Stepper motors move a fixed angle per step (unless they are
overloaded, and then they just vibrate). Servos move any amount based
on a feedback device that measures their position and reports back to
the control circuit. They can also be overloaded, but at least the
control circuit knows that they are not going to where they are
supposed to be.
With a smooth enough floor and round enough wheels and bearings, you
can move the space shuttle with two 4 volt .9 amp motors. With an
ordinary floor and Rc car wheels, you can move 20 pounds if you gear
the motors low enough. The motors have to overcome rolling friction,
air drag, and also have some force to spare to produce acceleration.
This is a very poorly defined problem as stated. For some actual load
and wheel situation, you probably need to do some experimenting to
better estimate the motive power required.
The shortest answer is that they are completely different and have
nothing to do with each other, and shouldn't be confused.
A servo is more than a motor: it's both a motor and a feedback
mechanism. You tell a servo "I want you to move to the following
position" and the servo just plain does it. Commercial servoes
typically have a fixed range of motion; they can turn their output
shaft something like 270 degrees lock to lock. This makes them good
for things like steering; you just say "go to straight ahead" and
quit worrying about it.
A stepper motor is, in some respects, the exact opposite of a servo.
You tell a stepper motor you want it to move a step, and it does so.
Keeping track of how many steps it's gone, and details like not giving
it too many steps too quickly, are up to you. A stepper can keep
going round and round, which is why it's good for drive wheels.
Notice that you can do a servo in software by using a stepper, and
counting how many steps you've given it.
Nowhere near enough information. How much torque do the motors
provide? How are they geared? What sort of acceleration do you want?
Joseph J. Pfeiffer, Jr., Ph.D. Phone -- (505) 646-1605
Department of Computer Science FAX -- (505) 646-1002
Thanks guys for clearing that up. Its much clearer now.
One more question :
What determines torque or turning power of a stepper motor - voltage
i.e. All other things being equal, will a 3 volt, 0.35 A stepper motor
deliver more torque than say a 5.3 volt, 0.17A motor?
All torque comes from the interaction of stator and rotor magnetic
fields. And magnetic fields are proportional to the current that
causes them (if they come from a coil and not a permanent magnet). So
torque is proportional to current (if the current energizes one
electromagnet) or current squared (in the case of a series wound
motor, where the same current magnetizes both the rotating part and
the stationary part, in series).
But it takes voltage to cause current to happen. And the faster a
motor is turning, the more voltage it generates. The applied voltage
must overcome the generated voltage before any current can be moved
through the motor. So voltage must be available (and more for higher
speed) but that voltage supply must have the ability to deliver
current to produce torque. Total power is proportional to volts times
amps (and to torque times speed).
I assume you know how steppers work.
Servo motors differ in that they are basically standard
armature type, either brushed PM or brushless motors. They
are necessarily feedback devices, using an encoder to
provide closed loop operation and position ref. Steppers can
be open loop, with no position ref from the motor itself.
Steppers have great low rpm torque, low max rpm. Good for
direct drive positioning. Geared servos (think R/C servos)
are good for your steering application.
Actually, a servo motor is not a kind of motor, but a feedback control
system that contain any sort of motor. I have seen servo systems made
with stepper, synchronous and induction motors, as well as with shunt
wound and permanent magnet motors. Somebody has probably made servos
out of series wound motors also, but stabilizing the control loop is a
bit harder because of the torque proportional to current squared
property of this type.
Yes, in the general sense you're right.
"Servo" referrs to a feedback technique for controlling position.
"Servo Mechanism" referrs to any mechanism using this technique.
"Servo Motor" referrs to any motor using it.
However, in hobby robotics, "servo" is generally used to mean
an R/C Servo Motor, rather than the more general concepts.
D. Jay Newman
Say u were to tell the servo motor (mechanism) to spin half way around
and i were to lightly grip the wheel to give it resistance. Would the
circuitry detect that and transfer more current to the motor to make
it overcome the resistance and spin to the desired location?
A stepper steps, a servo servoes. Servos seek a position that is
signalled to them by their data input, analog or digital, whereas
a servo can be made with a stepper or with a brush motor.
Slowly enough, and with good enough bearings, anything can move
-Steve Walz firstname.lastname@example.org ftp://ftp.armory.com/pub/user/rstevew
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