Can anyone help me add a linear encoder to my xy table?  As I research this issue, I know, in theory, how to get the job done, but I could use some advise/suggestions on what encoder to buy.

I am using a modified XY router table to fill a tray of chemical with a grid spacing of .3 inches and a hole size of .22 inches.  My filler is a bank of three nozzles .083 inches, spaced .3 inches apart.  I am using a Mach3 and a VB script to position, activate/deactivate a coolant relay to trigger my filler, and delay before moving to the next position.

This setup works for filling the 600 containers on my tray; however, without feedback, I cannot be sure that the gantry moved!  I have had couplings break or come loose and motor shafts break, so using a rotary encoder on the motor shaft doesn't give me the comfort I would like.

Any help with the electrical specs and hints on how to connect an encoder to Mach3 would be greatly appreciated.

Mach won't close the position loop. You can hook the encoders to mach and view their position however if you do have some failure on the servo end mach will still send pulses out the port and not know that there was a problem.

The encoders are for reference only and won't affect the output of the software.
The best way to ensure it is where it is supposed to be is by using servos. If your hardware is well built a good servo will guarantee it is where it is supposed to be. Steppers can loose position, by definition servos won't. 

Chad

OK, now I am really confused.  I was under the impression that the encoder could return a series of pulses that could be used to tell the software that a given movement took place.  Why would that be any different then the rotorary encoder on the back of a motor?  If the motor got a command to move .3" and the encoder moved .2" wouldn't the software issue additional pulses to the motor?

Mike

If the motor got a command to move .3" and the encoder moved .2" wouldn't the software issue additional pulses to the motor?

No.

(Servo- closed loop)
What we are talking about is a servo loop. Servo means the drive or amplifier knows the position of the motor and will correct the position if it isn't at the predicted position. All this is handled in the servo drive not inside of mach. if mach sends out pulses to go to .3 and the servo drive realizes it has only gone .2 the drive will all by it's self move to the .3 . Mach just assumes that that the position is correct. It can't look and say I am not there I need to go further. Mach lets the servo driver deal with that.

(Stepper- open loop)
Stepper motors 99% of the time don't have any sort of encoder on the shaft. Mach sends go one step, the driver sends one step to the motor, the motor goes one step. As far as mach is concerned it is where it should be. If you grab the shaft on the stepper and turn it mach will have know idea that the shaft has been turned and continue sending places out assuming it is in the right place.


I guess to summarize:
Stepper can loose position and there isn't anything you can do about (as far as mach is concerned)
Servos Rarely loose position because the servo driver is keeping track of the position and if it gets off by an encoder count it will correct for it on the fly.

In a servo system you want the encoder on the motor. If you put the encoder on the mechanics of the machine if there is ANY back lash or slop in the ball screw or the gear reduction/belt the servo drive will "hunt" and usually burn out your motor. 

Chad

Chad,

Thank you for your very detailed and understandable explaination and please accept my appologies for not fully understanding this technology, yet  .

Now a follow-up:  Since I am moving the filler using VB code, shouldn't I be able to "close the loop" by issuing a move command and then checking the DRO before issuing a fill (coolant on/off) command?  If I am not where I compute that I should be, I would, lets say, attempt to reposition 3 times before giving up.

Mike