中英文翻译直流电机的闭环控制和微机接口

1、Closed-loop con trol of DC drives AND Microcomputer In terface A basic scheme of the Closed-loop speed control system employing current limit control ,also know as parallel current, is shown in fig 4-2A-1. Fig 4-2A-1 driver with current limit control Wn* sets the speed referenee. A signal proportion

2、al to the motor speed is obtained from the speed sen sor. The speed sen sor output is filtered to remove the ac ripple and compared with the speed reference .the speed error is processed through a speed controller. The output of the speed controller Uc adjusts the rectifier firing angle make the act

3、ual speed close to reference speed. the speed controller is usually a PI(proportional and integral) controller and serves three purposesstabilizes the drive and adjusts the damping ratio at the desired value ,makes the steady-error close to zero by integral action, and filters out noise again due to

4、 the integral action. The drive employs current limit control, the purpose of which is to prevent the current from exceeding safe values .As long as IAIx, where Ix is the maximum permissible value of IA, the current control loop does not affect the drive operation. If IA exceeds Ix, even by a small

5、amount, a large output signal is produced by the threshold circuit, the curre nt con trol overrides the speed con trol, and the speed error is corrected esse ntially at a con sta nt curre nt equal to the maximum permissible value. When the speed reachesclose to the desired value, IA falls below Ix,

6、the current con trol goes out of action and speed con trol takes over. Thus in this scheme, at any give n time the operati on of the drives is mainly con trolled either by the speed con trol loop or the curre nt con trol loop, and hence it is also called parallel curre nt con trol. Ano ther scheme o

7、f closed-loop speed con trol is show n in Fig.4-2A-2. Fig 4-2A-2 driver with inner curre nt con trol loop It employs an inner current control loop within an outer speed loop. The output of the speed con troller ec is applied to a curre nt limiter which sets the curre nt refere nee la* for the inner

8、curre nt con trol loop. The output of the curre nt con troller Uc adjusts the conv erter firing an gle such that the actual speed is brought to a value set by the speed comma nd Wm*. Any positive speeder error, caused by either an in crease in the speed comma nd or an in crease in the load torque, p

9、roduces a higher curre nt refere nee la*. The motor accelerates due to an in crease in la, to correct the speed error and fin ally settles at a new la* which makes the motor torque equal to the load torque and the speed error close to zero. For any large positive speed error, the current limiter sat

10、urates and the current reference la* is limited to a value lam*, and the drive current is not allowed to exceed the maximum permissible value .The speed error is corrected an the maximum permissible armature current until the speed error becomes small and the current limiter comes out of saturation

11、.Now the speed error is corrected with la less tha n the permissible value. “ch A n egative speed error will set the curre nt refere nce la* at a n egative value. Since the motor curre nt can not reverse, a n egative la* is of no use .lt will however Pl controller. When the speed error becomes posit

12、ive the“ charge ” the Pl controller will take a Ion ger time to resp ond, caus ing unn ecessary delay in the con trol acti on. The curre ntLimiter is therefore arran ged to set a zero-curre nt refere nce for n egative speed error. Since the speed con trol loop and the curre nt con trol loop are in c

13、ascade, the inner current control is also known as cascade control. lt is also called current guided controlt is more commonly used than the current-limit control because of the followi ng adva ntages: 1. lt provides faster response to any supply voltage disturbance. This can be explained by conside

14、ring the response of two drives to a decrease in the supply voltage. A decrease in the supply voltage reduces the motor current and torque. In the current-limit control, the speed falls becausethe motor torque is less than the load torque that has not changed. The resulting speed error is brought to

15、 the original value by setting the rectifier firing angle at a lower value .In the case of inner current control, the decreasein motor current ,due to the decrease in the supply voltage, produces a current error which changes the rectifier firing angle to bring the armature current back to the origi

16、nal value. The transient response is now governed by the the electrical time constant of the motor. Since the electrical time constant of a drive is much smaller compared to the mechanical time constant, the inner current control provides a faster response to the supply voltage disturbances. 2. For

