Electric power steering for emerging markets

The automotive industry is facing a revolution in steering technology. The strong growth in electric steering, as opposed to hydraulic, is due to the fact that electric systems are more economical to run, lighter, recyclable and easier to package and install. In addition manufacturers estimate that electrically assisted steering delivers a 3% fuel savings over hydraulic systems. We see a trend to use DC brush motor for this low end Electric Power Steering (EPS) market.
This low cost EPS solution is aimed for vehicles up to 1200cc.

Today about 9 million systems are sold, mainly in the Asian and European car markets. According to market research, the growth of EPS demand in the Chinese market has an average 50% increase yearly.



Currently the Chinese market still heavily depends on the EPS system from overseas suppliers. Especially for mid range to high end vehicle requiring mid to high power motors. Chinese Tier1 today focus more on low end, low power solution.
The migration from conventional hydraulic system to EPS is mainly due to the advantages which can be realized. Some of these advantages include:

Additional functionality for the driver like:

· Compensation of one-sided forces (flat tire)
· Adaptation of the steering assist to different driving situations
· Additional steering corrections in emergency cases in combination with the electronic stability control
· Adaptation of the steering ratio to vehicle speed
· Fast adaptation of the system to different cars by software
· Avoidance of the use of toxic hydraulic oil
· The Tier 1 supplier can deliver a completely tested subsystem

This paper presents component improvement ideas which can increase the safety at system level without incurring a high cost.

Functional blocks of a DC MOTOR EPS ECU

Overview

The functions which are required in a DC motor EPS ECU system are shown in .

An EPS system consists of the following components:
· A torque sensor, which senses the driver’s movements of the steering wheel as well as the movement of the vehicle;
· An ECU, which performs calculations on assisting force based on signals from the torque sensor;
· A motor, which produces turning force according to output from the ECU;
· A reduction gear, which increases the turning force from the motor and transfers it to the steering mechanism.


Other vehicle system inputs to the control algorithm are derived from the car CAN bus (e.g. steering angle and car speed, etc).

Some additional information is also necessary to drive the motor, such as the rotor position of the motor (supplied by a rotor sensor) and the phase currents (supplied by current sensors).
All these sensors along with the microcontroller (µC) must be powered by a suitable power supply. The complete supply system plus the sensor signal conditioning logic is assembled on the EPS system PCB.

The motor is controlled by four MOSFETs. As the µC cannot directly drive the large gate capacitances of the MOSFETs an additional interface is required in the form of a Driver IC.
For safety reasons the complete motor control system must be supervised. This is integrated on the PCB and normally comprises a relay which is used as main switch to disconnect the motor from the ECU in case of a detected failure mode.

MICROCONTROLLER (µC)

The µC has to perform the DC brush motor control for the EPS system. Basically it consists in a current control loop depending on the requested torque coming from the steering wheel via torque sensor.

To perform this control efficiently the µC should have the following features:

· multiplication/Division Unit
· high-performance Analog-to-Digital Converter
· timers
· general purpose input output.

The handling of the different time critical tasks of the control requires a fast and efficient interrupt handling mechanism.

Additional peripherals like SPI, CAN, serial interfaces are needed as well as a high performance tool chain for compact and efficient code generation and debugging.

In case an industrial safety standard norm like IEC61508 has to be implemented, the work load of the µC will be increased due to the various diagnosis and self-check tasks it will have to perform. This will influence the selection of the right µC and determine if a 8 bit or 16 bit is required.

power stage AND TOPOLOGIES
The role of the power stage is to switch the motor current.
The power stage is divided into two functions: the driver IC to control and protect of the MOSFET and the MOSFET itself to switch the current. The number of MOSFET and the partitioning, i.e. the combination of driver IC and MOSFET in one or more devices, depends on the motor power.

DRIVER IC
The driver IC is used to give sufficient current to charge and discharge the gate of the MOSFET normally switched at 20kHz as well as to assure a high gate to source voltage Vgs to the MOSFET.




Another important function is to detect a short circuit condition to avoid the destruction of the MOSFET. The affected MOSFET will be switched off and a diagnosis is given to the µC.

MOSFET

MOSFETs are normally used in a multi ½ bridges topology and controlled by a driver IC. The MOSFET in this application has to fulfill dedicated requirements such as high current capability, low on-state resistance and low gate charge. It has to be capable of medium switching frequencies in the range of 20 kHz. Finally because EPS is a safety relevant application, the highest achievable reliability is required.

For a 12V power network, typically 40V MOSFET are used. Latest technologies offer Rds(on) value in the range of 2mohm in a TO263 package and less than 4mohm in a smaller TO252 package thus enabling the design of EPS systems with very high power densities and efficiencies. The following table is showing a comparison between Infineon’s latest trench and previously developed planar 40V MOSFET technology.

The decrease in on-state resistance means a reduction in the power losses in the power stage, which allows higher efficiency to be achieved within the same package size. This would result in lower junction temperatures of the MOSFET and accordingly higher reliability.

The difference in the current rating is due to the bonding of the chip. The trench MOSFET is built with three 500µm bond wires for the source connection whereas the planar device only uses three 350µm bond wires.

As introduced previously, different topologies are used depending on motor power. For 12V low end system and typically motors below a range of 600W to 800W, an H-bridge configuration (2 times ½ bridge) is used. For mid to high end systems and typically motor with more than 600W to 800W, 3 phase brushless DC motors controlled by a full bridge (3 times ½ bridges) are preferred.

TOPOLOGIES FOR POWER STAGE

The traditional topology consists in separating the driver IC from the MOSFETs.

Alternatively it is possible to integrate the driver function and one ½ bridge in one power device. The benefit of this solution is the reduction of the number of device and PCB area and thus complexity. In this case the microcontroller can directly be connected to the power IC. Protection features like for example short circuit detection, over temperature and current sensing are also provided. Infineon offers such product for motor control in a 12V power net for DC current of up to 10A – 15A depending on cooling condition. The device can operate down to 5.5V.











Conclusion

Electric power steering will gain more market share in the future, due to system benefits. The increasing requirements for safety and the pressure to reduce costs and size will drive the need to re-partition the system with special focus on the system safety. The target is to avoid fatal system errors by detecting every possible single cause failure and reach a safe system deactivation. A wide range of safety features is possible by leveraging the feature sets of the various semiconductor technologies. The right choice of these features for a system partitioning will dictate the overall costs of the system and must be carried out carefully.

It is necessary to combine the know-how of the system supplier, the ECU supplier and the semiconductor component supplier to reach a safe, low cost solution. The chip set must be fine tuned in such a way that every chip can supervise the overall system and can switch off the steering assist when an error is detected.

The component supplier should be involved in the early definition phase of the system, to reach the main targets:

· High technical performance / safety standards
· Small system design
· Lowest costs


Click here for Illustrations:

Figure 1, Figure 2, Figure 3, Figure 4, Table 1