difference between "industrial" servo motors and RC servo motors ?

Posted by pogo on February 14, 2006, 8:50 pm

What is the difference between "industrial" servo motors and RC servo motors
? A vendor on eBay recently told me that the industrial type are all
continuous rotation. So what makes them a servo motor ? Are they driven by
PWM ?

Thanks!

Posted by Gordon McComb on February 14, 2006, 9:40 pm

Wikipedia isn't a bad place to start.

http://en.wikipedia.org/wiki/Servomechanism

Note the mention of positional control. This applies to RC servos AND
servos used for motion control and other industrial applications. The
position may span only a few degrees for RC, and thousands of degrees
for motion control. Obviously 10,000 degrees means multiple rotations of
the shaft, but the idea is the same.

-- Gordon

pogo wrote:

>
> What is the difference between "industrial" servo motors and RC servo motors
> ? A vendor on eBay recently told me that the industrial type are all
> continuous rotation. So what makes them a servo motor ? Are they driven by
> PWM ?

Posted by D Herring on February 14, 2006, 10:58 pm

pogo wrote:

> What is the difference between "industrial" servo motors and RC servo motors
> ? A vendor on eBay recently told me that the industrial type are all
> continuous rotation. So what makes them a servo motor ? Are they driven by
> PWM ?

There are different ways to build/design/tune motors. The cheapest way
to build a high efficiency motor will result in a fast spinning, low
torque motor. This is not suitable for positioning tasks, which
generally require low speeds and high torques. Thus, industrial servo
motors use expensive magnets and such to achieve high torques at low
speeds, in contrast with general industrial motors which are better
tuned for continuous rotation.

Hobby servo motors are a whole different beast. They use relatively
cheap, high-speed motors; but they compensate for low motor torque with
a high gear ratio at the cost of low speeds and high friction. This
tradeoff is fine in most hobby equipment, but isn't acceptable for many
industrial tasks. For example, the productivity of a pick&place robot
arm is directly related to how fast it can move, and high friction may
lead to heat problems.

For a wow factor, these are the most expensive motors I know of:
http://www.motionvillage.com/products/motors/torquers/index.html

I found these for a walking robot project, but we selected different
motors after we got a price quote: over $30k each for a mid-range model!
Somebody must have a good use for them. We didn't get a quote, but
the last motor on the datasheet weighs 1400 pounds and draws over 6
kilowatts. I've heard similar motors are used to drive tank turrets.

Back on topic, a servo motor is just a motor with a position-feedback
controller attached.

Later,
Daniel

Posted by pogo on February 16, 2006, 12:13 am

Hmmm. Good info as usual guys - thanks !

So do Industrial servos still use the same PWM that hobby servos use ? Could
something like a Scott Edwards SSC be used to control a big industrial servo
motor (with appropriate power connections, of course).

Evolution Robotic's high priced Scopion robot uses servo motors instead of
the steppers the ER1 came with. Why would they choose that over lower priced
straight DC motors + speed controller ?

Thanks !

Posted by D Herring on February 16, 2006, 1:49 am

pogo wrote:

> So do Industrial servos still use the same PWM that hobby servos use ? Could
> something like a Scott Edwards SSC be used to control a big industrial servo
> motor (with appropriate power connections, of course).

No; not without adding significant sensor and amplifier circuitry.
Hobby servos use PWM to specify the target position of the motor. The
servo motor has a built-in sensor (potentiometer) and controller (analog
timer circuit) to move the motor to the desired position. An industrial
servo motor is just a motor - no sensor or controller.

Industrial servo controllers generally have embedded processors and use
digital position commands. Totally different than hobby servos. Here
are some links to industrial controllers (I have no experience with
these products).

http://www.motionvillage.com/products/controllers/
http://www.motionvillage.com/products/drives/

> Evolution Robotic's high priced Scopion robot uses servo motors instead of
> the steppers the ER1 came with. Why would they choose that over lower priced
> straight DC motors + speed controller ?

No idea. Maybe they wanted higher torque and more precise position control?

Daniel

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