17、certain firing schemes, the rectifier and the control circuit together have a constant gain under continuous conduction. The drive is designed for this gain to set the damping ratio at 0.707, which gives an overshoot of 5 percent. Under discontinuous conduction, the gain reduces. The higher the redu

18、ction is in the conduction angel, the greater the reduction is in the gain. The drive response becomes sluggish in discontinuous conduction and progressively deteriorates as the conduction angle reduces. If an attempt is made to design the drive for discontinuous conduction operation, the drive is l

19、ikely to be oscillatory or even unstable for continuous conduction. The inner current control loop provides a close loop around the rectifier and the control circuit, and therefore, the variation of their gain has much less affect on the drive performance. Hence, the transient responseof the drive w

20、ith the inner current loop is superior to that with the current-limit control. 3. In the current-limit control, the current must first exceed the permissible value before the current-limit action can be initiated. Since the firing angle can be changed only at discrete intervals, substantial current

21、overshoot can be occur before the current limiting becomes effective. Small motors are more tolerant to high transient current. Therefore, to obtain a fast transient response, much higher transient currents are allowed by selecting a large size rectifier. The current regulation is then needed only f

22、or abnormal values of current. In such cases because of the simplicity, current-limit control is employed. Both the schemes have different responses for the increase and decrease in the speed command. A decrease in speed command at the most can make the motor torque zero; it can not be reversed as b

23、raking is not possible. The drive decelerates mainly due to the load torque. When load torque is low, the responseto a decreasein the speed command will be slow. These drives are therefore suitable for applications with large load torque, such as paper and printing machines, pumps, and blowers. A mi

24、crocomputer interface concerts information between two forms. Outside the microcomputer the information handled by an electronic system exists as a physical signal, but within the program, it is represented numerically. The function lf any interface can be broken down into a number of operations whi

25、ch modify the data in some way, so that the process of conversion between the eternal and internal forms is carried out in a number of steps. This can be illustrated by means of an example such as that of Figure 18.1, which shows an interface between a microcomputer and a transducer producing aconti

26、nuously variable analog signal. Transducers often produce very small output requiring amplification, or they may generate signals in a form that needs to be converted again before being handled by the rest of the system. For example, many transducers have variable resistance which must be converted

27、to a voltage by a special circuit. This process of converting the transducer output into a voltage signal which can be connected to the rest of the system is called signal conditioning. In the example of Figure1, the signal conditioning section translates the range of voltage or current signals from

28、 the transducer to one which can be converted to digital form by an analog-to-digital converter. Figure 18.1 Input Iterface An analog-to-digital converter (ADC) is used to convert a continuously variable signal to a corresponding digital form which can take any one of a fixed number of a fixed numbe

29、r of possible binary values. If the output of the transducer does not vary continuously, no ADC is necessary. In this case the signal conditioning section must convert the incoming signal to a form which can be connected directly to the next part of the interface, the input/output section of the mic

30、rocomputer itself. .In the a The I/O section converts digital “ on/off v”oltage signal to a form which can be presented to the processor via the system buses. Here the state of each input line, whether it is “on” or “off ” ,is indicated by a corresponding“1”or“0 inputs which have been converted to d

31、igital form, the patterns of ones and zeros in the internal representation will form binary numbers corresponding to the quantity being concerted. The “ raw”number from the interface are limited by the design of the interface circuitry and they often require linearization and scaling to produce valu

32、es suitable for use in the main program. For example, the interface might be used to convert temperatures in the range -20 to +50 degrees, but the numbers produced by an 8-bit converter will lie in the range 0 to 255. Obviously it is easier from the programmer point of view to deal directly with tem

33、perature rather than to work out the equivalent of any given temperature in terms of the numbers produced by the ADC. Every time the interface is used to read a transducer, the same operations must be carried out to convert the input number into a convenient* form. Additionally, the operation of som

34、e interfaces requires control signals to be passed between the microcomputer and components of the interface. For these reasons it normal to use a subroutine to look after the detailed operations of the interface and carry out any scaling and/or linearization which might be needed。 Output interfaces

35、 take a similar form (Fig18.2), the obvious difference being that here the flow of information is in the opposite direction; it is passed from the program to the outside world. In this case the program may call an output subroutine which supervises the operation of the interface and programs the sca

