伺服电机原理和应用.doc

步进电机的发展、应用和种类简介 步进电机最早是在1920年代由英国人所开发。1950年代后期晶体管的发明也逐渐应用在步进电机上，对于数字化的控制变得更为容易。往后经过不断改良，使得今日步进电机已广泛运用在需要高定位精度、高分解能、高响应性、信赖性等灵活控制性高的机械系统中。在生产过程中要求自动化、省人力、效率高的机器中，我们很容易发现步进电机的踪迹，尤其以重视速度、位置控制、需要精确操作各项指令动作的灵活控制性场合步进电机用得最多。 步进电机依其构造上的差异可分为三大类： （下图一） 可变磁阻式（VR型）：转子以软铁加工成齿状，当定子线圈不加激磁电压时，保持转矩为零，故其转子惯性小、响应性佳，但其容许负荷惯性并不大。其步进角通常为15。 永久磁铁式（PM型）：转子由永久磁铁构成，其磁化方向为辐向磁化，无激磁时有保持转矩。依转子材质区分，其步进角有45、90及7.5、11.25、15、18等几种。 混和式（HB型）：转子由轴向磁化的磁铁制成，磁极做成复极的形式，其乃兼采可变磁阻式步进电机及永久磁铁式步进电机的优点，精确度高、转矩大、步进角度小。 （图一） 目前市场上所使用的工业用步进电机，以混和式（HB型）最为普遍。 步进电机的特征 高精度的定位： 步进电机最大特征即是能够简单的做到高精度的定位控制。以5相步进电机为例：其定位基本单位（分辨率）为0.72（全步级）/0.36（半步级），是非常小的；停止定位精度误差皆在3分（0.05）以内，且无累计误差，故可达到高精度的定位控制。（步进电机的定位精度是取决于电机本身的机械加工精度）位置及速度控制： 步进电机在输入脉冲信号时，可以依输入的脉冲数做固定角度的回转进而得到灵活的角度控制（位置控制），并可得到与该脉冲信号周波数（频率）成比例的回转速度。具定位保持力： 步进电机在停止状态下（无脉波信号输入时），仍具有激磁保持力，故即使不依靠机械式的剎车，也能做到停止位置的保持。动作灵敏： 步进电机因为加速性能优越,所以可做到瞬时起动、停止、正反转之快速、频繁的定位动作。开回路控制、不必依赖传感器定位： 步进电机的控制系统构成简单，不需要速度感应器（ENCODER、转速发电机）及位置传感器（SENSOR），就能以输入的脉波做速度及位置的控制。也因其属开回路控制，故最适合于短距离、高频度、高精度之定位控制的场合下使用。中低速时具备高转矩： 步进电机在中低速时具有较大的转矩，故能够较同级伺服电机提供更大的扭力输出。高信赖性： 使用步进电机装置与使用离合器、减速机及极限开关等其它装置相较，步进电机的故障及误动作少，所以在检查及保养时也较简单容易。小型、高功率： 步进电机体积小、扭力大，尽管于狭窄的空间内，仍可顺利做安装，并提供高转矩输出。 步进电机的速度转矩特性 速度-转矩特性取决于电机及驱动器，尤其与所搭配的驱动器有着极大的影响；使用的驱动器不同，特性上的差异也就会有明显的不同。 （图二） 步进电机速度-转矩特性曲线图（图二）说明： (1)激磁最大静止转矩：当运转脉冲速度等于0 Hz时，曲线与Y轴交接的点即称为激磁最大静止转矩。也就是指电机在通电但无输入脉冲信号的情况下，其所具备的保持转矩即称为激磁最大静止转矩。 (2)脱出转矩：又称最大转矩，为电机于运转时所能带动的最大负荷。 (3)最大响应频率：在无负载、负荷惯性为0时，电机所能够响应之最快的速度。 (4)最大自起动频率：电机在无载的状态下可以做到瞬时的起动而不失步的速度谓之最大自起动频率。 二相与五相步进电机的差异 步进电机主要是依相数来做分类，而其中又以二相、五相步进电机为目前市场上所广泛采用。二相步进电机每转最细可分割为400等分，五相则可分割为 1000等分， 所以表现出来的特性以五相步进电机较佳、 加减速时间较短、 动态惯性较低。二相/五相步进电机差异比较： 二相步进电机 五相步进电机 电机构造（请参照图三） 8个主极4相(2相)4极线圈 10个主极5相2极线圈 分解能 1.8/0.9（200、400分割/圈） 0.72/0.36（500、1000分割/圈）较二相步进电机高出2.5倍 分解能。 振动性 100-200PPS之间为低速共振领域， 振动较大 无显著共振点 低振动 速度转矩特性 于速度上不及五相步进电机 高速度、高转矩 （图三）二相/五相步进电机基本性能汇整比较： 优/良/稍差 分解能 振动 速度 角度精度 响应性 转矩 噪音 二相步进电机 五相步进电机 步进电机的驱动系统 步进电机在单单仅给予电压时，电机是不会动作的，必须透过脉波产生器提供位置（脉波数）、速度的脉波信号指令，以及驱动器驱动电流流过电机内部线圈、依顺序切换激磁相序的方式才能够让电机运转。所以欲使步进电机动作的必要系统组成有： 1.脉冲产生器：给予角度（位置移动量）、动作速度及运转方向之脉冲信号的电机驱动指令。 2.步进驱动器：依控制器所投入的脉冲信号指令，提供电流来驱动步进电机动作。 3.步进电机：提供转矩动力输出来带动负载。所以步进电机系统构成简单，不需要速度感应器（ENCODER、转速发电机）、位置传感器（SENSOR），即能依照脉冲产生器所输入的脉冲来做到速度及位置的控制。 步进电机的速度、位置控制 速度控制： 步进电机的运转速度会与输入的脉冲速度成等比例的关系，所以在脉冲的速度愈快时，步进电机的转速也会跟着加快；脉波速度愈慢时，电机的转速自然也跟着变慢。电机的运转速度（RPM）与脉冲速度（PPS，又称Hz）间的关系式如下：电机的运转速度（RPM） 脉冲速度（PPS或 Hz） 60 步进电机分割数/圈说明： 1.RPM为一般电机的速度单位，即 rev / min，为每分钟电机所转的圈数；PPS为步进、伺服电机的速 度单位，即pulse per second，为每秒所送出的脉冲数。 2.由于RPM与PPS的单位不同，所以于转换的过程中要先将PPS的秒钟乘以60变为分钟 。 3.步进电机分割数/圈，又代表要让电机转一圈所必须送出的脉冲数。 4.上述公式拆解后之单位表示为 rev/min = pulse/sec 60 1/分割数 实例：五相半步级角0.36时（即1000分割/圈） （1）电机的运转速度600RPM时，即相当于脉冲速度10,000PPS。 （2）脉冲速度3,000PPS，即相当于电机的运转速度180RPM。位置控制： 步进电机不需要位置传感器（SENSOR），就可依照输入的脉冲数决定移动量，并将负载顺利、正确的送达指定位置点上。而移动量的大小，是依照电机分辨率的大小与输入的脉冲数来决定。脉冲数（PULSE）与移动量间的关系式如下： 位置移动量（ ） 步进电机分辨率（ ） 输入脉冲数 实例：二相全步级角1.8时 当输入1000个脉冲数（即1000PULSE），此时之移动量会是1800，刚好为5圈。 步进电机疑难杂症处理 如何有效改善步进电机的温升问题？ 可依下列步骤作做检查及确认： 1.是否用于连续运转的场合？ （步进电机的特性并不适合于连续运转的场合下使用,在此场合下使用时一定会有较高的温升 产生。请重新确认机构动作需求条件并重新评估使用的电机。） 2.请确认机构动作频度、周期？ （走停的动作频度过高将可能因脉冲输入停止的时间过短而导致电流尚未下降就又重新激活， 故此时的温升一定会较高。建议您可将动作频度降低以改善温升问题。 3.将RUN电流调小情况可否改善？ （在转矩足够的情况下将驱动器的RUN电流调小将可有效的使温升降低。但若因扭力的关系 一定得使用到较大的电流时,则建议您可将电机更换为大一等级的电机后再将电流调低以改 善温升问题。） 4.将STOP电流调小情况可否改善？ （在保持力足够的情况下将驱动器的STOP电流调小将可于电机停止时有效的使温升降低。但 若因停止保持力的关系一定得使用到较大的STOP电流时,则建议您可将电机更换为大一等级 的电机后再将电流调低以改善温升问题。） 5.驱动器上的指拨开关是否打开电流自动降低档？ （若未打开,电机停止时电流将无法自动下降,温升会因此而较高。建议使用此功能， 将可避免步进电机及驱动器的温升问题。） 6.目前使用的速度是否界于温升较高(即电流较大)的范围内？(由特性曲线图中的电流曲线得知) （请尽量避开温升较高的速度范围使用,对于温升的降低将有帮助。） 7.周围环境温度如何?是否过高？ （电机温度环境温度电机温升，故环境温度较高时，电机的温度也会因此而较高。建议 以加装安装散热面板或散热风扇的方式来帮助散热。） 8.