Stepper or servo for positioning accuracy (Urgent)

[Elrick]

I am looking for a motor in order to position in the space vertically precisely a small load.
I am sorry I don't know much about motors.

So I am hesitating between stepper motor and servo motor.
Do you have any idea how could I chose the best one?

Stepper motor :
I have heard that the stepper motor is not good because that it can miss some steps. And I need to be precise in position (rotation angle)
However maybe it wouldn't happen to me because I need a low speed and low couple.

Servo motor :
Which kind of motor would be the best?
I will certainly add with it a absolute encoder.

Torque = 1.45Nm
Speed = 100U/min

So could you give me any advises where I could get more information?

 

[itsmoked]

If you properly size a stepper it will never skip. Steppers are good for that service and would likely cost less than servos. Steppers have lower maintenance than servos. Servos can move faster than steppers. They can have less electrical noise. Steppers can cause more vibration. Lots of trade-offs.

For a mundane application I'd probably reach for stepper. For a dynamic application like machining I'd reach for servos.

Can you say what exactly you are driving?

Keith Cress
kcress -

http://www.flaminsystems.com

 

[Elrick]

What do you mean by
"If you properly size a stepper it will never skip."?

I would use the motor to move a linear system (with a belt and a carriage : see linked files)

 

[tygerdawg]

If you try to drive a stepper too fast, the load inertia can't keep up with the commanded motion. Your motor will be commanded to turn, but it will "miss a step" meaning the motor rotor will become decoupled from the magnetic driving field in the stator.

Look for a "micro stepping" motor with a "stepper indexer" (stepper controller), from companies such as Oriental Motors or Parker-Compumotor. If you don't know much about motors, then it's time you learned. Most of the manufacturers produce catalogs and engineering manuals with the required selection + sizing methods in them. Steppers were originally about 200 steps/revolution, but there were a few problems with those. Modern steppers have achieved resolutions of 1000's of steps/revolution. Even further, if you gear down this motion through a planetary gearhead, you may achieve extremely precise rotary motion. Your rotational speed, however, will be reduced accordingly.

TygerDawg
Blue Technik LLC
Virtuoso Robotics Engineering

http://www.bluetechnik.com

 

[Elrick]

Thank you for the companies tygerdawg.

I have been looking to drivers, controllers and stepper motors all day.
I have found some more explanation about my problem :

http://www.amci.com/tutorials/tutorials-stepper-vs-servo.asp

So I would like to purchase a stepper motor.

But a problem remain not totally fixed.
How could I know my stepper motor won't go too fast and would miss some steps? How could I know the limit value of load and speed for each of these stepper motor so it doesn't miss some steps?

 

[Skogsgurra]

Those limits are usually in the manufacturer's data.

Do not concentrate on an almost non-existing problem. Concentrate on the task.

Missing steps are a problem when you either have resonances in your mechanical system or if you accelerate/decelarate too fast. Or if your load is more than the motor can handle. All these things can easily be designed out of the system.

Gunnar Englund

http://www.gke.org

--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[Elrick]

Okay.
Thank you Skogsgurra.

 

[cswilson]

I'm generally a servo guy myself, but I agree with the others that this looks like a stepper application.

IMO, one of the key (underappreciated) advantages of servos is they give you greater confidence that you can accomplish anything within your performance envelope -- that is, anything up to maximum speed and maximum torque. You don't have to worry about "no-go" zones within that envelope.

You don't have that blanket assurance with steppers (although good microstepping drives go a long way to alleviating that problem). Good stepper users will carefully evaluate their system at the particularly speeds and torques they will be using to ensure proper operation, and will not automatically assume they can do anything up to maximum speed and torque levels without proper characterization.

If your application is fairly simple, with well-known speeds and loads, this should not be a significant problem.

Curt Wilson
Delta Tau Data Systems

 

[itsmoked]

Check also

http://www.geckodrive.com/

these are very good drives at reasonable prices. I've used several.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[DaveAlford]

I have used closed loop stepper motors by Oriental Motors ( Alpha-Step series)They work very nicely, compensate if missed steps do occur, have selectable resolution, and "anti-jerk" adjustment to smooth out vibrations.


ps. let me know if I shouldn't be using Manufacturer's names on the forum...I'm not intending to promote any particular brand, just trying to pass on experience.

 

[itsmoked]

Tiz not a problem even if you are a vendor if you are directly answering a poster's question and divulge your association.

Keith Cress
kcress -

http://www.flaminsystems.com