36、ling numbers which may be needed for a digital-to-analog converter (DAC).This subroutine passes information in turn to an output device which produces a corresponding electrical signal, which could be converted into analog form using a DAC. Finally the signal is conditioned (usually amplified) to a

37、form suitable for operating an actuator. Fig 18.2 Output In terface Fig.2 Output Internee The signals used within microcomputer circuits are almost always too small to be conn ected directly to the“ outside world ” and some kinds of in terface must be used to tran slate them to a more appropriate fo

38、rm. The desig n of secti on of in terface circuits is one of the most important tasks facing the engineer wishing to apply microcomputers. We have see n that in microcomputers in formatio n is represe nted as discrete patter ns of bits.This digital form is most useful whe n the microcomputer is to b

39、e connected to equipment which can only be switched on or off, where each bit might represe nt the state of a switch or actuator. Care must be taken when connecting logic circuits to ensure that their logical levels and curre nts rati ngs are compatible. The output voltages produced by a logic circu

40、it are normally specified in terms of worst case values when sourcing or sinking the maximum rated currents. Thus VOH is the guaranteed minimum“ high ” voltage st when sourcing the maximum rated “high output current loH, while VOL is the guaranteed minimum “ low ” outpvoltage when sinking the maximu

41、m rated “ low ” output curre nt IOL. There are corresp onding specificati ons for logic in puts which specify the minimum in put voltage which will be recog ni zed as a logic“ high ” VIH, and the maximum in put voltage which will be regarded as a logic“ low ” state VIL. For in put in terface, perhap

42、s the main problem facing the desig ner is that of electrical noise. Small noise signals may cause the system to malfunction, while larger amounts of no ise can perma nen tly damage it. The desig ner must be aware of these dan gers from the outset. There are many methods to protect in terface circui

43、ts and microcomputer from various kinds of no ise. Follow ing are some examples: 1. In put and output electrical isolati on betwee n the microcomputer system and exter nal devices using an opt-isolator or a tran sformer. 2. Removing high frequency noise pulses by a low-pass filter and Schmitt-trigge

44、r. 3. Protect ing aga inst excessive in put voltages using a pair of diodes to power supply reversibly biased in no rmal direct ion. For output in terface, parameters Voh, Vol, Ioh and Iol of a logic device are usually much to low to allow loads to be conn ected directly, and in practice an exter na

45、l circuit must be conn ected to amplify the curre nt and voltage to drive a load. Although several types of semic on ductor device are now available for con trolli ng DC and AC power s up to many kilowatts, there are two basic ways in which a switch sca n be conn ected to a load to con trol it; seri

46、es connection and shunt connection as show n in Figure18.3. Load Shunt Switch Series Switch Fig 18.3 Series qndShunt Connection 丄小丄. Fig.3 Series and Shunt Connection With series connection, the switch allows current to flow through the load when closed, while with shunt connection closing the switc

47、h allows current to bypass the load. Both connections are useful in low-power circuit, but only the series connection can be used in high-power circuits because of the power wasted in the series resistor R. 直流电机的闭环控制和微机接口 闭环速度控制系统的一种基本模式是采用限流控制，即我们所熟悉的并联电路控 制，如图4-2A- 1所示。Wm*为给定速度参考值，从速度传感器获得的信号与 电机速

48、度成正比，速度传感器输出滤除了交流波动， 然后与给定速度相比较，得 到速度偏差通过速度控制器处理，速度控制器的输出Uc调速整流器触发角a 使实际的速度接近给定速度，速度控制器通常是一个PI控制器，它主要有三个 作用：使传动系统稳定和调整阻尼比在一个希望的范围内，通过积分作用使稳态 速度偏差接近0和滤除噪声。 传动装置采用了限流控制，它的目的使防止电路超过安全值，只要IAvlx,即lx 是IA的允许最大值，电流控制环节就失去作用。如果IA大于Ix，甚至是一 个很小的值，电路就会产生一个大的输出信号，电流控制超过速度控制，在恒定 电流等于允许最大值时，纠正速度偏差。当速度达到要求值时，IA减小于I

