Design of automotive EPS technology solution based on Infineon products

As the number of cars continues to grow, more and more attention has been paid to the pollution of automobile emissions to the environment, and the continuous introduction of increasingly stringent emission standards has made environmental protection an indicator that cars must achieve. In addition, as car consumption continues to mature, consumers are also paying more and more attention to the various functional internal indicators of cars. As a substitute for traditional hydraulic power steering ( HPS ) and electro-hydraulic power steering (EHPS) technologies, electric power steering ( EPS ) is being used in more and more vehicles due to its advantages. Table 1 is the forecast of the Chinese EPS market by consulting firm Strategy Analytics, from which we can clearly see this growing trend. It is expected that the annual compound growth rate of China's EPS market will reach 16.9% during 2010-2017.

This growing market trend is due to the many advantages of the EPS system itself, which are mainly manifested in:

EPS can provide optimal assistance under various driving conditions, reduce disturbances, improve the steering characteristics of the vehicle, and improve the steering stability of the vehicle when driving at high speeds, thereby improving the active safety of the vehicle.

EPS only provides assistance when turning , thus reducing fuel consumption. Statistics show that EPS can save about 0.3 to 0.5 liters of fuel per 100 kilometers compared with HPS.

The EPS motor is powered by the battery, so it can provide assistance even if the engine is turned off or malfunctions.

EPS eliminates the hydraulic structure and has significantly fewer parts than HPS, so it is lighter, more compact, easier to design and install, and can reduce noise.

EPS is easy to adjust and test, and can be quickly matched with different vehicle models by setting different programs, thus shortening the production and development cycle.

EPS does not have the problem of oil leakage, which can greatly reduce warranty costs, reduce environmental pollution, and improve environmental protection.

EPS has better low temperature working performance than HPS.

It can realize auxiliary functions such as automatic parking system.

The increase in demand for EPS has also prompted many companies and scientific research institutions to increase their investment in R&D and production, which has led to the emergence of many new EPS system manufacturers entering the market. This phenomenon is particularly evident in emerging markets such as China and India. However, these companies have a short history of producing EPS products, so a mature and reliable solution is particularly important for them. The following will briefly introduce the types, general structures and characteristics of EPS systems.

EPS Introduction

EPS is a system that directly provides auxiliary torque to the steering system through the power of the motor. It relies on detecting information such as the engine, steering wheel, and vehicle speed to determine and provide appropriate steering assistance, so that the steering process can be completed accurately, easily, and safely.

Generally speaking, based on the different motor installation positions and mechanical structures, EPS systems can be divided into column-assisted, pinion-assisted, double pinion-assisted and rack-assisted. Figure 1 gives a brief introduction to the mechanical structure, output torque and applicable vehicle models of various EPS systems.

The column-assisted EPS system installs the motor on the steering column and connects it to the steering shaft through a reduction mechanism. It is characterized by a compact structure, and the measured torque signal is in the same straight line as the steering wheel torque, so the responsiveness of the motor assistance control is good, but the noise and vibration requirements of the motor are high. In addition, since the motor, ECU and reduction mechanism are all arranged in the cockpit, the working environment of the motor and ECU is good, and it is convenient for the engine compartment layout. The pinion-assisted EPS system installs the power-assisting motor at the steering pinion and directly assists the pinion, so it can provide a larger power-assisting torque and is suitable for slightly larger models, but due to the existence of the universal joint, it will affect the accuracy of the power-assisting control characteristics. The double pinion-assisted EPS system can provide greater power assistance than the pinion-assisted type due to the addition of a pair of gear racks, but the cost is also slightly higher. The rack-assisted EPS system can provide greater power assistance, but the entire system has a complex structure and high cost, so it is suitable for luxury cars and business cars.

The working principles of various types of EPS systems are basically the same. As shown in Figure 2, an EPS system is usually mainly composed of steering components, sensors , motors, deceleration mechanisms, and electronic control units (ECUs). When the driver turns, the sensor detects the torque and angle of the steering wheel, converts them into digital signals and transmits them to the electronic control unit. The electronic control unit calculates the output torque that best suits the current driving conditions based on these signals and the pre-set algorithm, and sends a signal to drive the motor to work. The torque output by the motor is transmitted to the steering mechanism by the transmission system to assist.

In order to achieve this steering process smoothly, the EPS system needs to implement the following functions:

Power-assisted control: Provides a larger assist torque when the car is parked or driving at low speed, making the steering process quick and easy, and provides a smaller assist torque when the car is driving at high speed to keep the steering process reliable and steady.

Return control: Ensure that the return torque gradually increases when the car turns from a straight state to a large angle. In addition, ensure the straight-line driving performance of the car in a non-turning state, and prevent the car from returning to the center under or overshooting at different speeds.

