On Mon, 8 Jul 2002, Irek Rybark wrote:

>It looks that I was wrong.  I put a sprocket directly on the motor's shaft
>and unfortunately the torque was not enough big to move it.  Of course the
>first mistake I made was the size of this sprocket.  Replacing with the
>small one helped a lot.

You can measure the required torque/force using a piece of string a
plastic bag (to be filled with coins) and a smooth-edged table. Refer to a
physics textbook (for school 10th grade or less) for howto.

>And here comes the disappointment.  The robot should have a nickname "Jerky"
>because of how it moves!  I suspected that dynamic (continuous) vs. static
>(single step) stepper motor control is different but what is happening with
>it when the stepper runs below 10 steps per second is heart breaking!  The
>whole robot jerks probably because the motor is not enough precise (16 steps
>per revolution) and makes too BIG STEPS.  Sometimes the motor gets stuck and
>robot turns around.
>I know that this is cheap surplus motor but at this point of time I am not
>that big robot "enthusiast" to pay $$$ per motor to some Swiss company.

You can add viscous dampers or friction brakes. The latter are easier to
DIY. A simple one: tie a short bit of string to a short piece of elastic,
wrap the string (not the elastic) once round the axle and fix the two ends
of your brake to the chassis. Adjust the elastic tension as required. This
is a sort of rope brake (dictionary term for this type of device). The
string should be wide (shoelaces etc). It brakes more in one direction
than in the other.

>The second problem is that the motors suck current from battery like fresh
>orange juice.

You need to look into current chopping stepper controllers imho, with
current settable by the CPU.

>1.      Do you think that gearhead is necessary in such cases?

Must have.

>2.      Where can I find an explanation of the dynamics of the stepper motors
>without going for LONG studies?

Ha. Try to get a copy of B.C.Kuo's book on stepper controls.

>3.      How to decrease power consumption and increase torque, which are
>contradictive goals?

See above at current chopper.

>4.      Is there a source of SMALL (diameter << in.) & CHEAP (price << $50)
>stepper motors?

Pass.

>5.      Do you have any suggestions regarding using stepper motors in such
>construction?

Avoid them. You can get servos that small or hack your own using some
small motors and wakeup clocks or windup toy gearboxes.

>I am not afraid to take the robot to my "closet junkyard" and build a new
>one but I would like to avoid this type of probably basic mistakes in the
>future.

Do the basic physics on your robot ('coefficient of friction' without
motors, weight -> required starting torque reduced to motor torque by
gears) etc. Then it will be easy ;-)

Peter

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