49、x， 电流控制不会产生作用。同时速度控制器开始工作，因此在这种模式下，在任何 已知时间内，传动装置主要是被速度控制环节或电流控制环节控制，因此，它也 被叫做并联控制。 另一种闭环速度控制模式如图4 2A 2所示，它采用内部电流控制环节和外部 速度控制环节，速度控制器的输出 ec应采用一个电流限流器，它使给定电流值 Ia*作为内部电流环节。电流控制器的输出Uc调整逆变器触发角，使实际的速度 接近于速度给定 Wm*做确定的一个值，由速度给定或转矩转矩的增加所引起的 任何正的速度偏差，都会产生更大的参考电流值Ia*。由于Ia增加，电机加速来 校正速度偏差，最终产生一个新的Ia*,使电机转矩等于负载转

50、矩，同时，速度偏 差接近于0,对于任何大的正的速度偏差，电流限制饱和和电流参考值Ia*被限 制在lam*,系统的电流不允许超过允许值的最大值，在最大允许电枢电流下，纠 正速度偏差直到它变小和限流装置退出饱和状态，此时，被纠正的速度偏差 Ia 小于允许值。 负的速度偏差将使电流参考值la*是一个正值，因为电机的电流不会颠倒，这个 正的la*没有作用，它取决于PI控制器，当速度偏差变成正的，PI调节器将有很 长的时间来响应。 在控制作用中造成的不必要的延迟。 电流控制器因此被用来为 负速度转速偏差设置零电流参考值。 因为速度控制环节和电流控制环节是串联的， 内部电流控制也是串联控制， 所以 它被称

51、作电流引导控制，它比电流限制控制更普遍的应用，主要有如下的有点： 1它提供对任何电源电压骚动的更快的反应。 这可以通过考虑对于电枢电压的 减少的两个传动装置的反应来解释。 在电枢电压的减少降低了电动机的电流和力 矩。在电流限制控制中，速度下降，因为电动机力矩小于没改变的负荷力矩。结 果通过设定触发角在一个很小的值， 速度偏差恢复到原值。 就内部电流调节而论， 由于在电电枢电压的减少， 电动机电流的减少， 产生了电流偏差， 它可以通过改 变触发角是电枢电流回到原值。 瞬态响应现在取决于电动机的电时间常数。 既然 电动机的时间常数与机械时间常数相比小得多， 内部电流调节提供对电源电压扰 动的更快的

52、反应。 2对于一定触发电路图， 整流器和控制电路一起在连续的情况下有恒定的增益。 电机是为这个增益设计的来确定阻尼比在 0.707 ，超过百分之 5。 在不连续的 情况下，增益降低。 导通角减少越多，增益减小的越多，当导通角降低时，电 机反应在不连续的情况下变得缓慢并且逐渐恶化。 如果试图设计电机在不连续 的情况下， 则在连续的传导下电机很可能振荡或者甚至不稳定。 内部电流控制环 提供闭环控制在整理器和控制电路， 因此，他们增益的变化对电机的性能有很小 的作用，因此，内部的电流环节的电机的瞬态响应比电流限制控制的大的多。 3. 在电流限制控制中，在开始电流限制作用之前电流先超过允许值，因为触发

53、 角仅仅在离散的间隔中改变，实际的电流的超调量发生在电流限制作用之前。 小电机对高的瞬态电流更能容忍， 因此，通过选择大的整流器来获得快的瞬态响 应和大的瞬态电流。电流的异常值需要电流调整。在这种情况下，由于简单，采 用了限流控制。 两种方法在速度增减方面有不同的响应， 速度的减小至多使电机的转矩为零， 当 制动时它才能翻转。 电机减速主要由于负载转矩。 当负载转矩很小时， 对速度减 小的反应将减小。 这些电机因此适合应用在有大的负载转矩的装置中， 比如例如 印刷机，泵和吹风机。 微机接口实现两种信息形式的交换。 在计算机之外， 有电子系统所处理的信息以 一种物理信号形式存在， 但在程序中，

