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Simplify automotive power steering design

Posted: 15 Feb 2012     Print Version  Bookmark and Share

Keywords:microcontroller  electric power steering  control system 

An optional current control (bottom of figure 2) will be applied to the PWM output control to regulate the motor current. This inner loop is based on a comparator; and the feedback bus current will be compared with the reference current value that is provided by a 12bit DAC. Changing the DAC output will modify the output current value.

Sensor-based motor control/system limitations
A sensor-based brushless motor control uses a Hall sensor input to detect rotor position and thus control the motor movement, providing Hall sensor inputs to the microcontroller and working as a closed loop system.

Design challenges
A high-performance integrated microcontroller with a higher MIPS CPU core, faster ADC (≥500 kSPS @ 10bit), internal flash, SRAM memory, internal EEPROM, and integrated analogue and digital peripherals are required to perform key functions such as high-performance analogue measurements, operating a CAN interface, driving the three-phase motor control, LCD driving, low power operation, RTC, and interfaces with different external protocols.

A power MOSFET with low Ron and low gate capacitance is required for driving the three phase motor.

Designing the board with high power MOSFET driver circuitry and handling high on-board current from the battery input is a design challenge for board designer.

As this system involves electro-mechanical components, designing a compact and cost effective electro-mechanical solution for "ignition"/drive control of the electrical power steering system is another hurdle, as is certifying this electro-mechanical design with EMI/EMC standards.

A fault detection and recovery mechanism is also required for automotive applications. Power supply design with battery protection, over-current, overheating, start-up fail condition are needed as well. And it is advantageous to choose a microcontroller with one-time programmable features to prevent reverse engineering of firmware by competitors and hackers.

System limitations
PSoC MCUs also support capacitive-sensing technology which replaces mechanical buttons with a "touch" based keypad, reducing failure due to mechanical buttons, thus providing better product reliability. Implementation of touch screen-based design on the driver's front panel, instead of a separate LCD display and keypad, will provide a better user interface and flexibility.

Implementing interfaces for external devices like an iPod/iPhone enables communication to these devices through UART or USB. User then can control iPod/iPhone devices and charge them in the vehicle. However, increasing the number of internal and external interfaces on the board is going to increase the number of ways that an intruder can create havoc on the system. This is one of the single largest limitations of this embedded system.

"Ignition"/drive control in an electric power steering system used in automotive application is currently implemented using microcontroller-plus-ASIC-based solutions. PSoC is an integrated combination of the microcontroller and ASIC. Using PSoC-based control, one can reduce the complete product cost (by reducing BOM cost) and project cost (by implementing in the PSoC Creator).

About the author
Ronak Desai is a staff engineer at Cypress Semiconductor with nine years of industry experience. He has a BE in Electronics and Communication from Mumbai University, India. He is part of the Development Kits Group and is based out of Bangalore, India.

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