外文翻译----步进电机基础

资料翻译

英文资料

Stepper Motor Basics

[TieluoLin.Jianxun Zhang．DSP-based microstep controller of stepper motor．Intelligent Control and Automation, 2004．Fifth World Congress on Volume 5, 15-19 June 2004.]

A stepper motor is an electromechanical device which converts electrical pulses into discrete mechanical movements. The shaft or spindle of a stepper motor rotates in discrete step increments when electrical command pulses are applied to it in the proper sequence. The motors rotation has several direct relationships to these applied input pulses. The sequence of the applied pulses is directly related to the direction of motor shafts rotation. The speed of the motor shafts rotation is directly related to the frequency of the input pulses and the length of rotation is directly related to the number of input pulses applied.

Stepper Motor Advantages and Disadvantages

Advantages

1. The rotation angle of the motor is proportional to the input pulse.

2. The motor has full torque at standstill (if the windings are energized)

3. Precise positioning and repeatability of movement since good stepper motors have an accuracy of 3 – 5% of a step and this error is non cumulative from one step to the next.

4. Excellent response to starting/stopping/reversing.

5. Very reliable since there are no contact brushes in the motor. Therefore the life of the motor is simply dependant on the life of the bearing.

6. The motors response to digital input pulses provides open-loop control, making the motor simpler and less costly to control.

7. It is possible to achieve very low speed synchronous rotation with a load that is directly coupled to the shaft.

8. A wide range of rotational speeds can be realized as the speed is proportional to the frequency of the input pulses.

Disadvantages

1. Resonances can occur if not properly controlled.

2. Not easy to operate at extremely high speeds.

Open Loop Operation

One of the most significant advantages of a stepper motor is its ability to be accurately controlled in an open loop system. Open loop control means no feedback information about position is needed. This type of control eliminates the need for expensive sensing and feedback devices such as optical encoders. Your position is known simply by keeping track of the input step pulses.

Stepper Motor Types

There are three basic stepper motor types. They are :

? Variable-reluctance

? Permanent-magnet

? Hybrid

Variable-reluctance (VR)

This type of stepper motor has been around for a long time. It is probably the easiest to understand from a structural point of view. Figure 1 shows a cross section of a typical V.R. stepper motor. This type of motor consists of a soft iron multi-toothed rotor and a wound stator. When the stator windings are energized with DC current the poles become magnetized. Rotation occurs when the rotor teeth are attracted to the energized stator poles.

Figure 1. Cross-section of a variablereluctance(VR) motor.

Permanent Magnet (PM)

Often referred to as a “tin can” or “canstock” motor the permanent magnet step motor is a low cost and low resolution type motor with typical step angles of 7.5° to 15°. (48 –24steps/revolution) PM motors as the name implies have permanent magnets added to the motor structure. The rotor no longer has teeth as with the VR motor. Instead the rotor is magnetized with alternating north and south poles situated in a straight line parallel to the rotor shaft. These magnetized rotor poles provide an increased magnetic flux intensity and because of this the PM motor exhibits improved torque characteristics when compared with the VR type.

Hybrid (HB)

The hybrid stepper motor is more expensive than the PM stepper motor but provides better performance with respect to step resolution, torque and speed. Typical step angles for the HB stepper motor range from 3.6°to 0.9°(100 –400 steps per revolution). Thehybrid stepper motor combines the best features of both the PM and VR type stepper motors. The rotor is multi-toothed like the VR motor and contains an axially magnetized concentric magnet around its shaft. The teeth on the rotor provide an even better path which helps guide the magnetic flux to preferred locations in the

airgap. This further increases the detent, holding and dynamic torque characteristics of the motor when compared with both the VR and PM types.

The two most commonly used types of stepper motors are the permanent magnet and the hybrid types. If a designer is not sure which type will best fit his applications requirements he should first evaluate the PM type as it is normally several times less expensive. If not then the hybrid motor may be the right choice.

There also excist some special stepper motor designs. One is the disc magnet motor. Here the rotor is designed sa a disc with rare earth magnets, See fig. 5 . This motor type has some advantages such as very low inertia and a optimized magnetic flow path with no coupling between the two stator windings. These qualities are essential in some applications.

