Re: [CAD_CAM_EDM_DRO] steppers best?

At 01:33 AM 10/17/2007, you wrote:

>1 -Are steppers our best choice?

The first thing to learn in engineering is that there is usually more
than one best choice after looking at all the parameters. You
probably don't really care if you drop a count (1 in 10,000 is a
small error.) You probably care about a financial budget (multiple
laser interferometers are probably right out.) You probably have a
power budget.

If they will work (based on your analysis of how fast your robot will
travel and the power necessary to move your robot), steppers are a
good choice. Servos are a good choice, but much more difficult to
implement and more expensive.

I would try unipolar steppers (5 or 6 or 8 wire) and simple
drivers. By simple I mean two resistors, one transistor and two
diodes per driver with a current limiter on the common line if
needed. Need schematics?

>2 - Is it pretty easy to do the microcontrolling effort to choose a
>certain number of steps (DIP switches etc) and then stop, or is this
>more like an iceberg - a lot deeper than it looks?

Yes, it is deeper than it looks because there are timing issues on
the motors. A simple chip like the Basic Stamp (Parallax) will do
everything you need in a simple language (but slowly!) See
http://www.parallax.com/detail.asp?product_id=27120 for the board to
build your logic and a processor like this one
http://www.parallax.com/detail.asp?product_id=BS2-IC which is easy to
program. The phase control can be done in one small array and the
count logic is trivial. The board runs on a 9V battery, the steppers
on whatever you want to use.

>3 - Other considerations?

Heat, speed and power come to mind. Running on a surface plate will
take lots less power than on a carpet. Wheels may be more important
than you think. Do you need steering? How about avoidance
sensors? How about an emergency stop button? What are the vehicle parameters?


David G. LeVine
Nashua, NH 03060