Damping control: Use the motor's induced electromotive force to reduce the steering wheel shaking that occurs when the car is driving at high speed, eliminate the steering wheel vibration caused by uneven road surface, and add a certain amount of damping to the steering process when the car is driving at high speed to overcome the feeling of steering floating.

Analysis and diagnosis function: It should be able to monitor the operating status in real time and have fault alarm and prompt functions. When the fault cannot be automatically eliminated, the EPS will be shut down to make the vehicle enter the traditional mechanical steering mode.

Communication function: It should have the ability to communicate with other systems via CAN or LIN bus and have an interface that can change the main parameters (mainly for motor control).

To achieve the above functions, the entire EPS system must work together efficiently and quickly. Therefore, the performance of the following components of the system is particularly important:

Sensor: The sensor provides the EPS system with all necessary information about the vehicle's driving conditions. It is the sensory organ of the system, and its signal output should be as accurate, fast, and concise as possible. The EPS system usually requires signals from the steering wheel torque sensor, the angle sensor (optional), and the wheel speed sensor. Currently, the general sensors use analog signal processing, and the signal output accuracy is not high. The output signal needs to be further processed by the ECU, which increases the workload of the ECU. Digital signal sensors are used by more and more EPS systems because they can provide digital signals that are directly available to the ECU, reduce the occupied ECU resources, and improve the accuracy of the signal.

Control strategy: Traditional steering systems mainly provide steering assistance to the driver to reduce the steering burden. The EPS system also needs to solve problems that affect vehicle stability and safety, such as oversteering and self-centering during high-speed driving. Therefore, a set of efficient and complete control strategies is needed to ensure its performance.

Electronic Control Unit (ECU): With the continuous improvement of EPS control strategies, higher requirements are placed on the calculation and response capabilities of ECU. As the core component of the EPS system, ECU must have: the ability to process sensor signals at high speed, the ability to calculate at high speed, the ability to execute quickly and effectively, the ability to handle large currents, the ability to communicate with other systems, system diagnosis monitoring and failure protection performance.

Power-assisted motor: Due to the uncertainty of the driver when steering, the power-assisted motor of the EPS system must have high responsiveness and be able to execute the driver's steering intention in a very short time. Since the installation space of the EPS system is limited, the power-assisted motor should also have the characteristics of high specific power, light weight, and low noise. In addition, because EPS involves the safety application of the car, the power-assisted motor must be safe, reliable and have a long life.

Mechanical structure: Compared with traditional steering systems, the EPS system uses a worm gear reduction mechanism and has to withstand the huge torque output by the motor, so the system's mechanical structure is required to have higher efficiency, better wear resistance and mechanical strength.

In view of these requirements of EPS system, in order to reduce the difficulty of EPS system development for customers in Asia Pacific and shorten their development cycle, as the first-level agent of Infineon , Beijing Jingchuan Electronic Technology Development Co., Ltd. (Jingchuan) and Infineon Technologies decided to use Infineon's decades of successful experience in cooperation and development with major EPS system suppliers in the world and combine it with the actual situation in the region to jointly develop a set of efficient and practical reference solutions. Jingchuan and Infineon provide customers with this set of solutions for reference and secondary development. In the following chapters, we will introduce this solution in detail.

Automotive EPS Solutions Based on Infineon Products

As a first-tier agent of Infineon, Jingchuan has strong technical capabilities, especially rich experience in motor drive. The cooperation with Infineon mainly relies on Infineon's experience in cooperating with major EPS system suppliers in the world, and combines Infineon's comprehensive product line to develop an EPS system reference solution that meets the needs of China and the Asia-Pacific region. Since the low-end EPS system using DC brushed motors has a relatively mature solution in the Asia-Pacific region, the reference solution developed this time is mainly aimed at the high-end permanent magnet synchronous motor solution, and the motor power can reach 550 watts. The block diagram of the specific solution is shown in Figure 3. As can be seen from the figure, the entire solution is divided into several parts: sensors, ECU and motors. Below we will focus on the sensor and ECU parts.

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1. Sensor

Generally speaking, an EPS system that uses passive return control needs to collect at least steering wheel torque signal, vehicle speed signal, engine ignition signal and ignition switch signal. Since this solution is mainly for high-end applications, it adds active return control function, so it is also necessary to provide the system with steering wheel angle information.

At present, most domestic EPS systems use potentiometer torque sensors, which output analog signals and must be further processed and occupy the A/D conversion port of the microcontroller, thereby occupying the resources of the microcontroller. In this solution, we recommend using two Infineon linear Hall sensors TLE4998 as torque sensors. TLE4998 is an automotive-grade (-40℃-150℃) programmable linear Hall sensor that fully adopts digital logic structure (20-bit digital signal processing) and has digital temperature compensation function. It can output SPC (Short PWM Code), PWM or SENT (Single Edge Nibble Transmission) signals as needed, where the PWM signal has a 12-bit resolution, and the SPC and SENT signals have a resolution of up to 16 bits. In addition, TLE4998 also has various protections (anti-reverse connection, overvoltage, output short circuit, etc.) and online diagnosis (voltage, EEPROM error, etc.) functions, and has extremely strong stress resistance and anti-EMC performance.

