Electronics notes/Motors and servos

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This is for beginners and very much by a beginner. It's meant to try to cover hobbyist needs, and as a starting point to find out which may be the relevant details for you, not for definitive information.

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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)
  • 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)
    • more complex to drive than DC motor - will cost a few bucks.
  • Servomotor
    • low to medium torque (respectable for its size)
    • contains a small DC motor, gearbox, position feedback (often a potentiometer), and control circuitry that does the actual positioning
    • slow 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)
    • often three-phase motors
    • some may have hall sensors
    • 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
    • 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 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



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 positional motors, for example positioning ailerons in model airplanes, and some of the larger models can be useful in various other DIYing robots.

Since servos themselves adjust until arriving at the position you told them to go to, they are a convenient option when you need exact position control.


Servos have a given range of rotation and a mechanical limit. Trying to position them beyond there may damage the servo. In most situations you'll find that you don't need all the range, but if you do you may want to do some calibrating.

Continuous rotation servos exist, and can be hacked from various servos. They are functionally similar to stepper motors but typically with less torque.


In general, drawn current is proportional to load.


Humming can happen for various reasons:

  • have some some small force to act against fairly continuously
  • servo driven to its mechanical limit
  • digital servos seem more likely to hum, apparently largely because they respond faster
  • more?


Servos have control circuitry that do the actual work of positioning. You tell servos the angle you want by sending them a pulse of a specific length, on the order of 0.5-2ms long (details vary).


A typical servo expects a control pulse every ~20ms or so, which means it can be updated/repositioned at most ~40-50 times per second. Updating less often simply makes it more sluggish (verify), not updating basically means it stops attempting to change position.

Depending on design, updating faster than 50 times per second may short out the servo's H-bridge (Basic servo controllers work by proportionally stretching the control pulse, and put that onto an H-bridge that controls the motor), or, if protected against this somewhat obvious fragility, trying to update more often simply doesn't work.


Digital servos refer to the servo's internal control circuitry. They are controlled the same way, but drive their own motor more cleverly, which allows them to be a little more accurate, respond a little faster, and apply more power - but at the expense of a little more power consumption. You can often also update them faster(verify), but see their specs before you do.


A servo controller controller can be convenient when you don't want to dedicate a lot of time to doing the pulsing. Such a controller is a simple IC that remembers the position (typically communicated via serial communication), and does the pulsing work for you. This can be convenient for microcontrollers, particularly when you don't have hardware PWM, when controlling more than one or two servos, and when you don't want to spend a lot of time controlling servos.


See also:

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 (coming from the dynamo effect) from damaging your electronics. MOSFETs do it more efficiently, and there are also driver ICs to do all this for you. (link)

Speed control is done not through noncontinuous voltage (which sort of works, but not too predictably), but instead through pulsing full voltage. Simple H-bridges and driver chips will not do this themselves. They can probably take pulsed control signals, but depending on your means of control it may be convenient to get slightly more complex drivers that do this for you. These are usually ICs with a serial interface. (link)


Stepper motor controller notes

BLDC notes

The coils themselves act as hall sensors, having a small voltage as the motor is turned. Because there are three of them you'll see three sinusoids, 120 degrees apart. You can figure out speed and direction - so you can DIY these into jog wheels / rotary encoders kind of things.

See e.g. this project, which uses three comparators (one cheap LM324 quad op amp), and use a microcontroller to deal with its three binary-level inputs to figure out which direction it is spinning in, and how fast.


Synchronous motor notes