RAILMODEL ELECTRONICS (2016)

Servo motors and how to use them

operation

The servo motors position is controlled by a pulse at its input. This has to be a positive going pulse of 1.0 to 2.0 milliseconds which will turn the output arm through 180 degrees. Voltage required is 5 to 6 volts, up to 1 amp current

 

type economy

There are several types available. From precision engineered with metal gears to plastic geared , light weight and cheaply made that do not have a great life expectancy.

uses

Servos are used in model aircraft and boating, in model cars, drones and toys. Increasingly in the model railway field too. although the torque available is quite high they do not like being jammed out of position. They will attempt to get to the end point, possibly stripping the gears but definitely causing a large current drain on the battery.

 

control

control is usually by a variable input voltage (0 to 5volts) The servo arm being positioned at a point relative to the voltage. This ratio is usually adjustable. Also adjustable will the speed of movement.
There can be uncontrollled movement at switch on or sporadic movement if the controlling electronics is not robust.
The control pulse should occur at about 20 millisecond intervals and the output should be terminated to +5 volts through a 10 Kohm resistor, a low value capacitor may also help. across the output to the servo, 0.001 mfd will do.

 

life

Mean Time Before Failure (MTBF) is only published for good quality servos. Typical running time for model aircraft use is 40 to 80 hours - intermittent use.
The cheap ones, called 9g's ($6 worth) seem to be considered throw away's - they are expected to die.

Installation

Mount them firmly, on plywood in aircraft, plastic mounts or custom metal brackets. A 3 wire light cable connects them to the control electronics and this can be extended some metres.
The control arm comes with holes along its length that a wire can be put into that runs to the controlled item. Movement should be free or the battery might drain quicker. The servo must not be overdriven, chattering or gear damage will result.
Not usually a problem on model aircraft control surfaces but could be a problem with model railway switches (points) where the moving rail comes up against a solid rail, end point adjustments are usually provided to set the movement up.

There should be an omega loop or _A_ or Ü bend in the servo wire so that a small amount of blocking can be tollerated, but only a very small bit else the back pressure of the spring will work against the servo and make it chatter.
With 2 or more servos running from the same power source some chattering may be present if the supply capacity is not good enough.

links

Some servos from Adafruit
How do servos work
Robotic servo descriptions
Servo motor shop