Electronics notes/Motors and servos
Some motor types
- DC motor
- high speed - related to voltage, but there is no exact control or feedback
- low torque
- DC motor with gearhead
- lower speed and higher torque than the DC motor it's on
- Stepper motor
- slow speed, high torque
- but rotates almost freely unless held using the same power to rotate it
- control over individual steps (the only way to actually drive them)
- easily driven in both directions
- more complex to drive than DC motor - will cost a few bucks.
- Servomotor
- low to medium torque (for its size, which is usually small)
- contains a small DC motor, gearbox, position feedback (often a potentiometer), and control circuitry that does the actual positioning
- slowish speed (may take a few hundred milliseconds to sweep 180 degrees)
- can be positioned fairly exactly within a fixed range of rotation, such as 180 degrees, 90 degrees, 270 degrees, or sometimes a few rotations (varies between designs)
- most servos have three wires: power, ground, and control (where control is a pulsing signal)
- Brushless DC motor (BLDC motor)
- higher speed than basic DC. Seen in hard drives, some RC motors (e.g. quadcopters and such), and places where longer life without replacement is important
- no brushes required, because the rotor (the bit that moves) only has permanent magnets
- often three-phase designs
- some may have hall sensors for speed detection
- BLDC drivers need to know the rotational position to drive well. Some may sense the induction in the undriven coils, while others rely on existing hall sensors
- developed in the seventies, once semiconductors made the more complex driving feasible(verify)
- http://en.wikipedia.org/wiki/Brushless_DC_motor
- Universal motor
- Made to operate on either DC or AC (specific design can still vary, but not a lot)
- Mostly like a DC motor, but modified somewhat to allow AC operation
- AC speed control works a little differently
- they are high speed/torque for their size so see a lot of uses, from washing machines, but also much smaller tools as well
- but not as efficient as some other types
- https://en.wikipedia.org/wiki/Universal_motor
- Synchronous motor
- one you can run on AC (mains-voltage or lower)
- usually constant speed, in that rotation synchronizes with the frequency of the supply current (you could feed it inverter output but there are often better alternatives)
- there are drivers from DC, basically generating waveforms
- there are simpler, cheaper variants, often used in some simpler analog devices.
- there are also three-phase
https://en.wikipedia.org/wiki/Synchronous_motor#Non-excited_motors
TODO: expand list to include more common types
Cassette tape motors
Cassette tape motors are somewhat specialized variant
While typically a basic DC motor at their core, they need to run at well controlled speed to not introduce warbley pitch changes, even under slightly varying mechanical load, and in some cases battery voltage that lowers over time.
So these designs try to somehow aim for a constant RPM, (and will probably spec the motor with a bit more torque than typically needed (verify))
If there are no components on the PCB, towards a 2-wire motor,
it's likely that the motor is itself a constant-speed design.
One such motor design is electromechanical, basically uses one or two weighed leaf springs that itself carry the current, which at high enough speed pull away and electrically disconnects the motor. Considering how simple and cheap this feedback loop is, is it works surprisingly decently, and doesn't care about voltage level or ripple so much as long as the energy delivered is enough for the target speed.
It has various small flaws, though, such as speed still varying with temperature, and it's mainly seen in cheaper tape designs.
A 4-wire motor probably means the other two wires are likely tachometer (speed sensor), and the feedback and speed control circuits are likely external rather than part of the motor (controller) design.
Another good indication of this is that there is a trimpot on the PCB in the motor circuit (possibly two, if it does high speed dubbing), which will be speed calibration.
Probably the motor's torque is overspecced to make it easier to stay constant even when physical load varies somewhat.
Presumably there are also 2-wire motors where most of this is internal(verify),
which will then often expose something to tune the speed.
Servomotor notes
Servos are motors that move to a specific position, e.g. relatively small ones for ailerons in model airplanes, or stronger ones elsewhere.
Servos have a controller, which is trying to move the motor until it arrives at the communicated position, using an internal position sensor (cheaper servos are basically a DC motor, potmeter for a sensor, and that control).
One upside is that you don't need to add external position control to get decent position control.
Another upside is that under varying movement it will continually try to correct its position (it'll use current roughly proportional to load).
Most have a range of rotation, something like 180 degrees or 270 degrees, or sometimes a few rotations, after which they hit a mechanical limit.
Trying to position them beyond their mechanical range probably damages the servo.
Continuous rotation servos exist, and can be hacked from some types of servos. They are functionally similar to stepper motors but with less torque.
Position updates: Since servos have control circuitry, they just expect to be told the position.
This is communicated via the width of a pulse, on the order of 0.5-2ms long.
You can send these pulses no more often than every ~20ms or so, which means it can be updated/repositioned at most ~40-50 times per second. Updating it faster won't work, and in some designs can even damage the servo
Details vary, though, so always check spec sheets.
Digital servos work the same, but have cleverer-than-minimal cicruits,
meaning they can be a little more accurate, and apply more power, might move faster, might be updatable faster, at the expense of a little more power consumption.
Check the specs about all of that.
Current draw for small hobby servos may be a few mA idle, perhaps 100mA no-load, and something towards 1A for stall current.
A servo controller usually refers to an IC that remembers positions for multiple ICs for you, so that you can tell something else to do the job for you, handy for DIY where you don't have hardware to do that (some PWM is geared at servos, simpler variants are not) or need more than a few channels.
See also:
You may also care about how servo signals are communicated in RC aircraft control
DC motor controller notes
To control a regular DC electromotor to spin both ways you need circuitry to feed energy through it in both directions. Four relays will allow this, but four transistors (an H-bridge) is simpler and cheaper, and with four extra diodes you can prevent feedback spikes from damaging your electronics.
You may care about motor driver ICs to do this more easily, and possibly more efficiently.
(link)
Speed control is done not through varing voltage (which works, but not very predictably), but instead through pulsing full voltage, often specifically PWM.
Simple H-bridges and driver chips will not do this themselves, but you can feed them such a pulsed signal. This works out as a sort of convenient modular design.
Depending on your means of control it may be convenient to get a serially controlled PWM driver setup on a board.
Stepper motor controller notes
BLDC notes
Brushless motors use fixed magnets and somewhat more controlled EM fields to work against them, so that they can avoid brushes.
Brushless are a little more more complex, a little pricier, and a little more complex to contol, and lower torque at highest speeds(verify), and harder to fix, but have various physical/engineering upsides. (e.g. more torque per weight so smaller for the same gotfor, no brush sparking, not as EM-noisy, not as acoustically noisy, can cool a little better)
Since in various uses these also require fairly precise speed sensing - so controllers tend to have that built in.
One way is a hall effect sensor sensing a magnetic spot.
Another is using undriven coils as they act as induction sensors (back-EMF, really). (This is why you can e.g. this project turn a hard drive into a jog sensor, sensing speed and direction)