Living Fossil - moving stepper motors 6 times a pen's width.

[icamaraleret]

Apologies to those who I've already asked for help on this.. 

I'm attempting to build a machine that documents in real-time tectonic movement occurring within Earth. 
I've attached an image of how I picture the machine, the idea is that it would function as a tape recorder, unwinding one end and winding the other. 

Yesterday at the Not Just Arduino meeting, I got quite a lot of help as to the mechanical aspect and about what I need to make this work. I've got an arduino, two stepper motors (200 steps) for each roll, a linear rail with a motor for my pen to move along the top roll and a CDC shield with three A4988 drivers. 

The pen, is of a 0.05mm thickness and should move six times its' width each day for three months = 25 mm in total. Although the distance probably means I don't even need to hook up the rolls to motors, I am quite keen to learn how to make something that could (if powered up) go on forever. Or at least until the roll runs out. 

So whilst I'm trying to get the roll ready, is there any advice/libraries or tutorials that any of you have come across and think could be useful to start coding with provisional numbers?

I would be forever grateful!

[Nigel Worsley]

Using stepper motors on both rolls will give problems, especially as their diameters change. Use a standard DC motor for the takeup roll, their torque is directly proportional to current so is easy set. Feed the motor from a low voltage supply through a current limiting resistor and you are good to go, making a proper constant current regulator would make it easier to change the torque but isn't really necessary.

Nigle