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Explain servo versus stepper for me...

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#1 Etrsi_645

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Posted 25 May 2009 - 11:15 AM

please....

thank you,

#2 jrcrilly

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Posted 25 May 2009 - 11:21 AM

A servo system uses a freerunning DC motor with encoders added to provide feedback regarding its motion. A stepper system uses a motor that is commanded to move a precise amount. The controller knows how far it told the stepper to move and how many times so no feedback is required so long as no steps are ever skipped.

Early stepper systems used steps that were too large so tracking wasn't smooth. Modern systems don't have this problem. Servos rotate faster so they are geared down more and are generally noisier.

#3 jog

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Posted 25 May 2009 - 11:30 AM

But which is better?

#4 WirelessDude

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Posted 25 May 2009 - 02:17 PM

Depends on the application...

Servo for GOTO providing varying speed control. Stepper for focusers and rotators...

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#5 Jerry3672

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Posted 25 May 2009 - 02:23 PM

The High end mounts like Paramount use servo motors for the Goto.

#6 rsbfoto

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Posted 25 May 2009 - 02:41 PM

You can also use stepper motors with an encoder and so step skipping (loosing due to torque problem) is no problem ...

#7 Luigi

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Posted 25 May 2009 - 03:33 PM

The main advantage of DC servos is they can provide greater torque over a much greater range of speeds. This is needed in some applications but for an astro mount it doesn't matter much. Many really inepensive mounts use servo motors, like the Celestron $270 60SLT, which includes a 60mm refractor, red dot finder, diagonal, and a couple of EPs, all really cheezy compared to the mount. I bought the setup to use the mount for my solar scope and literally threw all the other stuff away. The CG5ASGT uses servomotors. The G11 I had used steppers. DC servomotors often use a lot of little spur gears which is what can make them noiser then stepper driven scopes.

#8 jrcrilly

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Posted 25 May 2009 - 03:47 PM

But which is better?


Either can be as good as the designers want it to be.

#9 jrcrilly

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Posted 25 May 2009 - 03:53 PM

The High end mounts like Paramount use servo motors for the Goto.


As do nearly all goto setups, including the lowest-end gear. Vixen used to be an exception but I suspect that current models use servos. The ServoCat (despite the name) uses steppers for tracking but for goto operation encoders must be added. The Dob Driver II uses steppers and no encoders for tracking and for goto operation. Whatever they call the EQ-6 these days uses steppers in its goto system.

#10 mtb.daily

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Posted 25 May 2009 - 05:21 PM

Both can do very high accuracy. It depends on the gear ratios involved in the design.

For example, I am designing a new mount for myself that will use standard 1.8 degree/step steppers.

With proper gear ratios, I will be able to get 0.05 ArcSec per step of the stepper motor. Unfortunately, this means my maximum slewing rate will be about 0.5 degrees per second which is OK for me.

Yes, you can get such resolution with servos but, they tend to cost considerably more than steppers.

with high torque steppers, you can almost guarantee 100% step movement. Keep in mind that you can get steppers the have 400-500 oz/in torque.

With gear ratios high enough, that amounts to an enormous lift amount.

-Jerry

#11 ccs_hello

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Posted 25 May 2009 - 09:14 PM

Either can be as good as the designers want it to be.


John C is right on the money :bow:.

For GOTO type, DC perm-mag motor based servo system is easier to implement and has been around for a long time. Some of them have been extremely cost-optimized thus can be considered as not so good. But some of them can be good.

For GOTO type stepper-based, assuming same fast slew speed is required (i.e., don't want to wait to take a tea break every time GOTOing), then the design is not trivial. These are high-torque low gear-down microstepping hybrid-stepper based. There may be some original/early brands/models using such system (sorry I am not familiar with them), then a model in AWR has it. And recently Synta's SynScan/SynTrek (EQ-G type) is using that.

This and this CN posts have some additional info.

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#12 mtb.daily

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Posted 26 May 2009 - 03:38 PM


One thing that is noted in the articles referred to by ccs_hello is that steppers can only move to a given step while servos have no such limit.

Well, consider this: Yes, steppers only move to specific points, or arcMin/arcSecs, but, consider that servo motor, via an encoder, will also ONLY move to a specific point just like steppers.

The smoothness is based on the gearing ratios.

jerry

#13 mtb.daily

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Posted 26 May 2009 - 04:17 PM

Opps... Forgot something

Servos can rotate very fast, 5000+ rpm is very common.

Steppers, can rotate very fast but, the speed depends on the applied Voltage and Current as well as the ramp up speed of the stepper controller.

I am getting 3600 rpm on my steppers and achieve 2 degrees per second slewing rate.


Jerry






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