五菱宏光s轎車變速器的設計-三軸式五檔手動變速器含5張CAD圖
五菱宏光s轎車變速器的設計-三軸式五檔手動變速器含5張CAD圖,五菱宏光,轎車,變速器,設計,三軸式,五檔,手動,CAD
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Soclet v
Copyright
910014
of AuromoT ive Engineers, Inc.
Development of Electric Power Steering
Yasuo Shimizu and Toshitake Kawai
Honda R&D Co., Ltd.
107
ABSTRACT
A new electric power steering (EPS) was developed which uses an electric motor to provide assistance. It is a system combinning the latest in power electronics and high power motor technologies. The development was aimed at enhancing the existing hydraulic power steering's energy efficiency, driver comfort as well as increasing active stability.
This paper describies the overall concept of EPS and outlines the components and control strategies using electronics. The EPS was tested on a front wheel drive vehicle weighing 1000kg in front axle load. The results showed a 5.5% improvement in fuel economy. The EPS has also achieved returnability that gives the driver more moderate feelings matching the vehicle in action as well as the active stability control strategy for high speed driving.
1.INTRODUCTION
Recently, electronics application to automobile technology is remarkably spreading. This means that the electronics are going to be used for the following objectives which the automobile technology inherently demands :
(1) Improvement in efficiency;
(2) Improvement in comfortability; and
(3) Improvement in safety.
The electronics application allows us to obtain very desirable effects in relation to those 3 objectives.
For example, the electronic fuel injection systems have given engines high output and remarkably less fuel consumption. Shift shock in the automatic transmissions has been very much reduced. The anti lock brake systems allow us to brake easily the vehicles running on snow covered slippery roads.
Concerning the power steering system, the hydraulic steering system is now the main steering system and widely used even in small cars. The hydraulic steering system has about a half century history and thought to be a completed technology. However, considering the objectives above mentioned, it is thought that a time has come to restudy the existing system.
This paper describes thorough solution of negative points owned inherently by the hydraulic power steering system, and outlines effects of the electric power steering system (EPS) developed newly to eliminate those negative points and further to add merits to the steering system.
2. DEVELOPMENT CONCEPT OF EPS
Considering the points described above, requirements for the next generation power steering which will take the place of the hydraulic power steering are thought to be as follows: the new technology shall make improvements over the hydraulic power steering, create new merits to be added to the existing system, and establish the technology as a basis of the steering system in future. We studied concretely on the new steering system to be applied to a front drive passenger car weighing 1000 kg in front axle load. Our targets were as follows:
* Improvement in actual fuel efficiency could be obtained.
* Drivers feeling on steering should be the same or better compared with that on the existing hydraulic power steering, and the new power steering should match easily dynamic performance of the car.
* The new system should have the same or higher drive safety compared with that of the existing system.
Further, design concept of the system and component were as follows:
* To enhance productivity, the system had to be composed simply.
* As the system had the most important components in view of drive safety, highly reliable design was required.
* The system should have high serviceability and maintenability.
3.SYSTEM OUTLINE
3.1 Construction and operation description
(1) System composition
As Fig.1 shows, EPS system consists of the following components:
*
Gear box containing rack and pinion mechanism and generating assist force based on signal issued by input from steering wheel operation and signal from vehicle speed sensor.
Two vehicle speed sensors to detect vehicle speed.
Relay to shut off power supply.
Fuse box containing fuse to be fused to shut off overcurrent.
Indicator light to issue failure warning and show kind of failure.
Indicator light
\ Vehicle speed sensor
— Vehicle speed sensor
Fuse & relay box
(2) Component
(2.1) Gear box
As shown in Fig.2, the followings are added to the traditional rack and pinion steering mechanism:
* Steering sensor to detect steering input.
* Control unit to calculate optimum assist force based upon main input including signal from the sensor described above and the vehicle speed sensor, to output the calculation results to a power unit and at the same time to diagnose condition of each function parts and the control unit itself.
* Power unit to drive a motor according to signal issued by the control unit.
* Assisting mechanism to reciprocate the rack shaft by thrust force to which motor torque is converted through a ball screw.
