Steer the wheels with microcontrollers

By Robert Weiss
Infineon Technologies AG

Today, electronic power steering (EPS) is one of the key arguments in CO₂ reduction. Thus, a wide range of different car types is equipped with an electromechanical power steering system. But what kind of functionality is involved in implementing electronic power-assisted steering systems?

EPS is an electromechanical power steering system that uses an electronically controlled electric motor in the place of conventional hydraulic steering assistance. An EPS system occupies less space in the engine compartment, is easier to assemble and reduces fuel consumption. It eliminates the need for toxic hydraulic oil in the steering system. In smaller vehicles, the electric motor is connected to the steering column via a gearbox. In mid-range vehicles, it is flange-mounted crosswise or lengthwise onto the gear rack and operates through a gearbox. When the driver turns the steering wheel, the electric motor applies power assistance to the steering.

EPS components
An EPS system consists of a control unit, a number of sensors and a brushless motor. The control unit controls the system and supplies the information needed by the electric motor. It receives information from sensors measuring the steering angle, driving speed and torque. Sensors that detect the motor position and motor current ensure that the motor is working at its optimum operating point. In the figure, the XC2300 operates as the master processor, controlling the servo motor and other components. A second smaller MCU or an ASIC is used as a supervision unit.

The actuator is a three-phase synchronous or asynchronous brushless motor. The motor's rotating field is generated electronically. A pulse-width modulated (PWM) signal with a signal frequency of about 20kHz affects the speed and the torque.

Rotary encoders or magnetic sensors—also called Giant Magnetic Resistors—provide the data identifying the rotor position. Typically, two-phase currents are measured by shunts or Hall sensors. These sensors have analog outputs that need to be amplified for processing. The forces acting on the steering column and the amount of assistance required from the motor are measured by the torque sensor. Signal processing is carried out in the control unit. The wheel sensor supplies information on the speed of the car, and the steering angle sensor delivers information on the current position of the steering wheel. Other control units process these signals. The data is transmitted over the CAN. Some of the evaluation logic can be integrated into the sensors, depending on the types of sensors chosen for the system. This improves precision and reduces the susceptibility to faults.

The control unit consists of a number of voltage regulators, CAN transceivers, signal processing circuitry, bridge drivers, power switches and MCUs.

The voltage regulators supply the different voltages needed by the sensors, MCUs and ASICs. The CAN transceivers act as a bridge between the CAN and the MCU. Depending of the sensor type, the data signal processing may be analog or digital. Because MCUs are not able to control the power switches (B6 bridge) directly, a bridge driver is needed. The bridge driver generates the gate voltage and the related currents needed to switch the transistors rapidly. Intelligent drivers also include diagnostic interfaces that can detect various problems including half-bridge short circuits, low phase voltages or high component temperatures. The MCU controls and monitors the motor and the entire system. It also has to perform diagnostics and to communicate with the network. An additional controller is used to detect faults and to activate emergency operating modes.

Need for algorithms
Because of their high demands in terms of motor dynamics and constant torque, EPS systems need complex algorithms like field-oriented control. This type of control acts directly on the motor's rotor field and requires considerable processing power, because it involves computing multiple coordinate transformations (Clark/Park transformations) and the regulation of both phase currents at 50?s intervals. A PWM signal is needed to control the motor using the space vector method. Thanks to the high performance of the MAC unit, the needed CPU load is less than 10 percent.

Infineon's XC2300 MCU is part of the company's XC2000 family. It offers a processor architecture that addresses the issue of system monitoring in detail, is capable of processing demanding control algorithms fast and has extensive built-in hardware support. To enhance reliability, sensitive data is verified with a CRC, which involves writing data twice and comparing it. The entire memory system is protected by a hardware error correction unit. To encapsulate different software modules a memory protection unit has been implemented.

The XC2000 architecture is based on an onward development of the C166 core. It offers control and DSP capabilities. In contrast to the C166 architecture, however, it can execute instructions in a single clock cycle. The XC2300 offers close to twice the processing power at the same clock speed. The XC2300 also has a multiply accumulate unit that enables matrix operations or FIR filter functions to be implemented easily.

An EPS system consists of a control unit, a number of sensors and a brushless motor.

The rapid processing of matrix operations (Clark/Park transformations) and the implementation of powerful PI controllers both play an important part in an EPS system. The XC2300 supports up to 128 interrupt sources on 16 interrupt levels. Besides classic interrupt handling, the processor also features a DMA transfer option in the form of a peripheral event controller, which enables large data blocks to be moved or copied easily within the 16Mbyte address space. The program memory access is 64bits wide and currently supports up to 576Kbytes of embedded flash. The flash memory is separated physically into multiple blocks and incorporates error correction and monitoring for greater operational reliability. Each flash memory area can be individually read- and write-protected with a password. There are also up to 50Kbytes of embedded SRAM that can be used to manage data. An additional protection mechanism can also be used to block unauthorized access to important CPU registers. For greater operational reliability, protective mechanisms are also triggered when restricted instructions are executed or the CPU stack is overwritten.

About the author
Robert Weiss is senior staff application engineer of automotive MCU unit at Infineon Technologies AG.