sizing for motor

[becma27]

Hi,

I am trying to choose a motor for a project and need some help choosing the corect one. I am given that a 3x3 foot table is to be rotated about an axis with the table weighing 200 lbs. The speeds are very, very slow and I am looking for a resolution of about 100urad (using an encoder). What info do I need to calculate to choose a motor for the application and what are the formulas?

Thanks for the help,
becma27

 

[becma27]

A couple of more things I think are needed. A flexure along the axis resists torque in either direction with 100 ft-lbs. This is for a positioning simulation so the motor will be required to rotate to a point, hold, and then rotate some where else. The companies I have looked at for motors are compumotor and kolmorgen.

Thanks

 

[SERVOCAM]

If you were sizing for a servo or stepper motor, we need to find the inertia of the load we are moving. If we assume you are driving it from the center of the square table we can use the following formula:

W
J = --- x (h^2 + w^2)
12g

J = inertia (lb-in-sec^2)
W = Weight(lbs)
g = gravity (386 in/sec^2)
h = height (in.)
w = width (in.)


Given:
W = 200lbs
h = 36 inches
w = 36 inches

200
J = ------ x (1296 + 1296)
148996

J = 3.5 lb-in-sec^2

Now that we found our system inertia, we will need to know your Motion Profile. This is the defined move you want to do. I call this How Far & How Fast or Distance in a time.
Example: I want to move 5 revs in 10 seconds and if known, accelerate to max speed in 1 second.

Torque for servo/stepper motors is based on acceleration torque and reflected torque (friction torque / running torque). Basically, Acceleration Torque = inertia x rotational acceleration.

We can't calculate anthing else until we know the motion profile. One thing to consider is that driving it from the center, you will need to add a gearbox due to the high inertia. Once you add the gearbox, the next thing you need to worry about is backlash. You may want to consider Rack & Pinion set up.

There are many sizing software packages out there that can help, the problem is that most won't offer sizing for blocks. Many Mfgs will give you the formuals in the back of their catalogs.

Give me the rest of the details on the app, and I will size it for you...that is if you want to go servo.
Cameron Anderson - Sales & Applications Engineer
Aerotech, Inc. -

http://www.aerotech.com

"Dedicated to the Science of Motion"

 

[becma27]

Servocam,

Thanks for the input. First, I guess I mistyped in saying it was a cube. It is square in both x and y, 36 inches, but height is probably only two or three inches. The other thing is the motion profile. It is a pointing system so I will input into a computer move up three degrees and then I need the motor to hold in position while aquiering or whatever and then give another comand. So it is not a continuous run application. I wish to move two to three degrees per second in a second maybe. Also, I need to pass a 1 1/2 inch optical beam through the middle of the motor if possible. Thanks.

becma27

 

[SERVOCAM]

becma27, because you need an open center, the tradition motor/gearbox driving from the center is out of the question. Did you have an idea of how you want to drive it?

You could do Belt & Pulley, Chain & Sprocket, Rack & Pinion...or purchase something already for your application. Take a look at my ART330 Rotary Worm Drive Stage with a large aperture or my ADRT, Diriec-Drive Rotary table,

http://www.aerotech.com

You could also look at Emerson CT's (Camco's) Flex-i-dex,

http://www.emersonct.com/mmeProducts/downloads/literaturePDF/Flexidex-v4.pdf

Because your acceleration rate will be so slow, your accel torque requirements will be nothing after the gearbox (if going Belt & Pulley or Chain). Because the speed is so slow, I suggest getting the GB ratio of 100:1 or higher to get the motor speed up.

You could easily get away using a 34 frame step motor w/ 100:1 gearbox to do the application. With the gearbox, your torque will be <5 oz-in. It would be better to get the gearbox ratio up even higher, it will help increase resolution, get the motor speed up higher, and also get you into a smaller motor (save money).

Also, on my equation above, I made a HUGE typo (sorry), your inertia for the system is:

200
J = ---- x (1296 + 1296)
4632

J = 111.92 lb-in-sec^2

I guess I didn't know how to multiply 12 x 386.

Also remember Backlash and how it will effect you positioning on the outer edge. Did you have an idea of what mechanical drive system you want to use?
Cameron Anderson - Sales & Applications Engineer
Aerotech, Inc. -

http://www.aerotech.com

&quot;Dedicated to the Science of Motion&quot;

 

[jbartos]

Suggestion: Please, would you clarify around which axis the table will rotate, and state the table dimensions.