Dc servo motors are normally used as prime movers in computers, numerically controlled machinery, or other applications where starts and stops are made quickly and accurately. Servo motors have lightweight, low-inertia armatures that respond quickly to excitation-voltage changes. In addition, very low armature inductance in these servo motors results in a low electrical time constant (typically 0.05 to 1.5 msec) that further sharpens servo motor response to command signals. Servo motors include permanent-magnetic, printed-circuit, and moving-coil (or shell) dc servo motors. The rotor of a shell dc servo motor consists of a cylindrical shell of copper or aluminum wire coils which rotate in a magnetic field in the annular space between magnetic pole pieces and a stationary iron core. The servo motor features a field, which is provided by cast AlNiCo magnets whose magnetic axis is radial. Servo motors usually have two, four, or six poles.
Dc servo motor characteristics include inertia, physical shape, costs, shaft resonance, shaft configuration, speed, and weight. Although these dc servo motors have similar torque ratings, their physical and electrical constants vary.
A servomechanism, or servo, is an automatic device that uses error-sensing negative feedback to correct the performance of a mechanism.
The term correctly applies only to systems where the feedback or error-correction signals help control mechanical position, speed or other parameters. For example, an automotive power window control is not a servomechanism, as there is no automatic feedback that controls position—the operator does this by observation. By contrast the car's cruise control uses closed loop feedback, which classifies it as a servomechanism.
A servomechanism may or may not use a servomotor. For example, a household furnace controlled by a thermostat is a servomechanism, yet there is no motor being controlled directly by the servomechanism.
A common type of servo provides 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. 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. This procedure is one widely used application of control theory.
Speed control via a governor is another type of servomechanism. The steam engine uses mechanical governors; another early application was to govern the speed of water wheels. Prior to World War II the constant speed propeller was developed to control engine speed for maneuvering aircraft. Fuel controls for gas turbine engines employ either hydromechanical or electronic governing.
Positioning servomechanisms were first used in military fire-control and marine navigation equipment. Today servomechanisms are used in automatic machine tools, satellite-tracking antennas, remote control airplanes, automatic navigation systems on boats and planes, and antiaircraft-gun control systems. Other examples are fly-by-wire systems in aircraft which use servos to actuate the aircraft's control surfaces, and radio-controlled models which use RC servos for the same purpose. Many autofocus cameras also use a servomechanism to accurately move the lens, and thus adjust the focus. A modern hard disk drive has a magnetic servo system with sub-micrometre positioning accuracy.
Typical servos give a rotary (angular) output. Linear types are common as well, using a leadscrew 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. However, these devices are not true servos, but rather mechanical amplifiers. (See also Power steering or Vacuum servo.)
In industrial machines, servos are used to perform complex motion.
DC Servo Motor Selection: The first selection approach is to choose a servo motor large enough for a machine that has already been designed; the second is to select the best available servo motor with a specific feature and then build the system around it; and the third is to study servo motor performance and system requirements and mate the two.
The final servo motor system design is usually the least sophisticated that meets the performance specifications reliably. Servo motor requirements may include control of acceleration, velocity, and position to very close tolerances. This says that the servo designer must define the system carefully, establish the servo motor's performance specifications, determine critical areas, and set up tolerances. Only then will the designer be able to propose an adequate servosystem and choose a servo motor type.
Dc servo motor characteristics include inertia, physical shape, costs, shaft resonance, shaft configuration, speed, and weight. Although these dc servo motors have similar torque ratings, their physical and electrical constants vary.
A servomechanism, or servo, is an automatic device that uses error-sensing negative feedback to correct the performance of a mechanism.
The term correctly applies only to systems where the feedback or error-correction signals help control mechanical position, speed or other parameters. For example, an automotive power window control is not a servomechanism, as there is no automatic feedback that controls position—the operator does this by observation. By contrast the car's cruise control uses closed loop feedback, which classifies it as a servomechanism.
A servomechanism may or may not use a servomotor. For example, a household furnace controlled by a thermostat is a servomechanism, yet there is no motor being controlled directly by the servomechanism.
A common type of servo provides 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. 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. This procedure is one widely used application of control theory.
Speed control via a governor is another type of servomechanism. The steam engine uses mechanical governors; another early application was to govern the speed of water wheels. Prior to World War II the constant speed propeller was developed to control engine speed for maneuvering aircraft. Fuel controls for gas turbine engines employ either hydromechanical or electronic governing.
Positioning servomechanisms were first used in military fire-control and marine navigation equipment. Today servomechanisms are used in automatic machine tools, satellite-tracking antennas, remote control airplanes, automatic navigation systems on boats and planes, and antiaircraft-gun control systems. Other examples are fly-by-wire systems in aircraft which use servos to actuate the aircraft's control surfaces, and radio-controlled models which use RC servos for the same purpose. Many autofocus cameras also use a servomechanism to accurately move the lens, and thus adjust the focus. A modern hard disk drive has a magnetic servo system with sub-micrometre positioning accuracy.
Typical servos give a rotary (angular) output. Linear types are common as well, using a leadscrew 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. However, these devices are not true servos, but rather mechanical amplifiers. (See also Power steering or Vacuum servo.)
In industrial machines, servos are used to perform complex motion.
DC Servo Motor Selection: The first selection approach is to choose a servo motor large enough for a machine that has already been designed; the second is to select the best available servo motor with a specific feature and then build the system around it; and the third is to study servo motor performance and system requirements and mate the two.
The final servo motor system design is usually the least sophisticated that meets the performance specifications reliably. Servo motor requirements may include control of acceleration, velocity, and position to very close tolerances. This says that the servo designer must define the system carefully, establish the servo motor's performance specifications, determine critical areas, and set up tolerances. Only then will the designer be able to propose an adequate servosystem and choose a servo motor type.
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