Servo step multiplier

[JerryFlyGuy]

Ok.. I've never stopped to think about it but.. now that I'm looking over servos.. why do servo drives come w/ step multipliers? I can understand the need for microstepping on a stepper.. but how does this work w/ a servo?? Or, is it the reverse and that for each step pulse the servo moves ten encoder counts [or whatever multiple is selected]??

I confuse easily..

Jerry

[Mariss Freimanis]

A step pulse multiplier on a servodrive is only marginally more useful than a screen door on a submarine. It's intended purpose is to help those afflicted with an overly-high encoder line-count and a wimpy maximum step pulse frequency. All it can do is turn a 1,000-line encoder into the equivalent of having a 100-line, 200-line or 500-line encoder.

Mariss

[JerryFlyGuy]

So being as I'm already running a G100 that shouldn't be an issue..

While I have your ear Mariss, I've a question for you on servo power vs Stepper power. I've been given [basically] a set of 34size servo's. They are 60VDC, 3000rpm, 225oz cont [@ 6.8A] x 1125oz peak [@ 38A] unit's and I'm trying to figure out how they would compare to the 640oz steppers I'm currently using. The steppers are performing very well, I can hit 800ipm rapid's very easily using a 10:1 planetary reducer and a 2" pinion.

I've worked through your Watts=IPM x Weight/531 and am not sure if I'm reading it all correctly.

At the end of the day, do you think these servo's would be an improvement over my steppers?

Thanks sir!
Jerry

[Mariss Freimanis]

225 in-oz at 3,000 RPM is 500 Watts (Watts = in-oz * RPM / 1351). Meanwhile 6.8A times 60VDC equals 408 Watts for electrical power going into the motor. Since it's unlikely you'll get 500 Watts coming out for 408 Watts going in, I'd suspect one set of your numbers is incorrect.:-)

A NEMA-34 step motor tops out at 250 Watts mechanical when driven hard. It seems you'll have in the neighborhood of twice as much power. Performance should double as well if the motor is properly geared to your load.

Mariss

[JerryFlyGuy]

And as long as I shoot to gear the servo to run at the higher side of the Rpm scale [up near the 3000rpm], the more "power" I'll have correct?

I'll get you a G320 order here in the next couple days..

Jerry

[Mariss Freimanis]

Power is speed times torque. It is power that gets things done. The continuous rated torque is a limit that's the same at 1RPM or 3,000 RPM. To get power from a servomotor means you have to maximize the product of torque and RPM. Since you can't change the torque limit, you can only maximize RPM to get the maximum power from a motor.

Mariss

[JerryFlyGuy]

Thanks Mariss, thats what I'd thought.

Jerry

[springlakecnc]

Hey Jerry,
Everything Mariss says is true. If you have 250 line encoders, and your servo motor makes 5 turn's per inch, and you are using your old dependable 486 pc to run your machine, you will only get about 25 inches per minuit rapid traverse. add a pulse multiplier set at times 5, and now your machine will go 125 inches per minuit, if that don't exceed the servo motor rpm's. A low grade pentimun may get u 60 inches per minuit, and your new computer may max out the rpm of the servo motor.

[JerryFlyGuy]

what if I'm running a G100? don't worry.. I can max out the servo by at least three times its rated speed,.. I just couldn't see where there was a need for a step multiplier.. Mariss cleared it up handily..