CHAPTER 2 LITERATURE REVIEW

Transcription

1 11 CHAPTER 2 LITERATURE REVIEW 2.1 INTRODUCTION Electromechanical actuators are mainly used in many industrial systems such as aerospace, automotive, underwater and domestic applications. In particular, the electromechanical actuators are best suited to the engine gimbal control launch vehicle mechanism. In selecting control actuators for engine gimbal control application there is a critical need to reduce the weight and size while providing better reliability. By incorporating functional redundancy, the actuator can tolerate up to two failures in its critical components. The direct connection of electrical motors to the movable nozzle without any mechanical coupling or gears helps to attain a remarkable level of reliability, efficiency and backlash free performance of the actuators. In recent years with the advancement of power electronics switching devices and better control capability, the advanced electrical motors are widely used for this electromechanical actuator. The permanent magnet brushless dc torque motor is the best choice of electrical motor for this application. The actuator mechanism shown in Figure 2.1 consists of brushless dc motor with roller screw mechanism. Roller screws are used to convert the rotary motion to linear for the mechanism. Electromechanical control actuators driven by electric motors have begun to displace hydraulic and pneumatic technology up to 12-ton class of requirements.

2 12 Figure 2.1 Electromechanical actuator with roller screw mechanism 2.2 EXISTING ISSUES AND PROBLEM STATEMENT Hydraulic and pneumatic systems are used previously for energy conversion in the aerospace industry. The electromechanical actuator replaces the hydraulic actuator for energy conversion in flight control mechanism due to the lack of mechanical reliability in the latter system. The functional reliability is improved in the electromechanical actuator by employing redundant technique. Conventional approach for incorporating redundancy in the actuator mechanism includes usage of multiple motors driving a roller screw through a set of gears and pinions. The primary concern regarding the reliability of multiple motor actuator systems is the probability of a jam or structural failure in the gearbox. Hence it is desirable to obtain the required redundancy with a multi-channel motor which can electromagnetically sum the torque of the individual quadrants on a single armature core as proposed in this thesis. The electrical motor used for this flight control electromechanical actuator must possess operational winding redundancy in order to meet the reliability requirement of the system. In all electrical motors the probability of winding faults is high in proportion compared to other kinds of electrical

3 13 machine faults. In order to increase the reliability of the system, multiredundancy advanced electrical motors are considered. For the above application, the electromechanical actuator requires a motor of high torque, high efficiency, long life, less weight and size. For this safety critical system application the conventional structured electrical motors are unable to satisfy the reliability requirement. The dc motors are not suitable due to its limited reliability because of the presence of commutator, brushes and electromagnetic interference. Replacing the electromagnet with permanent magnets and with the use of rotor position sensor and commutation electronics makes brushless dc motor suitable for the space environment. The permanent magnet brushless dc torque motor is best suited for this electromechanical energy conversion application due to its better controllability and superior performance compared to other motors. Hence a customized brushless dc motor with a single high energy rare earth permanent magnet rotor with four separate three phase windings arrayed in single stator quadrants around the periphery for quadruplex redundancy is studied for this application. 2.3 SCOPE AND ORIGINAL CONTRIBUTION OF WORK The main scope of this thesis is To introduce a new winding method in the stator assembly of brushless dc motor to isolate all four quadrants electrically and physically such that each quadrant behaves as a separate three phase star connected motor in a single stack leading to quadruplex redundancy. To design the frameless quadruplex winding redundancy permanent magnet brushless dc motor as per the specification.

4 14 To analytically calculate the electrical loading and magnetic required for the torque and speed output. To use two dimensional finite element analysis based electromagnetic simulation software to validate the analytical design and to plot the flux density profiles and torque output. To fabricate the prototype motors (integral slot stator and fractional slot stator and a common permanent magnet rotor) and experimentally test the performance of the motor having four quadrant windings. To select the motor configuration for the required application based on the performance output. To make the design changes in the selected motor configuration in order to improve the torque output. To study the performance of the brushless dc motor with Cobalt Iron alloy lamination material as stator magnetic core instead of conventional silicon steel lamination sheet for the same stator assembly and rotor assembly design configurations. This thesis presents the design and performance analysis of quadruplex winding redundancy permanent magnet brushless dc torque motor for the electromechanical actuator application in flight control mechanism. Two quadruplex winding redundant stator assembly configurations (fractional slot stator and integral slot stator) and a common permanent magnet rotor assembly configuration conforming to the stator quadruplex redundancy are developed as per the requirement. The prototype motors are tested for no-load speed, back-emf and torque output of each quadrant to study the suitability of four quadrant redundancy. The torque constant is improved by using new

