Motion Control for Electric Drives

Transcription

1 Motion Control f Electric Drives L. Romeral Departament d Enginyeria Electrònica. Universitat Politécnica de Catalunya Colón 1, Terrassa (Barcelona). Spain Tel.: , Fax.: romeral@eel.upc.es Abstract This paper present a general overview of the motion control system blocks, specially that related with mot considerations and tque control. Meover, some trends in mot control systems are also presented. 1. Introduction Everyone recognizes the vital role played by electrical mots in the development of industrial systems. The dc machine was the first practical device to convert electrical power into mechanical power, and viceversa. Inherently straightfward operating characteristics, flexible perfmance and efficiency encouraged the use of dc mots in many types of industrial drive application The later development of the low cost ac cage mots, and, recently, of electronic variable frequency control have displaced the dc mot to some extent, particularly in the lower kw range. The ac squirrel cage mot is the basis, universal wk hse of industry, converting me than 75% of all electrical power into mechanical energy. This type of mot retains some quite unattractive perfmance characteristics, such as instability and non linear load current characteristic, in spite of intensive development. However, modern electronic control technology, that generates the necessary variable voltage variable frequency supply that 3 - phase ac machine requires, is able not only to render the ac mot satisfacty f many modern drive applications, but also greatly to extend its application and enable advantage to be taken of its low capital and maintenance costs. New mots appearing, fully dependents of the power electronics, are changing the applications and possibilities of motion control systems. Permanent -Magnet Brushless drives, which need a sixtransist four quadrants output bridge, are classified in two principal types: - Brushless DC, o Switched PM Mots, with a switched trapezoidal stat field, - Brushless Synchronous, AC Servo, Synchronous PM Mot, with a unifmly rotating stat field as in an IM. The trapezoidal current drive is conventionally referred to as the Brushless Dc drive, but me crectly as the six-step drive because there are six-steps in the commutation f one electrical cycle, the objective being to commutate the bridge electronically to keep the current flow in any particular mot winding flowing in the same direction during commutation. So that the crect sequence of switching commutation can be achieved to create tque, it is necessary f the drive to know the position of the permanent magnet field of the rot. To achieve this, either two three position senss are used, the most common type of sens being Hall type sens, which is relatively inexpensive. F the Brushless Synchronous Drives, the circuit of the power section is exactly the same as that f the trapezoidal drive, but now the output of the power inverter is a sinusoidal wave whose amplitude, frequency and phase are controlled using PWM control. The mot rotates in phase with current phase and without slip, which means that the mot position must be maintained synchronized with the inverter current. To do this, the drive has to know the exact position of the rot, which is achieved by fitting a high resolution encoder, nmally a brushless resolver, to the mot. A sinusoidal mot drive system provides the highest rotation unifmity at any speed in open loop. This applies equally to conventional variable speed drives as well as ac servos. Another advantage of the sinusoidal drive is that only one feed back device is necessary to the proper control of the mot. The switched reluctance mot (SR) is a variable speed brushless mot of simple mechanical construction. Tque is produced by exploiting the tendency of a magnetic circuit to assume a position of least disttion of the paths of the lines of fce. Magnetic circuits are produced within the mot using current pulses that are electronically controlled to produce high efficiency over a wide speed range. Because of its simplicity, SR mot is not only inexpensive, but is inherently robust and reliable, and it is a good choice when very high speeds are required. Meover, the SR drive is web suited to digital control, and it is capable of fourquadrant operation with excellent responses and stability. Fig. 1 shows the general structure of a Motion System. Several blocks are shown, being the most imptant the electric mot, whose choice depends on the application type. After the mot is selected, a particular power electronic converter is specified, and then particular algithms of power converter, tque control and motion control end up as selected. Other imptant elements are the sens and interfaces.

