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What is a Servo?

servoA Servo is a small device that incorporates a two wire DC motor, a gear train, a potentiometer, an integrated circuit, and an output shaft. Of the three wires that stick out from the motor casing, one is for power, one is for ground, and one is a control input line. The shaft of the servo can be positioned to specific angular positions by sending a coded signal. As long as the coded signal exists on the input line, the servo will maintain the angular position of the shaft. If the coded signal changes, then the angular position of the shaft changes.

A very common use of servos is in Radio Controlled models like cars, airplanes, robots, and puppets. They are also used in powerful heavy-duty sail boats. Servos are rated for Speed and Torque. Normally there are two servos of the same kind, one geared towards speed (sacrificing torque), and the other towards torque (sacrificing speed). A good example of this is the HS-625MG servo and the HS-645MG servo.

Servos come in different sizes but use similar control schemes and are extremely useful in robotics. The motors are small and are extremely powerful for their size. It also draws power proportional to the mechanical load. A lightly loaded servo, therefore, doesn’t consume much energy.

A typical Servo looks like a rectangular box with a motor shaft coming out of one end and a connector with three wires out of the other end. The three wires are the power, Control, and Ground. Servos work with voltages between 4 and 6 volts. The control line is used to position the servo. The servo motor comes in different sizes, which affect the overall size of the servo. The gears of a servo vary from servo to servo. Inexpensive servos have plastic gears, and more expensive servos have metal gears which are much more rugged but wear faster. The potentiometer of a servo is the feedback device. The electronics of a servo are pretty much the same in all servos, but the output shaft bearing of a servo has either a plastic on plastic bearing that will not take much side load or a metal on metal bearings that stand up better under extended use, or ball bearings which work best. We highly recommend ball bearing servos if your application demands heavy side loads.

Servos are constructed from three basic pieces; a motor, a potentiometer (variable resistor) that is connected to the output shaft, and a control board. The potentiometer allows the control circuitry to monitor the current angle of the servo motor. The motor, through a series of gears, turns the output shaft and the potentiometer simultaneously. The potentiometer is fed into the servo control circuit and when the control circuit detects that the position is correct, it stops the motor. If the control circuit detects that the angle is not correct, it will turn the motor the correct direction until the angle is correct. Normally a servo is used to control an angular motion of between 0 and 180 degrees. It is not mechanically capable (unless modified) of turning any farther due to the mechanical stop build on to the main output gear.

The amount of power applied to the motor is proportional to the distance it needs to travel. So, if the shaft needs to turn a large distance, the motor will run at full speed. If it needs to turn only a small amount, the motor will run at a slower speed. This is called proportional control.

Gears - Nylon, Karbonite, Metal

Gear Material Strength ComparisonWe offer four different gear materials in our Hitec line of servos. Nylon is the base and is great for standard applications and offers excellent wear resistance to keep the gear mesh tight for years. Karbonite is 4 times stronger than Nylon and offers even better wear resistance and super quiet operation. Standard Metal is perfect for rugged applications and is 16 times stronger than Nylon but will wear much faster. Our Titanium gears are simply awesome. Virtually no wear after years of use and 48 times stronger than Nylon gears. If your application is critical and you can not afford a gear failure, Titanium is the way to go.

Neodymium Motor Magnets

Ever wonder how Hitec packs so much power into these little servo cases? All of their cored servo motors use incredibly strong Neodymium rare earth magnets. Neodymium magnets are also known as “supermagnets” so its worth the little extra cost for the increased performance gained from using these magnets. In the “servo world” you get what you pay for.