Stepper or servo motor

Thread Starter

mik3

Joined Feb 4, 2008
4,843
Hi guys,

I have a rotating disk (7.5cm radius) with three holes 120 degrees apart at 5cm from its center. The total mass of the disk is about 1Kg.

I want to rotate this disk by an electric motor and stop each hole at the same point, i.e rotate by 120 degrees, stop (first hole), rotate by 120 degrees, stop (second hole), rotate by 120 degrees, stop (third hole) and so on.

At its best, it has to complete a revolution per second. Thus, it has to start/stop fast and with an accuracy of 0.1mm.

What type of motor do you suggest, stepper or servo?
 

thatoneguy

Joined Feb 19, 2009
6,359
The torque would be a huge load for a stepper, but with a large enough one, it would do the job, probably a gearhead stepper to get the accuracy you need.

I'm not sure how you would sense position using a servo to get 0.1mm accuracy.
 

crutschow

Joined Mar 14, 2008
25,125
The big advantage of a stepper is that it's open loop so you don't have to worry about developing a compensated (and generally complicated) closed-loop servo system controller.

A gearhead stepper as thatoneguy suggested could have gear backlash which would be a problem for precise positioning. A large, direct-drive stepper motor with a micro-positioning controller would probably be best.
 

John P

Joined Oct 14, 2008
1,808
Yes, but the big DISadvantage of a stepper is that it's open loop! (sorry)

With a stepper, once you lose position, you are dead. The motor will never detect that it's running inaccurately. But it might be possible to detect the edges of metal blades that rotate with the disk. Then you'd have frequent opportunities to re-sync the counting of motor steps.

Better still would be a system which detects the holes, and stops when they're in the right place. That way you can use any motor you like, so long as you can control it slowly in the region where it needs to stop. Can you shine a light (probably from an infrared LED) through the hole, or reflect it off the surface of the disk?
 

crutschow

Joined Mar 14, 2008
25,125
Yes, but the big DISadvantage of a stepper is that it's open loop! (sorry)

With a stepper, once you lose position, you are dead. The motor will never detect that it's running inaccurately. But it might be possible to detect the edges of metal blades that rotate with the disk. Then you'd have frequent opportunities to re-sync the counting of motor steps.

Better still would be a system which detects the holes, and stops when they're in the right place. That way you can use any motor you like, so long as you can control it slowly in the region where it needs to stop. Can you shine a light (probably from an infrared LED) through the hole, or reflect it off the surface of the disk?
Certainly a stepper motor can lose it position. That's why most such systems have an accurate index point that is used to reset the zero position of the step counter. That way it always knows the location after no more than one revolution due to any reason (power outage, restart, misstep, etc.).

If you have a hole detector, how will you reliably know when your are close to the hole to "control it slowly in the region where it needs to stop"? :confused:
 

russ_hensel

Joined Jan 11, 2009
825
A 1kg disk positioned fast and with great accuracy is going to be a task. Can you reduce the mass of the disk, what is the disk doing besides rotating?
 

John P

Joined Oct 14, 2008
1,808
If you have a hole detector, how will you reliably know when your are close to the hole to "control it slowly in the region where it needs to stop"? :confused:
It's easy, you just use the right kind of hole detector.

I was just saying, if placing a feature in a certain location is the objective, it'll be most accurate if the feature itself can be sensed as part of the control system. Maybe in this case it's not possible, but we don't know much about the setup.

At least he doesn't want to find holes in almonds.
 

mcgyvr

Joined Oct 15, 2009
5,394
A 1kg disk positioned fast and with great accuracy is going to be a task. Can you reduce the mass of the disk, what is the disk doing besides rotating?
Its only 2 pounds.. Shouldn't be an issue. Most stepper manufacturers have calculators on their site to figure out the proper size for the speed/torque you need.

A servo will do the job too but at a much higher cost.
 

Thread Starter

mik3

Joined Feb 4, 2008
4,843
A 1kg disk positioned fast and with great accuracy is going to be a task. Can you reduce the mass of the disk, what is the disk doing besides rotating?
The mass is an approximation.

The disk starts, rotates 120 degrees, stops, waits for some time and repeats.

When it waits something moves through the hole.
 

Thread Starter

mik3

Joined Feb 4, 2008
4,843
It's easy, you just use the right kind of hole detector.

I was just saying, if placing a feature in a certain location is the objective, it'll be most accurate if the feature itself can be sensed as part of the control system. Maybe in this case it's not possible, but we don't know much about the setup.

At least he doesn't want to find holes in almonds.
Don't be that sure about the almonds!

Have you seen my almond drilling machine? ;)
 

shortbus

Joined Sep 30, 2009
7,761
Agree with Nerdegutta, the Geneva Motion is the way to do something like this. They are made in various number of moves and are controlled by the speed of the drum with the pin.
 
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