畢業(yè)設(shè)計(jì)中英文翻譯——直流電機(jī)的閉環(huán)控制和微機(jī)接口

1、Closed-loop control of DC drives AND Microcomputer InterfaceA 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.UcSpeed sensor IaU1WmMotorFiring circuitFilterThreshold circuitWm*TMFilterSpeed controller（PI）AC s

2、upplyFig 4-2A-1 driver with current limit controlWn* sets the speed reference. A signal proportional to the motor speed is obtained from the speed sensor. The speed sensor output is filtered to remove the ac ripple and compared with the speed reference .the speed error is processed through a speed c

3、ontroller. The output of the speed controller Uc adjusts the rectifier firing angle to make the actual speed close to reference speed. the speed controller is usually a PI(proportional and integral) controller and serves three purposes-stabilizes the drive and adjusts the damping ratio at the desire

4、d value ,makes the steady-error close to zero by integral action, and filters out noise again due to 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 IA<Ix, where Ix is the maximum permissible value

5、 of IA, the current control loop does not affect the drive operation. If IA exceeds Ix, even by a small amount, a large output signal is produced by the threshold circuit, the current control overrides the speed control, and the speed error is corrected essentially at a constant current equal to the

6、 maximum permissible value. When the speed reaches close to the desired value, IA falls below Ix, the current control goes out of action and speed control takes over. Thus in this scheme, at any given time the operation of the drives is mainly controlled either by the speed control loop or the curre

7、nt control loop, and hence it is also called parallel current control. Another scheme of closed-loop speed control is shown in Fig.4-2A-2.Motorrr otorWm*UceIIa*ewmWmecAC supplyCurrent Controller(PI)Firing circuitFilterMSpeed sensor Current limiterSpeed Controller(PI)Fig 4-2A-2 driver with inner curr

8、ent control loopIt employs an inner current control loop within an outer speed loop. The output of the speed controller ec is applied to a current limiter which sets the current reference Ia* for the inner current control loop. The output of the current controller Uc adjusts the converter firing ang

9、le such that the actual speed is brought to a value set by the speed command Wm*. Any positive speeder error, caused by either an increase in the speed command or an increase in the load torque, produces a higher current reference Ia*. The motor accelerates due to an increase in Ia, to correct the s

10、peed error and finally settles at a new Ia* 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 saturates and the current reference Ia* is limited to a value Iam*, and the drive current is not allowed to exc

11、eed 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 .Now the speed error is corrected with Ia less than the permissible value.A negative speed error will set th

12、e current reference Ia* at a negative value. Since the motor current can not reverse, a negative Ia* is of no use .It will however “charge” the PI controller. When the speed error becomes positive the “charge” the PI controller will take a longer time to respond, causing unnecessary delay in the con

13、trol action. The currentLimiter is therefore arranged to set a zero-current reference for negative speed error. Since the speed control loop and the current control loop are in cascade, the inner current control is also known as cascade control. It is also called current guided control .It is more c

14、ommonly used than the current-limit control because of the following advantages: 1. It provides faster response to any supply voltage disturbance. This can be explained by considering the response of two drives to a decrease in the supply voltage. A decrease in the supply voltage reduces the motor c

15、urrent and torque. In the current-limit control, the speed falls because the motor torque is less than the load torque that has not changed. The resulting speed error is brought to the original value by setting the rectifier firing angle at a lower value .In the case of inner current control, the de

16、crease in 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 original value. The transient response is now governed by the the electrical time constant of the motor.Since the electrical

17、 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 certain firing schemes, the rectifier and the control circuit together have a constant gain under continuous conduction.

18、 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 reduction is in the conduction angel, the greater the reduction is in the gain. The drive response becomes sluggish in disco

19、ntinuous 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 likely to be oscillatory or even unstable for continuous conduction. The inner current control loop provides a close loop

20、 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 response of the drive with the inner current loop is superior to that with the current-limit control.3. In the current-limit control, the curr

21、ent 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 overshoot can be occur before the current limiting becomes effective.Small motors are more tolerant to high transient cu

22、rrent. 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 for abnormal values of current. In such cases because of the simplicity, current-limit control is employed.Both the scheme

23、s 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 braking is not possible. The drive decelerates mainly due to the load torque. When load torque is low, the response to a de

24、crease in 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 microcomputer interface concerts information between two forms. Outside the microcomputer the information handled by an ele

25、ctronic 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 which modify the data in some way, so that the process of conversion between the eternal and internal forms is carried out i

26、n 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 a continuously variable analog signal. Transducers often produce very small output requiring amplification, or they may generate

27、 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 to a voltage by a special circuit. This process of converting the transducer output into a voltage signal which can be co

28、nnected 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 the transducer to one which can be converted to digital form by an analog-to-digital converter.Figure 18.1 Input Iterfac

29、eAn 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 number of possible binary values. If the output of the transducer does not vary continuously, no ADC is necessary. In this case

30、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 microcomputer itself.The I/O section converts digital “on/off” voltage signal to a form which can be presented to the processo

31、r 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” .In the analog inputs which have been converted to digital form, the patterns of ones and zeros in the internal representation will form binary numbers corresponding to th

32、e 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 values suitable for use in the main program. For example, the interface might be used to convert temperatures in the range

33、-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 programmers point of view to deal directly with temperature rather than to work out the equivalent of any given temperature in terms of the numbers produced by the ADC.

