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Open-loop Servomechanisms, Overshoot And Hunting, Mechanical Inertia And ServomechanismsEnabling servomechanisms

The name servomechanism means, quite literally, slave machine. A servomechanism is a physical device that responds to an input control-signal by forcing an output actuator to perform a desired function. Servomechanisms are often the connection between computers, electronics, and mechanical actions. If computers are the brains, servomechanisms are the muscles and the hands that do physical work. Servomechanisms use electronic, hydraulic, or mechanical devices to control power. Servomechanisms enable a control operator to perform dangerous tasks at a distance and they are often employed to control massive objects using fingertip control.

The power-steering assistance accessory on almost all automobiles is a familiar example of a servomechanism. Automotive power steering uses hydraulic fluid under great pressure to power an actuator that redirects the wheels of a car as needed. The driver gently turns the steering wheel and the power-assist servomechanism provides much of the necessary energy needed to position the wheels.

The Boeing 777 is the first heavy jet plane engineered to fly with all major flight-control functions managed by servomechanisms. The design of this revolutionary plane is based on the so-called "fly-by-wire" system. In normal flight a digital signal communicates the pilot's instructions electrically to control servomechanisms that position the plane's control surfaces as needed.

High-performance airplanes need special servomechanisms called flight-control systems to compensate for performance instabilities that would otherwise compromise their safety. The aerodynamic designs that optimize a plane's performance sometimes cause instabilities that are difficult for a pilot to manage.

A plane may have a tendency to pitch up and down uncontrollably, or yaw back and forth under certain conditions. These two instabilities may combine with a third problem where the plane tends to roll unpredictably. Sensors called accelerometers pick up these oscillations before the pilot is aware of them and servomechanisms introduce just the right amount of correction needed to stop the unwanted activity. The servos that perform this magic are called pitch dampers, yaw dampers, and roll dampers. Their effect is to smooth out the performance of a plane so that it does only what it should. Without servomechanism technology flight-control systems would be impossible and the large safe aircraft we take for granted would be impractical.

Various servomechanisms provide the enabling connection between data and mechanical actions. If all servomechanisms were to disappear from technology overnight, our world would be much less comfortable, much less safe, and certainly less convenient.

See also Computer, digital.



Asimov, Isaac. Understanding Physics. New York: Dorset, 1988.

Faillot, J.L., ed. Vibration Control of Flexible Servo Mechanisms. New York: Springer-Verlaag, 1994.

Johnson, Eric R. Servomechanisms. New York: Prentice-Hall, Inc., 1996.


Albus, James S., and John M. Evans, Jr., "Robot Systems." Scientific American (1976).

Tustin, Arnold. "Feedback." Scientific American September, 1952.

Donald Beaty


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—Information processed as encoded on or off data bits.


—Devices using active components to control power.


—A signal proportional to the servomechanism correction.


—Comparing output and input to determine correction.


—Repetitious failure of a servomech anism's response.


—Power transfer using fluid under great pressure.


—The tendency of an object in motion to remain in motion, and the tendency of an object at rest to remain at rest.


—Minimum, a zeroed condition.

Phase shift

—Change in timing relative to standard reference.

Pitch instability

—Cyclic up and down oscillation.

Roll instability

—A cylinder's tendency to oscillate about its long axis.


—A device that responds to temperature changes and can be used to activate switches controlling heating and cooling equipment.

Yaw instability

—Tendency to develop side-to-side rotational motions.

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