Re: [CAD_CAM_EDM_DRO] Motor Size /Joystick Question

While balance is certainly very important, there are still other factors
that need to be taken into consideration. Even with perfect balance, to
move an axis the motor needs to have sufficient torque to overcome friction
and it must provide torque to accelerate and then decelerate the load--this
is dependent on the inertia of the axis, basically the weight times the
moment arm. For any sort of rapid move this will be a major factor.

Bob

----- Original Message -----
From: "Andrey Lipavsky" <alipavsky@...>
To: <CAD_CAM_EDM_DRO@yahoogroups.com>
Sent: Sunday, August 28, 2005 4:03 PM
Subject: RE: [CAD_CAM_EDM_DRO] Motor Size /Joystick Question


Matt,
I have some things for you to consider in reference to your question of
required torque.

1) It is not so much the weight of the table that's important here. What's
actually more important is the weight distribution, or balance, around each
axis of motion. The motor will only need to move the portion of weight that
is unbalanced. If you balance the table with each axis centered, the table
than becomes its own counterweight. The required torque could be
theoretically zero if everything is perfectly balanced, in the best-case
scenario. In the worst-case scenario, it could also be more than the weight
of the table if you have the axis mounted via an offset lever that's
attached to the table. The net result is that the better you balance the
weight around each axis, the less weight each motor will need to move.
You'll also need to consider whatever object might be mounted to the table.


2) Both Xylotex.com and AutomationDirect.com sell NEMA-23, 270in/oz, 2.8amp
bipolar motors relatively cheap. Again, if you have the table balanced
well, this should be way more than enough power. Also consider that the max
recommended speed for motors of this size is about 300 RPM. That translates
into a direct-drive scenario of 0.2 seconds to move 180°. Keep in mind that
this speed would require a 6KHz pulse train for full-step and 60KHz for
1/10-step.

I hope this helps.
Andrey

-----Original Message-----
From: CAD_CAM_EDM_DRO@yahoogroups.com
[mailto:CAD_CAM_EDM_DRO@yahoogroups.com] On Behalf Of skerbitz
Sent: Saturday, August 27, 2005 2:52 PM
To: CAD_CAM_EDM_DRO@yahoogroups.com
Subject: [CAD_CAM_EDM_DRO] Motor Size /Joystick Question

I have an project that uses bipolar stepper motors to pan and tilt a
metal table weighing about 20 to 30 pounds. The steppers are not
controlled using a PC, but by using a joystick. I know that an
oscillator circuit (555 type) can be built to run the stepper motor
drivers in 'standalone' mode, but ususally this involves using
external switches and doesn't provide variable speed control with a
joystick, which is what I want. As I move the joystick I would like
the speed to increase or decrease just as it would say in a normal PC
game.

I've spent a LOT of time searching on google and found a few
interesting things, the closest being this :

http://www.imshome.com/osc.html

which plugs into IMS's bipolar stepper motor driver.

Can anybody think of something cheaper/better ?

And now for another issue, I don't have much of a feel for how
powerful the bipolar motors need to be to pan/tilt the table. Lets say
for the sake of argument that the motors and the table were geared,
like 2 cogs on a bicycle with a chain, to provide more torque at lower
speed and that I wanted the table to be able to turn 180 degrees in
roughly 2 to 5 seconds. Noting the weight above (20 to 30 pounds) what
would you recommend for motor size (other than bigger is better :)

Thanks,
Matt






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