During the system design, the following two layout plans had to be examined in view of reliability of the electric system:
a) The electric parts such as the control unit and power unit were installed in the cabin to protect them from being exposed to outside circumstances directly.
As a conclusion the plan b) was selected and both control unit and power unit were placed in the gear box. This construction is called "mechatronic combined construction". Adoption of this construction forced us to study how to design both the units in small size. However, the following greater merits could be obtained that made our study efforts worthwhile.
Gear box
Fig.1 System composition drawing
Control unit
Steering sensor
Assisting mechanism
Motor
Power unit
Rack and pinion mechanism
Joint
Ball screw
Helical gear
b) The electric system parts were placed in the gear box to simplify the system and further, the box could serve as a container having enough strength to protect the electric parts from the outside condition.
* Simple system composition allowed us not only to reduce its weight, but also to control the assist characteristics which included performance variation of the control unit and power unit. As a result, steady characteristics of the system could be obtained.
* As the plan b) did not require to lay lines of the power unit inside or outside of the cabin, signal lines were less affected by outer noise, and power line issued noise reduced.
* As heat generated by the power unit could be transferred to the gear box, no cooling fin was required for the power unit.
* Automatic assembly of the EPS and further, automatic attaching of the complete EPS to a car can be expected.
Now, explanation on each component is given.
1) Steering sensor
As Fig.3 shows, the steering sensor detects steering condition, and consists of a torque sensor, which senses rotation torque of a steering wheel, and a rotation speed sensor, which senses rotation speed of the steering wheel.
a) Torque sensor
The torque sensor consists of an input shaft, connected to the steering wheel, a pinion shaft, with pinion gear of the rack and pinion mechanism, a torsion bar, connecting the input shaft and the pinion shaft, a slider, with a movable iron core, a cam mechanism, converting relative torsional displacement between the input shaft and pinion shaft to axial displacement of the slider, and a linear variable differential transformer, converting displacement of the slider to electric signal. As Fig.4 shows, amount and direction of the steering wheel torque can be detected based on displacement and direction of the slider. In Fig.4, S3 shows output signal of torque and S1 and S2 show outputs for diagnosis use.
Gear train
Input shaft
(Rotation speed sensor output)
Differential transformer
Relative torsional displacement
Torsion bar
Fig.3 Steering sensor
Cam mechanisn
DC generator
Slider
Output shaft (pinion gear)R
Displacement
Output voltage (V)
(Torque sensor output)
Fig.4 Working diagram of torque sensor
The differential transformer type torque sensor has a high sensing accuracy and also good temperature characteristics since the output is obtained differentially. Further, as its electric system is composed in duplication, system failure can be detected easily and the system has high reliability.
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b) Rotation speed sensor
The rotation speed sensor consists of a gear train equipped on input shaft and a DC generator whose speed is raised by the gear train. This sensor senses rotation speed and rotation direction of the generator to know rotation speed and direction of the steering wheel. The signals indicating rotation speed and direction of the generator are issued to the control unit. S4 in Fig.3 is the rotation speed output.
2) Control unit
As Fig.5 shows, the control unit consists of the following major circuits:
* One chip, 8-bit microcomputer with built-in A/D converter and PWM unit.
* Interface circuit to adjust signals from various sensors.
* Driving circuit to drive the power unit(described later) through the PWM unit.
* Torque check circuit to check torque signal.
* Watch dog timer (WDT) circuit to check operation of the one chip microcomputer.
* Relay driving circuit to drive relays.
* Indicator lamp driving circuit to drive the indicator lamp on/flashing.
* Constant voltage circuit to supply constant voltage current to each element.
Further, failure diagnosis is performed on each sensor and the 1 chip microcomputer. If there should be any failure in them, the relay driving circuit stops to shut off the relay and consequently the motor power stops. At the same time, the indicator light turns to on and failure mode is stored, and the light can be flashed to indicate failure if necessary. Then, the EPS system can be operated only manually.
Table look up system allows the control unit to call instantaneously data from the predetermined data table based upon signals from various sensors so that the control unit can calculate those data to obtain the optimum assist force (motor assist torque data).
