Motion control in an electric vehicle with four independentlydriven in-wheel motors

We study methods of motion control for an electric vehicle (EV) with four independently driven in-wheel motors. First, we propose and simulate a novel robust dynamic yaw-moment control (DYC). DYC is a vehicle attitude control method that generates yaw from torque differences between the right and left wheels. The results of simulations, however, identify a problem with instability on slippery, low μ roads. To solve this problem, a new skid detection method is proposed that will be a part of traction control system (TCS) for each drive wheel. The experimental results show that this method can detect a skidding wheel, without any information on chassis velocity. Therefore, this method will be of great help during cornering or braking in a TCS. These methods will be integrated and tested in our new experimental EV     

Motion control with induction motors

Induction motors (IM) provide a very wide speed range, mechanically robust and relatively low cost motion control option. An up-to-date summary of the status of induction motor motion control technology is the subject of this paper. The topics which this paper includes are as follows: basic motion control system requirements; field orientation instantaneous torque control principles for induction motors (FO-IM); current regulators for induction motor motion control; flux and torque regulators for induction motor motion control; self commissioning and continuous self-tuning for field orientation. Technology advances based on modern control and estimation theory have the potential to further enhance the capability of this important class of servo drive systems     

Modeling, Analysis, and Neural Network Control of an EV Electrical Differential

This paper presents system modeling, analysis, and simulation of an electric vehicle (EV) with two independent rear wheel drives. The traction control system is designed to guarantee the EV dynamics and stability when there are no differential gears. Using two in-wheel electric motors makes it possible to have torque and speed control in each wheel. This control level improves EV stability and safety. The proposed traction control system uses the vehicle speed, which is different from wheel speed characterized by a slip in the driving mode, as an input. In this case, a generalized neural network algorithm is proposed to estimate the vehicle speed. The analysis and simulations lead to the conclusion that the proposed system is feasible. Simulation results on a test vehicle propelled by two 37-kW induction motors showed that the proposed control approach operates satisfactorily.     

A Loss-Minimization DTC Scheme for EV Induction Motors

This paper proposes a strategy to minimize the losses of an induction motor propelling an electric vehicle (EV). The proposed control strategy, which is based on a direct flux and torque control scheme, utilizes the stator flux as a control variable, and the flux level is selected in accordance with the torque demand of the EV to achieve the efficiency-optimized drive performance. Moreover, among EV's motor electric propulsion features, the energy efficiency is a basic characteristic that is influenced by vehicle dynamics and system architecture. For this reason, the EV dynamics are taken into account. Simulation tests have been carried out on a 1.1-kW EV induction motor drive to evaluate the consistency and the performance of the proposed control approach     

A Novel Traction Control for EV Based on Maximum Transmissible Torque Estimation

Controlling an immeasurable state with an indirect control input is a difficult problem faced in traction control of vehicles. Research on motion control of electric vehicles (EVs) has progressed considerably, but traction control has not been so sophisticated and practical because of this difficulty. Therefore, this paper takes advantage of the features of driving motors to estimate the maximum transmissible torque output in real time based on a purely kinematic relationship. An innovative controller that follows the estimated value directly and constrains the torque reference for slip prevention is then proposed. By analysis and comparison with prior control methods, the resulting control design approach is shown to be more effective and more practical, both in simulation and on an experimental EV.      

Iterative learning control of antilock braking of electric and hybrid vehicles

Hybrid electric vehicles (HEVs) use multiple sources of power for propulsion which provides great ease and flexibility to achieve advanced controllability and additional driving performance. In this paper, the electric motor in HEV and electric vehicle (EV) propulsion systems is used to achieve antilock braking performance without a conventional antilock braking system (ABS). The paper illustrates that the antilock braking of HEV can be easily achieved using iterative learning control for various road conditions. A vehicle model, a slip ratio model, and a vehicle speed observer were developed to control the antilock performance of HEV during braking. Through iterative learning process, the motor torque is optimized to keep the tire slip ratio corresponding to the peak traction coefficient during braking. Simulations were performed on a compact size vehicle to validate the proposed control method. The control algorithm proposed in this paper may also be used for the ABS control of conventional vehicles.     

