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1、SELECTING THE MOTOR THAT SUITS YOUR APPLICATIONMotion control, in its widest sense, could relate to anything from a welding robot to the hydraulic system in a mobile crane. In the field of Electronic Motion Control, we are primarily concerned with systems falling within a limited power range, typica
2、lly up to about 10HP (7KW), and requiring precision in one or more aspects. This may involve accurate control of distance or speed, very often both and sometimes other parameters such as torque or acceleration rate. In the case of the two examples given, the welding robot requires precise control of
3、 both speed and distance; the crane hydraulic system uses the driver as the feedback system so its accuracy varies with the skill of the operator. This wouldnt be considered a motion control system in the strict sense of the term. Our standard motion control system consists of three basic elements:F
4、ig. 1 Elements of motion control systemThe motor，This may be a stepper motor (either rotary or linear), a DC brush motor or a brushless servo motor. The motor needs to be fitted with some kind of feedback device unless it is a stepper motor.Fig. 2 shows a system complete with feedback to control mot
5、or speed. Such a system is known as a closed-loop velocity servo system.Fig. 2 Typical closed loop (velocity) servo systemThe drive，this is an electronic power amplifier that delivers the power to operate the motor in response to low-level control signals. In general, the drive will be specifically
6、designed to operate with a particular motor type you cant use a stepper drive to operate a DC brush motor, for instance.Application Areas of Motor TypesStepper MotorsStepper Motor BenefitsStepper motors have the following benefits: Low cost Ruggedness Simplicity in construction High reliability No m
7、aintenance Wide acceptance No tweaking to stabilize No feedback components are needed They work in just about any environment Inherently more failsafe than servo motors.There is virtually no conceivable failure within the stepper drive module that could cause the motor to run away. Stepper motors ar
8、e simple to drive and control in an open-loop configuration. They only require four leads. They provide excellent torque at low speeds, up to 5 times the continuous torque of a brush motor of the same frame size or double the torque of the equivalent brushless motor. This often eliminates the need f
9、or a gearbox. A stepper-driven-system is inherently stiff, with known limits to the dynamic position error.Stepper Motor DisadvantagesStepper motors have the following disadvantages: Resonance effects and relatively long settling times Rough performance at low speed unless a micro step drive is used
10、 Liability to undetected position loss as a result of operating open-loop They consume current regardless of load conditions and therefore tend to run hot Losses at speed are relatively high and can cause excessive heating, and they are frequently noisy (especially at high speeds). They can exhibit
11、lag-lead oscillation, which is difficult to damp. There is a limit to their available size, and positioning accuracy relies on the mechanics (e.g., ball screw accuracy). Many of these drawbacks can be overcome by the use of a closed-loop control scheme. Note: The Comp motor Zeta Series minimizes or
