Introduction to Servo Motors
What is a servo motor?
Servo motors (or servos) are self-contained electric devices (see Figure 1 below) that rotate or push parts of a machine with great precision. Servos are found in many places: from toys to home electronics to cars and airplanes. If you have a radio-controlled model car, airplane, or helicopter, you are using at least a few servos. In a model car or aircraft, servos move levers back and forth to control steering or adjust wing surfaces. By rotating a shaft connected to the engine throttle, a servo regulates the speed of a fuel-powered car or aircraft. Servos also appear behind the scenes in devices we use every day. Electronic devices such as DVD and Blu-ray DiscTM players use servos to extend or retract the disc trays. In 21st-century automobiles, servos manage the car's speed: The gas pedal, similar to the volume control on a radio, sends an electrical signal that tells the car's computer how far down it is pressed. The car's computer calculates that information and other data from other sensors and sends a signal to the servo attached to the throttle to adjust the engine speed. Commercial aircraft use servos and a related hydraulic technology to push and pull just about everything in the plane.
|Figure 1. This assortment of servos is available in stores and by mail order. Servos range in price and application.|
And of course, robots might not exist without servos. You see servo-controlled robots in almost every movie (those complex animatronic puppets have dozens of servos), and you have probably seen a number of robotic animal toys for sale. Smaller laboratory robots also use servos to move their joints. Hobby servos come in a variety of shapes and sizes for different applications. You may want a large, powerful one for moving the arm of a big robot, or a tiny one to make a robot's eyebrows go up and down. Figure 2 below shows two sizes you can find in a hobby store— an inexpensive common size and a more expensive miniature one.
|Figure 2. Two common servo sizes. The standard servo on the left can range in power or speed to move something quickly, or it can accommodate a heavier load, such as steering a big radio-controlled monster truck or lifting the blade on a radio-controlled earthmover toy. The miniature servo is about the size of a U.S. quarter and is intended for applications where smallness is a critical factor but a lot of power is not.|
How does a servo motor work?
The simplicity of a servo is among the features that make them so reliable. The heart of a servo is a small direct current (DC) motor, similar to what you might find in an inexpensive toy. These motors run on electricity from a battery and spin at high RPM (rotations per minute) but put out very low torque (a twisting force used to do work— you apply torque when you open a jar). An arrangement of gears takes the high speed of the motor and slows it down while at the same time increasing the torque. (Basic law of physics: work = force x distance.) A tiny electric motor does not have much torque, but it can spin really fast (small force, big distance). The gear design inside the servo case converts the output to a much slower rotation speed but with more torque (big force, little distance). The amount of actual work is the same, just more useful. Gears in an inexpensive servo motor are generally made of plastic to keep it lighter and less costly (see Figure 3 below). On a servo designed to provide more torque for heavier work, the gears are made of metal (see Figure 4 below) and are harder to damage.
|Figure 3. The gears in a typical standard-size servo are made of plastic and convert the fast, low-power motion of the motor (on the right) to the output shaft (on the left).|
|Figure 4. In a high-power servo, the plastic gears are replaced by metal ones for strength. The motor is usually more powerful than in a low-cost servo and the overall output torque can be as much as 20 times higher than a cheaper plastic one. Better quality is more expensive, and high-output servos can cost two or three times as much as standard ones.|
With a small DC motor, you apply power from a battery, and the motor spins. Unlike a simple DC motor, however, a servo's spinning motor shaft is slowed way down with gears. A positional sensor on the final gear is connected to a small circuit board (see Figure 5 below). The sensor tells this circuit board how far the servo output shaft has rotated. The electronic input signal from the computer or the radio in a remote-controlled vehicle also feeds into that circuit board. The electronics on the circuit board decode the signals to determine how far the user wants the servo to rotate. It then compares the desired position to the actual position and decides which direction to rotate the shaft so it gets to the desired position.
|Figure 5. The circuit board and DC motor in a high-power servo. Did you notice how few parts are on the circuit board? Servos have evolved to a very efficient design over many years.|
Imagine you are playing catch with a friend on a sports field. You stand at one end and want your friend to go out for a long throw. You could keep calling out "farther, farther, farther" until she got as far away as you wanted. But if she went out farther than you can throw, you would have to call out "closer" until she got back to the right spot. If she were a simple motor in a robot arm and you were the microprocessor, you would have to spend some of your time watching what she did and giving her commands to move her back to the right spot (this is called a feedback loop). If she were a servo motor, you could just say "go out exactly 4.5 meters" and know that she would find the right spot. That is what makes servo motors so useful: once you tell them what you want done, they do the job without your help. This automatic seeking behavior of servo motors makes them perfect for many robotic applications.
