外文翻译。步进电机和伺服电机的系统控制

Step Motor&Servo Motor Systems and Controls

Motion Architect? Software Does the Work for You... Configure ,Diagnose, Debug Compumotor’s Motion Architect is a Microsoft? Windows?-based software development tool for 6000Series products that allows you to automatically generate commented setup code, edit and execute motion control programs, and create a custom operator test panel. The heart of Motion Architect is the shell, which provides an integrated environment to access the following modules.

? System Con figurator—This module prompts you to fill in all pertinent set-up information to initiate motion. Configurable to the specific 6000 Series product that is selected, the information is then used to generate actual 6000-language code that is the beginning of your program.

? Program Editor—This module allows you to edit code. It also has the commands available through “Help” menus. A user’s guide is provided on disk.

? Terminal Emulator—This module allows you to interact directly with the 6000 product. “Help” is again available with all commands and their definitions available for reference. ? Test Panel—You can simulate your programs, debug programs, and check for program flow using this module.

Motion Architect? has been designed for use with all 6000 Series products—for both servo and stepper technologies. The versatility of Windows and the 6000 Series language allow you to solve applications ranging from the very simple to the complex.

Motion Architect comes standard with each of the 6000 Series products and is a tool that makes using these controllers even more simple—shortening the project development time considerably. A value-added feature of Motion Architect, when used with the 6000 Servo Controllers, is its tuning aide. This additional module allows you to graphically display a variety of move parameters and see how these parameters change based on tuning values.

Using Motion Architect, you can open multiple windows at once. For example, both the Program Editor and Terminal Emulator windows can be opened to run the program, get information, and then make changes to the program.

On-line help is available throughout Motion Architect, including interactive access to the contents of the Compumotor 6000 Series Software Reference Guide.

SOLVING APPLICATIONS FROM SIMPLE TO

COMPLEX

Servo Control is Yours with Servo Tuner Software

Compumotor combines the 6000 Series servo controllers with Servo Tuner software. The Servo Tuner is an add-on module that expands and enhances the capabilities of Motion Architect?.

Motion Architect and the Servo Tuner combine to provide graphical feedback of

real-time motion information and provide an easy environment for setting tuning gains and related systemparameters as well as providing file operations to save and recall tuning sessions.

Draw Your Own Motion Control Solutions with Motion Toolbox Software Motion Toolbox? is an extensive library of LabVIEW? virtual instruments (VIs) for icon-based programming of Compumotor’s 6000 Series motion controllers.

When using Motion Toolbox with LabVIEW, programming of the 6000 Series controller is accomplished by linking graphic icons, or VIs, together to form a block diagram. Motion Toolbox’s has a library of more than 150 command,status, and example VIs. All command and status VIs include LabVIEW source diagrams so you can modify them, if necessary, to suit your particular needs. Motion Toolbox als user manual to help you gut up and running quickly.

comprehensiveM Software for Computer-Aided Motion Applications CompuCAM is a Windows-based programming package that imports geometry from CAD programs, plotter files, or NC programs and generates 6000 code compatible with Compumotor’s 6000 Series motion controllers. Available for purchase from Compumotor, CompuCAM is an add-on module which is invoked as a utility from the menu bar of Motion Architect.

From CompuCAM, run your CAD software package. Once a drawing is created, save it as either a DXF file, HP-GL plot file or G-code NC program. This geometry is then imported into CompuCAM where the 6000 code is generated. After generating the program, you may use Motion Architect functions such as editing or downloading the code for execution.

Motion Builder Software for Easy Programming of the 6000 Series

Motion Builder revolutionizes motion control programming. This innovative software allows programmers to program in a way they are familiar with—a flowchart-style method. Motion Builder decreases the learning curve and makes motion control programming easy.

Motion Builder is a Microsoft Windows-based graphical development environment which allows expert and novice programmers to easily program the 6000 Series products without learning a new programming language. Simply drag and drop visual icons that represent the motion functions you want to perform.