请确认电机端的接线是否正确？ （相位接错将造成电机运转不顺的抖动现象，亦可能因此而产生温升较高的问题。） 若皆无上述原因问题时，此情况下电机温度应为正常，并未过热才是，请您直接以温度计测量电机确实温度。以我们的驱动器来说，因为有具备过热保护功能，故若温度过高,保护功能将开启，同时并将电机断电,让客户更能安心使用。伺服电机原理和应用Servomotors are available as AC or DC motors. Early servomotors were generally DC 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 servo amplifiers. Today a class of motors is designed for applica-tions 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 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 in-cludes the ability to operate 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 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 con-stant voltage source so that it will turn at maximum RPM at all times regardless of the speed 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 previously. Figure 11-83 shows a cutaway picture of a PM motor and Fig. 11-84 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 gear 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 11-83 Typical PM servomotors.FIGURE 11-84 Cutaway picture of a permanent magnet servomotor.11.11.5.1 Brushless Servomotors The 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 and off at appropriate times. Figure 11-85 shows three examples of the voltage and current waveforms that are sent to the brushless servomotor. Figure 11-86 shows an example of the three windings of the brushless servomotor. The main point about the brushless servomo-tor is that it can be powered by either ac voltage or dc voltage.Figure 11-85 shows three types of voltage waveforms that can be used to power the brushless servomotor. Figure ll-85a shows a trapezoidal EMF (voltage) input and a square wave current input. Figure ll-85b shows a sinusoidal waveform for the input voltage and a square wave current waveform. Figure ll-85c shows a sinusoidal input waveform and a sinusoidal current waveform. The sinusoidal input and sinusoidal current waveform are the most popular voltage supplies for the brushless servomotor.Figure 11-86 shows three sets of transistors that are similar to the transistors in the output stage of the variable-frequency drive. In Fig. ll-86a the transistors are connected to the three windings of the motor in a similar manner as in the variable-frequency drive. In Fig. 1 l-86b 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 transis-tor are shown in Fig. ll-86c. Figure ll-86d shows the back EMF for the drive waveforms.FIGURE 11-85 (a) Trap-ezoidal input voltage and square wave current wave-forms. (b) Sinusoidal in-put voltage and sinusoidal voltage and square wave output voltage wave-forms. (c) Sinusoidal in-put voltage and sinusoi-dal current waveforms. This has become the most popular type of brushless servomotor control.Servomotor 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 controllers and PLCs that may interface with the servo system. Figure 11-87 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 11-88 shows a diagram 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 tachometer 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-former 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. 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 connected 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 AC voltage that remains at a constant level, such as 110 volts AC or 240 volts AC. FIGURE 11-86 (a) Tran-sistors connected to the three windings of the brushless servomotor. (b) Waveforms of the three separate voltages that are used to power the three motor wind-ings. (c) Waveforms of the signals used to control the transistor se-quence that provides the waveforms for the previous diagram, (d) Waveform of the overall back EMF.FIGURE 11-87 Example servomotors and ampli-fiers.FIGURE 11-88 Diagram of a servo controller. This diagram shows the digi-tal (on-off) signals and the analog signals that are sent to the controller, and the signals the con-troller sends back to the host controller or PLC.