Development of a Clutch Control System for a Hybrid Electric Vehicle with One Motor and Two Clutches Kazutaka Adachi*, Hiroyuki Ashizawa**, Sachiyo Nomura***, Yoshimasa Ochi**** *Nissan Motor Co., Ltd., Atsugi, Kanagawa, 23-192, Japan (Tel: +1-6-27-1; e-mail: k-adachi@mail.nissant.co.jp) ** Nissan Motor Co., Ltd., Atsugi, Kanagawa, 23-192, Japan (Tel: +1-6-27-1; e-mail: h-ashizawa@mail.nissant.co.jp) *** Nissan Motor Co., Ltd., Atsugi, Kanagawa, 23-192, Japan (Tel: +1-6-27-1; e-mail: h-ashizawa@mail.nissant.co.jp) **** National Defence Academy, Yokosuka, Kanagawa, 239-66, Japan (Tel: +1-6-1-31; e-mail: ochi@nda.ac.jp) Abstract: This paper describes a newly developed hybrid electric vehicle system incorporating one motor and two clutches. The mechanical system is connected in a sequence of the engine, the first clutch, the motor, the second clutch and the mechanical gears. The system can transmit driving torque with high efficiency because the friction and pumping loss of the engine are removed by releasing the first clutch. It is difficult for the system to reach the same level of driving performance as a conventional vehicle with an automatic transmission. The nonlinear characteristics of the first clutch are described along with its stroke control system. Bench test results are presented to show that the nonlinear characteristics are removed to achieve the desired stroke performance. Test results confirm that smooth and fast vehicle acceleration is achieved by controlling the torque capacity of the first clutch using the stroke control system. Keywords: Mechanical systems, Hybrid vehicles, Control application, Hydraulic actuator, Nonlinear control. 1. INTRODUCTION A hybrid system with an internal combustion engine and an 1), 2) electric motor has been developed to improve fuel economy. This hybrid electric vehicle has a simple structure, with the mechanical system connected in a sequence of the engine, the first clutch, the motor, the second clutch and the mechanical gears. The motor works to drive the engine during firing, to accelerate the vehicle and to regenerate energy during vehicle deceleration by engaging or releasing the first clutch or the second clutch. The system can transmit the driving torque from the motor with high efficiency by releasing the first clutch to eliminate the friction and pumping loss of the engine. On the other hand, it is difficult for the system to attain the same driving performance of a conventional vehicle fitted with an automatic transmission in terms of acceleration, vibration and noise. During motor drive, the second clutch controls the driving torque and the motor controls the slipping rotational speed of the second clutch in preparing to fire the engine. When the driver depresses the accelerator pedal beyond the motor drive region, the first clutch engages the engine with the motor after the motor control system increases the slipping rotational speed. After the engine is started, the vehicle is accelerated by adding the engine torque to the motor torque. However, when the first clutch engages the engine with the motor, a shock occurs in the vehicle if the sum of the driving torque of the engine and that of the motor exceeds the maximum motor torque. At that time, if the first clutch slowly engages the engine with the motor so as to accelerate the vehicle smoothly, the system cannot achieve both the desired engine firing time and acceleration. In addition, the resultant high temperature may cause the durability of the first or second clutch to deteriorate. In this hybrid system, it is important for the first clutch to control the torque capacity. However, the mechanical design of the first clutch is intended to satisfy many requirements, including size, stroke speed and clutch torque capacity, with the result that the clutch has strong nonlinear characteristics. This paper describes the configuration of the hybrid system, the stroke control system of the first clutch based on bench test results, and vehicle driving test results. The test results indicate that the first clutch control system provides good control performance. 2. HYBRID ELECTRIC SYSTEM This hybrid system was configured by removing the torque converter with a lockup clutch from a conventional 7-speed automatic transmission and installing the first clutch and the motor in its place as shown in Fig. 1. The first clutch has been modified to a manual transmission by adding some functions such as hydraulically driven movement and stroke measurement. Copyright by the International Federation of Automatic Control (IFAC) 797
