Yeah!
Roman, thanks for the professional feedback.  I was hoping that there will
be someone with *robotic* experience.

> -----Original Message-----
> From: pic microcontroller discussion list
> [mailto:PICLIST@MITVMA.MIT.EDU]On Behalf Of Roman Black
> Sent: Tuesday, July 09, 2002 5:04 AM
> To: PICLIST@MITVMA.MIT.EDU
> Subject: Re: [EE]: stepper motor in TemUR microrobot
>
>
> Hi Irek, shame about the jerky problem. You really
> need more gearing, like a 1:5 pair of gears, as you
> probably won't get the performance you want with 1:1
> connecting the stepper to the chain, even with your
> small cog.

Ok. Then dreams about attaching stepper directly to the sprocket are really
unreal?
With the gearing the whole thing becomes too big for me.  I was thinking
about building a robot with the dimensions below 1". The robots currently
have ca. 2" and in the next generation should shrink rather than grow... :o)


> Steppers *will* vibrate quite a bit, and even with the
> right gearing you might find that because you have hard
> shiny chains it will still not get precise location on
> the ground. It is fairly important to have good gripping
> rubber between the stepper motor and the ground. :o)

Now I am aware that one of the problems is this "hard shiny chain" I was so
proud of.  Mechanically there is no way to have this chain *not skidding* on
the floor. There is no mechanical part, which can accumulate step's energy
and smoothly release it.  Probably a drive made with pulleys and rubber band
should help a lot. What do you think?


> My little stepperbot here:
> http://www.romanblack.com/deskbot.htm

This is a real beauty! :o) Especially impressive for me is that your robot
"has enough torque to push a full coffee cup without slowing, in fact it
will push about 1kg across a smooth table".


> has 1:7 gearing on the steppers, and rubber tyres.
> Without the rubber tyres it slides and does not drive
> in a straight line or get accurate distance etc.
> Even with the rubber tyres it loses/gains 1mm in every
> 50 or so, without rubber tyres it is more like 20mm
> error out of every 50!! Skid city.

My experiments with rubber grommets put on the sprockets' shafts acting as
tires helped.


> Another thing to think about is the h-bridges, they are
> normally 3v to 4v (saturation) when turned on, so you
> may only be getting 4v on your motor and losing 4v on
> the bridge. Better to use 4 fets and drive the motor
> unipolar (has half the coil resistance and 10x less
> loss on the drivers).

It sounds like another advice worth of case of *real* beer.  Thanks.


> You can get tiny stepper motors in any 3.5inch floppy
> drives, dead drives are available everywhere.

As I replied to Andy before, that I tried with the floppies.
http://teamur.netfirms.com/test/test_floppy_disk_drive.htm
Unfortunately it doesn't work for most of the contemporary drives. It was
probably fine in 5 1/4" drives era but not it looks that producers
integrated steppers into chassis. Besides it is not easy to find two
identical floppies...
Does anybody have different experience with the floppies?

Thanks!
It really helped to straighten things up in my head.
Irek

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