Fırçasız servomotorlar yapıları ve kontrol esasları

Abstract

1970'li yıllarda gelişim gösteren ve 80'li yıllarda ivme kazanarak günümüze kadar gelişmesine devam eden ve hala hızlı bir şekilde gelişmekte olan bilgisayar devrimi sayesinde kontrol sistemleri yapılarında da gözle görülür bir değişiklik görülmüştür. Kompleks yapıdaki kontrol algoritmalarının analog olarak gerçekleştirilme zorluğu gelişen mikroişlemci teknolojisi sayesinde dijital olarak gerçekleştirilmesi dolayısı ile ortadan kalkmıştır. Bu değişim aynı zamanda motor kontrol uygulamalarına da yansımıştır. Tabi ki motor kontrol uygulamalarında güç elektroniği teknolojisindeki gelişmelerin payını da hesaba katmak gerekir. 1956'da SCR(Silikon Kontrollü Doğrultucular) ve Tristörlerle başlayan ve 1958 yılında ilk ticari tristörün sunumu ile devam eden güç elektroniği evrimi çok sayıda güç yan-iletken cihazların ve dönüşüm tekniklerinin bulunması ile 1980'li yıllara kadar gelmiştir. Güç elektroniği ve mikroelektroniğin evliliği sonucu çok sayıda güç elektroniği entegreleri piyasaya arz edilmiştir. Güç elektroniği devrimi 1980'lerin sonlarında ve 1990'lann başlarına doğru gerçek momentumunu kazanmıştır. Aynı değişimi motor yapılan ve kontrollarında da görmekteyiz. Servo uygulamalarında vazgeçilmez sürücü elemanlar olarak gördüğümüz d.c. servomotorlarda fırçalar ve çalışma yapısından kaynaklanan problemler dolayısı ile fırçasız d.c. motorların geliştirilmesini zorunlu hale getirmiştir. Çalışma karakteristikleri uzun zaman evvel ortaya atılan bu yeni motor yapılan ancak teknolojinin gelişmesi ve bu teorik esasların pratiğe uygulanması suretiyle kullanım dahiline gelmiştir. 80'li yılların ortalarında piyasada ilk pratik uygulamalarını görmeye başladığımız ve birkaç büyük motor imalatçısının tekelinde olan bu motor teknolojisi geçen 10 yıllık süre zarfında büyük bir gelişme göstermiş ve sayılı bir kaç akademisyenin yazmış olduğu kitaplarla da bilim literatürüne dahil edilmiştir. Maalesef teknoloji transferi kolay olmamaktadır. Özellikle de bu teknolojiyi elinde bulunduran firmalar bu konuda piyasaya mal arzetmekte ise. Bir kaç yıllık gecikmeyle de olsa bu konu üzerinde duran ve motor yapılarını bilim literatürüne kazandıran T.KENJO, Y.DOTE, T.J.E. MİLLER, LEONHARD gibi akademis-yenlerin katkılarını burada belirtmekte yarar vardır. Fırçasız servomotorlar fırça elemanlarının olmaması dolayısı ile ark oluşumu, fırça bakımı, v.s. gibi problemlerden muaftır. Bu çalışmada fırçasız servomotorların genel yapılan ve çalışma esaslan üzerinde durulmuş, güç elektroniği, kumanda devrelerinden bahsedilmiş ve uygulama için gerekli olan uygun servomotor ve sürücü aksamının nasıl seçileceğine değinilmiştir. Ayrıca ikinci mertebeden bir sistem yapısında olan servomotor tranfer fonksiyonu üzerinde birim basamak cevabı hazırlanan bir program vasıtasıyla incelenmiştir.

