when
using 5V and below motors.  Best results you can get with motors around 2=
V
@ 4A.

I have some projects using Allegro chips, they work pretty well, but they
require good heatsink and they don't drive higher currents.  You really
need to implement discrete high current transistors.  8 x IRF530 is a goo=
d
solution for a 4 to 6A motor, but of course, it requires a current
management, using  0.05 ohm resistors, a LM339 and a reference voltage, y=
ou
can produce a current limiter or chopper and keep the motor working at it=
s
limit.

Whenever playing with stepper motors, keep your mind open to strange
results, write down every test for later comparison and consult, you WILL
get lost during all the experiments of pulse time, voltage, current, spee=
d,
torque, etc.

For the speed vs torque tests, the best is attaching a force drag to the
motor, something like a belt driving a fan underwater or if you are good =
in
mechanics, a felt type break system, a rotating aluminum disk sandwiched =
in
between two stationary felt break pads, like a car's disk brake system, b=
ut
using a felt or cloth to create the drag.  Using springs and some
adjustment screws you can produce and calibrate a precise drag to the
motor. Then, you can play with pulse timming and current and find out the
best torque and speed.

One of the tests you can do for "missing steps", is just programming your
microcontroller to do several back and forth steps, 20 FW, 135 BKW, 183 F=
W
and so on, but ending up in the same exactly point where started, I mean,
FW =3D BKW.  Then you glue a pointer to the shaft (toothpick?) and expect=
 it
to stop exactly where started, if not, the motor lost steps.  You WILL pl=
ay
for hours with this system, until you find out the best numbers for this
particular motor.

When you find out the motor limits for torque and speed, never make it wo=
rk
more than 85% toward those limits.

Wagner.

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