34、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 some interfaces requires control signals to be passed between the microcomputer and components of the interface. For thes

35、e 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 take a similar form (Fig18.2), the obvious difference being that here the flow of information is in the opposite dire

36、ction; 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 scaling numbers which may be needed for a digital-to-analog converter (DAC).This subroutine passes information in turn to

37、 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 form suitable for operating an actuator. Fig 18.2 Output InterfaceThe signals used within microcomputer circuits are a

38、lmost always too small to be connected directly to the “outside world” and some kinds of interface must be used to translate them to a more appropriate form. The design of section of interface circuits is one of the most important tasks facing the engineer wishing to apply microcomputers. We have se

39、en that in microcomputers information is represented as discrete patterns of bits.This digital form is most useful when the microcomputer is to be connected to equipment which can only be switched on or off, where each bit might represent the state of a switch or actuator. Care must be taken when co

40、nnecting logic circuits to ensure that their logical levels and currents ratings are compatible. The output voltages produced by a logic circuit 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

41、 when sourcing the maximum rated “high” output current IoH, while VOL is the guaranteed minimum “l(fā)ow” output voltage when sinking the maximum rated “l(fā)ow” output current IOL. There are corresponding specifications for logic inputs which specify the minimum input voltage which will be recognized as a

42、logic “high” state VIH, and the maximum input voltage which will be regarded as a logic “l(fā)ow” state VIL.For input interface, perhaps the main problem facing the designer is that of electrical noise. Small noise signals may cause the system to malfunction, while larger amounts of noise can permanentl

43、y damage it. The designer must be aware of these dangers from the outset. There are many methods to protect interface circuits and microcomputer from various kinds of noise. Following are some examples:1. Input and output electrical isolation between the microcomputer system and external devices usi

44、ng an opt-isolator or a transformer.2. Removing high frequency noise pulses by a low-pass filter and Schmitt-trigger.3. Protecting against excessive input voltages using a pair of diodes to power supply reversibly biased in normal direction.For output interface, parameters Voh, Vol, Ioh and Iol of a

45、 logic device are usually much to low to allow loads to be connected directly, and in practice an external circuit must be connected to amplify the current and voltage to drive a load. Although several types of semiconductor device are now available for controlling DC and AC power s up to many kilow

46、atts, there are two basic ways in which a switch scan be connected to a load to control it; series connection and shunt connection as shown in Figure18.3.Fig 18.3 Series and Shunt Connection With series connection, the switch allows current to flow through the load when closed, while with shunt conn

47、ection closing the switch 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. 直流電機(jī)的閉環(huán)控制和微機(jī)接口閉環(huán)速度控制系統(tǒng)的一種基本模式是采用限流控制，即我們所熟悉的并聯(lián)電路控制，如圖42A1所示。Wm*為給定速度參

48、考值，從速度傳感器獲得的信號與電機(jī)速度成正比，速度傳感器輸出濾除了交流波動，然后與給定速度相比較，得到速度偏差通過速度控制器處理，速度控制器的輸出Uc調(diào)速整流器觸發(fā)角，使實(shí)際的速度接近給定速度，速度控制器通常是一個(gè)PI控制器，它主要有三個(gè)作用：使傳動系統(tǒng)穩(wěn)定和調(diào)整阻尼比在一個(gè)希望的范圍內(nèi)，通過積分作用使穩(wěn)態(tài)速度偏差接近0和濾除噪聲。傳動裝置采用了限流控制，它的目的使防止電路超過安全值，只要IA<Ix,即Ix是IA的允許最大值，電流控制環(huán)節(jié)就失去作用。如果IA大于Ix，甚至是一個(gè)很小的值，電路就會產(chǎn)生一個(gè)大的輸出信號，電流控制超過速度控制，在恒定電流等于允許最大值時(shí)，糾正速度偏差。當(dāng)速度達(dá)

49、到要求值時(shí)，IA減小于Ix，電流控制不會產(chǎn)生作用。同時(shí)速度控制器開始工作，因此在這種模式下，在任何已知時(shí)間內(nèi)，傳動裝置主要是被速度控制環(huán)節(jié)或電流控制環(huán)節(jié)控制，因此，它也被叫做并聯(lián)控制。另一種閉環(huán)速度控制模式如圖42A2所示，它采用內(nèi)部電流控制環(huán)節(jié)和外部速度控制環(huán)節(jié)，速度控制器的輸出ec應(yīng)采用一個(gè)電流限流器，它使給定電流值Ia*作為內(nèi)部電流環(huán)節(jié)。電流控制器的輸出Uc調(diào)整逆變器觸發(fā)角，使實(shí)際的速度接近于速度給定Wm*做確定的一個(gè)值，由速度給定或轉(zhuǎn)矩轉(zhuǎn)矩的增加所引起的任何正的速度偏差，都會產(chǎn)生更大的參考電流值Ia*。由于Ia增加，電機(jī)加速來校正速度偏差，最終產(chǎn)生一個(gè)新的Ia*,使電機(jī)轉(zhuǎn)矩等于負(fù)載轉(zhuǎn)矩