Motion Builder is a complete application development environment. In addition to visually programming the 6000 Series products, users may configure, debug, download, and execute the motion program.

SERVO VERSUS STEPPER... WHAT YOU NEED TO

KNOW

Motor Types and Their Applications

The following section will give you some idea of the applications that are particularly appropriate for each motor type, together with certain applications that are best avoided. It should be stressed that there is a wide range of applications which can be equally well met by more than one motor type, and the choice will tend to be dictated by customer preference, previous experience or compatibility with existing equipment.

A helpful tool for selecting the proper motor for your applicat ion is Compumotor’s Motor Sizing and Selection software package. Using this software, users can easily identify the appropriate motor size and type.

High torque, low speed

continuous duty applications are appropriate to the step motor. At low speeds it is very efficient in terms of torque output relative to both size and input power. Microstepping can be used to improve smoothness in lowspeed applications such as a metering pump drive for very accurate flow control.

High torque, high speed

continuous duty applications suit the servo motor, and in fact a step motor should be avoided in such applications because the high-speed losses can cause excessive motor heating.

Short, rapid, repetitive moves

are the natural domain of the stepper due to its high torque at low speeds, good

torque-to-inertia ratio and lack of commutation problems. The brushes of the DC motor can limit its potential for frequent starts, stops and direction changes.

Low speed, high smoothness application s

are appropriate for microstepping or direct drive servos.

Applications in hazardous environments

or in a vacuum may not be able to use a brushed motor. Either a stepper or a brushless motor is called for, depending on the demands of the load. Bear in mind that heat dissipation may be a problem in a vacuum when the loads are excessive. SELECTING THE MOTOR THAT SUITS YOUR APPLICATION Introduction

Motion 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, typically 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 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 wouldn’t be considered a motion control system in the strict sense of the term.Our standard motion control system consists of three basic elements:

Fig. 1 Elements of motion control system

The 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 motor speed. Such a system is known as a closed-loop velocity servo system.

Fig. 2 Typical closed loop (velocity) servo system

The drive. This is an electronic power amplifier thatdelivers the power to operate the motor in response to low-level control signals. In general, the drive will be specifically designed to operate with a particular motor type –you can’t use a stepper drive to operate a DC brush motor, for instance.

Application Areas of Motor Types

Stepper Motors

Stepper Motor Benefits

Stepper motors have the following benefits:

? Low cost

? Ruggedness

? Simplicity in construction

? High reliability

? No maintenance

? 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 are 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 for a gearbox. A stepper-driven-system is inherently stiff, with known limits to the dynamic position error.

Stepper Motor Disadvantages

Stepper motors have the following disadvantages:

? Resonance effects and relatively long settling

times

? Rough performance at low speed unless a

microstep drive is used

? 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 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., ballscrew accuracy). Many of

these drawbacks can be overcome by the use of

a closed-loop control scheme.

Note: The Compumotor Zeta Series minimizes or

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 Motors

When the motor is driven in its full-step mode, energizing two windings or “phases” at a time (see Fig. 1.8), the torque 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. 1.9. Assuming the drive delivers the same winding current in each case, this will cause 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 performance—the available torque is obviously limited by the weaker step, but there will be a significant improvement in low-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 dissipate the motor because the manufacturer’s 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-phase-on state produces approximately equal torque on alternate steps (see Fig. 1.10).

Fig. 1.8 Full step current, 2-phase on

Fig. 1.9 Half step current

Fig. 1.10 Half step current, profiled

We have seen that energizing both phases with equal currents produces an intermediate step position half-way between the one-phase-on positions. If the two phase currents are unequal, the rotor position will be shifted towards the stronger pole. This effect is utilized in the microstepping 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-speed smoothness is dramatically improved. High-resolution microstep drives divide the full motor step into as many as 500 microsteps, 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 Fig. 1.11). 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. 1.11 Phase currents in microstep mode

Standard 200-Step Hybrid Motor

The 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-tooth displacement between the two sections is retained. The stator has 8 poles each with 5 teeth, making a total of 40 teeth (see Fig. 1.12).

