custom tool development...

[AWillette]

So over my winter vacation (one of the the perks of academia) I started sketching out an idea for custom tool that would require use of the A/B-axis, ideally with a small 12v stepper. Basically I saw some work people have done with a 6-axis arm (http://roboticarmfabrication.wordpress.com/2010/12/14/string-fab-wrap-up/) and knew I could do similar with only a 4-axis machine (because thats what I have access to). I figured it would be something fun to work on so decided to hit the ground running when we started up after the break.

I have a couple questions that I thought I'd post here for some advice. I did go through the indexer installation documents that came with the machine as I figure that would be the best place to look, but they're for an earlier version of the control box and I wasn't able to gleam too much from it.

1. I have a PRSstandard with the latest control box and opened up the cabinet first thing when I got into the shop today. I see there are 4 stepper controllers - are the two X-axis's controlled separately or is the 4th controller available for the A-axis? I'd like to avoid having to buy a new controller as I don't own the machine and don't have too much money to be throwing into this project.

Answered this with some digging through the archive. Since I'll have to buy a stepper controller this project will be need to be put off for a bit. I wonder if ShopBot sponsors projects...

2. What are the properties of the stepper pulses (voltage, etc.). I'm hoping that I would be able to step-down the voltage in-line without degrading the pulses, but would need to know what I'm dealing with before I start developing the circuit for this.

Any help on any of this would be appreciated!

Cheers!

[richards]

Aaron,

The Shopbot controller sends out TTL signals to the stepper driver. The stepper driver converts those signals to the proper voltages for the motors being used.

Some stepper drivers require a 15mA Active Low signal, others require a 2.5mA Active High signal. It all depends on the model of your machine and which stepper drivers you're using.

Driving a stepper motor requires a little bit of 'black magic'. Stepper motors must be ramped up from standstill to full speed. If you ramp the motor too fast, it will stall. If you ramp the motor too slowly, you'll waste time (and you may have jitter caused by resonance).

If you're going to develop a machine or if you're going to modify your existing machine, do yourself a great favor and build a test bench where you can experiment with motors, stepper drivers and power supplies until you understand how they work. It will cost some money (my two test benches cost about $2,500 each) and it will take some time (I've been 'testing' for about four years), but in the end, you'll know how things work and you'll be the 'expert' that others email for advice.