Motor control and torque coordination of an electric vehicle actuated by two in-wheel motors

In this research, an electric vehicle actuated by two in-wheel DC motors is developed. By properly coordinating the motor torques, both drive-by-wire and electrical steering can be achieved. Two critical issues respectively related to the design of motor controllers and the coordination of the two motor torques under control saturation are investigated in this study. Firstly, as for the in-wheel motors that are used for driving and steering simultaneously, their operation covers a wider dynamic range that forward acceleration (deceleration), and reverse acceleration (deceleration) may occur alternately. To perform driving and steering smoothly and efficiently, each motor should be switched to an appropriate mode to generate the torque demanded. Secondly, during the high-speed maneuvering, the high back-emf voltage in the motor coil substantially reduces the motor’s torque generating capability. Since the electrical steering depends on the differential torque of two wheels, when electrical steering is demanded in this case, torque/current saturation may occur in either one of the motors and the electrical steering performance could be seriously degraded. To address these issues, controllers of two levels are proposed. For the low-level controller (the motor controller), it operates the motor automatically in an appropriate mode for performance and efficiency consideration. An input transformation is introduced to cancel the nonlinearity in current dynamics so as to control the motor torque easily and precisely regardless of mode switching. For the high-level controller (the torque coordination controller), besides generating reference commands to the low-level controllers, during control saturation it can also properly re-distributes control signals to maintain consistent steering performance and provides compensation for integrator windup. The control system is implemented and the performance is experimentally and numerically validated.     

Analysis and Application of Particular Current Signatures (Symptoms) for Cage Monitoring in Nonsinusoidally Fed Motors With High Rejection to Drive Load, Inertia, and Frequency Variations

In this paper, some original and effective fault indicators for broken-bar detection in power squirrel-cage induction motors are presented. A motor phase-current signature analysis can be performed by evaluating the typical ratios $I_{(7 - 2s)f}/I_{5f}$ and $I_{(5 + 2s)f}/I_{7f}$, $I_{(13 - 2s)f}/I_{11f}$ and $I_{(11 + 2s)f}/I_{13f}$, etc., which appear in the phase-current spectrum of faulted motors fed by nonsinusoidal voltage sources. The main advantages of the new indicators are the following: 1) accentuate insensitivity to disturbs such as load torque, drive inertia, and frequency variations; 2) low dependence with respect to machine parameters (except the pole number); and 3) linear dependence on fault gravity. They can be directly applied on motors fed by open-loop low-switching frequency gate turn-off/thyristor converters. Railway traction drives are possible targets. Application to mains-fed motors can be tried too, if suitable harmonics are present in the plant supply. A detailed analytical formulation for fault indicators is furnished, based on the multiphase symmetrical component theory; theoretical results have been supported by experimental work, performed by using a motor with an appositely prepared cage, and successively, method validation was achieved on three other industrial motors.      

Current phase control methods for permanent magnet synchronousmotors

Three types of current phase control methods are examined for the interior magnet motor and the surface magnet motor: (1) the i _d=0 control method, (2) the cos φ=1 control method, and (3) the constant-flux-linkage control method. The control circuits for realizing these control methods were investigated and a drive test carried out. The most suitable current phase control method for the rotor geometry was examined by computer simulation and experimentation. It was found that in the i_d=0 control method, high-performance torque control can be obtained as the torque is proportional to the armature current. In the cos φ=1 control method, the torque per armature current is small and the torque characteristic is nonlinear. Therefore, high-performance torque control cannot be expected. The constant-flux-linkage control method is desirable for interior magnet motors as the torque characteristic is almost linear and the required inverter capacity is comparatively small     

High-performance adaptive robust control with balanced torque allocation for the over-actuated cutter-head driving system in tunnel boring machine

The tunnel boring machine(TBM) is a kind of large-scale underground equipment for the tunnel and subway excavations. The cutter-head driving system is one of the key components in hard rock TBM, which is driven by multiple motors to provide enough torque during the excavation. Synchronizing motions of the multiple driving motors and balancing the driving torques are two essential control issues to be addressed of cutter-head driving system. However, the existing synchronization control approaches usually focus on the pure motion synchronization of the multiple driving motors only. These methods will lead to the phenomena of uneven driving torques even when their motions are quite well synchronized, and may result in the shaft broken accident. Instead in this paper, an adaptive robust control law integrated with torque allocation technique scheme is proposed that achieves not only better motion synchronization of the driving motors but also simultaneous regulation of driving torques. Namely, the adaptive robust control (ARC) is introduced to deal with the negative effects from uncertainties and variable load acting on the cutter-head. And a torque allocation algorithm is proposed to distribute the driving torque of each motor evenly. Comparative simulations are carried out to verify the excellent performance of the proposed scheme.     

A fault-tolerant control architecture for induction motor drives in automotive applications

This paper describes a fault-tolerant control system for a high-performance induction motor drive that propels an electrical vehicle (EV) or hybrid electric vehicle (HEV). In the proposed control scheme, the developed system takes into account the controller transition smoothness in the event of sensor failure. Moreover, due to the EV or HEV requirements for sensorless operations, a practical sensorless control scheme is developed and used within the proposed fault-tolerant control system. This requires the presence of an adaptive flux observer. The speed estimator is based on the approximation of the magnetic characteristic slope of the induction motor to the mutual inductance value. Simulation results, in terms of speed and torque responses, show the effectiveness of the proposed approach.     