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 additional auxiliary signals are also connected through port 1. These signals include inhibit (INH), which is used to disable the drive from an ex-ternal controller, and forward and reverse commands (FAC and RAC), which tell the con-troller to send the voltage to the motor so that it will rotate in the forward or reverse direc-tion. 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 posi-tion so that it touches the overtravel limit switch, it will automatically energize the drive to begin 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 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 con-troller is sending the correct information to the motor. Input signals 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 applications that use DC brush-type servomotors. Figure 11-89 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 drive 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 con-nected 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 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, overtempera-ture, and overcurrent. A fourth signal is identified as SSO (system status output), which in-dicates 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 indicating 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 amplifier 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.Applications for Servo Amplifiers and Motors You will get a better idea of how servomotors and amplifiers operate if you see some typical applications. Figure 11-90 shows an example of a servomotor used to control a press feed. In this application 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 po-sition of the sheet material must be synchronized 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 loading a new roll of material. The operator panel could also be used to call up parameters for the drive that cor-respond 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 the correct cutoff points for each type of material or product that is run.FIGURE 11-89 Diagram of a pulse-width modulator (PWM) amplifier with a brush-type DC servomotor.FIGURE 11-90 Appli-cation of a servomotor controlling the speed of material as it enters a press for cutting pieces to size. 11.11.8.1 An Example of a Servo Controlled In-Line Bottle-Filling ApplicationA second application is shown in Fig. 11-91. 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 horizontally along the length of the ball-screw shaft. This movement will be smooth so that each of the nozzles can dis-pense product into the bottles with little spillage.The servo drive system utilizes a positioning drive controller with software that allows the position and velocity to be tracked as the conveyor line moves the bottles. A master encoder tracks the bottles as they move along the conveyor line. An auger feed system is also used just prior to the point where the bottles enter the filling station. The auger causes a specific amount of space to be set between each bottle as it enters the filling station. The bottles may be packed tightly as they approach the auger, but as they pass through the auger their space is set ex