5 15 high permeability magnetic material (HiperCo) for the stator magnetic core of the motor instead of conventional silicon steel lamination core. 2.4 LITERATURE STUDY Reliability is of great significance for the critical systems such as launch vehicles, satellites, robots and special vehicles. The structure and the operating environment influence the reliability of the electromechanical actuator system remarkably. The reliability of the launch vehicle system depends on the reliability of each part of the actuator system. The electromechanical system comprises of electrical motor, its drive electronics and mechanical interfaces. The normal structured motors are unable to satisfy the reliability requirements. Therefore motors with redundancy structures are developed to overcome this problem. In recent years, there has been some preliminary research results of electrical redundancy of brushless dc motor used for electromechanical actuator (Hemant Jaison et al. 2011). The highly redundant electromechanical actuation system comprising a relatively large number of actuation elements is controlled in such a way that faults in individual elements are effectively detected, identified and isolated. The electromechanical actuator includes the electrical part as a brushless dc motor and the mechanical arrangement as a ball screw. The motor is connected to the ball screw by means of suitable gearing system. Thus the quadruplex brushless dc motor provides very high redundancy for the actuator. Effective detection and isolation of winding faults in control actuation systems is thus essential for enhanced reliability. The performance of reliability improvement have been studied in hybrid four redundancy brushless dc motor which combines both the electrical and mechanical redundancy (Hong Guo et al. 2008). This system

6 16 has two armature stator assemblies and two rotor assemblies on the same shaft for redundancy. This system has larger size and volume of the motor and critical mechanical interface with the system. Wei Xing et al. (2007) have proposed double stator tri-redundant brushless dc motor that carries three sets of windings in two homocentric stators for functional redundancy. The double stator is composed of two independent stators which are homocentric to a single shaft. The cup rotor is composed of magnet poles and sleeve. Two types of magnetic circuits, that is parallel magnetic circuit where direct axial fluxes of outer stator and inner stator shunt across a mutual rotor. The other type is serial magnetic circuit, the outer stator and the inner stator is with the same direct axial flux are analysed. All this functional redundant structure has hybrid method having either two rotors or two stators which increases the weight and size of the motor which is the major criteria for the electromechanical actuator system. The proposed method in this thesis has four functional redundancy winding in single stator. This configuration reduces the size, weight and eliminates the mechanical interface needed to the electromechanical system. The hybrid three-redundant electromechanical actuator control system has been studied (Na Wang and Yuanjun Zhou 2009). This system controls the valve angle based on the instructions of the host computer in order to regulate the pressure of the cabin and the cockpit. At the normal time, one set of the automatic driver works obeying the direction of the host computer. When any problem occurs on the working driver, the host computer will cut to the other automatic driver system as soon as possible to confirm the system works normally. Only if both the two sets of the automatic drivers are out of control, the manual driver is started to regulate the anger of the valve according to the panel button.