Servomotors are classified as a.c. servomotors and d.c. servomotors and used as a key components of automated systems such as robots and NC machine tools by performing accurate positioning and speed regulation in response the commands from computers and sensors. In the field of factory automation(FA), FA equipment, numerical control(NC) machine tools, industrial robots, etc., are used to save energy and promote automation giving high productivity and producing high qulity products. As the microelectronic revolution groves they are becoming more and more intelligent rely on the control principles. Servomotor and motion-control technologies are based on mechatronics engineering. The recent remarkable progress in power electronics and microelec Ironies, more advanced servomotor and motion control systems is now available. Both microcomputers and digital signal processors(DSP) are being used as controllers and sensor signal processors owing to their fast computational capability and suitable architecture. The servomotor is an important component of actuators for FA and NC equipment. Servos are now lightweight, small, easily integrated, efficient, controllable, and maintanance-free. In the unmanned factories which are being used more of servomotors, easy motor maintenance is desirable. In such applications brushless servomotors have superior abilities to perform the required tasks. The current passing through the armature coil of a motor will be one of the alternating sine, square, trapezoidal, or other waveforms. In d.c. servomotors commutator switches(brushes) are usually mechanically installed on motors to generate those alternating currents. Conversly, alternating current can be generated by external semiconductor switching circuits this making the brushless servomotor. Brushless servomotors have the following advantages. 1. They have higher maximum speed. 2. They work in less favourable surrounding conditions. 3. They save maintenance labour and produce less noise. With the elimination of the brushes and the commutator there is also greater freedom in planning the usage of brushless motors. Motors can be made smaller or flatter; permitting easy integration with main equipment. The components necessary for the control of a servomotor are a main frame, sensors for angle, angular velocity, current, voltage, magnetic flux, and temperature, and a semiconductor power converter(power amplifier), including various analog or digital ICs for triggering control. In addition to these, a small motor-driven gear having a position and a speed sensor is also mounted on the motor shaft. A digital controller (DSP) is included too. Detectors need to be able to detect rotational position for position control and to detect the speed for response rate and/or speed control. The speed signal is calculated from the position(encoder) signal in most applications of digital control. For this purpose generally an encoder is used., and the encoder and small-sized motor are sometimes united into a single unit, the 'encoder motor', in order to make the system smaller. Semiconductor power converters have been developed as a major technique of power electronics. Bipolar transistors and MOSFETs having high power handling capability combined with the high speed switching characteristics are produced commercially. The techniques of their production are being improved worldwide. Variable-frequecy and PWM(pulse-width-modulation) inverters and converters have been produced using these high performance components. Some of them have working frequencies of 10-30 kHz. Highly advanced IC production techniques make possible analog and digital ICs, custom LSIs and one-chip microcomputers for use as a gate signal generators. These components give good control performance and reliability. IC techniques are also applied in the design of high speed/power semiconductor devices. Some of the ICs are in the market for controlling the small sized brushless servomotors. Owing to orthogonally controllable magnetic flux(current) and voltage/frequency constants, brushless motors have control performance at least equal to that of d.c. motors. In addition, things that that are impossible with brush motors have been made possible by brushless servomotor: for example sine wave output current control to reduce torque ripple, high speed operation to make a motor smaller and lighter, improvement of the efficiency of converters to save energy, and equivalent field-weakening control. Furthermore, semiconductor converters now have sequence control abilities such as starting and stopping, malfunction detection, self-diagnosis, and self-protection. The application of brushless servomotors became attractive for several reasons: reduction of price of power transistors, establishment of the technique of current control of PWM inverters, development of permanent magnet materials, development of highly accurate detectors, and manufacture of these components in a compact form. The function of the commutator of the d.c. brush motor is performed by a pole sensor and a semiconductor power converter in the brushless servomotors. The generated torque is proportional to the product of current and field flux that is orthogonal. The brushless servomotor is an a.c. motor in all respects and is in fact called the a.c. servomotor. Combined with a dedicated control device, the performance of XI the brushless servomotor is found to be equal to or superior to the performance of the high-performance d.c. servomotor. The rotational speed of the d.c. servomotor is generally varied by changing the voltage applied to tha armature. On the other hand, the rotational speed of the a.c. motor is generally varied by changing the frequency. However the frequency has its limits of variation. A wide range of speed variation, which is a feature of servomotors, can be obtained by using only a simple inverter. In the d.c. motor, torque variation is reduced by increasing the commutator segments; in the brushless motor, torque variation is reduced by making the coil three-phase and by transforming the curent of each phase into a sine wave. In the past, positioning using a servomotor hase been