Speed (rpm) Accelerator depression angle 1 st clutch Electric motor 2 nd clutch 7-speed automatic transmission is driven only by the motor, the first clutch is released to remove the engine load. A slip speed control system was applied to the second clutch to achieve a small speed ratio. Therefore, the second clutch torque capacity is equal to the vehicle driving torque. The engine is fired by the same motor when smooth vehicle acceleration is required. If the total torque capacity of the two clutches exceeds the maximum motor torque, the vehicle experiences shock and hesitation. To avoid that, the torque capacity of the two clutches must be controlled accurately. The method of controlling the second clutch will be explained in another paper at the 211 SAE World Congress. This paper describes the design method of the first clutch control system. 3. FIRST CLUTCH CONTROL SYSTEM 1 st clutch Electric motor Fig. 1. Hybrid electric system : Driving tire speed (Conversion to 2 nd clutch output speed) : Motor speed : Engine speed 2 nd clutch Motor drive mode Engine firing HEV drive mode Fig. 2. Concept of engine firing sequence Time Time Since the clutches of the automatic transmission are used for the second clutch, the clutch utilized for this purpose differs depending on the gear. The motor works to drive the vehicle, to regenerate energy during deceleration and to drive the engine during firing. Therefore, to achieve the same smooth acceleration performances of a conventional vehicle, the two clutches must cooperate. Figure 2 shows the three major drive modes of this hybrid electric vehicle. The vehicle takes off under motor power when the driver depresses the accelerator pedal a little. As the driver depresses the accelerator pedal more, the engine fires after the first clutch is engaged. The vehicle then accelerates rapidly because the engine torque is added to the motor torque. When the vehicle The first clutch has to be engaged within the torque capacity limit determined by the command generator during engine firing. It also has to be completely released during the motor drive mode and during energy regeneration. As explained below, the first clutch has strong nonlinear characteristics that are related to the stroke position determined by the spring force. There is a nonlinear region in the clutch release position and in the position for preparing for immediate engine firing, making it difficult to control the positions. Moreover, the relation between the stroke position and the first clutch torque capacity fluctuates due to manufacturing variability and aging, among other factors. The engaging torque capacity of the first clutch must be considered when firing the engine. The first clutch control system has been designed to take these issues into account. The first clutch control system consists of a command generator, a stroke position controller and an engine speed controller as shown in Fig. 3. The command generator determines the stroke position or the engine speed command based on vehicle state information received by CAN communication. The stroke position controller and the engine speed controller are explained below. c I Command generator p CC1 e C1 Engine speed controller p C1 Stroke position controller p CE1 p A1 1 st Clutch controller Hybrid electric control unit (Hydraulic pressure control) h C1 AT i 1 Controller 1 st Clutch Motor Hybrid unit Engine Stroke position : CAN communication bus c 1 : vehicle state information e C1 : torque capacity command h C1 : hydraulic pressure command p C1 : stroke position command i 1 : electric current for valve p A1 : actual stroke position p CE1 : stroke position command from engine speed controller p CC1 : stroke position command from command generator Fig. 3. Block diagram of 1 st clutch control system 79
Spring force [N] (Hydraulic pressure) Clutch torque capacity [Nm] 3.1 Plant Modelling 1) Plant of stroke position Figure shows the first clutch mechanical system which has been modified to a manual transmission. The first clutch is equipped with a stroke sensor and a stroke actuator driven by oil pressure. The first clutch torque capacity is related to and controlled according to the stroke position. The motion equation of the stroke position is expressed as m 2 CL1 s pa1 ( s) a CL1 where h C1 s e c CL1 LCL1s sp A1 m CL1 : mass of first clutch s k P p s CL1 A1 c CL1 : viscous coefficient of first clutch k CL1 : spring coefficient of first clutch p A1 : stroke of first clutch A1 a CL1 : pressure receiving area of first clutch piston h C1 : first clutch piston pressure L CL1 : lag time of hydraulic mechanism Figure 5 shows the first clutch stroke position versus the spring force. Strong nonlinear characteristics, caused by using a cone disc spring, are seen in the region of a long stroke. 1 st Clutch Controller (Hybrid Controller) Clutch Mechanical Part Clutch Disk Flywheel Hydraulic command Hydraulic Controller (AT Controller) Pressure Control Valve Electric Current Pump ホ ンフ (1) : CAN communication bus Oil Pressure Mechanical Part 2 2 Increase of force and stroke position Stable region : spring power [N] : clutch torque capacity [Nm] Decrease of force and increase of stroke position Unstable region Strong nonlinearity 2 6 1 Stroke position [mm] Releasing clutch Fig. 5. Characteristics of first clutch 2) Plant of engine rotational speed The engine is fired by using motor torque transferred via the first clutch. The motor torque used in firing the engine is controlled by the torque capacity of the first clutch. The relation between the stroke position and capacity of the first clutch is shown in the dash-dot line in Fig. 