What is the difference between open loop and closed loop? I have seen this mentioned a few times. Also what is the difference between a stepper and a servo? Do they use the same controllers and everything else? Can I go with the cheaper option and at some point in the future just swap the stepper and servo or would that require swapping everything?
Really long winded reply.
The fundamental difference between a step motor and more conventional brushed DC servo motor is how the winding and magnets are arranged. (AC Servos are a bit different again).
In a step motor the magnets are in the armature and the winding around the outside. There are normally 2 windings that you can think of as oriented 90 degrees to each other. (in simple terms).
Now to turn the armature you apply a voltage across the two sets of windings and that creates a magnetic field. (Pretend North is up and East is to the right). The magnetized armature orients itself and now is locked solid.
To now make the armature turn all you have to do is change the magnetic field of one of the windings. For example so now South is up and East is still to the right. The armature reorients itself to the new field by turning a bit. Do that again but with the East/west winding making the east west poles change. And again the armature moves a bit. And so on.
The magnetic poles in the armature and the windings are set up so there are 200 locked positions when both windings are energized in the various relationships to each other.
And there's the rub. To make the motor, turn the voltage across each winding has to be reversed in a timely manner.
Hopefully this information hasn't put you to sleep yet.
Now you need a bit of electronics knowledge for the next part which will explain why there are issues with stepper motors.
The windings have two ratings; resistance and inductance. The resistance determines what maximum voltage can be applied to the winding to create current flow that doesn't burn out the wires. Let's say it's a 12V motor and can handle 1 amp which works out to 12W of heat for each winding or 24W for both so it will get warm. Also the number of amps of current and the number of turns of wire (called amp-turns) is equivalent to the torque applied to the armature. So that 12V 1A winding might make 100 oz-in of torque and obviously then only 10 oz-in if 0.1A is going through the winding.
The second parameter is inductance. That's measured in milli-henri and is what restricts how quickly the current builds up to that 1A after the 12V is applied. It's not instant and the higher the inductance the longer it takes. Not only that since there is current flowing in say a clockwise direction through the winding changing the applied 12V to make it flow in the counter clockwise direction means first the existing current has to change to 0A and then build up again to 1A. That also takes time
Finally and this is the same thing that limits the speed of DC Brushed motors is when you make a motor act like a generator it creates voltage and current. So that motor that is turning through the magnetic field is also producing voltage of the opposite polarity to the applied voltage. Once that voltage matches the applied voltage the motor has reached top speed. Double the voltage, the motor doubles in speed. (more or less). This is called back emf for backwards electromotive force.
So that controller for the stepper motor has to keep changing the winding current fast enough to make the motor turn but if it changes to the next polarity before 1A is reached then the motor doesn't reach full torque and it's both the RPM and the inductance that restrict that.
And there's the reason stepper motor torque falls off the faster it goes.
One way around that is to wind the stepper motor coils to be say 1.2V instead of 12V. If you apply 12V to it odds are you'd melt the wires so you have to sense the current in the winding and switch it off it once the current reaches 1A. When the current drops below 1A the voltage is turned on again creating a chopping effect. The up side of all this is twofold. First with 12V applied the current climbs to 1A way faster so the steps to change current direction can happen faster and the motor turns faster while having torque at higher speeds.
Remember that bit about the generator and creating the reverse polarity voltage? Well if that motor is turning fast enough that the windings are creating 10V there's still 2 volts left over to try and push 1A into the windings. So where before the 12V motor stopped having current flow through the windings as the back emf increased we can now overcome that with the higher voltage.
I won't touch on micro-stepping in this long discussion. It's there not to make the motor more accurate but to make it run smoother.
The amazing upside of step motors is that you can run them open loop. The controller gets a step pulse with the direction pin set for clockwise and the motor moves 1/200 of a turn and stops. Send it 200 step pulses and you get a full turn. If the pulses aren't too fast then you can easily turn a lead screw and move the carriage without feedback.
The DC motors just have a voltage applied and the bushes route the current through the commutator into the windings so that the armature turns inside the magnetic field. Stall a DC motor and burn it out. So we know the applied voltage is way higher than the winding rating. But what limits the DC motor speed is that back EMF and you don't have any control to move it 1 turn and then stop.
That's why those types of motors require an encoder and feed back position into the controller so that it knows when the motor has turned 200 encoder pulses for one revolution and then stop the motor.
Again it's more complicated than that for both stepper and servo motors. To be able to get up to speed they can't get there instantly so they need an acceleration, steady speed and then deceleration. That's what the trajectory planner does with the CNC systems. It knows the distance is 400 steps or two revolutions of the motor. It accelerates up to speed in the first 50 steps or encoder pulses, then runs for 300 steps at that speed and finally decelerates slowing down the steps or applied voltage so the motor turns more slowly and then stops..
Anyway, because the stepper motors run within their parameter range and torque they provide a simple inexpensive method of machine control.