Size and Power

In addition to being classified by their step angle stepper motors are also classified according to frame sizes which correspond to the diameter of the body of the motor. For instance a size 11 stepper motor has a body diameter of approximately 1.1 inches. Likewise a size 23 stepper motor has a body diameter of 2.3 inches (58 mm), etc. The body length may however, vary from motor to motor within the same frame size classification. As a general rule the available torque output from a motor of a particular frame size will increase with increased body length.

Power levels for IC-driven stepper motors typically range from below a watt for very small motors up to 10 –20 watts for larger motors. The maximum power dissipation level or thermal limits of the motor are seldom clearly stated in the motor manufacturers

data. To determine this we must apply the relationship P=V×I For example, a size 23 step motor may be rated at 6V and 1A per phase. Therefore, with two phases energized

the motor has a rated power dissipation of 12 watts. It is normal practice to rate a stepper motor at the power dissipation level where the motor case rises 65°C above the ambient in still air. Therefore, if the motor can be mounted to a heatsink it is often

possible to increase the allowable power dissipation level. This is important as the motor is designed to be and should be used at its maximum power dissipation ,to be efficient from

a size/output power/cost point of view.

When to Use a StepperMotor

A stepper motor can be a good choice henever controlled movement is equired. They can be used to advantage in applications where you need to control rotation angle, speed, position and synchronism. Because of the inherent advantages listed previously, stepper motors have found their place in many different applications. Some of these include printers, plotters, highend office equipment, hard disk drives, medical equipment, fax machines, automotive and many more.

The Rotating Magnetic Field

When a phase winding of a stepper motor is energized with current a magnetic flux is developed in the stator. The d When a phase winding of a stepper motor is energized with current a magnetic flux is developed irection of this flux is determined by the “Right Ha nd

Rule” which states: “If the coil is grasped in the right hand with the fingers pointing in the direction of the current in the winding (the thumb is extended at a 90°angleto the fingers), then the thumb will point in the direction of the magnetic field.”

Figure 2 shows the magnetic flux path developed when phase B is energized with winding current in the direction shown. The rotor then aligns itself so that the flux opposition is minimized. In this case the motor would rotate clockwise so that its south pole aligns with the north pole of the stator B at position 2 and its north pole aligns with the south pole of stator B at position 6. To get the motor to rotate we can now see that we must provide a sequence of energizing the stator windings in such a fashion that provides a rotating magnetic flux field which the rotor follows due to magnetic attraction.

Figure 2 Magnetic flux path through atwo-pole stepper motor with a lag betweenthe rotor and stator.

Torque Generation

The torque produced by a stepper motor depends on several factors.

? The step rate

? The drive current in the windings

? The drive design or type

In a stepper motor a torque is developed when the magnetic fluxes of the rotor and stator are displaced from each other. The stator is made up of a high permeability magnetic material. The presence of this high permeability material causes the magnetic flux to be confined for the most part to the paths defined by the stator structure in the same fashion that currents are confined to the conductors of an electronic circuit. This serves to concentrate the flux at the stator poles. Thetorque output produced by the motor is proportional to the intensity of the magnetic flux generated when the winding is energized.

The basic relationship which defines the intensity of the magneticflux is defined by:

H = (N ×i) ÷ l where:

N = The number of winding turns

i = current

H = Magnetic field intensity

l = Magnetic flux path length

This relationship shows that the magnetic flux intensity and consequently the torque is proportional to the number of winding turns and the current and inversely proportional to the length of the magnetic flux path. From this basic relationship one can see that the same frame size stepper motor could have very different torque output capabilities simply by changing the winding parameters. More detailed information on how the winding parameters affect the output capability of the motor can be found in the application note entitled “DriveCircuit Basics”.

Stepping Modes

The following are the most common drive modes.

? Wave Drive (1 phase on)

? Full Step Drive (2 phases on)

? Half Step Drive (1 & 2 phases on)

? Micro stepping (Continuously varying motor currents)

For the following discussions please refer to the figure 3.

Figure 3 Unipolar and bipolar wound stepper motors.

In Wave Drive only one winding is energized at any given time. The stator is energized according to the sequence A B A B and the rotor steps from position

8 2 4 6. For unipolar and bipolar wo und motors with the same winding parameters this excitation mode would result in the same mechanical position. The disadvantage of this drive mode is that in the unipolar wound motor you are only using 25% and in the bipolar motor only 50% of the total motor winding at any given time. This means that you are not getting the maximum torque output from the motor.

