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1、原文说明原文说明的内容是：文章阐述了电机的工作原理、发展过程、以及伺服电机的工作控制原理。并且举例说明了伺服电机所适用的场合。题名Servomotors Elements and Applications作者 NEWMARKER来源 佳工机电网How Does a Motor Work?An electric motor converts electricity into mechanical motion. Electric motors are used in household appliances, electric fans, remote-controlled toys, and
2、in thousands of other applications. The electric motor grew out of one of the earliest discoveries in electric scienceAragos rotations. In 1824, Francois Arago discovered that a magnetic needle suspended over a copper disk would rotate when the disc was spun. The next year, computer pioneer Charles
3、Babbage and astronomer John Herschel showed that the action could be reversed: spinning a more powerful magnet above the copper disk would spin the copper disc. Then, in 1831, Michael Faraday conducted experiments that helped explain why this took place. While this laid the groundwork for the electr
4、ic motor, it was another half century before electric motors were doing useful work. Over the next few decades many inventors made improved devices for turning electricity into motion. One of these was Hippolyte Pixiis 1832 improvement called the commutator, which switched the flow of current betwee
5、n two or more sets of stationary electromagnets to keep a motor continuously rotating. Thomas Davenport was the first to build an electric motor large enough to be used in industry, and he was also the first to seek a patent on a motor. Soon electric motors were being used for such things as transpo
6、rtation. Moritz-Hermann De Jacobi used an electric motor on a boat on the Neva River, and Charles G. Page used one to build a small locomotive. After the appearance of commercial electric power systems in the 1880s, larger electric motors were possible. Edison encouraged the use of electric motors i
7、n industrial applications and designed several new electric motors for that purpose. An important change came in the later 1880s and 1890s, when electric power companies began considering the switch to alternating current. Alternating current was perfect for the distribution of electric power over l
8、ong distances, and it worked well with the Edison electric lamp, but no practical AC motor existed until the works of Galileo Ferraris in Italy and Nikola Tesla in the United States. Teslas contributions are remembered today more than Ferraris in part because Tesla was subsequently hired by the West
9、inghouse corporation, which used his patents along with many others to become one of the major producers of electric equipment. With a suitable AC motor available, AC power took off. It is still in use today.ServomotorServomotors are available as AC or DC motors. Early servomotors were generally DC
10、motors because the only type of control for large currents was through SCRs for many years. As transistors became capable of controlling larger currents and switching the large currents at higher frequencies, the AC servomotor became used more often. Early servomotors were specifically designed for
11、servo amplifiers. Today a class of motors is designed for applications that may use a servo amplifier or a variable-frequency controller, which means that a motor may be used in a servo system in one application, and used in a variable-frequency drive in another application. Some companies also call
12、 any closed-loop system that does not use a stepper motor a servo system, so it is possible for a simple AC induction motor that is connected to a velocity controller to be called a servomotor.Some changes that must be made to any motor that is designed as a servomotor includes the ability to operat
13、e at a range of speeds without overheating, the ability to operate at zero speed and retain sufficient torque to hold a load in position, and the ability to operate at very low speeds for long periods of time without overheating. Older-type motors have cooling fans that are connected directly to the
14、 motor shaft. When the motor runs at slow speed, the fan does not move enough air to cool the motor. Newer motors have a separate fan mounted so it will provide optimum cooling air. This fan is powered by a constant voltage source so that it will turn at maximum RPM at all times regardless of the sp
15、eed of the servomotor. One of the most usable types of motors in servo systems is the permanent magnet (PM) type motor. The voltage for the field winding of the permanent magnet type motor can be AC voltage or DC voltage. The permanent magnet-type motor is similar to other PM type motors presented p
