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Open-loop servomechanisms

Overshoot and hunting

Mechanical inertia and servomechanisms

Enabling 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 pilots instructions electrically to control servomechanisms that position the planes control surfaces as needed.

High-performance airplanes need special servo-mechanisms called flight-control systems to compensate for performance instabilities that would otherwise compromise their safety. The aerodynamic designs that optimize a planes 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.

Open-loop servomechanisms

Servomechanisms are classified on the basis of whether they depend upon information sampled at the output of the system for comparison with the input instructions. The simplest servomechanisms are called open-loop servomechanisms and do not feed back the results of their output. Open-loop servomechanisms do not verify that input instructions have been satisfied and they do not automatically correct errors.

An example of an open-loop servomechanism is a simple motor used to rotate a television-antenna. The motor used to rotate the antenna in an open-loop configuration is energized for a measured time in the expectation that antenna will be repositioned correctly. There is no automatic check to verify that the desired action has been accomplished. An open-loop servomechanism design is very unsatisfactory as a basis for an antenna rotator, just as it is usually not the best choice for other applications.

When error feedback is included in the design the result is called a closed-loop servomechanism. The servos output result is sampled continuously and this information is continuously compared with the input instructions. Any important difference between the feedback and the input signal is interpreted as an error that must corrected automatically. Closed-loop servo systems automatically null, or cancel, disagreements between input instructions and output results.

The key to understanding a closed-loop servomechanism is to recognize that it is designed to minimize disagreements between the input instructions and the output results by forcing an action that reduces the error.

A more sophisticated antenna rotator system, compared to the open-loop version described earlier, will use the principles of the closed-loop servomechanism. When it is decided that the antenna is to be turned to a new direction the operator will introduce input information that creates a deliberate error in the ser-vomechanisms feedback loop. The servos electronic controller senses this purposely-introduced change and energizes the rotators motor. The antenna rotates in the direction that tends to null the error. When the error has been effectively canceled, the motor is turned off automatically leaving the antenna pointing in the desired direction. If a strong wind causes the antenna turn more slowly than usual the motor will continue to be energized until the error is canceled. If a strong wind repositions the antenna improperly the resulting error will cause the motor to be energized once again, bringing the antenna back into alignment.

Another example of a simple closed-loop servomechanism is a thermostatically-controlled gas furnace. A sensor called a thermostat determines that heat is required, closing a switch that actuates an electric circuit that turns on the furnace. When the buildings temperature reaches the set point the electric circuit is de-energized, turning off the fuel that supplies the flame. The feedback loop is completed when warmed air of the desired temperature is sensed by the thermostat.

Overshoot and hunting

A gas-furnace controller example above illustrates a potential problem with servomechanisms that must be solved when they are designed. If not properly engineered, closed-loop servomechanisms tend to be unstable. They must not overcontrol. The controller must be intelligent enough to shut down the actuator just before satisfaction is accomplished. Just as a car driver must slow down gradually before stopping at an intersection, a servomechanism must anticipate the effects of inertial mass. The inertia may be mechanical or it may be thermal, as in the case of the gas furnace. If the furnace flame were to continue to burn until the air temperature reaches the exact set point on the temperature selector, the residual heat in the furnace firebox would continue to heat the house, raising the temperature excessively. The room temperature will overshoot the desired value, perhaps uncomfortably. Most space-heating furnace control thermostats include a heat-anticipation provision designed to minimize thermal overshoot. A properly-adjusted anticipation control turns off the furnaces flame before the room temperature reaches the desired set point, allowing the temperature to coast up to the desired value as the furnace cools.

Mechanical inertia and servomechanisms

There is a similar overshoot problem that requires compensation by mechanical servomechanisms. If a servo is used to manipulate a massive object such as a radar antenna weighing 1,000 lb (454 kg ) or more, the actuator must anticipate the antennas approach to a newly-selected position. The inertial mass of the antenna will otherwise cause it to overshoot the desired alignment. When the feedback signal is compared with the input and the control electronics discovers the overshoot, the antenna will reverse direction in an attempt to correct the new error. If the antenna overshoots again this may lead to a continuing oscillation called hunting where the antenna continually seeks a null but always turns too far before shutting down, requiring a continuing series of corrections. The resulting oscillation is very undesirable.

Servomechanisms must use very sophisticated electronic circuits that act as electronic anticipators of the loads position and speed to minimize instability while simultaneously maintaining a fast response to new instructions. Better servomechanism designs adjust the timing of error signals to provide just the right amount of anticipation under varying circumstances. The electrical phase-shift network needed to produce a stable servomechanism must be designed with great care.

Enabling servomechanisms

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.


Digital Information processed as encoded on or off data bits.

Electronic Devices using active components to control power.

Error A signal proportional to the servomechanism correction.

Feedback Comparing output and input to determine correction.

Hunting Repetitious failure of a servomechan-isms response.

Hydraulic Power transfer using fluid under great pressure.

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

Null Minimum, a zeroed condition.

Phase shift Change in timing relative to standard reference.

Pitch instability Cyclic up and down oscillation.

Roll instability A cylinders tendency to oscillate about its long axis.

Thermostat 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.



Cassidy, David, Gerald Holton, and James Rutherford. Understanding Physics. New York: Springer Publishing, July 2002.

DeSchutter, Joris, Johan Baeten. Integrated Visual Servoing and Force Control New York: Springer Publishing. November 2003

Holton, Gerald, Stephen G. Brush. Physics, The Human Adventure. 3rd ed., Chapel Hill, NC: Rutgers University Press, March 2001.

Huang, Jie. Nonlinear Output Regulation Theory and Applications (Advances in Design and Control ). Melville, NY: Society for Industrial and Applied Mathematics, November 2004.

Youngkin, George W. Industrial Servo Control Systems. 2nd ed. Boca Raton, FL: CRC Publishing, October 2002.

Donald Beaty

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