5. Therefore the stroke of the first clutch can be controlled, the capacity of the clutch can be controlled. The equation of motion of the engine speed is expressed as shown below. E E L s s CE E s tc s e 1 1 fe C J s (2) where J E : moment of inertia of engine rotating parts C E : viscosity of engine rotating parts t C1 : first clutch torque capacity L C1 : lag time of CAN communication f E : friction of engine rotating parts 2 (Engine Side) Fig.. First clutch system (Transmission Side) Stroke Sensor Piston (Cylinder) Diaphragm Spring Pressure Plate Clutch Cover 3.2 Design of first clutch control system 1) Stroke position control The stroke position control system is configured as shown in Fig. 6 by using a two-degree-of-freedom control method 3). The feedback compensator maintains the stability of the control system against disturbances or parameter variations. The reference model sets the response characteristic of the control system. This model is designed as a second-order lag system, based on a study of the order of the plant. The feedforward controller compensates the response characteristics set by the reference model and is composed of the reference model and the inverse system of the plant. 799
Piston stroke Feedforward compensator G RS (s)/p S (s).1 Bench Tests. EXPERIMENTAL RESULTS p C1 Reference model + G RS (s) - Feedback compensator G BS (s) + + h C1 Plant of stroke position P S (s) p A1 1) Stroke position control performance To confirm the performance of the stroke position control system, tests were conducted to verify the half-engaged clutch position for engine firing, the preparation position for engine firing and the completely released clutch position. The results are shown in Fig.. Fig. 6. Block diagram of stroke position control system 2) Engine speed control The engine speed control system is designed to provide dead time compensation during engine firing. Dead time occurs when the characteristics of the stroke position versus the engaging torque capacity are reduced due to manufacturing variations or other factors. Therefore, the plant characteristics have to be maintained by the engine speed control system. However, if the feedback compensator of the engine speed control system has strong integration characteristics, the engaging torque capacity would exceed the maximum value. As a result, the hybrid electric vehicle would experience a shock because the motor would generate more torque than the maximum value. The engine speed control system consists of only a robust compensator because the system does not need any unnecessary compensation. The robust compensator has been designed to ensure the necessary stability ), 5), 6). First, position control performance in the releasing direction was confirmed as shown Fig. (A). The target value was changed from 1 to 3 mm in a step-like manner from the completely engaged clutch position to the half-engaged position for engine firing. The results show that the actual stroke position response tracked the reference response well. In addition, the hydraulic pressure was increased proportionally because the spring coefficient has linear characteristics in this region. The target value was changed from 3 to 5 mm in a step-like manner from the half-engaged clutch position for engine firing to the preparation position for engine firing. The actual stroke position response tracked the reference response well, although the characteristics changed from linear to nonlinear. The hydraulic pressure was not changed in this test because the spring force was nearly equal between 3 and 5 mm. The target value was changed from 5 to 7 mm in a step-like manner from the preparation position for engine firing to the fully released clutch position. The actual stroke position response tracked the reference response well, although the spring coefficient showed a negative value because of instability. H E (s) Robust Compensator + - H E (s) P E (s) Figure (B) shows the control performance in the engaging direction. The target value was changed from 7 to 5 mm in the unstable region in a step-like manner. The actual stroke position response tracked the reference response reasonably well, although the steady-state value of the actual response displayed a little error. However, the error did not influence the first clutch control performance. t C1 t CR1 + + t CH1 Plant of engine speed E P E (s) The actual stroke position response tracked the reference response well when the target value was changed from 5 to 3 mm from the nonlinear to the linear region in a step-like manner. Look up table t CH1 p C1 Stroke position control system h C1 1 st clutch P S (s) t c (Pa 1 ) Engine E The target value was changed from 3 to 1 mm in the linear region in a step-like manner, and the actual stroke position tracked the reference response well. Clutch capacity Plant of engine speed Fig. 7. Block diagram of engine speed control system These results confirmed the controllability of the desired stroke position in relation to the reference response and without a steady-state value, regardless of the nonlinear characteristics of the plant.