2 Harmonic losses Commutation & Conduction losses Ce, Coppel & Mechanical losses Electric Power Source Power Electronic Converter ELECTRIC MOTOR Load Electric Senss Electric Senss Observers Motion Senss Observers Power Electronic Tque Motion DRIVE CONTROLLER Power Source Perfmance Commands Motion Commands User Interface Fig. 1. General structure of a Motion System 2. General Blocks of a Motion System Apart from the mot, others blocks appear in a general Motion System: 2.1 Power Converter All ac and dc drives use power semiconduct devices to convert and control electrical power. The devices operate in the switching mode (either ON -OFF states), which cause the losses to be reduced, and energy conversion efficiency to be improved. The Power Converter built up with the power semiconduct devices modifies the electric power from the mains to another voltage-frequency relationship able to supply the electric mot as control loop decides. Several classes of power converters can be found in Motion Control Drives: a) AC-AC Converters: f DC variable speed electronic drives Thyrist (phase control): from kw to MW Transist drives (choppers): low power specific applications b) DC-DC Converters: f AC variable speed electronic drives Induction mots Brushless Permanent Magnet Brushless DC Mot Brushless Synchronous (AC Servos) c) Switching Converters: f Switched Reluctance Mot Drives 2.2 Motion s Motion controllers include all the closed loops and related controllers that drive the mot to the desired reference (Fig. 2), being the most imptant the tque controller. Many adjustable-speed drive (ASD) applications require medium- high-bandwidth tque control in der to achieve adequate control perfmance. What this means is that if a drive system can function as a controllable source of tque, with fast dynamic response, it should be able to implement the desired motion control operation. Pri to 1980, ASDs implemented with dc mots had significant advantages over other types. The primary advantage of dc ASDs over ac ASDs, f example, was that dc drives could readily provide tque control through direct control of the armature current. In contrast, ac drives up to that time usually used squarewave converters and were me suitable f simple speed control. Without tque control, the dynamic perfmance of a drive is at best slow and oscillaty, and in the wst case, unstable.

3 The inner the loop, the faster the response Response time ms - s ms s ns n* + - Speed * Tque v * i * Inverter IM mec Load i v Sens Observer n, Sens Observer Fig. 2. Closed Loops in Motion Control A dc mot controller provides rapid tque control because the commutat maintains a fixed (and nearly ideal) tque angle at all times. The instantaneous tque is proptional to the product of the armature current and the field current, and these are dynamically decoupled. Dynamic decoupling means that field current and armature current can be adjusted independently without interference. In a typical application, the field current (which changes slowly because of high winding inductance) is set and held at a target constant value. The armature current is then adjusted to provide control of the shaft tque. The dynamic response can be very fast: even large dc machines have armature L/R time constants in the range of about 20 ms, so the tque can be adjusted at bandwidths of a few tens of hertz. The challenge of a dc ASD is that it must be able to adjust the armature voltage and control armature current of a machine with minimal power loss. An early dc ASD, the Ward-Leonard system, controlled the dc voltages to the armature and the field with a simple algithm, in which the desired tque and the magnetizing field strength served as command inputs. In this system, a separate dc generat provided the adjustable dc source f the armature. Me recent dc drives use power electronics either a phasecontrolled rectifier a dc-dc converter from a separate dc supply to provide the armature control. An ac mot, on the other hand, involves complex, nonlinear relationships between voltages, currents, fluxes, and tque. High-fidelity control of ac currents in these machines generally require fast power electronic components, which were not readily available until the 1970s. F these reasons, virtually all medium- highperfmance ASD's installed befe 1980 were dc drives. Many of these are still in operation today, but they are being supplanted rapidly by the new generations of ac drives. The early generation of ac ASDs, which appeared mainly in the 1980s, used pulse-width modulation (PWM) to provide adjustable-frequency sinusoidal currents to ac machine stats. The best of these drives provide excellent speed control but do not have direct tque capability. Typically, the ac mot is treated as a "speed source" in which the actual speed is determined primarily by the stat input frequency. The voltage is adjusted approximately so as to keep the no-load current fixed as the frequency changes. Since this results in a frequency-proptional voltage, the method is often called "constant volts per hertz" control. Commercial constant volts per hertz drives have become very sophisticated. They permit adjustment of acceleration and deceleration rates, suppt wide speed ranges, and adjust f resistive drops to suppt tight regulation of speed. Since drives of this type do not require any special sensing, and need only speed infmation f high perfmance, they are generally termed scalar control drive systems. Scalar drives are popular in a wide range of ASD applications, and represent the most common type of ac drive. They have the advantage that little infmation about the mot is needed to make them operate properly. However, Speed control, while it suppts a wide range of applications, does not cover the true high-end applications that require a tque source. With the development of faster-switching and me easily controlled power electronic components such as the gate turn-off thyrist (GTO), the power bipolar junction transist (BJT), and most recently the insulated-gate bipolar transist (IGBT), researchers realized that new control schemes could be implemented which would provide high-bandwidth current control.

