Inexpensive servos?

[balsaman]

Have a look at the surplus DC motors available at The Surplus Center:

http://www.surpluscenter.com/sort.as...rch=dc%20motor

I like the 1/8 hp 24 volt DC ones for $7.95. Take off the brake, add an encoder and you are good to go! 4:1 belt drive sounds perfect (or 2:1 with a fine pitch screw?).

There are other nice ones there too.

Eric

[ezland00]

Nice find. I don't know much about motors. How does it work?

[Mr.Ed]

But why take off the brake? It brakes when power is shut off. Wouldn't that be nice in an emergency situation? You hit the Emergency switch and it stops instantly.

I would consider buying one if i could be pursuaded

Would it have good vertical bearing capability? (that is, if you run it straigh attached to an end-mill.

Give us your thoughts on this DC engine.

Ed.

[NeoMoses]

Very nice find, indeed! I'm looking @ some of those 1/4 HP, low RPM motors for a BPort, and possibly some of the 1/8 HP for a router. Now, I just need to find some more money.

[Mr.Chips]

From a NewBie
You mentioned adding a "Encoder". Could you tell me or give me a link that explains this.
Are these "Stepper" motors?
Thanks

[deft]

what would one of these 1/8 hp motors be equivalent to in a stepper motor?

[kong]

Stepper motors move in steps, usually about 2 degrees, so one "pulse" from your PC would move the motor one step. Servo motors have no such stepping, they are either on or off. The encoder is a device that is connected to the servo motor and feeds-back the amount the motor has turned to the motor driver (interface device between your pc and the motor). The motor driver can then interpret the pulses from your pc, and turn the motor on, until the encoder has sent back the correct number of pulses back to the controller to indicate it has moved the required amount. It is like the wheel on your mouse, a small plastic disc with lines on it, read by an optical sensor. Each line can indicate one degree (depending on the resolution of the encoder) and sends that info back to the controller. Hence, servo motor controllers are more complex and expensive than stepper controllers.
That's probably as plain as mud!

[deft]

great explaination, of how servo motors work but would the 1/8 hp servos be equivalent to a 100 oz/in stepper, 200 oz/in stepper (or something else)?

[ehiebert]

Here's an simplified explanation for the newbies to help understand servo motors.


The holding torque of the servos whould me much greater than the steppers. Just look at the power dsifference....

100 oz. in. Stepper
------------------------
5.2V and 1.2 A/ph = about 6 W

1/8 HP Servo
----------------
1 HP = 746W so these are 93.25 W



Even and extremely inneficient servo motor will probably have more holding torque than a 100 oz. stepper.

Servo motion electronics are much different than the stepper. It is a controlled motion system. The encoders are used to tell the controller where the motor currently is. The controller compares this position to where the motor is supposed to be. The difference between the two readings is the position error signal (feedback signal). The greater the error signal is, the more current the controlle applies to the motor to bring it into position.

Conversely, a stepper driver just applies the full power to the motor with each required step. The stepper solution is not encoded so the controlling program has no idea where the motor is. It just assumes that the motor has not missed a pulse. It is up to the designer of a stepper system to ensure that holding torque limits have not been exceeded while running the toolpath. An example of how this can happen is if your router hits an very hard knot in a wooden workpiece, and the user used a feedrate very close to the motor torque limits.

For a heavy duty precision mill I would want servo motors... especially for the precision part. However, be aware that the motor drivers/controller is going to mucho more expensive than a simple stepper controller.

[balsaman]

I am currently using 1/8 hp DC motors (115 volts, 1700 rpm). Running on 75 volts I am getting 150 imp rapid moves and plenty of "push" at cutting speeds. I am using direct drive.

Eric

[High Seas]

Like maybe - I should race out and get the last of these!
After your great explaniations and experience maybe I'll wanna upgrade before they're all sold out.
As an aside - if this kind of great info was available (well it was I know - but I din't know what I 'didn't know) maybe I'd have gone a different route - alas - I now have a PPPI.. (pre planned product improvement)!
Jim
BTW - how much more is 'mucho" 2x, 3x?

[kong]

A 3 axis stepper controller from xylotex will set you back about $125, but for servo's, you will need something like three Gecko 320's @ $114 each + encoders.
Sorry, linkies:
http://www.xylotex.com
http://www.geckodrive.com