How Servo Motors Work
Servo motors have been around for a long time and are utilized in many applications. They are small in size but pack a big punch and are very energy-efficient. Electronic enthusiasts looking to learn more about these electronic components may wonder what a servo motor is. Servo motors are specialized motors with a feedback mechanism allowing precise control of angular or linear position, speed, and acceleration. They’re used to operate remote-controlled or radio-controlled toy cars, robots, and airplanes. These motors are also used in industrial applications, robotics, in-line manufacturing, pharmaceutics, and food services.But how does a servo motor work? The servo circuitry is built inside the motor unit and has a positionable shaft, usually fitted with a gear (as shown below). The motor is controlled with an electric signal, which determines the amount of movement of the shaft.
Inside a Servo Motor
You must look under the hood to fully understand what a servo motor is and how it works. Inside there is a pretty simple set-up: a small DC motor, potentiometer, and a control circuit. Gears attach the motor to the control wheel. As the motor rotates, the potentiometer's resistance changes, so the control circuit can precisely regulate how much movement there is and in which direction.When the motor shaft is at the desired position, the power supplied to the motor is stopped. If not, the motor is turned in the appropriate direction. The desired position is sent via electrical pulses through the signal wire. The motor's speed is proportional to the difference between actual and desired positions. So, if the motor is near the desired position, it will turn slowly. Otherwise, it will turn fast. This is called proportional control. This means the motor will only run as hard as necessary to accomplish the task at hand, a very efficient little guy. That explains what a servo motor is and the mechanics behind its operation.
Servo Motor Control
Servos are controlled by sending an electrical pulse of variable width, or pulse width modulation (PWM), through the control wire. There is a minimum pulse, a maximum pulse, and a repetition rate. A servo motor can usually only turn 90° in either direction for a total of 180° movement. The motor's neutral position is defined as the position where the servo has the same amount of potential rotation in both the clockwise or counter-clockwise direction. To better understand what a servo motor is, the PWM sent to the motor determines the position of the shaft, and based on the duration of the pulse sent via the control wire, the rotor will turn to the desired position. The servo motor expects to see a pulse every 20 milliseconds (ms), and the length of the pulse determines how far the motor turns. If you’re still wondering how does a servo motor work, here’s an example - a 1.5ms pulse will make the motor turn to the 90° position. Shorter than 1.5ms moves it in the counter-clockwise direction toward the 0° position, and any longer than 1.5ms will turn the servo in a clockwise direction toward the 180° position.
When these servos are commanded to move, they will move to the position and hold that position. If an external force pushes against the servo while the servo is holding a position, the servo will resist moving out of that position. The maximum amount of force the servo can exert is called the torque rating of the servo. Servos will not hold their position forever, though; the position pulse must be repeated to instruct the servo to stay in position. Understanding the technical aspects of a servo motor and how it works is vital before choosing the right type for your applications.
Types of Servo Motors
Servo motors come in various types, shapes, and sizes. Joseph Farcot used the term "servo" for the first time in 1859 when he used steam to control the rudders on a ship. A servo has three parts: a motor and feedback device, and control electronics. A servo motor is one of these three parts. If you need feedback, you can use a potentiometer, a Hall-effect device, a tachometer, a resolver, an encode, a linear transducer, or any other sensor that fits your needs. The last parts of the servo system are the control electronics that power the motor, and the feedback data and the command reference are compared to ensure the servo motor is working as it should. This introduces the concept that underpins what a servo motor is.Still, how does a servo motor work? It’s vital to understand that these motors come in two types: AC and DC. AC servos can handle more current surges and are usually used in industrial machines, so they are more likely to be found there. DC servos aren't built to take a lot of power surges and are usually better for smaller applications. In general, DC motors are cheaper than their AC counterparts. Some of these have been built for continuous rotation, making it easy to get your robot moving. They have two ball bearings on the output shaft to reduce friction and make it easier to get to the potentiometer that changes the rest point.
Servo Motor Applications
To fully understand what a servo motor is, it’s also important to understand its applications. Servos are used in radio-controlled airplanes to position control surfaces like elevators, rudders, walking robots, or grippers. They are small, have built-in control circuitry, and have good power for their size.In food services and pharmaceuticals, the tools are designed to be used in harsher environments, where the potential for corrosion is high due to being washed at high pressures and temperatures repeatedly to maintain strict hygiene standards. Servos are also used in in-line manufacturing, where high repetition yet precise work is necessary.
Of course, you don't have to know how a servo works to use one, but as with most electronics, the more you understand what a servo motor is, the more doors open for expanded projects and projects' capabilities. Whether you're a hobbyist building robots, an engineer designing industrial systems, or constantly curious, where will servo motors take you? Want to learn more about electronic components? Explore Jameco Electronics.
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Servo Motor Buyer GuideAbout the Author
Ryan Winters serves as Product Manager at Jameco Electronics, an organization based in the San Francisco Bay Area of California, where he was born and raised. A self-directed learner, Mr. Winters dedicates his personal time to refining practical skills through hands-on work with automobiles, computers, and electronic devices, as well as experimental robotics projects. Through these diverse areas of technical exploration and problem-solving, he further develops the analytical and creative abilities that serve him well in his professional role, where he applies expertise gained from years of independent study and tinkering to benefit Jameco Electronics and their customers.