Re: small servo motors keep stalling

--- thanks for the help and suggetstions with my motor problem....I am
gonna try to get it goin next week with some tchnical assistance from the
place i bought the setup from...He also wanted me to post an e-mail he
sent to me with explanation of the torque ....here it is.......It is correct that
The motor in question is a gearmotor with a 5.9 to one gear reduction.
Approx free speed for the gearhead output shaft is around 1000 rpm.

The CSP-1 description on the website references a 250Lb max thrust.
Notice first that this is a maximum not a continuous value.
That parameter should be asterisked. If you continue reading down the
page, The footnote references that this number is achieved using a 5 TPI
ballscrew .

Let's do a little math. Kt= 5.17 ozin/A for that motor . Note the pwmamp-3
can continuously output 3A with 6A peak current capability.

Pre gearhead Torque = 5.17 x 3= 15.51 ozin (before the gear head)

Now lets multiply that number by the gear ratio to get the torque at the
output
shaft and don't forget the gearhead efficiency

Constant torque= 15.51 X 5.9 x (.95 )= 86.93 ozin


A ballscrew with a .200" lead ( 5tpi) generates about 1.765 pounds of
thrust
for every ozin of torque applied

So my continuous thrust at ( 3 Amps ) is

Thrust = 86.93 x 1.765 = 153 Lbs

If I push the amp for a short interval to it's peak current rating of 6A I could
generate

300 lbs of thrust. Notice this peak is well above the maximum thrust called
out in our literature.

This level of thrust on a small machine is more than adequate for most
applications.
I am referring here to Jon's comment that the system is under powered
even with a ballscrew.

On the issue of backlash. There is backlash compensation in the
software for any or all axes. With the above motor there is typically about
0.001 " lash when using a
0.200" lead ballscrew.

It amazes me that a person with no experience with this system ( Jon ) is
so quick to
malign it in a public forum. We have sold many of these systems as well as
machines based on these motors and amps since 1996. Many of these
systems
run 8 hours a day, day in day out .

> feck123 wrote:
>
> >yes after looking at the motors it says 5:9 ratio gear motor...is that good

or

> >bad ? the 1/2-20 is bad too, buti gotta try to work with it for now...i

know of

> >lots of people that have no problems with this setup using stepper
> >motors..is the servo setup a lot less tolerant of low pecision
> >machine/screw setups?..i do not know specs on the drivers and

power

> >supply..i bought the whole setup from place called jra co.....website
> >jraco.com...the csp-1 system...it is a nice setup and should be ok --if i

can

> >ever figure out how to mount my motors correctly ! :) --- thanks for the
> >help ! Mike
> >
> >
> Well, so it appears that the motor has a 1.8:1 reduction. The problem
> is that the
> gears presumably have backlash. Where is the encoder? Right on the
> motor shaft?
> Then, the backlash is between the encoder and the leadscrew, which

is

> not good
> for accurate positioning.
>
> Without some info on the motors, most especially the Kt or torque
> constant, it is
> very hard to know what to do. But, it is pretty clear that the
> available torque is
> way too low. My Bridgeport design delivers 1000 Lbs of linear force to

the

> table. This is not enough for very rapid acceleration, but is enough for

my

> uses. it sounds like, after the gears and the high-friction screw, you
> have just a few
> pounds of force left. I can't imagine this outfit can get their system
> to work
> on anything. They claim 200 Lbs linear force, but that is probably

assuming

> a ballscrew, with 90% efficiency. Your screw is probably 20% efficient,

and

> it gets worse as more load is applied. I am assuming you have their
> SERVOMO-3 and the PWMAMP3. Note that to get full rated torque on
> the SERVOMO-3, you need 8 amps. but, the PWMAMP3 only supplies 3

Amps!

> So, you are only getting 3/8 of the rated torque, instead of 175 Oz-In,
> you are
> getting 65.5 Oz-In. That would be unsuitable for even a ballscrew

machine.

> Note, also, that they say PWMAMP-3 can be used with ballscrew-

equipped

> machines. If you have a larger capacity amp, then maybe things aren't

as

> bad as this. The CSP-1 sales doc doesn't mention which amps are

included

> with that package. You certainly spent a LOT of money for what looks

like

> a pretty marginal system, at best, and without seeing it first.
>
> The SERVOMO-3 is rated at 1043 RPM, which works out to 52 IPM.
> So, putting in a belt reduction ratio would severely limit the top speed.
> I think you need to get more amps to the motor, or exchange for

ballscrews.

>
> Jon