毕业设计中英文翻译——直流电机的闭环控制和微机接口.doc

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 supplyFig 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 controller. 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 desired 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 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 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 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 current 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 current 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 angle 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 speed 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 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 .Now the speed error is corrected with Ia less than the permissible value.A negative speed error will set the 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 control 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 commonly 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 current 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 decrease 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 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. 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 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 likely 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 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 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 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 for 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 braking is not possible. The drive decelerates mainly due to the load torque. When load torque is low, the response to a decrease 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 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 which 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 a continuously 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 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 the transducer to one which can be converted to digital form by an analog-to-digital converter.Figure 18.1 Input IterfaceAn 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 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 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” .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 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 values 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 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. 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 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 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 scaling 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 form suitable for operating an actuator. Fig 18.2 Output InterfaceThe signals used within microcomputer circuits are almost 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 seen 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 connecting 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 when sourcing the maximum rated “high” output current IoH, while VOL is the guaranteed minimum “low” output voltage when sinking the maximum rated “low” output current IOL. There are corresponding specifications for logic inputs which specify the minimum input voltage which will be recognized as a logic “high” state VIH, and the maximum input voltage which will be regarded as a logic “low” 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 permanently 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 using 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 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 kilowatts, 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 connection 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42A1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，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IAIx,即Ix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Ix，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42A2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Iam*,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Ia*是一个正值，因为电机的电流不会颠倒，这个正的Ia*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，它是