INDEXER-2

Thanks all,

I am still a bit confused :) It is clear that I didn't know all
of the things this word meant.

But, the thing I had in mind, was the electronic step system. I
still don't quite understand this:

Leslie's description sounds like a stand alone step controller
system.

Quote Jon:

> The other thing is more completely described as an indexing

head.

> As used with a general, 3-axis CNC machine, it looks kind of

like

> a dividing head (which has a crank on the side, and each

revolution

> of the crank turns a lathe chuck so many degrees (often 9

degrees)).

> Instead of the crank, there is a button which can be pressed by

a

> tool in the spindle of the CNC, and it advances the lathe chuck

by

> a predetermined angle.

What is this used for ?


Back to the electonics :

Jon again:

> One is a device, usually connected to a stepper driver, that

makes the stepper

> step through so many steps when given one pulse. Modern ones do

> accel/decel on those steps, as well. This is an

electronics-only box.

I am not sure if I follow this: Let say you have a 200 step pr.
rev. motor. Let us say this "one" indexer puls do execute 10
steps.
Then you will have the motor go 20 step pr. rev. ? Is this right
? Sounds like you downgrade the whole thing ?


But, my reason for asking about this, is just that many seems to
want a readback from steppers. As things are, you would get some
trouble with integrating it into software. So, what I just don't
quite understand is, - why isn't there just some simple solution
to this ?

Here is what ( I could make a simplified schematic design - to
make it a bit clearer ):

1. Attach an encoder to a step motor. This could maybe be just a
wheel from a mouse. It needs to be exactly positioned and have
the same resolution as the steps/rev.

2. Have an encoder counter, just a simple one. Just to keep it
simple, say 4 bits long. I will call this counter B.

3. Have a second counter, I call this counter A.

4. Add some gates and a one-shot timer.

5. Now we add everything together:

You have this circuit inserted between the parallel port, and the
step motor driver. Let us say you have a gate connected to the
step signal, and on the falling flank of the pulse, you switch the
input of the step motor drive, to the rest of the circuit. After a
predefined time, it will connect back to the parallel port.

Then each step/dir to the motor is also used to count up/down the
counter A.

The encoder will do the same thing on counter B.

Compare this with a 4-bit magnitude comparator ( 7485 - if I
remember correctly ). This has 3 outputs:

Inputs from counter A and counter B - is the same.
A is less than B, and A is greater than B.

For not equal, then issue a step signal. ( we have a switch, or
multiplexer that will give us access to the stepper driver )

If the motor loses a few steps, this circuit will make sure to
correct it again. You have other magnitude comparators that is 8
bits wide, and it is possible to chain them together. The whole
thing could also be put into a micro controller, say a PIC device.

The idea here, is not to make any software changes, do any
changes to the step motor amplifier. Just insert it between them.

Add more logic to it, and it could signal your computer or do
other stuff - in case it can't correct the loss of steps. Sure
there is
some timing stuff to consider, and other problems.

You could also take care of the whole stepping - let this circuit
do all the step signaling, at a defined rate, having a better
timing,
but then you need some memory and other stuff as well.

So I wondered if this in fact is something similar to an "INDEXER"
?

//ARNE