To build our animatronic figures, we will be using servo motors as actuators. The term “servo motor” refers to electric motors which can be controlled in some fashion. We will be using a specific class of servo motor made for use in radio-controlled model planes, boats and cars. These are usually called “R/C servos” or just “servos” for short.
 
There are some basic things you need to know about servos. Servos are more than just motors. They are complete little control systems in a box. In addition to the motor, there are a series of gears which decrease the rate of rotation, while drastically increasing the torque, a sensor for measuring the current position of the shaft, and some circuitry to run the motor the right direction until it arrives at the commanded position.
 
The piece which rotates is replaceable, and is called a “servo horn”. Servos typically come with a selection of different horns, as well as some basic mounting hardware. In most modeling applications, you attach to the servo horn via rods through the holes. In robotic and animatronic applications, we generally attach metal pieces to the horns via screws. The horn itself fits on to a small gear on the output shaft. Sadly, different manufacturers use different gears, so horn sets are generally not interchangeable across different brands. The rubber pieces provide some protection from shock - a real problem when crashing a model airplane. We will not be using them.
 
In addition to their size specifications, servos are typically rated by their torque and speed. Torque is a measure of how much rotary force the servo can deliver, typically in ounce-inches (abbreviated “oz-in”). A servo which is rated for a torque of 48 oz-in can hold up 3 lbs. (48 oz. = 3 lbs.) one inch away from the shaft. Three inches away, it could only lift 1 lb. Note that it can hold it, but can’t move it. If you want to move a 1 lb. weight located three inches from the shaft, you will need significantly more torque than 48 oz-in.
 
Servo speed is specified as the time in seconds to turn 60 degrees (1/6 of a turn). Typical servos take about two tenths of a second to go 60 degrees. Most servos will turn a maximum of about 180 degrees. To go the full range will take at least three times as long, or about 0.6 seconds. There are some specialty servos which are designed to do multiple turns. Sail winch servos have a pulley wheel rather than a servo horn, and are designed to real in a cord over about three turns.
 
Both torque and speed depend on the voltage you run the servo at. Most servos (but not all) are designed to work from 4.8 volts to 6.0 volts. For convenience and longevity, we will be running ours at 5 volts. As such, you should expect the performance to be closer to the 4.8 V numbers rather than the 6.0 V numbers. (“V” is the abbreviation for volts.)
 
Servos have three wires - ground (or negative), power (or positive), and control. These are typically colored black or borwn, red, and white or yellow respectively. These go to a 3-pin connector. Although different manufacturers use slightly different connectors, almost all of them will work with standard servo controller boards. (Airtronics brand servos have power and ground reversed and will need to be modified before use.) This works because the boards don’t have any housing around the pins. The down side is that this means you can easily plug in the servos backwards or shifted by a pin, which can damage them and the controller. Make certain you have all your servos plugged in the right way before applying power!
 
Servos