16、reviously. Figure-1 shows a cutaway picture of a PM motor and Fig.-2 shows a cutaway diagram of a PM motor. From the picture and diagram you can see the housing, rotor and stator all look very similar to the previous type PM motors. The major difference with this type of motor is that it may have ge
17、ar reduction to be able to move larger loads quickly from a stand still position. This type of PM motor also has an encoder or resolver built into the motor housing. This ensures that the device will accurately indicate the position or velocity of the motor shaft.FIGURE 1-1 Typical PM servomotorsFIG
18、URE 1-2 Cutaway picture of a permanent magnet servomotorBrushless ServomotorsThe brushless servomotor is designed to operate without brushes. This means that the commutation that the brushes provided must now be provided electronically. Electronic commutation is provided by switching transistors on
19、and off at appropriate times. Figure 1-3 shows three examples of the voltage and current waveforms that are sent to the brushless servomotor. Figure 1-4 shows an example of the three windings of the brushless servomotor. The main point about the brushless servomotor is that it can be powered by eith
20、er ac voltage or dc voltage. FIGURE 1-3 (a) Trapezoidal input voltage and square wave current waveforms. (b) Sinusoidal input voltage and sinusoidal voltage and square wave output voltage waveforms. (c) Sinusoidal input voltage and sinusoidal current waveforms. This has become the most popular type
21、of brushless servomotor control.Figure 1-4 shows three sets of transistors that are similar to the transistors in the output stage of the variable-frequency drive. In Fig. l-4a the transistors are connected to the three windings of the motor in a similar manner as in the variable-frequency drive. In
22、 Fig. l-4b the diagram of the waveforms for the output of the transistors is shown as three separate sinusoidal waves. The waveforms for the control circuit for the base of each transistor are shown in Fig. l-4c. Figure l-4d shows the back EMF for the drive waveforms. FIGURE 11-86 (a) Transistors co
23、nnected to the three windings of the brushless servomotor. (b) Waveforms of the three separate voltages that are used to power the three motor windings. (c) Waveforms of the signals used to control the transistor sequence that provides the waveforms for the previous diagram, (d) Waveform of the over
24、all back EMFServomotor Controllers Servomotor controllers have become more than just amplifiers for a servomotor. Today servomotor controllers must be able to make a number of decisions and provide a means to receive signals from external sensors and controls in the system, and send signals to host
25、controllers and PLCs that may interface with the servo system. Figure 1-5 shows a picture of several servomotors and their amplifiers. The components in this picture look similar to a variety of other types of motors and controllers. FIGURE 1-5 Example servomotors and amplifiersFigure 1-6 shows a di
26、agram of the servomotor controller so that you can see some of the differences from other types of motor controllers. The controller in this diagram is for a DC servomotor. The controller has three ports that bring signals in or send signals out of the controller. The power supply, servomotor, and t
27、achometer are connected to port P3 at the bottom of the controller. You can see that the supply voltage is 115-volt AC single phase. A main disconnect is connected in series with the LI wire. The LI and N lines supply power to an isolation step-down transformer. The secondary voltage of the trans-fo
28、rmer can be any voltage between 20 and 85 volts. The controller is grounded at terminal 8. You should remember that the ground at this point is only used to provide protection against short circuits for all metal parts in the system. The servomotor is connected to the controller at terminals 4 and 5
29、. Terminal 5 is + and terminal 4 is - . Terminal 3 provides a ground for the shield of the wires that connect the motor and the controller. The tachometer is connected to terminals 1 and 2. Terminal 2 is + and terminal 1 is - . The shield for this cable is grounded to the motor case. The wires conne
30、cted to this port will be larger than wires connected to the other ports, since they must be capable of carrying the larger motor current. If the motor uses an external cooling fan, it will be connected through this port. In most cases the cooling fan will be powered by single-phase or three-phase A
31、C voltage that remains at a constant level, such as 110 volts AC or 240 volts AC. FIGURE 1-6 Diagram of a servo controller. This diagram shows the digital (on-off) signals and the analog signals that are sent to the controller, and the signals the controller sends back to the host controller or PLC.
