This dissertation deals with integrated stability program (ISP) on electric vehicles (EVs). A novel vehicle motion control is achieved by utilizing electric motors' advantages. The proposed method is robust against both environmental disturbance and model error by applying state estimation and two-degree-of-freedom (2DOF) control.

Controllability is the most distinguishing feature of electric motors against internal combustion engines (ICEs). EVs' advantages are summarized as following four points; 1) motor torque response is 10-100 times faster than that of an internal combustion engine. This property enables high performance adhesion control, skid prevention and slip control, 2) motor torque can be measured easily by observing motor current. It can be used for road condition estimation, 3) since an electric motor is compact and inexpensive, it can be equipped in each wheel. This feature realizes high performance three dimensional vehicle motion control including rolling stability control (RSC) and yawing stability control (YSC), 4) there is no difference between acceleration and deceleration control. This actuator advantage enables high performance traction and braking control.

Recently, electronic stability control (ESC) has been installed on internal combustion engine vehicles (ICEVs). Especially in the United States, obligation of ESC installation has started since 2008. ESC is yaw moment control on a two dimensional surface and utilizes differential torque of traction force or braking force of right and left wheels. However, as ICE is based on chemical explosion and braking force is based on PWM modulation, these forces cannot be known precisely; and they also contain dead time and time-delay. In case of all-in-wheel motor EVs, high performance vehicle motion control can be achieved utilizing the advantages mentioned above. On the other hand, EVs are relatively small and wheel width is narrow but height of the vehicle is maintained due to cabin space. So danger of rollover is comparability higher and drivability is different. Therefore, chassis technology should be established taking roll stability control (RSC) and yaw stability control (YSC) into consideration.

In addition to the actuator advantages, robust RSC is designed by utilizing robust state estimation method and 2-DOF control based on disturbance observer (DOB). As motor torque can be known precisely, unmeasurable vehicle and environmental state variables can be estimated by Kalman filter. Cornering stiffness adaptation algorithm contributes to the robust estimation against model error. 2-DOF control based on DOB, which fulfills tracking capability performance to reference value and disturbance suppression performance at the same time, achieves robust RSC. Besides, integrated stability control (ISP), which is a combination of RSC and YSC is proposed.

DOB compensates not only disturbance, but also error between actual model and nominal model of controller. It is called model nominalization. General control hierarchy of vehicle system consists of electric motor's current control, wheel slip/skid prevention control, vehicle motion control and vehicle network control such as platoon or autonomous control. If the minor controller is designed properly, it decreases load of upper controller design and stable and robust performance can be anticipated. The design methodology is a basic architecture for modern control system designs.

To verify the proposed method, three dimensional vehicle simulation software and experimental EV are developed. Simulation model is set to be the same as the experimental vehicle model. Implementation load decreased and analytical validation for experimental results became possible because of the simulation. The experimental vehicle has two in-wheel motors (ISPSM) on rear wheels. For experimental EV, time step for sampling and calculation of motor torque is 1ms to utilize motor's fast torque response. As cost limitation is very important for development of commercial cars, only local and inexpensive sensors such as accelerometers, gyro sensors, steering angle sensor and wheel velocity sensors are used instead of GPS and INS sensors.

Although vehicle driven by electric motors are gaining popularity these years, recognition of EVs was not enough when the author started to do this research. The situation has changed due to global warming and the global financial crisis in October of 2008. Electric driven vehicles are anticipated by society not only because of environmental reasons but also the creation of new business market. In addition, younger generations have less interest on cars in Japan. Some of the reasons are the change of consumer minds and products that lack individuality are sold in the market. Few would argue with the electrification of the transportation, the problem is after the technology and how to keep attracting consumers. The author confirms that active safety technology described in this thesis will be the next-generation technology.