(Control unit)
(Steering sensor)
A
aQetzolu-
(Vehicle speed sensor) ,
czb —
(Battery voltage ) f
(Indicator light
Indicator driving circuit
a
I
Torque CHK circuit
十.
Relay driving circuit
WDT WDT
I
o)
0
IC-miconi
| Constant voltage circuit \
Photo 1 Control unit
Relay
Current sensor i
(Power|unit)
Reliability design had to be given to the control unit as the mechatronic combined construction, as mentioned before, required the unit to be small in size and further to have a high resistance against environmental stress. During the development, discrete circuits were used for the control unit. However, after its specification had been determined, high density design using LSI was done. As a result, the LSI whose size is a little smaller than of a tobacco case was obtained. Considering vibration resistance, water proof and heat resistance, the control unit construction is epoxy resin molded one. Photo 1 shows its appearance (the mold treatment is omitted to show its circuit).
3) Power unit
The power unit consists of the following major components as shown in Fig.5:
* FET bridge circuit to drive the motor.
* Circuit to drive the bridge circuit based on signal from the control unit.
* Current sensor to detect current supplied to the power unit.
* Relay to turn on/off motor current.
The power unit drives the motor based on instruction from the control unit. The current at that time is monitored by the control unit via the current sensor and if an abnormal condition is found, the relay shuts off motor current
In our initial development concept, a vehicle weighing 1000 kg in front axle load was supposed. Then, the max. current of 90 A had to be driven by the FET bridge circuit. On the other hand, to meet requirements of the mechatronic combined construction, the power unit had to be small in size and have a high reliability as mentioned above. To satisfy the requirements, FET chip is installed on a ceramics base plate and they are sealed with gel to be a module type (see Photo 2). This design allows the power unit to have a high capacity, but still its size is a little smaller than that of a tobacco case.
Progress of heat analysis technology permitted the chip surface temperature to be visible, and the optimized junction temperature could be achieved. The power unit was integrated to the aluminum casted gear box as one body, and it was confirmed that heat from the unit was dissipated effectively by this method.
Photo 2 Power unit
4) Assisting mechanism
As Fig.2 shows, the assisting mechanism consists of the following components and rotation torque is converted smoothly and effectively to reciprocating movement of the rack shaft.
* Motor arranged coaxially with the rack shafts to generate rotation torque.
* Helical gear to transmit rotation torque to the ball screw.
* Ball screw to convert rotation torque to thrust force through the helical gear.
* Joint giving thrust force obtained from the ball screw to the rack shaft.
As equation (1) shows, when a thrust conversion ratio i/J2 is large, or motor torque TM is high, strong thrust force can be achieved.
P = 2 ?丸(1) where, P : thrust force i : gear ratio 門:efficiency Tm : motor torque 6 : lead of ball screw
Considering degree of freedom in steering feeling setting, smaller conversion ratio is desirable. To make motor torque as high as possible,a large motor as far as the layout permits is used, i.e., the motor has an outer diameter of 94mm and 6-pole ferrite magnet. At first brushless motor was planned in view of maintenance free, but as torque of the same size brushless motor was about 2/3 of that of brush motor, we were forced to give up the brushless motor at that stage . The motor was designed to satisfy the following 3 items according to required performance of the EPS:
* High torque output at low rotation speed.
* Low torque ripple.
* Low friction.
Motor torque is determined by outer diameter of its rotor. Therefore, a rotor center part is needed only to have an enough thickness to form magnetic circuit.
The hollow rotor through which the rack shaft was placed was designed to reduce rotor weight.
As the ball screw was required to convert torque to thrust force and vice versa smoothly and efficiently, it was designed considering the following two points:
* Nut should be moment free.
* To make lead angle larger.
To reduce deflection influence on the rack resulting from bending moment given by tie rod, the rack and ball screw were arranged in parallel and the joint was designed to absorb deflection of the rack.The design allowed us to have the minimum screw diameter and to make the lead angle larger.
(2.2) Vehicle speed sensor
The vehicle speed sensor senses vehicle speed and issues pulse rows with frequency proportional to the vehicle speed to the control unit. Two vehicle speed sensors are used to diagnose failure of them.