Hydraulic pressure [kpa] Clutch stroke [mm] Hydraulic pressure [kpa] Engine speed [rpm] Clutch stroke [mm] 7 6 5 3 2 1 12 6 : command : reference : actual 1 2 3 5 6 7 9 1 11 12 13 1 15 : command : actual capacity of the clutch was reduced by 2% from the nominal level by the engine speed control system. The results coincide with the solid line. This confirmed that the engine speed control system can effectively compensate the lag time. 2 2 : nominal characteristics : 2% varied stroke position control : 2% varied engine speed control 2 1 2 3 5 6 7 9 1 11 12 13 1 15 7 6 5 3 2 1 6 2 (A) Releasing direction : command : reference : actual 1 2 3 5 6 7 9 1 11 12 13 1 15 : command : actual 1 2 3 5 6 7 9 1 11 12 13 1 15 (B) Engaging direction Fig.. Bench test results (1) 2) Engine speed control performance A test was conducted to verify whether the lag time of engine firing caused by a reduction of the clutch engaging torque capacity relative to the stroke position could be compensated by the engine speed control system. The results are shown in Fig. 9. The dashed line shows the control results for the first clutch to the position for engine firing, when the engaging torque capacity of the clutch was reduced by 2% from the nominal level due to the stroke position control system. The solid line shows the corresponding control results when the engaging torque capacity of the first clutch was set at the nominal level. The dashed line shows the results when the engine firing time was retarded by ms compared with the solid line results at an engine speed of rpm in both cases. The dotted line shows the control results for the first clutch to the position for engine firing, when the engaging torque.2..6. 1. 1.2 1. 1.6 1. Fig. 9. Bench test results (2).2 Driving Tests A test was conducted to confirm whether the engine firing algorithm could satisfy the target performance in actual driving. Figure 1 shows the experimental results obtained for firing the engine by increasing the accelerator pedal depression angle following vehicle takeoff without firing the engine. During motor drive under a small accelerator pedal depression angle, the target slip speed was maintained at 5 rpm and the driving torque was controlled by the torque capacity of the second clutch. Because engine firing was demanded by increasing the depression angle of the accelerator pedal, the engine was fired by increasing the slip speed and engaging the first clutch. Judging from the first clutch stroke command value and the engine speed, the required time was about.6 sec. to fire the engine. The motor torque variation caused by engine firing did not affect vehicle acceleration, and the vehicle accelerated appropriately in response to the driver's accelerator input. To be effective, the engine firing algorithm must allow the first clutch control system to be activated. Therefore, a test was conducted to confirm whether this algorithm can accommodate a fast vehicle takeoff where motor drive is used for only a very short period. The experimental results obtained for vehicle takeoff under an accelerator pedal depression angle of 3 are shown in Fig. 11. Good performance was obtained, as the engine was fired quickly within.5 sec. of the driver s demand for acceleration and the vehicle accelerated without any undesirable variation in speed. However, vehicle acceleration was delayed about.25 sec. during engine start because the driving torque was limited to 12 Nm. If the driving torque was not limited, the sum of the driving torque and the engine firing torque would exceed the maximum motor torque, causing the vehicle to vibrate greatly. 1
