How to start a stepper?

[chuckknigh]

OK, I've been thinking about the simplest possible driver circuit, thinking about the open source project. That would be a 4 transistor (per axis) driver, using wave control under TurboCNC. A few transistors, some diodes, and some resistors wired up to the parallel port should suffice, though I welcome other input.

My reason for asking is, I'm working on a <$20 "proof of concept" model with a single axis. A CNC Primer unit, if you will...which can function as a Z axis when completed. I intend for it to be an "I can actually do this!" type of project...simple, cheap, and easily understandable.

But, the more I think about it, the worse it gets. (Of course, I have a tendency to overanalyze everything)

OK...we have 4 coils...numbered 0, 1, 2, and 3. The motor is "stopped" at a random coil, let's say coil 2, and power is removed / computer is rebooted. That means the next step would require a pulse on coil 1 or 3, depending on the direction...that would be the next step.

But, since there's no feedback as to the position of the rotor, how does TCNC know where to start the pulses when it's first started? It could be, depending on the orientation, as much as 2 steps off at the start.

Extending this logic, it would seem this problem would exist in all stepper driver circuits. Any time power is removed, do the chips "zero" themselves back to the initial step? Seems like that's how it would work.

So, how is this situation handled? I really can't figure it out. Help?

-- Chuck Knight

[marvinstov]

Nice going Chuck. You electronics guys just amaze me. You all say " All you need is a small circuit board, throw some chips, resistors, a few diodes at the board and what ever it is you need will stick to the board and you're done." Just amazing, wish I could do that. Keep going, I'm listening.

Marv

[drawbar]

In my experience, if I've just booted my computer up and turned my control power on, I can count on having to 'zero' my axes, because I have no idea how they've moved while it was down. For some machines, that means using homing limit switches. I don't have those installed, so I home manually (the way a machinist with a DRO does). But in any case, having just jogged the axis before zeroing, the step control is now properly somewhere in the middle of its phase sequence, holding steady and waiting for the next step that you command.

Also, when you energize one phase of the motor, the rotor will snap to that position, no matter what position it was in before. It may jump across those 2 or 3 magnetic 'detents' to get there, but I assure you, it will get there! That always happens on my control when I first turn it on. It goes to 'phase 1' from whatever phase it was on before. And as long as I accept the fact that I have to zero the axis then, I'm OK with that.

Brian

[chuckknigh]

Originally Posted by marvinstov Nice going Chuck. You electronics guys just amaze me.

The *true* electronics guys amaze me, too. I'm a hack, relatively speaking. Found a basic circuit in one of my electronics books, and am working from that.

In fact, I'm going to an electronics surplus store, today, to get some parts. Living in a small town has its advantages, but parts availability is not one of them.

-- Chuck Knight

[marvinstov]

Originally Posted by chuckknigh OK...we have 4 coils...numbered 0, 1, 2, and 3. The motor is "stopped" at a random coil, let's say coil 2, and power is removed / computer is rebooted. That means the next step would require a pulse on coil 1 or 3, depending on the direction...that would be the next step. But, since there's no feedback as to the position of the rotor, how does TCNC know where to start the pulses when it's first started? It could be, depending on the orientation, as much as 2 steps off at the start. Extending this logic, it would seem this problem would exist in all stepper driver circuits. Any time power is removed, do the chips "zero" themselves back to the initial step? Seems like that's how it would work. So, how is this situation handled? I really can't figure it out. Help? -- Chuck Knight

Chuck,
Check out the Xylotex data sheet. It mentions on there how the Xylotex board handles this I think. http://www.xylotex.com/XS35V.pdf There is a table on page 4 and some more explanation on the bottom of page 6.

Marv

[pminmo]

Chuck,
The motor goes to what ever state the driver reset to. The sequence is how to make it rotate in steps and direction. Think of a magnet on a table by itself. Each time you put it on the table its poles are oriented randomly. If you put a bar magnet to it it will alway orient to the attacting poles unless it's already oriented that way.

Phil

[signIT]

Hi,Look at this, including one stepper http://www.bde.se/bsart/bb002n.htm I have three cards with three small ~800mA steppers i will use them for a cutting machine, cutting metal foil only one stepper driver and stepper down and up wil be manual controlled.
Price card and one stepper SEK340 which is about USD45 that's nice I think that the PCB's and components can be bought separate.

SIgnIT

[chuckknigh]

Very nice. But that's a mail order stepper driver, and not what I'm aiming for.

The purpose of this project is to be a stepping stone into the world of computer control. It's not meant to be a truly practical driver design (it actually is, but there are better ones) but merely to allow someone to get to the stage where they admit to themselves, "I can do this!"

To this end I'm designing it from components that should be available at any Radio Shack or RS Components. Darlington transistors, some decent sized diodes, and some pull up resistors. By keeping it simple, I'm trying to make sure people can *easily* build this first step.

That's a big step...advancing from a pipe dream, to a piece of equipment moving under computer control...even if it is only a single axis, and primarily for proof of concept.

My intention is to make a single axis (a CNC primer unit) which, when complete, can function as a reasonable Z axis for a full machine. I figure this is the most "constantly" sized axis of any of them -- relatively speaking the movement is less than a foot, the structure can be easily made strong, and the mounting of a router spindle, or a Dremel tool takes approximately the same "vertical" space.

A relatively small project that can be put to immediate and practical use.

Seems like a good first step for the CNC newbie.

-- Chuck Knight

[signIT]

Hi,

This link has some schematics for stepper driver, low tech components and cheap, I haven't tried it but someone should, the site owner run this design in many applicarion single axiz or more.

What I can understand is this a very basic but working design meant for people that will try CNC technology but don't spend too much.

It is a freebie but it is not so user friendly instructions.

http://www.cnconabudget.com./freebies.html

Cheap is Good.

SignIT