7 17 The modeling of bldc motor and its drive with closed loop position control system with redundancy technique has been studied (Hemant Jaison et al. 2011). The simulation work is done for different failure modes of the drive and the impact of failure modes on system are analysed. The system has dual redundancy actuator which prevents performance loss during the occurrence of any failure. The system is composed of two chains, a prime chain and a redundant chain. Both the chains have similar type of control system. If any type of fault occurs in the prime chain, the prime chain is isolated from the system and switches to the redundant chain. Thereafter redundant chain will be driving the system. Ma Ruiqing et al. (2005) also proposed balanced current control method for dual redundancy brushless dc motor. The reliability analysis of the permanent magnet brushless dc motor for aerospace and automobile application has been widely studied. Villani et al. (2010) have studied the reliability improvement in motor and drive for electromechanical actuator by introducing multi-phase motor driven by multiphase power converters there by both the motor and electronics satisfy the fault tolerant requirement. The design details and prototype issues of five phase fault tolerant permanent magnet brushless dc drive has been presented. The fault tolerant power stage employs five independent H-bridge converters with separate bulk capacitor and phase current sensor. The control unit is based on the TMS320F2806 Digital Signal Controller, which computes the brushless dc motor control law according to the information provided by two groups of Hall effect position sensors. Farhad Aghili (2011) proposed fault tolerant controller for brushless dc motor that can maintain accurate torque production. The faulty phases are detected from the covariance of the estimation error and the phase currents of the remaining phases are optimally reshaped so that the motor

8 18 accurately generates torque as requested while minimizing power loss. The controller is applicable for brushless dc motors with any phase waveform. There are many literatures for the study of brushless dc motor for industrial applications. The design and analysis of enclosed rotor radial flux Halbach array permanent magnet brushless dc motor for spacecraft applications has been investigated (Praveen et al. 2010). The torque ripple in the motor is undesirable in Control Moment Gyroscope application that requires positional accuracy. The ripple leads to speed oscillations which cause deterioration in the performance and excites resonances in the mechanical portion of the drive system producing acoustic noise. The most important design consideration in the choice of low speed high performance torque motors for spacecraft applications is to obtain high torque density and efficiency and also to minimize the torque ripple and its related harmonics. Here the enclosed rotor Halbach array motor configuration has been suggested for zero cogging torque, better positional stability, high torque density, high torque to inertia ratio and zero magnetic stiction suiting space requirements. Initial design of major parameters and airgap magnetic flux density are estimated using the analytical model. The proportion of the Halbach array in the machine is optimized using finite element analysis to obtain near trapezoidal flux pattern. Jinyun Gan et al. (2000) investigated a new five phase surface inset permanent magnet brushless dc motor drive for electric vehicle application. The considered method has high power density and high efficiency characteristics of a brushless dc motor drive as well as the advantages of high starting torque and the wide constant power operating range of a dc series motor drive for electric vehicle propulsion. The motor drive has advantages of both the permanent magnet brushless dc motor drive and the dc series motor drive. The airgap flux of the motor is generated by both the permanent magnet

9 19 excitation and the specially controlled stator currents under the same permanent magnet pole. The torque of the motor is composed of permanent magnet torque produced by interaction between the permanent magnet excitation and the two phase stator currents and the stator currents of other phases. As this part of torque is approximately proportional to the square of phase current, it offers the characteristics of a dc series motor. Following similar trend in aerospace and automotive industries, electrical actuators are also becoming very popular to be used in remotely operated underwater vehicles and autonomous underwater vehicles. These vehicles are intensively being used in the offshore oil and gas exploration, production, underwater pipeline monitoring and maintenance, surveillance and rescue operation, military operation, marine and environmental impact assessments, salvage operation and hydro-graphic survey. For underwater vehicle, six units of electrically actuated propellers are fitted onboard, two at the back for forward and reverse movements, another two for the left and right turnings, while the other two for the up and down lifts. Of these six propellers, the two units positioned at the back are the biggest in terms of rated torque and power. Ishak et al. (2010) have investigated the permanent magnet brushless dc motor for electromechanical actuator application for underwater vehicle. Here the design considerations such as the operating temperature, operating pressure, suitable materials used for motor housing, bearings, motor topology, number of stator slots, number of rotor poles, cogging torque ripples, operating speed range, maximum torque and power and other mechanical constraints are taken into account. Finite element analysis on the motor design carried out and the prototype motor has been developed to validate the design. Jiaxin Chen et al. (2007) have proposed the design of permanent magnet brushless dc motor employing advanced techniques for driving high