12、reduces many of these different stepper motor disadvantages. There are three main stepper motor types: Permanent Magnet (P.M.) Motors Variable Reluctance (V.R.) Motors Hybrid MotorsWhen the motor is driven in its full-step mode, energizing two windings or “phases” at a time (see Fig. 3), the torque
13、available on each step will be the same (subject to very small variations in the motor and drive characteristics). In the half-step mode, we are alternately energizing two phases and then only one as shown in Fig. 4. Assuming the drive delivers the same winding current in each case, this will cause
14、greater torque to be produced when there are two windings energized. In other words, alternate steps will be strong and weak. This does not represent a major deterrent to motor performancethe available torque is obviously limited by the weaker step, but there will be a significant improvement in low
15、-speed smoothness over the full-step mode.Clearly, we would like to produce approximately equal torque on every step, and this torque should be at the level of the stronger step. We can achieve this by using a higher current level when there is only one winding energized. This does not over dissipat
16、e the motor because the manufacturers current rating assumes two phases to be energized the current rating is based on the allowable case temperature). With only one phase energized, the same total power will be dissipated if the current is increased by 40%. Using this higher current in the one-phas
17、e-on state produces approximately equal torque on alternate steps (see Fig. 5).Fig. 3 Full step currentFig. 4 Half step currentFig.5 Half step current, profiledWe have seen that energizing both phases with equal currents produces an intermediate step position half-way between the one-phase-one posit
18、ions. If the two phase currents are unequal, the rotor position will be shifted towards the stronger pole. This effect is utilized in the micro stepping drive, which subdivides the basic motor step by proportioning the current in the two windings. In this way, the step size is reduced and the low-sp
19、eed smoothness is dramatically improved. High-resolution micro step drives divide the full motor step into as many as 500 micro steps, giving 100,000 steps per revolution. In this situation, the current pattern in the windings closely resembles two sine waves with a 90 phase shift between them (see
20、Fig. 6). The motor is now being driven very much as though it is a conventional AC synchronous motor. In fact, the stepper motor can be driven in this way from a 60 Hz-US (50Hz-Europe) sine wave source by including a capacitor in series with one phase. It will rotate at 72 rpm.Fig. 6 Phase currents
21、in micro step modeStandard 200-Step Hybrid MotorThe standard stepper motor operates in the same way as our simple model, but has a greater number of teeth on the rotor and stator, giving a smaller basic step size. The rotor is in two sections as before, but has 50 teeth on each section. The half-too
22、th displacement between the two sections is retained. The stator has 8 poles each with 5 teeth, making a total of 40 teeth (see Fig. 7).Fig.7 200-step hybrid motorIf we imagine that a tooth is placed in each of the gaps between the stator poles, there would be a total of 48 teeth, two less than the
23、number of rotor teeth. So if rotor and stator teeth are aligned at 12 oclock, they will also be aligned at 6 oclock. At 3 oclock and 9 oclock the teeth will be misaligned. However, due to the displacement between the sets of rotor teeth, alignment will occur at 3 oclock and 9 oclock at the other end