32、The command signal is sent to the controller through port PI. The terminals for the command signal are 1 and 2. Terminal 1 is + and terminal 2 is - . This signal is a type signal, which means that it is not grounded or does not share a ground potential with any other part of the circuit. Several add
33、itional auxiliary signals are also connected through port 1. These signals include inhibit (INH), which is used to disable the drive from an external controller, and forward and reverse commands (FAC and RAC), which tell the controller to send the voltage to the motor so that it will rotate in the f
34、orward or reverse direction. In some applications, the forward maximum travel limit switch and reverse maximum travel limit switch are connected so that if the machine travel moves to the extreme position so that it touches the overtravel limit switch, it will automatically energize the drive to beg
35、in travel in the opposite direction. Port PI also provides several digital output signals that can be used to send fault signals or other information such as drive running back to a host controller or PLC. Port PI basically is the interface for all digital (on-off) signals. Port P2 is the interface
36、for analog (0-max) signals. Typical signals on this bus include motor current and motor velocity signals that are sent from the servo controller back to the host or PLC where they can be used in verification logic to ensure the controller is sending the correct information to the motor. Input signal
37、s from the host or PLC can also be sent to the controller to set maximum current and velocity for the drive. In newer digital drives, these values are controlled by drive parameters that are programmed into the drive. PWM Servo Amplifier The PWM servo amplifier is used on small-size servo applicatio
38、ns that use DC brush-type servomotors. Figure 1-7 shows a diagram for this type of amplifier. From the diagram you can see that single-phase AC power is provided to the amplifier as the supply at the lower left part of the diagram. The AC voltage is rectified and sent to the output section of the dr
39、ive that is shown in the top right comer of the diagram. The output section of the drive uses four IGBTs to create the pulse-width modulation waveform. The IGBTs are connected so that they provide 30-120 volts DC and up to 30 A to the brush-type DC servo-motor. The polarity of the motor is indicated
40、 in the diagram. The remaining circuits show a variety of fault circuits in the middle of the diagram that originate from the fault logic board and provide an output signal at the bottom of the diagram. You should notice that the fault output signals include overvoltage, overtemperature, and overcur
41、rent. A fourth signal is identified as SSO (system status output), which indicates the status of the system as faulted anytime a fault has occurred. A jumper is used to set the SSO signal as an open collector output with a logic level 1 indicating the drive is ready, or as a normally closed relay in
42、dicating the drive is ready. The input terminals at the bottom right part of the diagram are used to enable or inhibit the drive, and to select forward amplifier clamp (FAC) or reverse amplifier clamp (RAC). The inhibit signal is used as a control signal, since it inhibits the output stage of the am
43、plifier if it is high. The FAC and RAC signals limit the current in the opposite direction to 5%. The input signals are shown in the diagram at the upper left side. The VCS (velocity command signal) requires a +VCS and a -VCS signal to provide the differential signal. FIGURE 1-7 Diagram of a pulse-w
44、idth modulator (PWM) amplifier with a brush-type DC servomotorApplications for Servo Amplifiers and Motors You will get a better idea of how servomotors and amplifiers operate if you see some typical applications. Figure 1-8 shows an example of a servomotor used to control a press feed. In this appl
45、ication sheet material is fed into a press where it is cut off to length with a knife blade or sheer. The sheet material may have a logo or other advertisement that must line up registration marks with the cut-off point. In this application the speed and position of the sheet material must be synchr
46、onized with the correct cut-off point. The feed-back sensor could be an encoder or resolver that is coupled with a photoelectric sensor to determine the location of the registration mark. An operator panel is provided so that the operator can jog the system for maintenance to the blades, or when loa
47、ding a new roll of material. The operator panel could also be used to call up parameters for the drive that correspond to each type of material that is used. The system could also be integrated with a programmable controller or other type of controller and the operator panel could be used to select
48、the correct cutoff points for each type of material or product that is run. FIGURE 1-8 Application of a servomotor controlling the speed of material as it enters a press for cutting pieces to size.An Example of a Servo Controlled In-Line Bottle-Filling ApplicationA second application is shown in Fig
49、. 1-9. In this application multiple filling heads line up with bottles as they move along a continuous line. Each of the filling heads must match up with a bottle and track the bottle while it is moving. Product is dispensed as the nozzles move with the bottles. In this application 10 nozzles are mounted on a carriage that is driven by a ball-screw mechanism. The ball-screw mechanism is also called a lead screw. When the motor turns the shaft of the ball screw, the carriage will move horizon