Evaluation of soft switching for EV and HEV motor drives

Soft switching has the potential of reducing switch stresses and of lowering the switching losses as compared to hard switching. To understand the effectiveness of the soft-switching technique, when applied to electric vehicle (EV) and hybrid electric vehicle (HEV) systems, it may be necessary to first evaluate their system requirements and performance. This evaluation process would require knowledge of the vehicle dynamics. The vehicle load requires a special torque-speed profile from the drivetrain for minimum power ratings to meet the vehicle's operational constraints, such as initial acceleration and gradability. The selection of motor and its control for EV and HEV applications are dictated mainly by this special torque-speed requirement. As a consequence, this requirement will have a strong influence on the converter operation. This paper makes an attempt to evaluate EV and HEV running in both standard Federal Test Procedure 1975 city driving and highway driving cycles. A simplified analysis is carried out for several of the most commonly used electric motors operating on the optimal torque-speed profile. Special attention is given to the converter conduction and switching losses, by analyzing the switching losses, and by assuming that an ideal soft-switching scheme will have zero switching losses, one can evaluate the improvement in the system efficiency if a soft-switching control is used. The relative significance of soft switching for EV and HEV systems is then established     

High energy efficiency oriented-control and design of WFSM based on driving condition of electric vehicle

The purpose of this paper is to increase the energy efficiency of an electric vehicle (EV) with a wound-field synchronous motor (WFSM). Therefore, methods are proposed to estimate and improve the energy efficiency of the EV as well as the performance of the WFSM. The following contributions are provided: 1) EV model as well as the mathematical model of the electric motor are explained considering the common Artemis driving cycle (CADC); 2) a control method for maximizing the energy efficiency of the electric motor is proposed; 3) analysis methods for calculating the circuit parameters (resistance, inductance, and flux linkage) and losses (ohmic, iron, and mechanical loss) are described. The efficiency of the machine is accurately determined using the proposed analysis method; 4) based on the proposed methods, the design process of the WFSM is proposed to improve the energy efficiency considering the vehicle system and driving cycle; and 5) the proposed methods are verified through tests of the prototype and improved motor. As a result, through the proposed design and control method, even though the volume of the improved motor was 7.8% smaller than that of the prototype, the efficiency of the improved motor was higher than that of the prototype in all regions. In addition, to confirm the effectiveness of the proposed methods, the performance of the electric vehicle considering the driving cycle was analyzed according to the characteristics of the electric motor. The energy loss of the improved motor with the proposed control and design method was 63.3% less than that of the prototype. Accordingly, the energy efficiency of the vehicle system and the energy consumption of the battery increased by 7.8%p and decreased by 2.0 kWh, respectively.     

Nonlinear MPC-based slip control for electric vehicles with vehicle safety constraints

This paper presents a new slip control system for electric vehicles (EVs) equipped with four in-wheel motors, based on nonlinear model predictive control (nonlinear MPC) scheme. In order to ensure vehicle safety, wheel slip stable zone is considered as time-domain constraints of the nonlinear MPC. Besides, the motor output torque is limited by the motor maximum torque, which varies with motor angular velocity and battery voltage, so the motor maximum output torque limitation is considered as system time-varying constraints. The control objectives include: vehicle safety, good longitudinal acceleration and braking performance, preservation of driver comfort and lower power consumption. This paper utilizes nonlinear MPC to solve this complex optimization control problem subject to the constraints, and the vehicle safety objective is achieved by wheel slip stable zone constraints, the other objectives are realized by adding additional cost functions. In addition, a penalty on the slack variables is also added to ensure that the state constraints (wheel slip) do not cause infeasible problems. The effectiveness of the proposed controller is verified in the off-line co-simulation environment of AMESim and Simulink, and a rapid control prototyping platform based on Field programmable gate array (FPGA) and dSPACE is completed to evaluate the real time functionality and computational performance of the nonlinear MPC controller.     