10 20 speed embroidery machine. The motor performance has been investigated with the fundamental components of the applied phase voltage, back-emf and current phasor diagram with the real voltage, back-emf and current waveforms by a simulink based simulation model. The finite element analysis has been carried out to accurately compute the key motor parameters such as the airgap flux, back-emf, inductance and magnetic field. Based on the numerical magnetic field distribution, a modified incremental energy method has been applied to effectively calculate the self and mutual inductances of the stator windings. The developed motor prototype has been fabricated and validated with the experimental results. The brushless dc motors drive employing Hall effect sensors for rotor position detection has been widely used in many applications. The assumption is that the Hall sensors are ideally placed 120 electrical degrees apart. However, this assumption is not always valid. The sensor placement may be significantly inaccurate in the stator assembly of bldc motor. The unbalanced sensors lead to unequal conduction intervals among the motor phases and subsequently undesirable low frequency harmonics in developed torque that degrades the overall drive performance of the motor (Nikolay Samoylenko et al. 2008). The studies were conducted in wide range of operating conditions including steady state operation and transients below and above the ratings of the motor. The errors in Hall sensors cause unbalanced operation of the inverter leading to unequal conduction intervals among the motor phases. Jung Moo Seo et al. (2011) considered the design of a small-sized slotless bldc motor based on finite element analysis. The two pole three phase motor with a double layered short pitch winding has been designed for the study. Through numerical analysis, the basic dimensions are ascertained and the winding features are determined. The induced voltage is calculated and

11 21 the variation of the rotational speed and current with respect to the torque are estimated through analytic equations. The designed motor has been fabricated and the output characteristics are compared with those of the simulated one. Ring shaped high energy permanent magnets (NdFeB) were used and a distributed winding composed of nine single coils are fixed on the stator core. For achieving a high torque density, the size of the permanent magnet is maximized as much as possible and the flux density of the stator core is estimated with respect to various magnet volumes. Through back-emf calculation, the winding area is roughly determined on the basis of the number of coil turns. The simplified analytical method to design a small two-pole three phase slotless brushless dc motor has been studied according to the given specification (Miroslav Markovic and Yves Perriard 2006). It provides a simple solution to the designer for obtaining the motor design analytically. With few approximations, the motor analytical model is formulated to generate a system of equations. By applying an approximation, the motor model is transformed so that it generates a system of equations to solve. For a desired efficiency, by varying two free parameters, the system is analytically solved. The solution corresponding to a minimal motor mass is assumed as the optimum for given efficiency and the same procedure is performed for other values. Jae-Hak Choi et al. (2010) proposed the design process for brushless permanent magnet dc motors with segment rectangular wires in order to find the proper slot opening dimension and slot-pole combinations necessary to achieve high power density per volume with similar efficiency. A surface mounted permanent magnet motor design has been presented which focuses on motor power density. Multi-slot and closed slot types are used to increase the output torque and to reduce the cogging torque. The detailed

12 22 review of important design parameters have been performed by finite element analysis to investigate their effect on the motor performance. The cogging torque depends mainly on the length of the slot opening. The efficiency and output power are decreased by the magnetic flux leakage of teeth in the case of motors with a closed slot opening. The new airgap profile for single phase permanent magnet brushless dc motor has been proposed to improve starting torque and to reduce cogging torque (Mohammed Fazil and Rajagopal 2010). Parametric analysis were carried out to study the effect of variation of profile dimensions on cogging torque and starting torque. The profile is defined by dip, dip angle, leading edge, trailing edge, and inter magnet space. Dip is the point which is farthest from inner diameter of magnet and dip angle is the angle between trailing edge and dip. In this study, dip and dip angle are treated as variables to generate different airgap profiles. By varying dip and dip angle, a number of airgap profiles are generated. Finite element software with transient solver was used for calculating EMF constant and cogging torque. The influence of material magnetic properties on the motor iron loss of a permanent magnet brushless motor has been investigated both experimentally and by finite element calculations using various non-oriented electrical steel sheets as stator lamination material (Hiroaki Toda et al. 2005). The motor iron loss and the flux density in a tooth body were measured by applying torque load. The stator iron loss was calculated from flux density waveforms by time stepped finite element analysis with an iron loss model which accounts for hysteresis, eddy current and the excess loss component. Three kinds of conventional non-oriented electrical steels fabricated in conformity with Japan Industrial Standards and two kinds of electrical steels which have lower iron losses and higher flux densities after stress-relief annealing were used as stator lamination.