Fig. 1.12 200-step hybrid motor

If 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 number of rotor teeth. So if rotor and stator teeth are aligned at 12 o’clock, they will also be aligned at 6 o’clock. At 3 o’clock and 9 o’clock the teeth will be misaligned. However, due to the displacement between the sets of rotor teeth, alignment will occur at 3 o’clock and 9 o’clock at the other end 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. 1.12, the north poles at 12 and 6 o’clock attract the south-pole teeth at the front of the rotor; the south poles at 3 and 9 o’clock attract the north-pole teeth at 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 there 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 sets 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 step 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 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 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. 2.19 Digital servo drive

Digital Servo Drive Operation

Fig. 2.19 shows the components of a digital drive for a servo motor. All the main control functions are carried out by the microprocessor, which drives a D-to-A convertor 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 generates a pulse stream from which the processor can determine the distance travelled, 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 programmed with a mathematical model (or “algorithm”) of the equivalent analog system. This model predicts the behavior of the system. In response to a given input demand and output position. 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 equations 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 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 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.

In some applications, the load inertia varies between wide limits – think 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 the appropriate point in the operating cycle.

步进电机和伺服电机的系统控制

运动的控制者---软件：只要有了软件，它可以帮助我们配置改装、诊断故障、调试程序等。数控电动机的设计者会是一个微软窗口——基于构件的软件开发工具，可以为6000系列产品设置代码，同时可以控制设计者与执行者的运动节目，并创造一个定制运营商的测试小组。运动建筑师的心脏是一个空壳，它可以为进入以下模块提供一个综合环境。

1.系统配置——这个模块提示您填写所有相关初成立信息启动议案。配置向具体

6000系列产品的选择，然后这些信息将用于产生实际的6000 - 语言代码，这是你的开始计划。

2.程序编辑器——允许你编辑代码。它也有可行的“帮助”命令菜单。A用户指

南提供了相关的磁盘指南。

3.终端模拟器——本模块，可让您直接与6000系列产品互动。他所提供的“帮

助”是再次参考所有命令和定义。

4.测试小组——你可以使用本模块，模拟程序，调试程序，并跟踪检测程序。

运动建筑师已经将所有的6000系列产品都运用在了步进电机和伺服电机的技术上。由于丰富的对话窗口和6000系列语言，使得你能够从简单到复杂的解决问题。

运动建筑师的6000系列产品的标准配置工具，能够使得这些控制器更加简单，相当大的缩短项目开发时间。它的另外一个增值特点是使用6000伺服控制器的调谐助手。基于调谐价值观，这个额外的模块可以以图形化的方式为你展示各种参数。看看这些参数是如何让变化的。用运动的建筑师，你可以一次性打开多个窗口。举例来说，无论是程序编辑器和终端模拟器窗口，你都可以打开运行程序，得到信息，然后改变这一程序。运动建筑师可以利用在线帮助，在整个互动接触内容中为

数控电机6000系列软件做参考指南。

从简单到复杂的解决应用

伺服控制是你用伺服调谐器软件控制。数控电机与6000系列伺服控制器相结合并应用伺服调谐器软件。伺服调谐器是一个新增功能模块，它扩展和提高运动建筑师的能力。议案建筑师与伺服调谐器结合起来，以提供图形化的反馈方式，反馈实时运动信息并提供简便环境设置微调收益及相关制参数以及提供文件操作，以保存并记得微调会议。

请你用运动工具箱软件解决自己的运动控制。运动工具箱实际上是一个为数控电机和6000系列运动控制器而设计的广泛应用的虚拟图标式编程仪器。

当使用运动工具箱与虚拟编程仪时，编程6000系列控制器实质上是完成连接图形图标，或加上形成框图使之可见。运动工具箱中包含了1500多条命令，状态栏，实例等。所有的命令、状态栏、实例都包括可视的来源图表，使您可以修改他们，如果有必要，可以满足您的特殊的需要。运动工具箱同时还具有一个可视窗口，基于安装程序和一个全面的用户手册，可以帮助您运行得更好更快。