mainly performed by the so-called serial pulse train method. This is a method in which the number of command and encoder pulses are compared and the difference between them becomes the motor speed command. However today positioning control has been performed by the microprocessors CPU, as the techniques of microcomputers have been applied more widely. When the servodriver receives analogue signals as inputs, the results of the CPU's operation are converted from digital to analogue and used as inputs. Therefore the total system performing positioning control, the CPU can directly control the part where the servo-driver receives analog signals as input. Brushless servomotors (SM type) are called brushless d.c. servomotors because their structure is different from that of d.c. servomotors. Brushless servomotors rectify current by means of a transistor, instead of a commutator used in d.c. servomotors. On the other hand, brushless servomotors are also called a.c. servomotors because brushless servomotors of synchronous type with a permanent magnet rotor detect the position of the rotational magnetic field to control the three- phase current of the armature. Then they obtain the same torque characteristic d.c. motors by making the field and the current meet at right angles. The main circuit of a brushless servomotor is equipped with semiconductor power devices capable of self-switching off and high-speed switching. They have been developed by applying the techniques of power electronics and microelectronics and are classified into bipolar transistors, MOSFETs, GTOs, and SITs. They enable not only frequency control but also very rapid voltage and current wave generation very rapidly. These power semiconductor switching devices are produced in the modular form to be used in the main circuit. Bipolar transistors are widely used in the region of 700-1000V or below. They have lower ON-state resistance and shorter switching time that GTOs. Bipolar transistors are connected by means of Darlington connection and packaged together with diodes. This is the modular structure. XII MOSFETs are used as power semiconductor devices at very high speed with meidum to small capacity(more than 20 kHz). (jTOs are more suitable tor use with high voltage and large current than bipolar transistors. In addition GTOs permit high current density. In spite of these advantages, they are less useful than bipolar transistors. They are two types of operation region of power transistors. The linear and ON/OFF region. In the linear region the transistors are opareted in linear region. On the other hand, in the ON/OFF region the transistors are operated as ON/OFF bases which is knowm as PWM.. Power semiconductor devices are triggered and controlled by means of PWM(pulse width modulation). PWM is superior in respect of input power factor, efficiency and dynamic response. The main drive circuit of brushless servomotors consist of the converter(having six diodes) converting alternatig current into the direct current and the inverter having six transistors and six free-wheeling diodes. The main circuit is equipped with a resistance for commutation, a capasitor, and a snubber circuit that controls the surge voltage. Some of the base drive circuits of power transistors use Darlington connection. The mathematical control model of brushless servomotor is of the second- order system. Control of brushless servomotors requires the main frame of the motor, angle and angular velocity detectors, currenct, voltage and magnetic flux detectors, a transistor PWM inverter, and a semiconductor power converter including analogue and digital lCs for controlling those equipments. In addition brushless servomotor driver requires position, velocity, and force controllers* motion control) for controlling the whole system. The semiconductor power converter performs a sinusoidal output current control, orthogonal control of magnetic flux and current, equivalent field-weakening control, and so on. Servomotors are often accelerated and decelarated according to load demands. In incrementeal motion applications the load have to be accelerated/decelerated frequently. Therefore choosing of a servomotor according too load requirements is of prominent role. There are two types of operating regions in servomotors. 1. Continious Region 2. Intermittent Region. In continious region the servomotor is oparated in continious basis. The region is determined by the continious current of the motor. In intermittent region the peak current determines the region boundaries. Servomotors are generally operated in the intermittent region. In order to determine the correct motor for the application the motion profile has to be determined and the torque calculations according to that motion profile has to be carried out into the motor shaft. Generally there are four types of loads reflecting the motor shaft. 1. Inertia Loads Xlll 2. Friction Loads 3. Viscous Loads 4. Effective Load The inertia load is effective during acceleration/deceleration. During deceleration the friction load contributes to the motor torque reducing the inertia load. Due to that the acceleration load has to be determined for inertia load. The inertia load is the product of the acceleration rate and the total moment of inertia reflecting the motor shaft. The friction loads comes from the friction on moving components relative to each other. Generally it is determined by measurment. During determination of the friction load in linear servo appliacations the gib fastening force has to be considered. The viscous load is generally neglected in calcualtions. It is proportional the speed of the motor. The effective load is the load to drive the mass attaches into the motor shaft. In determining this load has to be calculated reflecting the motor shaft. The RMS value of the calculated torque has to be in the continious operating region of the servomotor. In drive system of CNC machine tools generally used ballscrew & servomotor system thus obtaining the one linear axes of the machine. During choosing the required drive gear of the servomotor the backlash between mechanical components has to be considered also.