This paper's content ranges from basic steps to vehicle motion control, the vehicle model definition, the model identification, the state estimation method and the robust vehicle motion control. In chapter 1, background and purpose of the research is described. Proposed ISP consists of state estimation system and vehicle motion control system, which are explained chapter 4 and 5. In chapter 2, research trends on vehicle motion control for IMEV are introduced. In chapter 3, vehicle dynamics which are tire dynamics, three dimensional vehicle dynamics taking into consideration on disturbance are explained. In chapter 4, state estimation method is proposed. Vehicle and environmental states which are not measured directly by actual sensors are estimated. In chapter 5, vehicle motion control, from tire control to robust ISP are proposed. Validity of the proposed methods is shown in chapter 6. In chapter 7, summary and future works are described.

本論文は,「Research on Advanced Motion Control for Electric Vehicles(電気自動車の先進的運動制御技術に関する研究)」と題し,電気モータの高度な制御性を利用し,車両運動に関する状態変数の推定と高性能制御法を提案して,シミュレーションと実験的な検証によってそれらを実証した結果をまとめたもので,英文で記述された7章より成る。

第1章「Introduction」は序論である。研究の背景と目的について述べ,電気モータの制御性のよさを利用したロバストな車体運動制御法の概要について述べている。提案する車体運動制御法は状態推定機構および,運動制御機構から構成されており,これらは第5,6章で詳しく述べるとしている。

第2章「Active Safety Technology for Automobile」では,本研究の位置づけを明確にするために,これまでの研究概要を説明している。電気自動車とハイブリッド車や燃料電池車との構造上の違いなどを説明し,車両運動制御の研究動向を解説しながら,次に本研究の位置づけを明確にしている。

第3章「Vehicle Dynamics」では車両モデルについて説明している。まずタイヤで発生する前後力および横力が,タイヤのスリップ率と横滑り角の非線形関数になることを述べ,次に車両の横方向運動とヨー運動を解析して,二次元平面での車両運動方程式を記述している。さらにロールに関する運動方程式を示し,ロールオーバのメカニズムを説明している。さらに,本研究では外乱に対するロバスト性に注目しているため,外力に対する車両運動の記述も行っている。

第4章「Identification of Vehicle Parameters」では車両パラメータの決定法および同定法について述べている。状態推定や運動制御には車両のモデルパラメータが必要となるが,その中でも非線形性が強いコーナリングスティフネスの適応同定手法や,ダンパとスプリングで構成されるサスペンションモデルの同定に関して詳述し,実際に試験車両で行った結果を示している。

第5章「Vehicle and Environmental State Estimation」では状態推定機構を述べている。すなわち,センサでは直接計測できない車両状態および環境状態の,ロバストな推定法を提案している。とくにバンク角度を考慮したヨーとロールの運動方程式にもとづき,車両の各状態変数とバンク角度を同時に逐次推定する方法を示している。また,非線形性を持つコーナリングスティフネスについても,前章で述べた適応同定手法を用いた,モデル変動にロバストな状態推定法を提案し,その効果をシミュレーションによって示している。

第6章「Vehicle Motion Control」では,車両運動制御機構について述べている。まず,タイヤのスリップ/スキッド制御について,実験結果を交えて説明している。次に,指令値追従性能と外乱抑圧性能を独立に設計できる二自由度制御をロール安定化制御に適用し,その効果を確認している。さらに,ヨー安定化との協調制御の必要性を述べ,これを実現する方法を提案し,それぞれについて実験とシミュレーションによって検証した結果を述べている。

第7章「Conclusion」は結論であり,本研究の成果をまとめ将来展望について述べている。

以上これを要するに,本論文は,電気モータの制御性の良さを活用した電気自動車ならではの高性能運動制御の追求において,特に,車両運動に重要な状態変数や外乱の推定情報を活かして,モデル変動や外乱にロバストな車両運動制御法の構築を行い,シミュレーションと自ら製作した電気自動車を用いた走行実験によってその有効性を実証したもので,電気工学,自動車工学,制御工学に貢献するところが少なくない。

よって本論文は博士(工学)の学位請求論文として合格と認められる。