Types of servo motors
Servos come in many sizes and in three basic types: positional rotation, continuous rotation, and linear.
Positional rotation servo: This is the most common type of servo motor. The output shaft rotates in about half of a circle, or 180 degrees. It has physical stops placed in the gear mechanism to prevent turning beyond these limits to protect the rotational sensor. These common servos are found in radio-controlled cars and water- and aircraft, toys, robots, and many other applications.
Continuous rotation servo: This is quite similar to the common positional rotation servo motor, except it can turn in either direction indefinitely. The control signal, rather than setting the static position of the servo, is interpreted as the direction and speed of rotation. The range of possible commands causes the servo to rotate clockwise or counterclockwise as desired, at varying speed, depending on the command signal. You might use a servo of this type on a radar dish if you mounted one on a robot. Or you could use one as a drive motor on a mobile robot.
Linear servo: This is also like the positional rotation servo motor described above, but with additional gears (usually a rack and pinion mechanism) to change the output from circular to back-and-forth. These servos are not easy to find, but you can sometimes find them at hobby stores where they are used as actuators in larger model airplanes.
Selecting a servo motor
When starting a project that uses servos, look at your application requirements. How fast must the servo rotate from one position to another? How hard will it have to push or pull? Do I need a positional rotation, continuous rotation, or linear servo? How much overshoot is allowable? The less you pay for the servo, the less mechanical power it will have to muster and the less precision it will have in its movements. You can pay a bit more and get one that moves quickly, but it may not have a lot of power. You can also buy one that will pull or push large loads, but it may not move quickly or precisely. Manufacturers' websites and online hobby guides will have a lot of this information you can use to compare models. You will also find that hobby stores have a selection of servos and can usually help you decide which one is right for your project and budget.
Controlling a servo motor
Servos take commands from a series of pulses sent from the computer or radio. A pulse is a transition from low voltage to high voltage which stays high for a short time, and then returns to low. In battery devices such as servos, "low" is considered to be ground or 0 volts and "high" is the battery voltage. Servos tend to work in a range of 4.5 to 6 volts, so they are extremely hobbyist computer-friendly.
Have you ever picked up one end of a rope that was tied to a tree or held one end of a jump rope while a friend held the other? Imagine that, while holding your end of the rope, you moved your arm up and down. The rope would make a big hump that would travel from your end to the other. What you have done is applied a pulse, and it traveled down the rope as a wave. As you raise your hand up and down, if you keep your hand in the air longer, someone watching this experiment from the side would see that the pulse in the rope would be longer or wider. If you bring your hand down sooner, the pulse is shorter or more narrow. This is the pulse width. If you keep your end going up and down, making a whole bunch of these pulses one after another, you have created a pulse train (see Figure 6 below). How often did you raise and lower your end? This is the frequency of your pulse train and is measured in pulses per second, or Hz (abbreviation of "hertz").
Note: The microprocessor in your computer uses pulses from special clock circuitry to get the job done. Have you heard of your computer speed referred to as something like 1.7 gigahertz (GHz)? This is a way of saying that the pulses are coming at 1.7 billion pulses per second, or 1,700,000,000 Hz. Imagine trying to move your rope that fast!
|Figure 6. An example of a pulse train you might generate to control a servo, as shown in a screen capture from an inexpensive digital oscilloscope, an instrument for observing voltages). Here, a pulse is generated once every 20 milliseconds, or at about 50 Hz. In this example, the pulse width is about 2 milliseconds, which would have a servo rotate almost all the way to one end of its rotation. An oscilloscope is incredibly useful for testing and debugging systems that use servos.|
Your servo must be connected to a source of power (4.5 to 6 volts) and the control signal must come from a computer or other circuitry. Each servo's requirements vary slightly, but a pulse train (as in Figure 6 above) of about 50 to 60 Hz works well for most models. The pulse width will vary from approximately 1 millisecond to 2 or 3 milliseconds (one millisecond is 1/1000 of a second). Popular hobbyist computers such as the ArduinoTM have software commands in the language for generating these pulse trains. But any microcontroller can be programmed to generate these waveforms. A system that passes information based on the width of pulses uses pulse width modulation (or PWM) and is a very common way of controlling motor speeds and LED brightness as well as servo motor position.
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