A servomechanism 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 theconstant speed propeller was developed to control engine speed for maneuvering aircraft. Fuel controls forgas 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, andradio-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 alinear 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. Servo Motors Servo motors are used in closed loop control systems in which work is the control variable, Figure 9. The digital servo motor controller directs operation of the servo motor by sending velocity command signals to the amplifier, which drives the servo motor. An integral feedback device (resolver) or devices (encoder and tachometer) are either incorporated within the servo motor or are remotely mounted, often on the load itself. These provide the servo motor's position and velocity feedback that the controller compares to its programmed motion profile and uses to alter its velocity signal. Servo motors feature a motion profile, which is a set of instructions programmed into the controller that defines the servo motor operation in terms of time, position, and velocity. The ability of the servo motor to adjust to differences between the motion profile and feedback signals depends greatly upon the type of controls and servo motors used. See the servo motors Control and Sensors Product section. Three basic types of servo motors are used in modern servosystems: ac servo motors, based on induction motor designs; dc servo motors, based on dc motor designs; and ac brushless servo motors, based on synchronous motor designs. Figure 9 - Typical dc servo motor system with either encoder or resolver feedback. Some older servo motor systems use a tachometer and encoder for feedback. A servomotor (servo) is an electromechanical device in which an electrical input determines the position of the armature of a motor. Servos are used extensively in robotics and radio-controlled cars, airplanes, and boats. You will be using an Airtronics 94102 Precision Heavy-Duty Standard Servo. The position of the armature (in the Figure) is determined by the duty cycle of a periodic rectangular pulse train. The duty cycle of a rectangular pulse train is expressed in %: It is the ratio of the pulse duration to the pulse period times 100% (See Figure for examples.) Figure: Illustration of Servomotor Identifying the Armature Figure: Examples of Duty Cycle Calculation The receiver sub-circuit of the lab project outputs a DC voltage. Therefore it is necessary to convert the DC voltage produced by the receiver into a rectangular pulse train whose duty cycle is determined by the DC level. In this lab exercise you will investigate how the servo controller circuit accomplishes this conversion from a DC voltage level to a rectangular pulse train with a specific duty cycle. Components of the Servomotor Controller Circuit The servo controller circuit consists of several op-amps and Transistor- Transistor Logic (TTL) components. Its purpose is to convert a DC voltage into a rectangular pulse train whose duty cycle is determined by the DC level. The servo responds to variations in the duty cycle of a 50 Hz rectangular pulse train. The controller circuit is designed to produce a 50 Hz rectangular pulse train with a duty cycle determined by the DC voltage level at its input. The path of the signal through the circuit is shown in the block diagram of Figure . A summary of each component is provided in the following section. Control of standard servo motor Servos are controlled by sending them a pulse of variable width. The control wire is used to send this pulse. The parameters for this pulse are that it has a minimum pulse, a maximum pulse, and a repetition rate. Given the rotation constraints of the servo, neutral is defined to be the position where the servo has exactly the same amount of potential rotation in the clockwise direction as it does in the counter clockwise direction. It is important to note that different servos will have different constraints on their rotation but they all have a neutral position, and that position is always around 1.5 milliseconds (ms). The angle is determined by the duration of a pulse that is applied to the control wire. This is called Pulse width Modulation. The servo expects to see a pulse every 20 ms. The length of the pulse will determine how far the motor turns. For example, a 1.5 ms pulse will make the motor turn to the 90 degree position (neutral 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 from moving out of that position. The maximum amount of force the servo can exert is 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. When a pulse is sent to a servo that is less than 1.5 ms the servo rotates to a position and holds its output shaft some number of degrees counterclockwise from the neutral point. When the pulse is wider than 1.5 ms the opposite occurs. The minimal width and the maximum width of pulse that will command the servo to turn to a valid position are functions of each servo. Different brands, and even different servos of the same brand, will have different maximum and minimums. Generally the minimum pulse will be about 1 ms wide and the maximum pulse will be 2 ms wide. From NEWNES ELECTRICAL POWER ENGINIEERS HANDBOOK. Page 291 10.6 The brushless servomotor A synchronous machine with permanent magnets on the rotor is the heart of the modern brushless servomotor drive. The motor stays in synchronism with the frequency of supply, though there is a limit to the maximum torque which can be developed before the rotor is forced out of synchronism, pull-out torque being typically between 1.5 and 4 times the continuously rated torque. The torque–speed curve is therefore simply a vertical line. The industrial application of brushless servomotors has grown significantly for the following reasons: ● reduction of price of power conversion products ● establishment of advanced control of PWM inverters ● development of new, more powerful and easier to use permanent magnet materials ● the developing need for highly accurate position control ● the manufacture of all these components in a very compact form They are, in principle, easy to control because the torque is generated in proportion to the current. In addition, they have high efficiency, and high dynamic responses can be achieved. Brushless servomotors are often called brushless dc servomotors because their struc-ture is different from that of dc servomotors. They rectify current by means of transistor switching within the associated drive or amplifier, instead of a commutator as used in dc servomotors. Confusingly, they are also called ac servomotors because brushless servo-motors of the synchronous type (with a permanent magnet rotor) detect the position of the rotational magnetic field to control the three-phase current of the armature. It is now widely recognized that brushless ac refers to a motor with a sinusoidal stator winding distribution which is designed for use on a sinusoidal or PWM inverter supply voltage. Brushless dc refers to a motor with a trapezoidal stator winding distribution which is designed for use on a square wave or block commutation inverter supply voltage. The brushless servomotor lacks the commutator of the dc motor, and has a device (the drive, sometimes referred to as the amplifier) for making the current flow accord-ing to the rotor position. In the dc motor, increasing the number of commutator seg-ments reduces torque variation. In the brushless motor, torque variation is reduced by making the coil three-phase and, in the steady state, by controlling the current of each phase into a sine wave.