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(3) Operation description
As Fig.6 shows, assisting operation (shown with black arrows) is added to manual steering (shown with white arrows) to reduce steering effort.
(3.1) Manual steering operation
Steering wheel input is transmitted to the pinion shaft of the rack and pinion mechanism via the universal joint. Rotation displacement of the pinion shaft is converted to axial displacement of the rack, and thrust arises. The axial thrust is transmitted via the tie rods to the knucles which support rotating tyres and the thrust is converted to oscillating displacement of tyres. Therefore, steering wheel input can finally make the tyres to oscillate to turn vehicle direction.
(3.2) Assisting operation
Other than the manual operation mentioned above, steering wheel input (torque and speed) is sensed by the steering sensor and signal is issued to the control unit. The control unit receives the signals along with vehicle speed signal from the vehicle speed sensor. The unit determines assisting force from predetermined table so that the optimum steering characteristics can be obtained and calculates output data. At the same time, the unit detects steering condition based on signals from various sensors and decided modes (normal, return and clamper). The output data and the modes switching signals are output to the power unit. The power unit drives the motor based on those signals. Motor torque generated is transmitted to the ball screw via the gear. Then, motor torque is converted smoothly and efficiently through the ball screw to assisting force working along axis of the rack shaft. As a result, the thrust acts on the rack shaft and reduce another thrust obtained by the rack and pinion mechanism as mentioned above, i.e., the thrust generated by the motor is fed back to the steering sensor to reduce driver's effort on the steering wheel.
Steering sensor
Oscillating displacement
Steering input
Assisting force
(Speed meter)
(Vehicle speed signal)
/ Control unit
Power unit
(Motor driving signal) Ball screw
Oscillating displacement
Rack and pinion Motor Gear C Torque/rotation speed signal)
Fig.6 System process diagram
3.2 Control principle and basic control
(1) Control principle
Fig.7 shows equivalent circuit of the motor. Where, motor terminal voltage is VM f inductance is L, resistance is R, induction voltage constant is k, rotation speed is N, current is i, and time is t, the following equation is obtained:
Vm = L (d i / d t) + R ? i + K ? N = R ? i + K ? N (2) Current i is proportional to motor torque Tm and when Kt is a proportional constant, the following equation is obtained:
Vm = Kt,Tm+K?N (3)
Control unit
Rotation speed sensor
Fig.8 Control principle diagram
Signal issued by the torque sensor sensing steering torque and signal issued by the rotation speed sensor sensing steering wheel rotation speed are input to the control unit as shown in Fig.8. In the control unit, motor torque data and its rotation speed data are determined respectively responding to the data table being predetermined so that the optimum steering characteristics can be achieved. As a result, the motor can be controlled directly by adding both data.
Fig.7 Motor equivalent circuit
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Fig.9 Control block diagram
Torque sensor
1.0；
Vehicle speed measurement .\
H——VsJ
Tm
■0.—Vs-
.0.
LO——N」
Torque gain table
Vehicle speed sensor
Torque offset table >
Torque measurement
Rotation speed data table
/ Rotation speed 「measurement -
Rotation speed sensor ■
.0 T
Basic torque data table
(Power unit )
Current sensor
Mode switching
a b
Output data
M
Rotation direction
Vehicle speed (Control unit )
Torque ! direction
6
■c
in
When signal issued by the vehicle speed sensor sensing vehicle speed is input to the control unit to reduce motor torque data, steering torque can be increased with vehicle speed increase to be able to give driver reaction feeling of the steering wheel.
(2) Basic control
This description covers an important technology to determine steering feeling. As steering is controlled directly by driver, its feeling requires smoothness instead of on-off like. For this purpose, the following technology was developed by us.
* Operation time for one loop was 1 msec or less.
* Driving the motor with PWM (Pulse width modulation) method of 20 KHz or more.
The first item was achieved by adopting the table looking up system that could process data instantaneously instead of complicated data processing with software. The second item was achieved using the enhancement type power MOS FET which could drive the motor with a high curren
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