24、 of the rotor.The windings are arranged in sets of four, and wound such that diametrically-opposite poles are the same. So referring to Fig. 7, the north poles at 12 and 6 oclock attract the south-pole teeth at the front of the rotor; the south poles at 3 and 9 oclock attract the north-pole teeth at
25、 the back. By switching current to the second set of coils, the stator field pattern rotates through 45. However, to align with this new field, the rotor only has to turn through 1.8. This is equivalent to one quarter of a tooth pitch on the rotor, giving 200 full steps per revolution.Note that ther
26、e are as many detent positions as there are full steps per rev, normally 200. The detent positions correspond with rotor teeth being fully aligned with stator teeth. When power is applied to a stepper drive, it is usual for it to energize in the “zero phase” state in which there is current in both s
27、ets of windings. The resulting rotor position does not correspond with a natural detent position, so an unloaded motor will always move by at least one half steps at power-on. Of course, if the system was turned off other than in the zero phase state, or the motor is moved in the meantime, a greater
28、 movement may be seen at power-up.Another point to remember is that for a given current pattern in the windings, there are as many stable positions as there are rotor teeth (50 for a 200-step motor). If a motor is de-synchronized, the resulting positional error will always be a whole number of rotor
29、 teeth or a multiple of 7.2. A motor cannot “miss” individual steps position errors of one or two steps must be due to noise, spurious step pulses or a controller fault.Fig. 8 Digital servo driveDigital Servo Drive OperationFig.8 shows the components of a digital drive for a servo motor. All the mai
30、n control functions are carried out by the microprocessor, which drives a D-to-A converter to produce an analog torque demand signal. From this point on, the drive is very much like an analog servo amplifier.Feedback information is derived from an encoder attached to the motor shaft. The encoder gen
31、erates a pulse stream from which the processor can determine the distance traveled, and by calculating the pulse frequency it is possible to measure velocity.The digital drive performs the same operations as its analog counterpart, but does so by solving a series of equations. The microprocessor is
32、programmed with a mathematical model (or “algorithm”) of the equivalent analog system. This model predicts the behavior of the system. It also takes into account additional information like the output velocity, the rate of change of the input and the various tuning settings.To solve all the equation
33、s takes a finite amount of time, even with a fast processor this time is typically between 100ms and 2ms. During this time, the torque demand must remain constant at its previously-calculated value and there will be no response to a change at the input or output. This “update time” therefore becomes
34、 a critical factor in the performance of a digital servo and in a high-performance system it must be kept to a minimum.The tuning of a digital servo is performed either by pushbuttons or by sending numerical data from a computer or terminal. No potentiometer adjustments are involved. The tuning data
35、 is used to set various coefficients in the servo algorithm and hence determines the behavior of the system. Even if the tuning is carried out using pushbuttons, the final values can be uploaded to a terminal to allow easy repetition.Some applications, the load inertia varies between wide limits thi