In this solution, we recommend using two Infineon TLE5011s to measure the phase difference to measure the steering wheel turn. TLE5011 is a magnetic sensor designed by Infineon using GMR (Giant Magneto Resistance) technology specifically for angle measurement. Sensors manufactured using this technology have the characteristics of high accuracy, small magnetic wheel gap, and can be made into a particularly compact sensor module due to the parallel placement of the sensor and magnet. TLE5011 can measure an angle range of 0-360°, and uses SPI signals to directly output 16-bit sine/cosine values. Its 3-wire SSC interface has a maximum communication rate of 2Mbps.

The vehicle speed signal required by the EPS system can be obtained by the wheel speed sensor in the ABS system, and the engine ignition signal and ignition switch signal can also be directly obtained through the LIN bus.

Since the output of TLE5011 is SPI signal, we design the torque sensor and the rotation angle sensor in one module in the scheme design, and use Infineon's 8-bit single-chip XC886LM as the operation unit in the module to process the sensor model and directly obtain the torque and angle information, because Infineon's XC88x series processor contains a coprocessor called CORDIC, which is specially used to calculate trigonometric functions, linear functions and hyperbolic functions, and has extremely high operation efficiency for the calculation of torque and angle. The result of the module operation will be transmitted to the ECU main controller through the LIN bus.

2. Electronic Control Unit (ECU)

ECU is the core component of the system, responsible for processing sensor signals, executing control strategies, outputting control signals to drive motors, system monitoring and diagnosis, and communication. Since this solution is to control a permanent magnet synchronous motor, its control algorithm needs to adopt a space vector control algorithm, which places high demands on the calculation accuracy and speed of the controller. In order to ensure that it can complete the task accurately and efficiently, in this solution, we use Infineon's most advanced XC2000 series products as the main microcontroller. XC2000 is a 16-bit microcontroller series designed by Infineon for automotive electronics based on 130nm technology and has the ability to execute certain 32-bit instructions. It uses Infineon's mature C166S-V2 architecture and has been improved, with the highest clock frequency reaching 80MHz. This architecture uses multi-channel data bus technology, most instructions can be completed within one clock cycle, and also supports DSP technology. The powerful functions and numerous peripherals of XC2000 make engineering developers more comfortable when designing systems. The XC2000 series products are divided into three sub-series, among which the XC2300 series is specially developed for security applications, featuring rapid response, high redundancy, high flexibility, and stable reliability.

The XC2336B used in this solution is the latest microcontroller from Infineon. It has a high-performance CPU with a maximum clock frequency of 80MHz and a five-stage pipeline and a Memory Protection Unit (MPU) to protect data from illegal access. It has different types of on-chip storage modules (8k stand-by RAM, 2k dual-port RAM, up to 16k data SRAM, up to 16k program/data SRAM and up to 320k program flash memory), and uses hardware CRC detection and ECC code to detect data errors and correct single-bit errors. Its numerous peripheral modules can flexibly meet various different needs, with 2 10-bit resolution synchronous A/D converters that can be expanded to 9 channels, 16-channel general capture/compare units, two capture/compare units with PWM generators, 5 timers, 40-channel general I/O interfaces, multiple CAN interfaces, etc., and on-chip debugging can be performed through DAP and JTAG interfaces. In addition, it also has a watchdog and a crystal watchdog to ensure the normal operation of the controller. The chip is very small, uses a 64-pin green LQFP package, and has an energy-saving wake-up mode.

In addition to the main microcontroller, this solution also uses an Infineon 8-bit microcontroller XC836MT as a backup microcontroller. This is a microcontroller with an 8051 core. Its main function is to monitor the main microcontroller and cut off the EPS to put the car into mechanical steering state when the main microcontroller fails, avoiding the car from losing control and causing danger.

In this solution, the power chip is a low voltage drop output regulator TLE42764D V50. Its output voltage is between 2.5V and 20V, adjustable or 5V fixed output with an accuracy of 2%, and the maximum output current is 400mA. It has a series of protections such as over-temperature, reverse connection, short circuit, etc., and can be shut down by Enable to reduce the shutdown current to less than 10μA.

In order to meet the requirements of the sensor in the system, the solution uses the voltage follower TLE4250-2G as the power supply of the sensor. It has a low cost and can effectively prevent the sensor short circuit from impacting the microcontroller. The chip uses a very small SMD package. It has a wide input voltage range, can output a maximum current of 50 mA, and has protection against over-temperature, reverse connection, short circuit, etc.

