Servomechanism

Information about Servomechanism



A servomechanism, or servo, is a device used to provide control of a desired operation through the use of feedback. An example of a servomechanism is a home furnace controlled by a thermostat.

A common type of servo provides is position control. Servos are commonly electrical or partially electronic in nature, using an electric motor as the primary means of creating mechanical force. Other types of servos use hydraulics, pneumatics, or magnetic principles. Usually, servos operate on the principle of negative feedback, where the control input is compared to the actual position of the mechanical system as measured by some sort of transducer at the output. Any difference between the actual and wanted values (an "error signal") is amplified and used to drive the system in the direction necessary to reduce or eliminate the error. An entire science known as control theory has been developed on this type of system.

Servos are found in many applications. They operate the throttle of engines that use a cruise control. CNC machines use servos to make the motion axes of a machine tool follow the desired tool path. Fly-by-wire systems in aircraft use servos to actuate the control surfaces that control the aircraft. Radio-controlled models use RC servos for the same purpose. Many autofocus cameras also use a servomechanism to accurately move the lens, and thus adjust the focus.

Typical servos give a rotary (angular) output. Linear types are common as well, using a screw thread or a linear motor to give linear motion.

Another device commonly referred to as a servo is used in automobiles to amplify the steering or braking force applied by the driver. In this form this device is not a true servo, but rather a mechanical amplifier.

In industrial machines, servos are used to perform complex motion.

RC servos

RC servos are servos typically employed in industrial robotics, automation, and radio-controlled models. They are also used to provide actuation for various mechanical systems such as the steering of a car, the flaps on a plane, or the rudder of a boat.

RC servos are comprised of a DC motor mechanically linked to a potentiometer. Pulse-width modulation (PWM) signals sent to the servo are translated into position commands by electronics inside the servo. When the servo is commanded to rotate, the DC motor is powered until the potentiometer reaches the value corresponding to the commanded position that it's ordered.

Due to their affordability, reliability, and simplicity of control by modern microprocessors, servo motors are often used in small-scale robotics applications.

The servo is controlled by three wires: ground (black/orange), power (red) and control (brown/other colour) and will move based on the pulses sent over the control wire. This wiring sequence is not true for all servos, for example the S03NXF Std. Servo is wired as brown(negative), red (positive) and orange (signal). The pulses sent over the control wire set the angle of the servo horn. The servo expects a pulse every 20 ms in order to gain correct information about the angle. The width of the servo pulse dictates the range of the servo's angular motion.

A servo pulse of 1.5 ms width will set the servo to its "neutral" position, or 90°. For example a servo pulse of 1.25 ms could set the servo to 0° and a pulse of 1.75 ms could set the servo to 180°. The physical limits and timings of the servo hardware varies between brands and models, but a general servo's angular motion will travel somewhere in the range of 180° - 210° and the neutral position is almost always at 1.5ms.

Servo motors are often powered from nickel-cadmium battery packs common to most RC devices. Voltage ratings vary from product to product, but most servos are operated at 4.8 V DC or 6.0 V DC (a 4 or 5 cell battery).

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For control theory in psychology and sociology, see .


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servomechanism, or servo, is a device used to provide control of a desired operation through the use of feedback. An example of a servomechanism is a home furnace controlled by a thermostat.

A common type of servo provides is position control.
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In physics, force is an action or agency that causes a body of mass m to accelerate. It may be experienced as a lift, a push, or a pull. The acceleration of the body is proportional to the vector sum of all forces acting on it (known as net force or resultant force).
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