In Full Step Drive you are energizingtwo phases at any given time.The stator is energized according to the sequence AB A B A B AB and the rotor steps from position 1 3 5 7 . Full step mode results in the same angular movement as 1 phase on drive but the mechanical position is offset by one half of a full

step. The torque output of the unipolar wound motor is lower than the bipolar motor (for motors with the same winding parameters) since the unipolar motor uses only 50% of the available winding while the bipolar motor uses the entire winding.

Half Step Drive combines both wave and full step (1&2 phases on) drive modes. Every second step only

one phase is energized and during the other steps one phase on each stator. The stator is energized according to the sequence AB B A B A A B B A B A and the rotor steps from position 1 2 3 4 5 6 7 8. This results in angular movements that are half of those in 1- or 2-phases-on drive modes. Half stepping can reduce a phenomena referred to as resonance which can be experienced

in 1- or 2-phases-on drive modes.

The excitation sequences for the above drive modes are summarized in Table 1. Table 1. Excitation sequences for different drive modes

In Microstepping Drive the currents in the windings are continuously varying to be able to break up one full step into many smaller discrete steps. More information on microstepping can be found in the microstepping chapter.

Single Step Response and Resonances

The single-step response characteristics of a stepper motor is shown in figure 4.

Figure 4 Single step response vs. time.

When one step pulse is applied to a stepper motor the rotor behaves in a manner as defined by the above curve.The step time t is the time it takes the motor shaft to rotate one step angle once the first step pulse is applied. This step time is highly dependent on the ratio of torque to inertia (load) as well as the type of driver used.

Since the torque is a function of the displacement it follows that the acceleration will also be. Therefore, when moving in large step increments a high torque is developed and consequently a high acceleration. This can cause overshots and ringing as shown. The settling time T is the time it takes these oscillations or ringing to cease. In certain applications this phenomena can be undesirable. It is possible to reduce or eliminate this behaviour by microstepping the stepper motor. For more information on micro stepping please consult the microstepping note.

Stepper motors can often exhibit a phenomena refered to as resonance at certain step rates. This can be seen as a sudden loss or drop in torque at certain speeds which can result in missed steps or loss of synchronism. It occurs when the input step pulse

rate coincides with the natural oscillation frequency of the rotor. Often there is a resonance area around the 100 – 200 pps region and also one in the high step pulse rate region. The resonance phenomena of a stepper motor comes from its basic construction and therefore it is not possible to eliminate it completely. It is also dependent upon the load conditions. It can be reduced by driving the motor in half or micro stepping modes.

中文译文

步进电机基础

[林铁国，张建勋．基于DSP的微控制器的步进电机控制和自动化， 2004 。第五次世界代

表大会第五册15-19日， 2004年6月．]

步进电机是一种机电设备，它把电气转换成脉冲离散机械动作。当电器指挥脉冲以正确的顺序应用时，轴或主轴步进电机旋转一步离散增量。电动机转动对这些应用输入脉冲有几种直接联系。应用脉冲的序列与电机轴旋转方向直接相关。电机轴旋转的速度与输入脉冲的频率直接相关，旋转的长度与应用输入脉冲的数字输入直接相关。

步进电机的优点和缺点:

优点：1. 电机旋转的角度与脉冲输入成正比;

2. 如果绕组被加强，电机转距会完全静止;

3.因为良好的步进电机有3%-5%的准确率而且即使有错误，也不是从一步

累计到另一步的，所以步进机能够精确定位并且可重复性运动;

4.对启动/停止/旋转有良好的反应;

5. 因为电机上没有连接点，所以它非常可靠。电机的寿命是由轴承的寿命决定

的;

6.电机对数字输入脉冲的反应提供提供开放环路的控制，这使得电机控制起来比

较容易而且不那么昂贵;

7.在直接地结合对轴的装载时，它是可能达到非常低速的同步自转的;

8. 当速度与输入脉冲的频率成比例时，就可以实现大范围的旋转速度

缺点：1. 如果不恰当地控制可能会发生共鸣;

2. 如果速度太快就不易操纵;

开环操作：

步进机的一个显著优势就是它的功能能在开环系统中被准确控制。开环控制意味着对需要的位置是没有反馈信息的。这类型的控制系统消除了对昂贵感应与反馈设置例如对光学编码器等的需要。通过跟踪输入脉冲就能很容易地知晓您所处的位置。

步进机类型：

有三种步进机类型，它们是：

1.可变磁阻型;

