Re: Servo motor selection

Hi,

Two ways to go here:

(1) I have a motor, now what can I do with it?
(2) I know what I need done, now how do I pick a motor to do it?

(1) Let's pick the Globe motor as an example. What can it do? Let's
start with its output power. Take the 60 oz-in @ 2600 RPM. Multiply
both togeter and divide the result by 1351. The answer is 115.5 Watts
of mechanical power.

OK, what can 115 Watts do? Well, 1 Watt is 531 inch-lbs per minute.
So 115 Watts is 61,311 inch-lbs per minute. If you need to move at
say 300 IPM, then you will get 61,311/300 or 204 lbs of force (or
push) at that speed with this motor.

Now how about gearing? Let's say you have a 5 turns per inch
leadscrew. It has to turn 5 times 300 or 1,500 RPM to get 300IPM. The
motor meanwhile is turning 2,600 RPM. This means gear ratio of
2600/1500 or about 1.75:1 reduction from motor to leadscrew.


(2) Going the other way. Say you need to move at 720 IPM and have
a "push" of 550 lbs. The motor power necessary is 720 times 550
divided by 531. This works out to 746 Watts of power needed.

You now look at motor data sheets and find a motor that develops 750W
of power. Let's a suitable motor would have a continuous torque
rating of 280 in-oz at 3,600 RPM. Let's also say the motor delivers
this speed at 72VDC and requires 12.5A to get the 280 in-oz torque.

This would determine your power supply reqirements. Your gear ratio
would be 3,600 divided by (720 times 5) or 1:1 meaning you could
drive the leadscrew direct.

Mariss

--- In CAD_CAM_EDM_DRO@y..., Drew Rogge <drew@p...> 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
>
> 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
>
> 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.
>
> Does anyone want to speak up about either of these choices,
recommend
> a better motor/encoder combo from C&H or recommend some other place
to
> find used servos? I've been watching ebay for a while and have seen
some
> servos go by but a lot of them sound like pretty huge motors.
>
> The page with C&H's servo motors is at:
>
> http://www.aaaim.com/cgi-
local/shop991/shop.pl/SID=13784101471/page=DCME.htm
>
> Thanks,
> Drew
>
> --
> Drew Rogge
> drew@p...