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Shift-by-Wire System for Lexus RWD Vehicles.

INTRODUCTION

The functions of shift operation systems include shifting the parking mechanism of the vehicle, and shifting the driving force direction of the vehicle. Historically, this shifting mechanism has mostly been installed inside the transmission, and carries out shifting directly using a mechanical connection with the shift lever. In a shift operation system with this configuration, the shift pattern, operation load/displacement, and shift operation device layout are mostly determined by mechanical requirements. This creates styling and sensory quality limitations which were not compatible with targets set for the next generation of cockpits for Lexus vehicles.

Therefore, a shift-by-wire system was introduced that eliminates the mechanical linkage between the shift lever, judges driver shift inputs using sensors installed on the shift operation devices, and performs shifting using an actuator attached to the transmission. This system enabled dramatic improvements to the layout and styling of shift operation devices, and much greater design flexibility for sensory perception. In addition, because shifting is performed by an actuator, this system is also compatible with future autonomous driving or autonomous parking vehicle systems. It can also respond to human errors such as mistaken operations by the driver.

With this new technology, functions that were once ensured by mechanical linkages in conventional shift operation systems could become a source of new failure modes, such as electrical faults and loss of power. To ensure reliability, the shift-by-wire system must address these new failure modes.

This paper describes the design concept of the shift-by-wire system for Lexus RWD vehicles, and the functional components that realize this system.

SYSTEM OUTLINE

Requirements of System Configuration

Since the shift-by-wire systems are required to shift both the parking mechanism and the driving force direction, the powertrain system has a major effect on the system configuration. The system configuration was designed in consideration of the following points.

* The Lexus powertrain lineup contains both conventional gas vehicles and hybrid vehicles without a source of hydraulic pressure. Assuming use with fuel cell vehicles or electric vehicles, as well as conventional gas vehicles with a start-and-stop system, a shift system that uses an electric motor has a simpler configuration than a hydraulic system.

* Existing hybrid vehicles perform mechanical shifting for only the parking mechanism. Shifting of the driving force direction is performed electrically by an electronic control unit (ECU) in the powertrain. In contrast, conventional gas vehicles shift both the parking mechanism and the driving force direction mechanically.

* Although this system had to be compatible with the wide powertrain lineup used by Lexus, it was also necessary to minimize the effects on existing powertrain ECUs using conventional shift levers.

After considering these key points, the shift-by-wire ECU (SBW ECU) was introduced to perform judgment of shift inputs and parking mechanism shifting, and the shift function for driving force direction was allocated to the powertrain ECU.

System Configuration

Figure 1 shows the configuration of the shift-by-wire system for Lexus RWD vehicles.

* Shift operation device:

The shift operation device consists of a shift lever that inputs the driving force direction requests from the driver (i.e., the R, N, D, and M positions), and a parking switch that inputs requests to hold the vehicle stationary (i.e., the P position).

* SBW ECU:

The SBW ECU judges the shift request from the driver and whether to perform a shift based on the vehicle state. Shifting operations to hold the vehicle stationary are controlled by the parking switching device via the SBW ECU, which also transmits the driving force direction commands to the powertrain ECU. The SBW ECU also transmits the current shift position to the shift position displays.

* Powertrain ECU:

The powertrain ECU shifts the driving force direction based on commands from the SBW ECU.

* Shift position displays:

The shift operation method and current shift position are displayed close to the shifter and in the instrument cluster.

* Parking switching device:

The parking switching device shifts the parking mechanism. The switching device actuator consists of a switched reluctance (SR) motor, reduction gears, and rotation angle sensor. This device also includes an absolute angle sensor to identify the state of the parking mechanism and a manual parking release mechanism.

* Power supply systems:

Electric power is supplied to the shift-by-wire system by two sources; the auxiliary battery and sub-battery.

As described above, the shift-by-wire system consists of a large number of elements that must coordinate with each other.

The next sections describe details of the shift operation device, parking switching device, and system failsafe design. These were key items in the development of the shift-by-wire system for Lexus RWD vehicles.

SHIFT OPERATION DEVICE

Operation System Requirements

One of the goals for the next generation of cockpits for Lexus vehicles was to realize a functional space that allows the driver to concentrate on driving. To achieve this goal, the operational systems directly related to driving were laid out around the driver based on the position of the steering wheel, to minimize changes in driver posture and gaze (Figure 2). In addition to functional requirements, cockpit sensory perception was also emphasized by, for example, creating an attractive comfortable layout that allows the driver to perform a coherent series of operations, and ensuring a refined operation feel.