50、，同時(shí)，速度偏差接近于0，對于任何大的正的速度偏差，電流限制飽和和電流參考值Ia*被限制在Iam*,系統(tǒng)的電流不允許超過允許值的最大值，在最大允許電樞電流下，糾正速度偏差直到它變小和限流裝置退出飽和狀態(tài)，此時(shí)，被糾正的速度偏差I(lǐng)a小于允許值。負(fù)的速度偏差將使電流參考值Ia*是一個(gè)正值，因?yàn)殡姍C(jī)的電流不會顛倒，這個(gè)正的Ia*沒有作用，它取決于PI控制器，當(dāng)速度偏差變成正的，PI調(diào)節(jié)器將有很長的時(shí)間來響應(yīng)。在控制作用中造成的不必要的延遲。電流控制器因此被用來為負(fù)速度轉(zhuǎn)速偏差設(shè)置零電流參考值。因?yàn)樗俣瓤刂骗h(huán)節(jié)和電流控制環(huán)節(jié)是串聯(lián)的，內(nèi)部電流控制也是串聯(lián)控制，所以它被稱作電流引導(dǎo)控制，它比電流限制控制

51、更普遍的應(yīng)用，主要有如下的有點(diǎn)：1它提供對任何電源電壓騷動的更快的反應(yīng)。 這可以通過考慮對于電樞電壓的減少的兩個(gè)傳動裝置的反應(yīng)來解釋。在電樞電壓的減少降低了電動機(jī)的電流和力矩。在電流限制控制中，速度下降，因?yàn)殡妱訖C(jī)力矩小于沒改變的負(fù)荷力矩。結(jié)果通過設(shè)定觸發(fā)角在一個(gè)很小的值，速度偏差恢復(fù)到原值。就內(nèi)部電流調(diào)節(jié)而論， 由于在電電樞電壓的減少，電動機(jī)電流的減少，產(chǎn)生了電流偏差，它可以通過改變觸發(fā)角是電樞電流回到原值。瞬態(tài)響應(yīng)現(xiàn)在取決于電動機(jī)的電時(shí)間常數(shù)。既然電動機(jī)的時(shí)間常數(shù)與機(jī)械時(shí)間常數(shù)相比小得多，內(nèi)部電流調(diào)節(jié)提供對電源電壓擾動的更快的反應(yīng)。2對于一定觸發(fā)電路圖，整流器和控制電路一起在連續(xù)的情況下有

52、恒定的增益。電機(jī)是為這個(gè)增益設(shè)計(jì)的來確定阻尼比在0.707 ，超過百分之5。 在不連續(xù)的情況下，增益降低。 導(dǎo)通角減少越多，增益減小的越多，當(dāng)導(dǎo)通角降低時(shí)，電機(jī)反應(yīng)在不連續(xù)的情況下變得緩慢并且逐漸惡化。 如果試圖設(shè)計(jì)電機(jī)在不連續(xù)的情況下，則在連續(xù)的傳導(dǎo)下電機(jī)很可能振蕩或者甚至不穩(wěn)定。內(nèi)部電流控制環(huán)提供閉環(huán)控制在整理器和控制電路，因此，他們增益的變化對電機(jī)的性能有很小的作用，因此，內(nèi)部的電流環(huán)節(jié)的電機(jī)的瞬態(tài)響應(yīng)比電流限制控制的大的多。3. 在電流限制控制中，在開始電流限制作用之前電流先超過允許值，因?yàn)橛|發(fā)角僅僅在離散的間隔中改變，實(shí)際的電流的超調(diào)量發(fā)生在電流限制作用之前。小電機(jī)對高的瞬態(tài)電流更能容忍，因此，通過選擇大的整流器來獲得快的瞬態(tài)響應(yīng)和大的瞬態(tài)電流。電流的異常值需要電流調(diào)整。在這種情況下，由于簡單，采用了限流控制。兩種方法在速度增減方面有不同的響應(yīng)，速度的減小至多使電機(jī)的轉(zhuǎn)矩為零，當(dāng)制動時(shí)它才能翻轉(zhuǎn)。電機(jī)減速主要由于負(fù)載轉(zhuǎn)矩。當(dāng)負(fù)載轉(zhuǎn)矩很小時(shí)，對速度減小的反應(yīng)將減小。這些電機(jī)因此適合應(yīng)用在有大的負(fù)載轉(zhuǎn)矩的裝置中，比如例如印刷機(jī)，泵和吹風(fēng)機(jī)。微機(jī)接口實(shí)現(xiàn)兩種信息形式的交換。在計(jì)算機(jī)之外，有電子系統(tǒng)所處理的信息以一種物理信號形式存在，但在程序中，它