4 Fig. 3. Indirect Vectial Control In 1972, F. Blashke published an approach to ac induction mot control known as field ientation. In field ientation, the mot input currents are adjusted to set a specific angle between fluxes produced in the rot and stat windings in a manner that follows from the operation of a dc machine. Me fmally, the approach applies direct-quadrature (D-Q) two-axis analysis methods directly to the tque control problem. When the dynamic equations f an induction mot is transfmed by means of wellknown rotating transfmation methods into a reference frame that coincides with rot flux, the results become similar to the dynamic behavi of a dc machine. This allows the ac mot's stat current to be separated into a flux-producing component and an thogonal tqueproducing component, analogous to a dc machine field current and armature current. The key to field-iented control is knowledge of the rot flux position angle with respect to the stat. In Blashke, the rot flux was measured by sensing coils. Approaches in which rot flux is sensed are now generally termed "direct field ientation" (DFO) methods. It is also possible to compute the angle from shaft position infmation, provided other mot parameters are known. This approach is now generally termed "indirect field ientation" (IFO), and is the basis of most commercial drives based on the field ientation concept (Fig. 3). The position sens is often considered an undesired element, and various ways to develop "sensless" field-iented drives are active research topics today. Whatever the field-ientation approach, once the flux angle is known, an algithm perfms the transfmation from three-phase stat currents into the thogonal tque- and flux-producing components. Control is then perfmed in these components, and an inverse transfmation is used to determine the necessary three-phase currents voltages. In principle, field ientation provides similar decoupled control of tque and flux which is inherently possible in the dc machine. There are certain fundamental differences, however: fieldientation provides what can be termed "asymptotic decoupling" the tque and field producing elements are decoupled if the field current is fixed. This is a min concern in most applications, since the field current is not usually adjusted rapidly in a drive application. A me significant difference is the need f precise mot infmation. The transfmations and control algithms presume ideal knowledge of various resistances and inductances within the machine. In a dc machine, small parameter errs will alter the output tque but not the

5 decoupling. In an ac machine, parameter errs alter the transfmation and can cause tque ripple and other problems. A mot must be carefully characterized and measured f use with a field-iented drive a process called commissioning that is often automated in commercial units. Field-iented tque control opened up many high-end applications f ac ASDs beyond those that can be served by speed control. In addition to the many benefits of the ac induction mot over dc machines (notably lower cost, better reliability, and higher efficiency), ac ASDs were found to offer faster tque response compared to dc drives. This is achieved through the lower inertia of an ac mot compared to a dc mot of similar rating, and through the faster L/R time constants in many ac machines compared to dc machines. Recent improvements in IGBTs and in the latest controlled thyrists has now boosted sustainable switching frequencies into the multi-kilohertz range. Highperfmance field-iented drives now reach ratings well above 100 kw. Pulse-width modulation and cost reductions in digital signal processs (DSPs) have made it much easier to compute transfmations rapidly and enfce the desired current wavefms. These developments are leading to widespread industrial application of field-iented ac induction mot drives in high-end applications. Drives of this type are generally termed vect drives to distinguish them from adjustable-speed scalar drive methods. Me recently, a new control method appeared in the industrial mot control field, the Direct Tque Control (DTO). This method is a technique similar in concept to field ientation, in which induction mot voltages currents are set to values as close as possible to the ones needed to generate a desired tque. Typically, the operation is perfmed with less intensive computation that f various field-iented methods. DTO is often implemented through space-vect modulation (SVM), in which the drive switches are set to alternate among just two of the possible output voltages so as to approximate closely the desired voltage vect. 3. Conclusions Vect control provides real time processing of the stat phase current and rot positioning. It provides four quadrants operation and excellent dynamic behaviour to maintain optimum efficiency and speed response time. A new high-end AC drive provides closed loop vect control of mot speed, which combined with Windowsbased configuration and hardware modularity including plug-in industrial netwking, gives the user the highest funcionality. These tools provide intuitive applications programming, using a template approach with a library of function blocks that can be parameterised and interconnected in minutes using point-and-click operations. In addition to blocks f standard application requirements such as ramps, speed loops and I/O control, the library includes advanced functions such as PID process control, as well as a suite of logic, arithmetic and decision operations to suppt complex applications programming. Meover, many drives come with a suite of preprogrammed applications software f such applications that can be controlled via a rugged human-machine interface (HMI). [1] Referencias [2] Drives and Servos, Year book Control Techniques, ISBN X [3] R. Krishnan, Criteria f the Comparison of Mot Drive systems in Motion Control, in Proceedings of International Conference on Intelligent Control and Instrumentation, SICICI '92. Singape, Volume 1, pp February 1992.