These goals were achieved in the development of the shift operation systems by the adoption of a shift-by-wire system with a high degree of design flexibility and high-quality operation. Consequently, the shift operation device was designed considering the points introduced in following sections.

Operation Method

The shift operation method combines the momentary "h" pattern shift lever and the push-type parking switch, which has a long history of use in Toyota's hybrid vehicles (Figure 3).

"h" Pattern Shift Lever: R, N, D, and M Position Shifting

The distinguishing characteristics of the "h" pattern shift lever are described below.

* When the driver selects the desired shift position, there is only one operation target. The operation start position and operation method are always the same. Therefore, the driver can operate the shift lever without being aware of the current shift position.

* The shift operation destination is always a dead-end and there is no need to stop the shift lever in the middle of a shift. This means that the driver can make shifts without concern of moving the shift lever too far. This also helps to prevent mis-shifts.

* Shifting between forward and reverse requires two actions, which require an intentional operation by the driver.

Push-Type Parking Switch: P Position Shifting

The distinguishing characteristics of the push-type parking switch are described below.

* Shifting operations to hold the vehicle stationary are performed using a push switch, located in a separate position from the shift lever used for the driving force direction. This prevents mis-shifts into the P position when shifting the driving force direction.

Holding Load and Layout Design

The holding load and layout of the "h" pattern shift lever and the push-type parking switch were designed focusing on the following points.

"h" Pattern Shift Lever

The shift lever is located on the center console close to the driver's hand. Therefore, a fairly high holding load is set to make the shift lever harder to operate accidently by the driver resting a hand on it or by touching it by mistake, as well as to help the driver notice an unintentional operation. An equivalent or greater selection load as a D to M shift with a conventional shift lever was set (Figure 4).

Push-Type Parking Switch

The parking switch was designed focusing on the following points.

* The design and styling of the parking switch must be grouped with the shift lever as a shift operation device.

* The parking switch must be laid out so that it cannot be operated mistakenly by a hand placed on the console or shift lever.

* Other switches that can be easily mistaken for the parking switch must not be laid out close by.

* The parking switch must be laid out in a visible position from the driver's gaze direction.

Operation Feeling and Operation Judgment Range

Operation systems that create an emotional driving experience must have a smart operation feeling and intuitive feedback to inform the driver that the operation has been performed. The following sections describe the load-displacement characteristics and sensor judgment range design of the "h" pattern shift lever and push-type parking switch.

"h" Pattern Shift Lever

The shift lever was designed focusing on the following points.

* To achieve a smart operation feeling, the lever is designed with a short displacement and solid operation feeling.

* Load characteristics with large feedback were designed to inform the driver intuitively that the operation has been performed.

* The sensor judgment range was set so that input judgment is always made after the detent is reached, and not before.

These points are illustrated in Figure 5.

Push-Type Parking Switch

The parking switch was designed focusing on the following points.

* An appropriate switch displacement for a driving operation device was set.

* A unified operation feeling as a shift operation device was created by following the load characteristics of the shift lever.

* The sensor judgment range was set so that input judgment is always made after the detent is reached, and not before.

These points are illustrated in Figure 6.

PARKING SWITCHING DEVICE

Requirements for Parking Switching Device

The parking switching device was developed considering the following performance requirements.

* Output torque:

In a comparatively heavy RWD vehicle, the actuator torque must ensure that the parking mechanism is released on a gradient.

* Response:

In normal use, the actuator must shift comfortably. It must also have the response to engage the parking mechanism reliably when the driver shifts into the P position on a gradient.

* Mountability:

The parking switching device must be compact and thin for installation in the tight space between the transmission and floor tunnel.

* Reliability:

The parking switching device must be sufficiently reliable for the lifetime of the vehicle in the usage environment of a RWD vehicle.

* Serviceability:

In the event of a system fault or loss of power, a manual P position release mechanism is provided so that the parking mechanism can be disengaged.

External view of the parking switching device is illustrated in Figure 7, and the installation environment is illustrated in Figure 8.

Hardware Configuration

The following sections describe the main new items developed for the parking switching device.

Thickness Reduction and Torque Improvement of Actuator

The parking actuator was based on the previous hybrid shift-by-wire system actuator, which features a combination of an SR motor and cycloid reduction gear. Since RWD vehicles are comparatively heavier than front-wheel drive (FWD) vehicles, the torque required for shifting the parking mechanism tends to be larger. In contrast, since the actuator installation environment is the tight space between the transmission and the floor tunnel, it is also important to reduce the size of the actuator. Therefore, while the motor has a larger diameter than the previous actuator, it is thinner and has higher torque due to two-stage reduction process using an additional spur gear. The same shifting response as the previous actuator was achieved by increasing the rotor rotation speed.