2.永久磁场型;

3.杂交复合型;

可变磁阻型：

这种步进电机已存在了很长的时间.从结构的观点来看它可能是最容易理解的。图1显示了典型的可变磁阻型步进机的截面。这种电机是由一个软铁多齿转子和创伤定子构成的。当定子绕组电源被直流电强化，电机的两极就被磁化了。当转子齿被吸引到磁化的两极时，电机就转动了。

图1 可变磁阻型步进机的截面

永久磁场型：

这种电机通常被称作“罐”或“倾斜”电机。这种永久磁场型电机是一种低成本和低分辨率类型的电机，它典型的分辨角度是7.5°到15°。永久磁场型步进电机顾名思义有永磁材料添加到电机结构中。与可变磁阻型电机相比，这种类型的电机转子不再有齿，取而带之的是转子随着电机南北两极交替位于一条直线平行于转子轴而磁化。这些磁化的转子级为电机提供一种增加的磁性涨潮强度，也因此这种类型的步进机与可变磁阻型步进机相比具有更高级的转距特征。

杂交复合型：

混合型步进机比永久磁场型步进机要贵，但是因为考虑到了步距，转距以及速度，它的表现要比永久磁场型步进机表现优良。对于杂交复合型步进机而言，典型的步距角度是从 3.6°到 0.9°。混合式步进机中和了可变磁阻型步进机和永久磁场型步进机的优良特征。转子像可变磁阻型步进机一样是多齿型的，并含有磁化轴同心圆磁铁绕其轴。转子上的齿提供了一条更好的路径，它有助于有助于引导磁通量为首选地点气隙。与以上两种类型的步进机相比，这进一步增加了定位、控股及动态转矩特性。

两种最常用类型的步进机是永久磁场型步进机和混合型步进机。如果一个设计师不知道哪种步进机是最适合其应用需求的，他应该首先评估挑选永久磁场型的，因为它通常是最便宜的。如果永久磁场型步电机不行的话，那么混合型的也许会是合适的选择。

同样，还存在一些特殊类型的步进机。一种是盘式永磁步进机。这里的转子是用稀土永磁材料制成的，设计得像一张唱片。（见图2）这型发动机具有一些优势，例如很低的惰性和没有围绕在两定子之间的优化磁流路。这些品性在某些应用中是必不可少的。

图2 磁通通过两杆步进电机的转子和定子之间

尺寸和功能：

除了按自己的步距角度分类之外，步进机也按与电机直径相关的整体尺寸来分类。例如一个尺寸为11的步进机直径大约为1.1英寸。同样大小为23的步进机直径为2.3英寸( 58毫米)。整体大小相同的步进机由于属于不同的机子在机身长度上可能会有变化，作为一般规则，某一特定尺寸电动机的可供输出力矩随机身长度的增加而增加。

功率级集成电路驱动步进电机一般由对非常小电机的1瓦特到对大型电机的10-20瓦特。在汽车制造商的资料里，电机的最高功耗水平和热量限制很少明确表示出来。为了证明这一点，我们必须运用关系式P=V×I，例如：大小为23的步进电机,每阶段可在额定的6V和1A，因此,两相电源的电动机的额定功耗为12瓦特。测量步进机的热量挥发水平，这是通常的做法。在静止的空气中，步进机的热量上升到65°。如果电动机可以挂载到一个散热器也常常能够增加允许功耗水平，这一点很重要，因为步进机的设计要求它应该从它的最高功耗、尺寸输出功率或者尺寸输出成本的角度来加以使用。

何时使用步进电机

当需要控制运动时，步进电机可以成为很好的选择。当你需要控制旋转角度,速度,位置和同步时，它们能够在应用中发挥优势。因为自身固有的优势，步进电机在多种不同的应用种都找到了它们的位置。其中一些项目包括打印机、绘图仪、精品办公设备、电脑硬盘、医疗设备、传真机，汽车和更多。

旋转磁场

当一相绕组的步进电机电源电流与磁是发达的定子，电流的方向是由“右手定则”决定的。“右手定则”规定：“若磁力线垂直进入右手，四指所指方向为导线中感应电流的方向，则大拇指所指的方向就是磁场的方向。”

图5显示了磁通路径发展，B阶段随着所示绕组电流的方向而加强。转子控制自己使反向流量最低。在这种情况下将电机顺时针转动,使南极配合北极的定子B在位置2和北极配合南极定子B在位置6 ，获得电机轮换。为了让步进电机转