软件电脑辅助运动应用软件compucam

compucam是基于微软的编程包，它能从CAD程序、示波器文档、数控程序和产生6000系列数控电机密码相兼容的运动控制器中输入几何图形。购买数控电机是可行的，因为compucam是一个附加模块，是运动建筑师的菜单栏，它是作为公用部分而被引用的。程序从compucam开始运行CAD软件包。一旦程序被起草创作，它就会被保存为DXF文件，或惠普-吉尔段文档，或G代码数控程序。这些几何图形然后输入compucam中，产生6000系列代码。在程序运行之后，你可使用的运动建筑师功能块，如编辑或下载代码等执行程序。

运动执行者软件可轻松编程6000系列

运动执行者革命性控制运动编程。这一具有创新意义的软件允许程序员以他们

所熟悉的- 流程图式的方法编程。运动执行者降低了学习曲线，并使运动控制编程变得相当容易。运动执行者是一套微软软件，基于图形化窗口的发展，让专家和新

手程序员容易学习计划6000系列产品新的编程语言。简单地拖放代表议案职能的

视觉图标，你可以随时的进行你所需要的操作。运动执行者是一个完整的应用开发

环境的软件。除了视觉编程6000 系列产品，用户还可以配置，调试，下载，策划和执行的议案计划。

电机类型及其应用

下一节将会给你介绍一些的适用特别场合的电机，而某些应用是最好避免。应当强调说，在一个广范的应用范围，电机是可同样满足一个以上的汽车类型，而选择往往是由客户偏好、以往经验或与现有的设备的兼容性决定的。一个非常有用的工具箱，可供你选择适当的运动，为你选择电机与选择软件包是compumotor软件包。使用这个软件，使用户可以轻松找出适当的电机大小和类型。

高扭矩，低转速

连续脉冲适宜于步进电机时，在低速时，就相对于扭矩输出规模和输入功率而言，它是非常高效率。微步，在低速应用，可以用来提高平滑度。如可作为计量泵驱动非常精确的流量控制。

高扭矩，高转速

连续脉冲适应于伺服电机时，其实步进电机应避免使用在这种情况下。这是因为高速可导致负荷。

简捷，快速，重复性动作

仅是自然域的步进电机由于其在低速时高转矩，因而存在惯性比例大，及缺乏折算的问题。直流电动机的电刷可限制其潜在的频繁开始，停止和方向的改变。

低速，高光滑的应用

这是最适合于微步或直接驱动伺服电机。

适用于危险环境或在真空中可能不能够使用电刷电机。步进或无刷电机是无所谓的，靠的是对负荷的需求。牢记当负载过高时，热耗散可能是个问题。

选择适合你的电机

导言

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运动控制系统的组成元件

混合式步进直流

伺服无刷电机电机，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

转速表

电压反馈

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。

步进电机有以下好处：

（1）成本低廉（2）坚固耐用（3）结构简单（4）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倍连续力距，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。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：compumotor系列能很多的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，在其全步模式，给两个绕组通电时或"2相"通电的时候（见图1.8 ），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，我们交替改变两相电流，如图1.9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，但在全步模式时，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1.10 ）

图1.8 图1.9 图1.10

我们已经看到，给两相都通与平等电流产生的一个中间步进，居于每一相的中

间位置。如果两相电流是不平等的，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1.11步进电机的相电流

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。

该半齿位于两部分之间。定子每5个齿有8个极，完整的共有40个齿（见图1.12 ）图1.12 200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1.12所示，北极在转子前面的12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，定子场模式旋转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齿全面接轨定子齿时相对应。当通电给步进驱动器时，它通常是"零阶段"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2.19数字伺服驱动

图2.19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D A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100 ms和2 ms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。