Position control without an encoder

[lauram]

I am relatively new to motor control and would like to move a mirror between 2 positions approx 90 degrees apart using a BLDC motor.
I want the mirror to flip between the 2 positions within a couple of seconds.
I hope to use limit switches to enable my control circuitry to know that it has almost reached either of the 2 positions.
At both of the positions/end points I plan to use stoppers. However, I want the motor to remain at the end point and not be disturbed by any vibration it may experience from the rest of the system.
At present, I am aware that I will need to reduce the current to the motor to stop it burning out, once it has reached an end point. However, I am unclear whether a minimal current is all I need to maintain this desired position, or whether there is an alternative approach.
I am reluctant to use a complete position feedback circuit, since I feel it is over-kill for what I am trying to achieve. Although, if you suggest otherwise..?
Also, for the application I am proposing, would a stepper motor be better, since they have a better holding torque?

 

[waross]

How about a torque motor or an induction motor driven by a VFD in current/torque limit mode?
If your mechanical load is light, you may be able to use a damper motor. You can get faster action by using a linkage that uses only part of the stroke. It will hold your device tight on the stop at each end. No limit switches will be needed.
yours

 

[blacksea]

You need any amp in velocity mode with activation both limit switches as forward (FLS) and reverse (RLS) ones. Using current/torque mode as suggested above may damage your mirror due to absence of velocity limit (up to max velocity).

 

[IRstuff]

Another alternative is a Geneva mechanism, since it'll give you both velocity profiling and some degree of position stability.

TTFN

 

[sreid]

For what you want to do, I think that a stepping motor would be a better choice. No position feedback is required and the motor will hold position without current (if the stopping position is on a detent). The limit switches wull give you an absolute position when you go through a home routine at startup.

 

[cswilson]

I agree with sreid. This is a classic stepper motor application. You would hold position against disturbances either by maintaining some current through the phase(s) at the desired position, or by relying on the natural detent torque of the motor at that position, or both.

If you wanted to use a BLDC motor without position feedback to do this, you would actually have to run it like a (low-pole-count) stepper motor.

Curt Wilson
Delta Tau Data Systems

 

[nbucska]

why don't you use a solenoid/spring/adjustable stops ?


Plesae read FAQ240-1032
My WEB: <

http://geocities.com/nbucska/>

 

[MikeHalloran]

You could use an overcenter spring to hold the mirror against either stop, and pretty much any old motor to push it away from the stop and past the center.





Mike Halloran
Pembroke Pines, FL, USA