Novel permanent magnet motor drives for electric vehicles

Novel permanent magnet (PM) motor drives have been successfully developed to fulfil the special requirements for electric vehicles such as high power density, high efficiency, high starting torque, and high cruising speed. These PM motors are all brushless and consist of various types, namely rectangular-fed, sinusoidal-fed, surface-magnet, buried-magnet, and hybrid. The advent of novel motor configurations lies on the unique electromagnetic topology, including the concept of multipole magnetic circuit and full slot-pitch coil span arrangements, leading to a reduction in both magnetic yoke and copper, decoupling of each phase flux path, and hence an increase in both power density and efficiency. Moreover, with the use of fractional number of slots per pole per phase, the cogging torque can be eliminated. On the other hand, by employing the claw-type rotor structure and fixing an additional field winding as the inner stator, these PM hybrid motors can further provide excellent controllability and improve efficiency map. In the PM motors, by purposely making use of the transformer EMF to prevent the current regulator from saturation, a novel control approach is developed to allow for attaining high-speed constant-power operation which is particularly essential for electric vehicles during cruising. Their design philosophy, control strategy, theoretical analysis, computer simulation, experimental tests and application to electric vehicles are described     

Control Strategy of PWM Inverter Drive System for Electric Vehicles

This paper presents an advanced ac drive system used in electric vehicles (EV's). The system consists of an induction motor, a PWM transistorized inverter, a variable ratio gear box and a controller. The special features of the drive system are discussed, with emphasis on the control strategy which can optimize the performance of the EV.     

Modular integrated longitudinal and lateral vehicle stability control for electric vehicles

In this paper, the problem of integrated longitudinal and lateral vehicle stability control is addressed using a modular optimal control structure. The optimization process of the high level model predictive control (MPC) controller determines required longitudinal force and yaw moment adjustments to minimize the error between vehicle longitudinal and lateral vehicle stability dynamic states with respect to the target courses. The low level controller is designed to optimally regulate torque at each wheel based on the control inputs of the high level controller, and distribute required torque between the wheels via actuation system. The actuation system that is utilized to implement the proposed control structure functions based on all-wheel drive technology that can provide active control of both traction and yaw moment control with differential torque. The multi-layered structure of the control system allows modularity in design. The performance of the control structure is investigated by conducting experimental tests. The experimental tests have been performed on an electric Chevrolet Equinox vehicle equipped with four independent motors. The results show that the integration of the vehicle longitudinal and lateral dynamics preserves vehicle stability in a planar motion and improves the vehicle dynamic response, especially in challenging driving maneuvers.     

A Modular and High-Precision Motion Control System With an Integrated Motor

This paper describes the design and control of an integrated direct-drive joint suitable for applications that require a high-precision motion control system. The joint integrates a direct-drive synchronous motor with axial air gap, a torque sensor, and a high-resolution position sensor. The key design aspects of the integrated joint, like the motor's armature, cooling system, motor housing, bearing arrangement, sensors, and power electronics are detailed. We also present a number of advanced implementations in motor torque control, optimal joint torque sensory feedback, and motion control using positive joint torque feedback. Experimental results illustrate an outstanding performance regarding thermal response, torque ripple, reference trajectory tracking, torque disturbance rejection, and joint stiffness.     

The sweep-extend mechanism: A 10-bar mechanism to perform biologically inspired burrowing motions

A new type of mechanism has been designed to move an output point through a motion akin to a swimmer’s breast-stroke. The mechanism is designed to clear loose debris from around small mobile search and rescue robots. However, it has potential applications in many other domains that require controllable cyclic motion. This paper describes the mechanism concept and presents mathematical analysis of its design parameters. The properties of the mechanism were examined to understand how the mechanism trajectory can be altered mid-cycle whilst maintaining the condition of continually rotating motors. Furthermore, equations to calculate the mechanism torque ratio were derived. The analysis reveals limitations in the trajectory variations that can be implemented whilst maintaining the condition of continually rotating motors. The mechanism was manufactured to validate the mathematical predictions. It was found that the predicted torque ratios are within 90% of the experimentally obtained torque output. The mechanism was controlled using proportional-derivative control (PD) and demonstrated to track several desired trajectories without reversing the direction of motor travel, with tracking error less than ±4 mm.     

A torque estimator using online tuning grey fuzzy PID for applications to torque-sensorless control of DC motors

DC motors are used indispensably in industrial applications because they provide such advantages as small size, high speed, low construction cost, and safe operation. A major area of research in DC motors is to determine a better method to measure the torque of motor shaft. The traditional way to measure the mechanical torque of a rotating shaft is attachment a torque transducer in the transmission system between the driving motor and the load. However, this technique requires additional parts for the transmission system, which makes the design more complicated, time consuming, costly in construction, and in many cases impossible to achieve.The purpose of this paper is to present a new method for estimating the load torque of a DC motor shaft by using a novel modelling method based on an adaptive control technique, named as online tuning grey fuzzy PID (OTGFPID). A test rig using a DC motor is setup to investigate the torque behaviour as well as to evaluate the developed estimator. Firstly, mathematical model is developed for the motor. Secondly, the experimental speed-torque data and the optimized motor model is used to optimize the torque estimator. Then the optimized estimator is used to estimate accurately the load torque. Finally, the capability of the optimized torque estimator has been validated with the practical experiments in comparison with a typical estimation method.