Motor torque (Nm) Vehicle acceleration (G) 1st clutch stroke (mm) Speed (rpm) Accelerator depression angle ( ) Motor torque (Nm) Vehicle acceleration (G) 1st clutch stroke (mm) Speed (rpm) Accelerator depression angle ( ) 5. CONCLUSION 3 2 1.2..6. 1. 1.2 1. 1.6 1. 2..2..6. 1. 1.2 1. 1.6 1. 2..2..6. 1. 1.2 1. 1.6 1. 2...2 : engine, : 2 nd clutch output, (rear wheel speed corresponding) : 2 nd clutch input (motor speed corresponding) : command, : reference, :actual.2..6. 1. 1.2 1. 1.6 1. 2. 3 2 1-1.2..6. 1. 1.2 1. 1.6 1. 2. Time (s) 3 2 1 Fig. 1. Driving test results (1).2..6. 1. 1.2 1. 1.6 1. 2..2..6. 1. 1.2 1. 1.6 1. 2..2..6. 1. 1.2 1. 1.6 1. 2...2 : engine, : 2 nd clutch output, (rear wheel speed corresponding) : 2 nd clutch input (motor speed corresponding) : command, : reference, :actual 3.2..6. 1. 1.2 1. 1.6 1. 2. 2 1-1.2..6. 1. 1.2 1. 1.6 1. 2. Time (s) Fig.11 Driving test results (2) The Nissan hybrid system described here was developed by removing the torque converter from a conventional 7-speed automatic transmission and inserting one clutch and one motor in the resultant vacant space. This hybrid system achieves both high environmental performance and fast response to the driver s demand for acceleration. In this system, it is essential to start the engine quickly without unwanted variation in vehicle acceleration during motor drive because the single motor both starts the engine and provides driving torque. The first clutch has strong nonlinear characteristics owing to manufacturing variability or aging, making it difficult for the control system of this clutch to achieve the desired performance of quick engine firing within the limits of the maximum motor torque. The first clutch control system consists of an engine speed controller and a stroke position controller. The stroke position controller compensates for the nonlinear characteristics, and the engine speed controller eliminates the effects caused by manufacturing variability or aging. Bench test results showed that this control system achieved the target performance. Driving tests were then conducted to evaluate the effectiveness of the engine start algorithm. The results showed that the engine was started quickly during motor drive without any undesirable variation in vehicle acceleration. Furthermore, even for a fast vehicle takeoff with a markedly short motor drive period, the results showed that this algorithm provided satisfactory performance. These results confirmed that this engine start algorithm effectively resolves all the major issues in this hybrid system in situations where the engine is started during motor drive. REFERENCES 1) S. Sasaki, T. Takaoka, H. Matsui and T. Kotani, (1997). Toyota s Newly Developed Electric-Gasoline Engine Hybrid Powertrain System, Proc. of 1 th EVS. 2) Y. Yoshioka and H. Sugita, (21). Noise and Vibration Reduction Technology in Hybrid Vehicle Development, SAE Paper No.21-1-115. 3) A. Higashimata, K. Adachi, S. Segawa, N. Kurogo and H. Waki, (2). Development of a Slip Control System for a Lock-Up Clutch, SAE Paper No. 2-1-1227. ) R. Tagawa, (196). Model matching and robust control, Computer and Applications, Vol. 13, Corona Publishing Co., Ltd., Japan, pp. 53-5 (in Japanese). 5) K. Nonami, H. Nishimura and M. Hirata, (199). Control System Design Using MATLAB, TDU Press, Tokyo (in Japanese). 6) K. Adachi, T. Wakahara and H. Ashizawa, (199). Design of a Robust Model-Matching Gear Ratio Servo System for a Belt-Drive CVT, 32 nd ISATA, Automotive Electronics and New Products 219/226, 99AE5. 2