54、它是用数字表示的。 任一接口的功能都可 分为以某种形式进行数据变幻的一些操作， 所以外部和内部形式的转换是由许多 步骤完成的。 用图 18.1 所示的情况为例加以说明， 图中展示了为计算机和产生连续变化信 号的传感器之间的接口。 传感器产生的信号常很小， 需要放大， 或者产生的信号 和形式被系统的的其它部分处理之前需要再次转换。 举例来说， 许多传感器具有 电阻变化，这必须有一专门电路转换成电压。 这种将传感器输出转换成电压信号， 并于系统的其它部分相连的过程，成为信号处理。如图 18.1 所示例子中，信号 调理部分将原子传感器的典雅或电流信号范围转换成可用模拟-数字转换器变成 数字形式的信号

55、范围。 一模拟-数字转化器（ADC）用来将连续变化信号变幻成相应的数字量，这 数字量可是可能的二进制数值中的一固定值。如果传感器输出不是连续变化的， 就不需要模拟 -数字转换。在这种情况下，信号调理单元必须将输入信号变幻成 另一信号， 也可直接与接口的下一部分， 即微型计算机本身的输入数出单元相连 接。 输入 /输出单元间数字 “开/关”电压信号转换成能通过系统总线传送到计算机 的信号形式。这里每一根线的状态，无论是 开”或者是 关”用相应的1 ”或 “0” 表示。对于已经转换成数字形式的模拟输入量， 内部表示中用1和0组成的排列 形式形成预备转换量相对应的二进制数。 从接口得到的原数值会受到

56、接口电路设计的限制， 而且常需要线性化和量程调整 才能形成适合于在主程序中使用的数值。举例来说，借口可用于转换范围为 -200C至+500C温度，而8位转换器所产生的数值会在范围0至 255之间。显然， 从程序员的观点， 对温度进行直接的处理要比使用由 AD 所产生的与一给定温度 相一致的值要容易。 接口总是用于读取传感器的值， 同时还要将输入数值转换成 更容易的形式。而且，接口操作需要将控制信号在微机和接口元件之间进行传送。 根据这些理由， 通常使用子程序来监视接口的具体操作， 并完成任何所需的量程 调整和或线性化。 输出接口采用相似的形式 （图 18.2）明显的差别在于信息流的方向相反；

57、是从程 序带外部世界。 这种情况下， 程序可称为输出程序， 它监视接口的操作并完成数 字-模拟转换器（DAC ）所需数字的标定。该子程序依次送出信息给输出器件， 产生相应的电信号，由 DAC 转换成模拟形式。最后信号将调理（通常是放大） 以形成适应于执行器操作的形式。 在微型机电路中使用的信号几乎总是太小而不能直接地连接到 “外部世界 ”，因而 必须用某种形式将其转换成更适宜的形式。 接口电路部分的设计是使用微机的工 程师面临最严重的问题之一。 我们已经了解到微机中， 信息已离散的位形式表示。 当微机要与只有打开或关闭的设备相连时， 这种数字形式是最有用的， 这里每一 位都可表示一开关或执行器

58、的状态。 连接逻辑电路时， 必须小心翼翼， 以保证他们的逻辑电平和电流逻辑值是兼 容的，由逻辑电路产生的输出电压通一拉出或灌入最大额定电流时， 按最弱情况 下数值所定义。这样 VOH 适当拉出最大额定 “高”输出电流 IOH 时的允许最小 “高”电压，而 VOL 则是当灌入最大额定 “低”输出电流 IOL 是允许的最 “低”电压。 对逻辑输入也有相应的参数， 规定最小输入电压为逻辑 “高”状态 VIH ，以及最大 输入电压为逻辑 “低”状态 VIL 。 对于输入接口， 也许设计所面临的主要问题是电噪声， 小噪声信号会引起系统工 作不良，而大量的噪声会造成永久性损坏。 设计这必须从一开始就清楚这些危险。 有许多方法保护接口电路和微机不受各种各样噪声影响，下面是一些例子： 1. 使用光电隔离或变压器实现微机系统和外部器件之间的输入输出电信号隔离