Re: [CAD_CAM_EDM_DRO] Servo motor selection

Drew Rogge wrote:

> Hi All,
>
> I've been looking into building a small CNC XY table using one of the
> Phase II XY tables, servos and Gecko 340 drives. The reason I'm planning
> on using servos is because I may go to a bigger machine in the future
> and having the Geckos sounds like it would be a good thing.
>
> The problem I'm having is that I don't know a whole lot about servo
> motors and how to size them. I've located a few motors on C&H Sales'
> web site and two of them look like they might be promising. The first
> is described as:
>
> 1550 RPM PITTMAN, #14203D244-R1. Permanent magnet, reversible. Ball
> bearing.
> Rated 38 VDC. At 38 VDC input the no-load rpm is 3440 with a no-load cur
> rent of 0.12 amp. Tested with 24 VDC input the no-load speed is 2190.
> No-load current 0.10 amp. With a 30 oz-in load the rpm is 1550 @ 2.2
> amps.
> The encoder mounted on the rear of the unit is a Hewlett Packard
> HEDS-9000
> Series with 500 counts per revolution. The encoder output is two phase
> quadrature. Outputs are TTL logic compatible. Input power to encoder is
> 5
> VDC nom. @40 mA. Shaft: 1/4" dia. x 7/8" long. Dimensions: 2-3/4" max.
> wide
> (across encoder) x 2-1/8" dia. (motor body) x 4-3/8" long (including
> encoder,
> but excluding shaft). Stock #DCME9701

Really, you have very little information, here. What is the peak and
continuous
current rating on this motor? No indication. probably it can handle 2.2 A
safely, but that isn't a lot.

>
> The second is:
>
> 2600 RPM GLOBE MOTORS, #537A274. 30.3 VDC permanent magnet reversible
> motor.
> The motor has ball bearings and has an optical encoder mounted on the
> rear.
> The no-load speed is 3780 rpm, no-load current is 0.2 amp. With a load
> of 60
> oz-in, the rpm is 2600 @ 5.6 amps. This is a very powerful cool running
> motor. The encoder employs a Hewlett Packard HEDS 9100 read head with a
> custom encoder disc. The encoder has a two-channel quadrature output
> with
> 512 counts per revolution. The encoder requires 5 VDC at approx. 40 mA
> for
> operation. The shaft is 1/4" dia. x 7/8" long. The shaft has a flat
> running
> full length. The motor has four tapped mounting holes on the shaft end
> of
> the motor face. The holes are tapped for #6X32 tpi screws. The bolt
> centers
> are 1-1/16" apart. Electrical connections are made via 3/16" quick
> connect
> tabs connected to the brush bosses. Dimensions: 2-1/8" dia. x 5" long.
> Stock #DCME9950
>

Well, this is a little better, as the motor probably can handle 5.6 A
I don't think it makes a whole lot of sense to hook servos up to a table
with Acme screws. It just won't give you the performance you are hoping
to get, unless you can defeat the backlash, somehow.

> The criteria I'm basing my picks on is that I think I want somewhat low
> rpm motor and normal old quadrature output on the encoder.

Yes, a motor optimized for lower speeds is the best choice, unless you have
a high thread count on the leadscrew. Otherwise, you need a high belt
reduction
ratio.

You don't give us any idea what you plan to do with this table, so that puts
us at a disadvantage in figuring out what would work best in your application.
Will you be milling steel, or engraving plastic nameplates? it makes a big
difference. You need to figure out what sort of linear force will be applied
to the
cutting tool, and what speed of traverse is needed. With this information,
you can work back to the motor and figure what speed and torque are
required from it. Then, you can select a suitable motor to fit the
characteristics
of the servo amp.

Jon