Re: new project


<Keiichi.McGuire@xxxxxxxxx> wrote in message
news:1134047189.282280.120300@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
> well, we're using the nema17 stepper motor shown on that page.
> We already have most of the physical hardware done and it fits well
> inside of the maze cell and leaves plenty of room for maneuverability.
> I tried DC motors before and I felt that it didn't give us the accuracy
> desired;

Why do you need high accuracy ? The mouse needs to be guided by its
environment (ie walls). You need some feedback to make it go in a reasonable
straightish line, but this is usually done with a simple encoder disc on the
wheels. The other properties of DC drive include high torque for starting
and stopping, lack of resonance problems that are one of the curses of
steppers, and finally a lower power consumption - the steppers will always
have at least one phase on.

and also dc motors that are geared and have a feedback tach on
> it is a bit pricy and out of our budget...

You don't need a fancy tacho feedback (see above). You don't need a
gearmotor if you can make a simple 'gear' train. In fact, a toothed belt
reduction drive is often used for steppers or dc motors.

> But yea, any thoughts on the IC's that I'm planning to use? Any other
> recommendations?

You need to establish what current you need through the steppers to obtain
the drive forces you require. If the motor is spec'd at 4v 0.9A that means a
maximum of 0.9A per phase, and also implies that each phase is about 4.5
ohms. However, you cannot necessarily actually use it for longish periods at
that drive level because of heating.

The data*** you haven't got will tell you more hopefully. Look for a
figure for the 'holding torque'. This essentially gives you the maximum
torque that can be applied before the motor 'slips'. You can use this number
(perhaps 5 milli Newton-meters for your motors) to work out how much drive
you can get, and hence how fast you can go - and also how fast you can stop
or change direction.

Power dissipation could indeed be a problem for the drivers. The output
drivers on the '67 are transistors and will be dropping something like 0.6v
(lo side) and 2v (hi side) at a per phase current of (say) 800mA. So you are
looking at some 2W per phase. That is, as you say, a lot of heat to shift
from a smallish package.

I would look for a switchmode driver/translator with DMOS or FET outputs. If
these drivers are 0.5 ohms Rds, then the same current (800 mA) will generate
some 800mW per phase.

Note the total power consumption is quite high - average 1.6A, maximum 3.2A,
assuming the same 800mA per phase.

If you want to go 'fast', you will need to apply much more than 4V.

Stepper drives look simple at first glance, but in reality DC motors are far
easier to 'tame'

Have fun !

Dave


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