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On Fri, 21 Dec 2018 21:17:08 +0000, D.M. Procida wrote:

> I'm fairly new to servos.
>
> I see that they come in two basic types: continuous rotation (their
> speed of rotation is determined by the pulse width)
I know nothing about these rotation except that they may be intended for
use on models that require a winch, e.g. ships/yachts and all-terrain
vehicles.

> and fixed travel (usually 180 degrees, sometimes a bit more).
>
These are intended to operate control surfaces, throttles, etc on models
to move things that have a fixed travel and need to have consistent
positioning for a given signal. The signal is analogue, a constant height
pulse train at about 50 Hz where the pulse width determines the servo
position and is normally 1mS to 2ms wide. They all use three-wire
connections (ground, +ve and control signal) but the plugs and pin
assignments may still be manufacturer-specific, so check. Same for
operating voltage, originally a nominal 5v to suit a 4 cell NoCd stack,
though now most will be happy on a single Li-ion cell (3.7-4.2v).

These servos were originally developed for radio-control model aircraft
and originally were not mix and match between manufacturers who sold
complete systems, Now that servos are frequently made independently of
receivers and transmitters, their connections, voltages and pulse widths
have become a lot more standardised.

Also be aware that in this application (moving rudders, elevators and
ailerons), while accurate positioning under fairly light loads is
important, the ability of a servo to apply a controllable force can at
least important as precise positioning.

> They come in various standard (not sure how standard these actually are)
> sizes - micro, sub-micro, standard.
>
That depends mostly on the speed and size of model the servo was designed
for: sub-micro servos are used for slow, light indoor models, micro for
small outdoor models (park fliers, F3K gliders) and standard servos for
'Sport' RC models, typically 1-5kg and with a 2.5-10cc IC motor or
equivalent electric motor (i.e 0.25 - 1.5 hp).

> And they can have plastic or metal gears.
>
Plastic equals light and cheap, not very durable, but less positioning
accuracy. Metal is used in competition models where positioning accuracy
is important for both very fast models and for aerobatic competition
models. Metal gears are also a lot more vibration-resistant, so I'd be
surprised to see plastic gears used on helicopters unless they are very
small and light, i.e indoor models or park flyers.

You match any servo to what it will be used for and pay accordingly.

> What else should one be aware of?
>
The amount of torque you require. The higher the load the servo must work
against, the more torque it must output, both to move the control and
then to hold it stationary against external loads: remember that the load
on a control surface varies as the square of airspeed and, to a first
approximation, is proportional to model weight, IOW a 200mph pylon racer
with a piped racing 8cc motor is going to need high torque, metal geared
servos while a light-weight sheet foam indoor aerobatic model wants the
smallest, lightest servos you can find while retaining good accuracy.

> I have an application in which I'd
> like reasonable accuracy and precision, and for a 5-degree command (say)
> to have a 5-degree output.
>
If you use servos, your program and hardware needs to be capable of
generating accurate pulse widths that can resolve at least 100 steps
across the 1 - 2 mS pulse width range. The accuracy of the pulse width
determines the positional accuracy of the servo.

If this isn't a good match for your requirements, consider using geared
stepper motors, with or without a gray-scale for measuring the motor's
current position rather than keeping book on steps taken in either
direction from a known starting position.

> I've been fooling around with a pair of HiTec HS-55s - they're OK, but
> they are not what I'd call precise. One of them also takes the slightest
> excuse not to respond (possibly my abuse has damaged it).
>
Cheap and cheerful: intended for use on small, fairly slow and light
outdoor foamies: IOW 'park flyers'. 'Foamie' means the model's body and
flying surfaces are solid blocks of lightweight moulded plastic foam.
This is the sort of stuff used to package printers, laptops, etc. It will
have holes moulded in the body to take batteries, motor, receiver and
servos.

> If I wanted something stronger and more accurate, and don't need more
> speed, what sort of thing should I be looking at?
>
Think about:

- the loads the servo has to work against without stalling.
  High loads mean a bigger servo with a more powerful motor and
  metal gears and less accurate positioning when working against an
  external load.

- vibration. Use metal rather than plastic gears if there's
  usually vibration.

- space available for the servo. Small space, vibration, and high
  loads mean small, powerful servos with metal bodies and gears.
  These ain't cheap. Lots of space and no vibration may mean that
  a physically big, cheaper plastic geared servo can be used.

That said, you may be able to use something like the HS-55 for
prototyping and program development and only swap in more expensive/
specialised servos at the end.


--
Martin    | martin at
Gregorie  | gregorie dot org

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