13 23 The effect of unsymmetric magnetization distribution on the performance of the hard disk drive spindle motor has been investigated (Seung-Chan Park et al. 2000). Time stepping finite element analysis was carried out to examine the effect of the unbalanced magnetic force on torque ripple and cogging torque. The unbalanced magnetic force as a source of mechanical vibration is very sensitive to the unbalance of magnetization distribution and torque ripple can be reduced by an optimum magnetization distribution. The permanent magnet brushless dc motor can be modelled accurately in finite element software with static and transient solvers which can facilitate and accelerate the design and optimization procedure. A comprehensive computer aided design procedure for a radial flux surface mounted permanent magnet brushless dc motor has been developed (Parag R. Upadhyay and Rajagopal 2005). The procedure involving the derivation of the output equation and two self corrective loops; one for the efficiency and the other for the airgap flux density were discussed. The design variables such as airgap flux density, slot electric loading, winding factor, stacking factor, stator current density, slot space factor, magnet fraction, slot fraction, flux density in the stator back iron, etc., are assumed. The basic output equations are derived and used for the design algorithm. Three designs, one each in fractional, low, and medium horse power categories are arrived using the developed program giving the targeted torque within an acceptable tolerance but with high efficiency. The computer based designs are validated using the finite element analysis. The major disadvantage of brushless dc motor is the generation of undesired cogging torque and ripple torque along with the excitation torque. Many authors discussed the origin, effect and methods to mitigate the cogging torque and ripple torque generated in the motor.

14 24 The techniques adopted in designing a surface permanent magnet motor to eliminate or minimize the cogging torque have been investigated (Nicola Bianchi and Silverio Bolognani 2002). This author gives the tools for choosing the more appropriate techniques or a combination of them to be adopted in the design of a smooth torque motor. A well known method to reduce the cogging torque is by skewing the stator slots or permanent magnets. The permanent magnet pole arc width can be selected to reduce or eliminate some harmonics. Also a multi-pole machine can be designed with permanent magnet of different arc widths to reduce the cogging torque. Another technique to reduce the cogging torque is by introducing number of notches in the stator teeth and dummy slots in the stator. The cogging torque reduction method has been suggested where the distribution of the residual magnetic flux density of the permanent magnet is modified by putting auxiliary slots on the pole face of the magnetizing yoke (Chang Seop Koh and Jin-Soo Seol 2003). The auxiliary slots are introduced on the pole face and not on the armature core. The conventional cogging torque reduction methods can be classified into two methods. One method changes the number of poles of the permanent magnet or the slots of the armature core so that the greatest common measure of them may be one. This method, although very effective from the viewpoint of cogging torque reduction, generates unbalanced radial force. The other method modifies the distribution of the residual magnetic flux density of the permanent magnet or the shape of the armature core. Hwang et al. (2001) proposed cogging torque reduction method by controlling the harmonic components of the square of airgap permeance function and flux density function of a permanent magnet motor. A unified analysis of the different design techniques by controlling airgap permeance function and flux density function has been investigated. To control airgap

15 25 permeance function, teeth width design, teeth pairing design, teeth notching design and inter-pole design are discussed. To control flux density function, magnet arc design, magnet edge shaping, permanent magnet pole pairing and permanent magnet shifting technique are studied. 2.5 SUMMARY The permanent magnet brushless dc motor is considered for the electromechanical actuator application in space mechanism. Advancement of rare earth permanent magnet materials and power electronics switching devices improves the suitability of permanent magnet brushless dc motor for this high critical environment. The electromechanical system with advanced electrical motor replaces the hydraulic and pneumatic system in the launch vehicle actuator mechanism. The issues in the existing system and the need for reliability improvement in the present electromechanical actuator are studied. The main scope and contribution in the proposed thesis is illustrated. The literature of brushless dc motor developed for aerospace, automobile, underwater rover and domestic application are reviewed. The reliability improvement by incorporating redundancy in the motor configuration is investigated. The various design techniques to mitigate the cogging torque is also studied.