Stepper motor specs

Hi all,

I've been reading about stepper motors and need some help to understand
some of the specs and how to choose the step motor that is best for my
application.

1- What is the difference btw Detent torque and Holding torque ? Which one
is of more importance?

2- I need to get two step motors for my robot (as wheels). The robot is
about 3-4 lbs. I don't need it to go fast, but i want the step motors
to be strong enough to move the robot. I think a reasonable voltage
is btw 9-12v. I don't want to use more than 12 v. I'm not sure about
the Amps i found step motors with 40 mA, 80 mA, 500 mA, ..etc.

How do I know what Amps is best for me? Also what type and value for
torque will i be needing ( 200 g-cm, 400 g-cm, 1000 g-cm, ...)?

3- I've read about Unipolar and Bipolar. What is the major difference btw
them? what things can I do with one that i cannot do with the other?
Which one is easier to use?

thanks in advance

 

1- What is the difference btw Detent torque and Holding torque ? Which one

OK, I'll guess on this one... someone will correct me if I'm wrong... detent
torque is with coils off, and holding torque is with coils energized?

Get a fishin scale and see how many pounds you need to pull it at the speed you
want. Torque is this force x the wheel diam (adjust units from SI to English as
required...)

I always get these confused... unipolar has 2 centertapped coils (6 wires) and
you hook the centertaps to the batt and step the coils with transistors.
Bipolar has 4 wires... 2 coils... and you energize them with either an H-bridge
(one side goes up to +v, the other goes down to gnd), or +-volt supplies. The
1st type only has half a coil energized, 2nd type has the whole coil
energized.. more torque, but more complex drive. 1st type can 'half step'
though, doubling the steps. Tradeoff.

 

It does seem counter-intuitive, with the "uni-" having more wires and
more individually-addressable windings. What worked for me was to
silently replace the -polar with -directional, since during operation
the current goes two directions in each bipolar winding but only one
direction in each unipolar.

Nothing to say that you can't step with two coils in a unipolar setup;
instead of A - !B - !A - B, one would use A/B - B/!A - !A/!B - !B/A.