36、nk of an arm robot that starts off unloaded and later carries a heavy load at full extension. The change in inertia may well be a factor of 20 or more, and such a change requires that the drive is re-tuned to maintain stable performance. This is simply achieved by sending the new tuning values at th
37、e appropriate point in the operating cycle. 步进电机和伺服电机的系统控制运动控制，在其最广泛的意义上说，可能与任何移动式起重机中焊接机器人液压系统有关。在电子运动控制领域，我们的主要关切系统范围内的有限功率的大小，通常高达约10hp（7千瓦），并要求在一个或多个方面有严格精密。这可能涉及精确控制的距离或速度，但很多时候是双方的，有时还涉及其它参数如转矩或加速率。在以下所举的两个例子中，焊接机器人，需要精确的控制双方的速度和距离;吊臂液压系统采用驱动作为反馈系统，因此，它的准确度会随着操作者的技能的不同而不同。在严格意义上来说，这将不会被视为一项运动控
38、制系统。 我们的标准运动控制系统由以下三个基本要素组成：图1运动控制系统组成元件电机，可能是一个步进电机（要么旋转或线性），也可能是直流无刷电机或无刷伺服马达。电机必须配备一些种回馈装置，除非它是一个步进电机。图 2显示了一个完善地反馈控制电机转速的系统。这样一个具有闭环控制系统的速度伺服系统。图2 典型的闭环（速度）伺服系统驱动器是一个电子功率放大器，以提供电力操作电动机来回应低层次的控制信号。一般来说，驱动器将特别设计，其操作与特定电机类型相配合。例如，你不能用一个步进驱动器来操作直流无刷电机。不同电机适应的不同领域步进电机：步进电机的好处。（1）成本低廉（2）坚固耐用（3）结构简单（4）
39、高可靠性（5）无维修（6）适用广泛（7）稳定性很高（8）无需反馈元件（8）适应多种工作环境（9）相对伺服电机更具有保险性。因此，几乎没有任何可以想象的失败使步进驱动模块出错。步进电机驱动简单，并且驱动和控制在一个开放的闭环系统内。他们只需要4个驱动器。低速时，驱动器提供良好的扭矩，是有刷电机同一帧大小5倍连续力矩，或相当于无刷电机一倍扭矩。这往往不再需要变速箱。步进驱动系统迟缓，在限定的范围内，可以更好的减少动态位置误差。步进电机弊端。步进电机有下列缺点：（1）共振效应和相对长的适应性（2）在低速，表现粗糙，除非微驱动器来驱动（3）开环系统可能导致未被查觉的损失（4）由于过载，他们消耗过多电流
40、。因此倾向于过热运行。（5）亏损速度比较高，并可产生过多热量因此，他们噪音很大（尤其是在高速下）。（6）他们的滞后现象导致振荡，这是很难抑制的。对他们的可行性，这儿有一个限度，而他们的大小，定位精度主要依靠的是机器（例如，滚珠丝杠的精确度） 。许多这些缺点是可以克服的，通过使用一个闭环控制方案。注：comp motor系列很多地减小或降低了这些不同的步进电机不利之处。主要有3类步进电机：（1）永磁式步进电机 ，（2）可变磁阻式步进电动机，（3）混合式步进电机汽车。当电动机驱动，给两个绕组通电时或2相通电的时候（见图 3），扭矩可于每一个步将是相同（除极少数的变异和传动特性）。在半步模式下，我们
41、交替改变两相电流，如图4所示。假设该驱动器在每种情况下提供了相同的绕组电流，再通电时，这将导致更大的转矩。换句话说，交替的步进距将时强时若。对电动机表现来说，这并不代表着一个重大的威慑。扭矩明显受制于较弱的一步，低速平滑有一个显着的改善。显然，我们想在每一个步骤实现约相等扭矩对时，这扭矩应该在水平较强的一步。们可以实现这个，当只有一个绕组通电时，通过用高电流水平。这并不过度消耗电机，因为该电机的额定电流假定两个阶段被激活（目前的评级是基于许可的情况温度） 。只有一相通电，如果目前是增加了40的功率，同样的总功率将会消散。利用这种更高的电流在一相中产生大致相等的扭矩，在交替的步进距中。（见图5
42、）图3半步电流图4 全步电流 图5 侧面全步电流我们已经看到，给两相都通与相同的电流产生的一个中间的步进，居于每一相的中间位置。如果两相电流是不相等的，转子位置将转向更强的一极。这种作用是利用细分驱动，其中细分的大小基于两个绕组中的电流的大小。以这种方式，步长是减少了，而低速平滑度得到大幅度提高。高细分驱动电动机细分整步步进到多达500个细分步，转一圈可细分十万步。在这种情况下，绕组中的电流极为相似的两个正弦波有90相移。（图1.11）电机被驱动好像转换成了交流同步电机。事实上，步进电机可被驱动，从60赫兹美（50赫兹-欧洲）正弦波源头起，包括电容器系列的一相。它将旋转72转。图6步进电机的相
43、电流标准200步混合电机标准步进电机运行在同就如同我们的简单模式，但有一个更大的数目齿数在转子和定子中，从而有了一个较小的基本步长。转子有2部分，但每部分有50个齿。该齿轮位于两部分之间。定子每5个齿有8个极，完整的共有40个齿（见图1.12）图7 200步混合标准电机如果我们想象一个齿，是摆在2个定子极点每一齿隙中，假设定子共有48个齿，少于转子齿数两个。因此，如果转子和定子的齿排列一整圈，他们同样也可以排列半圈。1/4和3/4圈也同样可以排列。然而，由于转子的齿排列位置，在另一端的转子，排列将发生在1/4和3/4位置处。绕组4个一组，并对角线方向的极性相反。如图7所示，北极在转子前面的12
44、点和6点位置，吸引着在在背面3时和9时的南极。通过开关第二组线圈的电流，定子模式旋转45。不过，要配合这个新的领域，转子只转过1.8。相当于转子，这只转过了四分之一的齿间距，每一次旋转要200个全步。注意到，每一次旋转全部时这儿有很多定位点位置，通常是200个。该定位点的位置与转子的齿能全面接轨定子齿时相对应。的当通电给步进驱动器时，它通常是零状态时最活跃，也就是两套绕组都通电。因此产生的转子位置并不符合转子自然定位点的位置。因此，空载时，一旦通电电机将至少步进半步。当然，如果系统关机，或在零相位位置，电机一旦通电将步进一大步。另一点要注意的是，对于一个给定电流的绕组，有很多稳定的位置，正如转
45、子齿（200步进电机有50个齿）。如果电机是同步电机，导致位置误差将永远是一个整体倍转子齿或能被7.2整除。电机不能细分，如个别一个或两个位置误差，是由于噪声，错误脉冲或控制器故障造成的。图8数字伺服驱动图8显示为伺服电机的数控驱动。所有的主控制功能是微处理器，驱动为DA模拟转换器，以产生一个模拟扭矩需求信号。从这个角度上，这台机器非常很像一个模拟伺服放大器。反馈的信息是来自隶属该电机轴的一个编码器。编码器生成脉冲流可确定传输路程，并通过计算脉冲频率，是可以测定转速的。 数码驱动通过求解一系列的方程式，履行同样类似的功能。微处理器是与数学模型（或“算法）的等效的编程模拟系统。这模型预测系统的行
46、为。它响应一个给定输入的信号并产生速度。它同样也考虑到额外信息如输出速度，速率转变中的投入和各种调校设定。解决所有方程需数额需有限的时间，即使是一个快速的处理器一次处理通常也是100ms和2ms之间。在此之间，在改变输入或输出，先前的计算值将有没有回应时，扭矩要求必须保持恒定。因此更新时间成为数字伺服和一台高性能系统关键的因素，它必须保持及时更新。调试数字伺服电机可按钮或从一个计算机或终端调试。电位器调整是涉及的。调试数据是设置在伺服算法的各种系数，因此，它决定了系统的性能。即使如果调谐进行使用按钮，终值也可以上传到终端，让其进行简单的重复。在某些应用中，因负载惯量各异，例如一个机器手臂卸载后又带有沉重的负荷。改变惯性可能是一个系数为20或以上，而这样的变化需要该驱动器重新调整，以保持其稳定。这只不过是在操作系统的适当点通过发送新的调试参数来实现的。
