[Robotgroup] Question ... servo or stepper motor ?

[Glenn Currie]

Hope these comments are helpful.
Several things come to mind.

If by servo, you mean a Futaba type Remote Control plane type servo,
they are fairly inexpensive, and have a standard method of control.
(a variable width pulse, is proportional to the position of the servo.)
You can take them apart and replace gears, the feed back resistor
etc.  They are usually made of mostly plastic, to reduce mass
(which translates to weight, if you have gravity in the vicinity).
They were made light to limit the weight in model aircraft.
Mass is the enemy if you want to change directions quickly,
and want to fly at all.
Futaba type RC servos, have been built much larger, and much
smaller sizes than the original devices.  You can get tiny ones, the size of
a grain of rice, (used in micro flying robots) or you can get fairly
big ones,  Check the Tower Hobby web site to see who has the
biggest/smallest RC servo... today.  Also, one of the main ratings for a
servo, is the torque it can produce.  See if that fits within you
design needs, and try to avoid the pearls of "servo envy".

Stepper motors tend to be more massive and can generally be more
precisely controlled.  There are many types of stepper motor.  One
key spec is, how many steps do you want to break a 360 degree
circular rotation into?  How much torque do you need?  How long
does it need to last?  Does mass (weight) make a difference in
you design.  There are plenty of controller chips that help in simplify
stepper motor design, but the controller can be more complex and
expensive (and precise) that a RC servo. Sometimes you end up having to 
have separate
driver transistors to handle the current the motor requires.  This adds
expense, mass and complexity.

There are always new variations on stepper motors, and RC servos
but in general RC servos have a relatively simple control system,
(one that generally works OK over a radio data link), and they
are relative low mass (few are made of lead).  Stepper motors
usually have a more complex control system and usually have
metal parts, instead of plastic.   Stepper motors, properly
controlled, can be quite durable, lasting for years.  RC servos
are built so you can rebuild them, WHEN  not if,  you strip out the gears.
Few people attempt rebuilding stepper motors.
RC servos, even big ones with lots of torque, can be very cheap
compared to what a high quality stepper motor and control system
cost.

... more on the Internet of course.

Cheers,

-Glenn

Def Egge wrote:
> Under what circumstances would one choose 1) to modify a servo for 
> 360-degree rotation over 2) a stepper motor?
>
> Inquiring minds want to know.
>
>
>
> All the best....
>
> Mike
>
>
>
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