Addition of Absolute Angle Sensor

A new absolute angle sensor was added to identify the state of the parking mechanism. The previous actuator only had a relative angle sensor and could not identify the current position of the parking mechanism before initial actuator learning was complete. However, the new absolute angle sensor enables disclosure of the current shift position immediately after the system starts up. In addition, since it is a noncontact sensor, there is no increase in friction even at cold temperatures, which helps to improve actuator position controllability and shifting response. Figure 9 shows the cross-section of the parking actuator.

Addition of Manual P Position Release Mechanism

Since the shift-by-wire system is an electrical system, it is not possible to shift out of the P position without turning on the ignition. This means that it may become impossible to move the vehicle out of a stationary position if a parking actuator or SBW ECU fault occurs, or the battery fails. Since vehicle servicing is often carried out in these situations, a manual P position release mechanism is provided on the parking switching device, to enable the vehicle to be moved out of a stationary position from the cabin, even when the ignition is off. This manual P position release mechanism does not operate when the actuator performs shifting. The actuator mechanism only pulls the shift position out of P when the cable is pulled. This design ensures that the normal shifting performance of the actuator is not affected.

Actuator Control Method

The parking actuator that shifts the parking mechanism of the transmission uses a leaf spring to shift a detent mechanism. This detent mechanism has a restricted rotation direction out of the P position (Figure 10). The actuator is driven and rotates the detent lever. The detent mechanism controls the position in either the P or not P positions by turning off the current within the retraction range of the detent mechanism (i.e., the range in which the retraction load of the detent is higher than the friction of the parking mechanism and the actuator itself) (Figure 11). The actuator drive ensures sufficient response by selecting the optimum speed control using advance angle control, while providing feedback of the current position using an encoder.

FAILSAFE DESIGN

Failsafe Concept

The following failsafe concept was designed for the functions that hold the vehicle stationary and shift the driving force direction, which are fundamental to the role of the shift-by-wire system.

* If an initial fault occurs:

The fundamental functions of the system should be capable of continuing. The driver should be notified to prevent a potential failure and encouraged to have the vehicle inspected and repaired.

* If a second fault occurs:

The functions may be stopped if safety cannot be ensured. The driver should be informed of the vehicle state and necessary actions.

The following sections outline the design of each function.

Failsafe Design for Function that Holds Vehicle in Stationary State

Even if a system failure occurs, the system has functions to hold the vehicle stationary using either the parking mechanism of the transmission or the electric parking brake.

* If the shift-by-wire system fails:

The vehicle can be held stationary using the electric parking brake.

* If the electric parking brake system fails:

The vehicle can be held stationary using the shift-by-wire system.

* If vehicle power fails:

The vehicle can be held stationary using the shift-by-wire system and sub-battery.

This failsafe design is achieved by constant inputs from two power sources (the auxiliary battery and sub-battery) to the SBW ECU. Relays that connect each battery and the parking actuator are triggered to make the switch. In this configuration, the relay connected to the auxiliary battery is driven in a normal state. If the auxiliary battery cannot be used because of a fault, the relay connected to the sub-battery is driven to switch the parking actuator (Figure 12).

Failsafe Concept of Driving Force Direction Switching

The design ensures that the vehicle can at least be driven in the current shift position even if a system failure occurs.

* Shift sensors:

Output is judged using four sensors. The shift input operation can be judged without error, even if one of the sensors fails. If two sensors fail, the driving force is released if there is the possibility of mistaken shift input judgment.

* Communication between SBW ECU and powertrain ECU: Communication path redundancy ensures that the required communication for shifting can continue on a sub-path if the main path has failed (Figure 1).

* If the SBW ECU crashes:

Although the position cannot be shifted by the SBW ECU, the powertrain ECU continues control if the vehicle is in the D position so that the driver can perform failsafe driving.

SUMMARY/CONCLUSIONS

A new shift-by-wire system has been developed to help realize the next generation of cockpits for Lexus vehicles. Shift operation systems that create an emotional driving experience appropriate for Lexus were achieved though optimum layout of the shift operation devices, and through operation feeling design. Selection of the optimum system configurations and functional component designs helped to create a highly adoptable and reliable system.

Yusuke Nakade, Atsushi Kamada, and Koki Ueno

Toyota Motor Corporation

Mikine Kume and Kouji Sakaguchi

DENSO Corporation

doi:10.4271/2017-01-1094
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Article Details
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Author:Nakade, Yusuke; Kamada, Atsushi; Ueno, Koki; Kume, Mikine; Sakaguchi, Kouji
Publication:SAE International Journal of Engines
Date:Apr 1, 2017
Words:2709
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