动，我们现在可以看到，我们必须提供一组序列定子，这组定子可以提供一个旋转磁场，由于磁吸引力，带动定子转动。

扭矩代转矩控制取决于若干因素:

.步距;

.绕组驱动电流;

.驱动设计或类型;

在一个步进电机中，当定子和转子的磁流量彼此取代时，扭距才发生变化。定子是由高渗透磁性物质组成的，这种高渗透磁性物质的存在导致磁流量被部分地限定，这有助于磁流量集中在定子两极。当绕组加强时，电机的扭力输出与磁流量产生的强度成比例。

界定磁流量的基本项：

H = (N ×i) ÷ l

N ：匝数

i ：电流

H：磁场强度

L：磁流量路径长度

这种关系表明磁场强度与扭距同匝数和电流成正比，与磁流量路径长度成反比。从这一基本关系可以看出可以看出同样的磁流量路径长度，不同的步进电机通过改变绕组参数可以有不同的输出力矩。更详细的资料关于绕组参数如何影响步进机输出量可以在题为“驱动电流基础”的应用说明中找到。

步进模式

下列各项是最通常的驱动模式

1. 波动驱动（在1 状态）

2. 半步驱动（在2状态）

3. 全步驱动（在1和2状态）

4. 细分步进（不断地改变电机的电流）

对于下列的讨论如图3所示

图3 单极步进电机

在波动驱动中只有一个线圈转动在任何接通时间。按照A → B → A → B的顺序驱动和转子的转动为8 → 2 → 4→ 6。对于和相同的参数的单极和有两极的电机，这一个脉冲模式会运行相同的机械位置。这一个驱动模式的缺点是在单极的电机只有在只有25% 被使用和在有两极的电机中在任何的运行时间的只有 50% 的总计电机转动。这表示你没有在从电机运行中得到最大的转力矩输出。

在全步驱动时在任何接同时间有2个脉冲周期。按照AB → A B→ AB→A B和转子的转动为1 → 3 → 5 → 7。全部驱动在驱动方式和1状态相同有角动但是机械的移动位置全部被弥补一半。因为单极的电机使用，单极电机的转力矩输出比两极的电机 (对于和相同的参数马达) 低只有 50% , 有两极的电机可以用全周期。

半步驱动整合了波动和全部驱动 (在1和2 脉冲)的驱动形式。每两个脉冲逐步运行被激活并在其他运行周期在每个固定的状态。按照AB → B → AB → A→ AB → B → AB → A和转子转动为1 → 2 → 3→ 4 → 5 → 6 → 7 → 8结果在角运动,有一半是在1或2阶段--驱动方式. 半步驱动,可以减少的现象称为共振,可以在经历了1或2阶段的驱动方式。

上述驱动方式激发触发顺序如表1

表1 驱动方式激发触发顺序

步进驱动电流绕组不断改变形成许多较小的离散步骤. 更多信息细分,可以发现在步进。

单步响应

步进电机单步响应特性如图4所示

如图4 步进电机单步响应特性

当脉冲采用了步进电机转子的运行方式到上述步骤曲线时间t的时候,它采取的电机轴转动一个步距角,第一步是脉冲应用. 这一步的时间是高度依赖比率转矩惯性(负载) ,以及使用的驱动种类。由于扭矩是一个函数的位移所以接下来的加速度也将. 因此,当移动大型梯级递增高转矩发达,因此一个高加速度. 这可引起了长鸣所示. 沉降时间T是要花时间,因此这些振荡或铃声停止. 在某些应用这一现象可不可取. 它可以减少或消除这种行为的细分步进电机. 由于扭矩是一个函数的位移所以接下来的加速度也将. 因此,当移动大型梯级递增高转矩发达,因此一个高加速度. 沉降时间T是要花时间,因此这些振荡或周期停止. 在某些应用这一现象. 它可以减少或消除这种行为的细分步进电机.

步进电机往往能表现出的现象称为共振, 这可以看作一个突然丧失或下降时转矩转速一定能够导致漏步骤或失去同步. 当它发生时,输入阶跃脉冲率刚好与自然振荡频率有关. 共振现象的一个步进电机来自其基础设施建设,因此不可能消除. 这也是取决于负载的情况. 它可以减少电机驱动在半步或微步进模式。