For the most important motor control in the EPS system, we use Infineon's TLE7183QU, a dedicated high-current three-phase motor driver chip, to drive the permanent magnet synchronous motor in this solution. The three high-side and three low-side output stages of the TLE7183QU can control 6 to 12 external MOS tubes by outputting 0-100% duty cycle PWM waves up to 30kHz. It has various protection and analysis functions such as overcurrent, overtemperature, short circuit, and has two modes of shutdown and sleep. The entire product uses a 48-pin TQFP package with an external heat sink, which ensures good heat dissipation performance and welding convenience of the system. In order to accurately control the motor using the vector control algorithm, we use two sampling resistors to collect the two-phase current of the motor, and use the TLE7183QU's built-in operational amplifier for overall current protection.

With the continuous promotion of high-end applications such as automatic parking systems, the communication between EPS systems and other systems has become increasingly close. To meet the needs of inter-system communication, this solution uses CAN transceiver TLE6250 (or TLE6251) and LIN transceiver TLE6258-2G. Among them, TLE6250 has industry-leading electrostatic protection and anti-electromagnetic interference in addition to the common functions of CAN transceivers. TLE6251 has extremely comprehensive failure analysis detection functions and extremely low sleep power consumption. TLE6258-2G is compatible with 1.2, 1.3 and 2.0 LIN specifications, and has low sleep power consumption.

In addition, in this solution, we also used some passive components of EPCOS to achieve some auxiliary functions. Specifically, we used its electrolytic capacitor B41866C6107M000 and inductor T6174 to make the power filter circuit to achieve the stability of the input power supply, and used the electrolytic capacitor B41695A7228Q007 as the ripple absorption capacitor of the motor power supply bus. In addition, the EPCOS thermistor B57702-M103-J was glued and fixed to the MOS tube to measure its temperature to prevent failures caused by system overheating.

From the perspective of the entire system, the chips used in this solution are specially designed for automotive applications, with an application temperature range of at least -40 to 125°C, and strong electrostatic protection and anti-electromagnetic interference performance. In addition, most of these chips are made of lead-free materials and are environmentally friendly green chips.

This solution is specially developed for the emerging markets in the Asia-Pacific region, balancing various factors such as performance, cost, and development difficulty to meet the needs of customers in this region. Globally, EPS systems are experiencing rapid development, and some of the world's leading suppliers have launched more advanced solutions, which we will briefly introduce below.

The development direction of new EPS system

The development direction of EPS system is miniaturization and high integration, so integrating EPS controller into EPS motor becomes a universal solution. At present, almost all leading EPS system suppliers in the world have developed such a solution. Its main feature is that all components are arranged on one or two ceramic plates in the form of bare chips, and the ceramic plates are placed in the back cover of the drive motor. Figure 4 is a schematic diagram of the structure of an integrated solution developed by Infineon for a customer.

We can see that in this solution, all the chips are arranged on two ceramic plates. One plate houses all the power supplies, microcontrollers, MOS tube driver chips, and other chips, while the other plate houses the MOS tube and an inductor, and has an enhanced heat dissipation design. The advantages of this solution are obvious. The entire electronic control unit is integrated into the motor back cover, which is small in size and has few connecting wires. It fully utilizes the heat dissipation of the motor back cover and has good system reliability. However, for customers in emerging markets in the Asia-Pacific region, since their terminal customer base is generally small and their own technical strength is relatively weak, the implementation of this solution is still quite difficult, mainly in the following aspects:

This solution requires the cooperation of chip manufacturers and motor manufacturers, as the customization of chips and motors requires a large investment. For most customers in the Asia-Pacific region, this means high initial R&D expenditures. Without the support of a large customer base, the project risk is too high. Since it involves the customization of chips and motors, developers are required to have strong system knowledge and often involve the participation of vehicle manufacturers. However, most customers in the Asia-Pacific region have entered the industry not long ago and their own technical accumulation is not enough to participate in such designs. This solution requires ceramic chip technology and bare chip lead technology. Currently, few chip manufacturers in the Asia-Pacific region have such mature processing technology.

It is precisely considering these factors that the EPS solution based on Infineon products introduced in this article is a practical and cost-effective solution.

Conclusion

The use of EPS is becoming more and more widespread, especially in the emerging markets of Asia Pacific, where its total market volume and annual growth rate are becoming more and more eye-catching. The EPS technology solution based on Infineon products introduced in this article has the advantages of comprehensive functions, reliable performance and high cost performance, which can help automotive system suppliers shorten the R&D process, reduce R&D costs, enter the market at a faster speed and occupy the market with convincing products. As a first-tier agent of Infineon, Jingchuan Company will do its best to help customers achieve this grand goal together with Infineon.