Motion Control and Motor Drives

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stepper motor

Hi. Actually i not sure that whether i post this question in this place is correct or not. If wrong pls recommend me the correct place. 

 

Stepper motor is open loop system. Why we stil can know the position of the motor?

 

Besides, does stepper motor with encoder allow velocity feedback? Then, what is the different stepper motor with encoder and servo motor?? i am naive. So, apologize if asking silly question.. Thanks

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Search "motion control tutorial" on the NI website, and then select the "tutorials" section.  Look through those, and they will help greatly.  If you have further questions, post back to this section.
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As long as your motor doesn't loose steps (proper sizing, load and acceleration not too high), the motion control board knows the position of the motor by counting the steps it has generated. If for some reason the motor looses steps or if the axis is moved by external forces, you need to do a reference move (e. g. move to home switch) to get back to a known position.

 

Typically stepper motors don't require velocity feedback. During a move the motion control board outputs a velocity profile (trajectory) and without step loss the axis will follow this profile. There is no good way to compensate step losses during a move, so in closed loop mode most motion control devices only track the following error during the move and execute pull-in moves at the end of the trajectory.  In contrast a servo motor always needs to run in closed loop mode and it's capable of compensating following errors and load changes during the move.

 

I hope that helps,

Jochen

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"Typically stepper motors don't require velocity feedback. During a move the motion control board outputs a velocity profile (trajectory) and without step loss the axis will follow this profile. There is no good way to compensate step losses during a move, so in closed loop mode most motion control devices only track the following error during the move and execute pull-in moves at the end of the trajectory. "

 

Just to confirm what i understand from this paragraph above:

1. whole paragraph explain abt stepper motor?

2. if Yes, what do you mean by closed loop mode motion control devices for stepper motor?

 

Thanks

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  1. The quotation is related to stepper motors only. 
  2. Closed loop stepper = stepper with encoder feedback

Does this answer your questions?

 

Jochen

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It answer my question. 

 

Then, do you have the experience that a servo motor is used as stepper motor?  If yes, when and why to use? Thanks

 

jason

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Jason,

 

I don't understand exactly why you are mixing up stepper and servo motors. Typically these motor types require very different control approaches with one exception.

Some special servo drives (e. g. from Yaskawa) accept a step and direction signal like a stepper drive. This mode is called P-command mode. In such a setup you can control servo motors like stepper motors. Is this your goal?

 

Jochen

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Actually i have seen another group is using a servo motor as a stepper motor.  I dono the reason(s) behind and want to find it out. 

 

Is there any advantage and disadvantage in a application?  Thanks

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Jason,

 

what exactly do you mean by "using a servo motor as a stepper motor"? Are you referring to P-Command mode? I mean physically a servo stays a servo and a stepper stays a stepper and they will always differ in terms of their physical behavior regardless of the type of control that is used. A stepper provides static holding torque without the need of external control, but this holding torque is limited and if an external force pulls the stepper motor out of position, there is no way to compensate that.

 

In contrast a servo motor doesn't provide static holding torque and external (PID) control is required to keep it in position if a force is applied, but the control algorithm will bring it back in it's intial position if the force is reduced again, regardless of the displacement the force has caused before.

 

There are many other differences, e.g. discrete steps vs. continuous motion, linearity, accuracy and so on. The application areas for steppers and servos overlap widely, but there are also many applications with requirements that can only be met by one of the two types - changing the type of control doesn't change this fact.

 

Regards,

Jochen

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In my case i mentioned, it sends step and direction signal to servo motor like stepper. However i am not sure that the case mentioned is P-command mode.

So, what is the advantage and disadvantage to send step and direction signal to servo motor and send +10V and -10V to servo motor? This info can useful in purchasing servo motor.

Thanks
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