Electronics notes/Motors and servos

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This is for beginners and very much by a beginner.

It's intended to get an intuitive overview for hobbyist needs. It may get you started, but to be able to do anything remotely clever, follow a proper course or read a good book.


Some basics and reference: Volts, amps, energy, power · Ground · batteries · resistors · changing voltage · transistors · fuses · diodes · varistors · capacitors · inductors · transformers · baluns · amplifier notes · frequency generation · skin effect


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Actuators: General actuator notes, circuit protection · Motors and servos · Solenoids

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Audio notes: See avnotes

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See also Category:Electronics.


This article/section is a stub — probably a pile of half-sorted notes, is not well-checked so may have incorrect bits. (Feel free to ignore, fix, or tell me)

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
    • 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 (respectable for its size, anyway)
    • 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


  • 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 motors 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.

These are some sort of constant-RPM designs, (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 varying with temperature, and it's mainly seen in cheaper tape designs.

See e.g. [1] [2]


A 4-wire motor probably means the other two are likely tachometer, 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

This article/section is a stub — probably a pile of half-sorted notes, is not well-checked so may have incorrect bits. (Feel free to ignore, fix, or tell me)

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 is that under varying forces it will try to correct its position (it'll use current roughly proportional to load).


Most have a range of rotation like 180 degrees, 270 degrees, or sometimes a few rotations, after which they hit a mechanical limit. (Continuous rotation servos exist, and can be hacked from some types of servos. They are functionally similar to stepper motors but with less torque)

Trying to position them beyond their mechanical range may damage the servo.


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 no faster than every ~20ms, 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 clever rather than minimal cicruits, meaning they can be a little more accurate, faster, and apply more power, at the expense of a little more power consumption. Some you can also also update more often. Check the specs.


Current draw for small hobby servos may be a few mA idle, perhaps 100mA no-load, and much of the way 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.

link

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)

Synchronous motor notes