Stepper motor as encoder??

Hi,

I'm wondering if it is possible to get enough back EMF from a stepper motor running at low speed, to process thru a pic, the number of steps it has gone thru to display the results to a 5X7 segment LED display.

I'm basically trying to avoid the coupling of an encoder to the motor's shaft.

Would it be accurate??

System is to be used on CNC system with visual display so operator has visual confirmation of machining progress.

Thanks.

Back EMF would be produced if the motor took a partial step and failed to move completely between poles...
This would generate an incorrect reading.
My feeling is that a shaft encoder is the way to go...
Have you the option of using a hall-sensor?

Thanks McGuinn,

I understand what your saying.

I,m not too familiar with hall sensors and how they can be used on a linear slide.
Transducers are too expensive.

Trying to get accurate position of cross slide to display without any physical change to motor or mounts.

I have tried optical encoder but too sensitive to moisture or dust.

Maybe in the end I'll have to go with shaft encoder.

P.S. slide measures 140cm.

Thanks.

When you say it's a 'slide' what do you mean? A conveyer system?
Is there a point that you could add a magnet that would rotate at a constant rate?

A hall sensor is a simple device, three pinned and cheap, it picks up the change in magnetic flux. They are frequently used in older floppy drives.
Looks like a transistor for all accounts...

Hi McGuinn,

The movement i am trying to read is that of a table moving on linear bearing/shaft combo.

Imagine moving your computer desk left or right and recording how much it has moved.
Under this table there is a 30cm X 140cm cavity where i can locate sensors.( transducers or optics or Hall sensors), I guess.

How can I make one hall sensor read the magnetic flux changes on 140cm of travel?

Will this require 1 or many hall sensors along the length of the table?

This seems very interesting and may be the way I go, especially if accuracy can be constant.

Thanks again I hope this gives you an idea of what I am trying to do.

P.S. this will be used for an educational commercial cnc milling machine.

Ah... OK...

My thoughts would be that if given dust free conditions (not possible in your case) you would use an optical sensor.
A derivative of an optical sensor for a dusty environment is a linear phonic wheel (or strip), like what ABS systems use on the hubs of cars and the flywheel.
This is where you have notches made in a ferrous strip attached to the linear plane that you wish to measure, which pass a static sensor. I'm not sure if this is a Hall sensor, or another similar object.
Maybe you could obtain an ABS sensor from a scrap merchant? The ferrous strip could be a mechano strip...???

Maybe you could test different materials for suitability?
A problem will arise if this lathe is producing metallic shavings... I hope it's a wood lathe!