Hello.
If I want to move a wheel only at some specific angles , which would be the best solution? A (continous) servo and a stepper could achieve the same goal, couldn't they? Why choose one or another? I can’t see the difference in its usefulness...
Thanks!
You cannot control the position with a continuous rotation servo - only speed and direction. If you only need a max of 3 revolutions a sail winch servo will give position control. I think there is one that can do 6 revolutions.
A stepper motor is designed to give position control. You may find some useful stuff in Stepper Motor Basics
You can also attach a rotary encoder to a shaft to provide position feedback but the programming will be much more complex.
...R
They are nothing like each other. Perhaps you should say what you want to power and ask
the pros and cons of each motor for that application?
Sorry, I mispelled the question. I wanted to say a NON-continous servo (i.e a "standard" servo).
Both devices (these t"standard" servos and stepper motors) can be moved to a specific position, hence my doubt.
The only difference I can see is while standard servo only can move 180º, and stepper can move 360º but I don't think that's all. I know to the way of programming is different, but that's no important for me: my question is about why choose one or another if I want to control the position of a shaft.
Thanks a lot for your patience
While you can technically tell a stepper motor to move to a certain position (by a certain number of steps) - there are problems with this approach:
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With no feedback of position, you have no way to really know when you have reached the proper angle.
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Steppers can miss steps - depending on a number of factors (but mainly due to excess load and over acceleration). Without feedback, you won't know this.
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Steppers can gain steps - mainly by trying to decellerate a mass, but not stopping it quite in time...
In short - without some way of measuring the absolute angle of the shaft on the stepper, you won't know which angle it is really at. If the stepper is paused, and something nudges the mass to rotate the shaft to a different angle, without feedback, you won't know this has happened.
So - you need to add that feedback in. For certain tasks (and designs), this can be just some simple "homing switches" that the stepper (or whatever it is moving, most likely) can actuate - to tell the controller "I am at my 'zero' location". You typically see this on CNC machines (like CNC routers and 3D printers, for instance); in this case, what is typically done is to specify and design the machine so that the steppers are much beefier than what they are ultimately moving (or gearing them down in some manner to increase torque and help to prevent back-driving). Doing this (and adding "homing") keeps the chances of missing a step to a minimum (and hopefully zero).
In the case of a true non-continuous servo - it has a feedback mechanism (using a potentiometer and a voltage comparator circuit, in an analog servo - and something more akin to an analog input and a microcontroller in many digital servos - some of which, btw, can potentially be hacked). Usually, though, you have no way to know what the angle the servo actually is at - you just send a command, and hope that the servo goes to the right spot (which in general - to a certain margin of error - it will).
So-called "Smart Servos" allow you to get true position information from them; in fact, it is possible to move them around, read the values, then "play back" the motion to them - in a relatively easy fashion. This capability, though, comes at a fairly steep cost (just so you are aware).
It is also possible to directly control a motor and cause it to swing to a desired angle (usually through monitoring coil voltages of unpowered coils - typically in brushless DC motors) - I have seen videos of it, but nobody has revealed the secrets of it yet (likely because it is very difficult, and also a very valuable form of IP if you know how to do it - akin to knowing how to implement software-defined traction control and differential capability in an electric automobile).
Other ways of monitoring shaft angles - you can use a variable capacitor, or a light sensor (and a radially shaded wheel), or a continuous rotation precision potentiometer (brand new - these are very expensive, tho). For instance - galvonometers for laser shows are essentially low-mass motors that are fixed in maximum swing angle - but move VERY quickly - they generally use a capacitive plate position sensor, or a reflective light sensor to achieve this.
Hope this gives you some help - or at least some ideas to research.
Osqui:
Sorry, I mispelled the question. I wanted to say a NON-continous servo (i.e a "standard" servo)......
Thanks a lot for your patience
Patience is certainly needed when you make a mistake like that.
Did you read what I said about sail-winch servos?
As @cr0sh has said, a servo know where it is. A stepper motor does not.
...R
Osqui:
Hello.
If I want to move a wheel only at some specific angles , which would be the best solution? A (continous) servo and a stepper could achieve the same goal, couldn't they? Why choose one or another? I can’t see the difference in its usefulness...
Thanks!
The simple answer is you choose what can deliver the requirements - but that's a very vague
set of requirements (without any numbers). And if you say "best" that also means you have a set
of costs to consider - ie you have to be clear which requirements matter most. So I don't think
there is any answer to the question you asked.
What wheel?
Which angles?
How fast?
How much torque?
What power is available?
How quiet should it be?
How much inertia and friction?
How accurate do the angles need to be?
What is the budget?
For some answers to these questions only a very expensive (> $1000) industrial servomotor could work,
for others a hobby servo is fine, for others a stepper would be better, and for others there is no solution
because its physically impossible.
Ooooh, thanks a lot!!!
@crOsh , your wonderful explanation is what I needed! I now understand a lot better this topic. Thanks, thanks thanks!! THANKS!!
@Robin2 , sorry, it was a so stupid error, I really apologize for your time. And yes, I've read (and printed!) your fantastic article about Stepper Basic...it's very very complete and I'll return to read it more times for complete understand it.
Thanks a lot again.
You are all the best.
Oooh, I've just seen you, @MarkT, have posted another answer !! What synchonicity!! Thanks for pointing out these considerations!