This article discusses the design of a control system featuring a microcontroller plus ASIC or microcontroller plus programmable logic SoC for an automotive electric power steering system. Such a system receives the "ignition" (steering "drive" command) input from the user plus vehicle inputs through a CAN transceiver and drives a three-phase brushless motor.
Using available processing headroom, the MCU can also perform battery monitoring, temperature sensing, direct drive LED or LCD display with temperature, battery status, speed value, and distance and error/warning messages. This feature discusses design techniques as well as design challenges for such an automotive electric power steering control system.
The "ignition"/drive control in the power steering system includes the following blocks:
Figure 1 "Ignition" control block diagram for electric power steering and accessory system
- Microcontroller: An ultra-low power microcontroller is required to conserve power for the operation because it is battery operated system. In addition to handling the "ignition"/drive system, motor, and other system features, the MCU can also be used for the central locking system as well as communication with different external devices used in the vehicle.
- CAN transceiver: The transceiver is used for receiving vehicle input and communicating it to the microcontroller.
- Steering motor: This is typically a brushless motor, either sensor-based (Hall Effect) or sensorless and needs to be reliable and efficient for an automotive application.
- Rechargeable battery: A variety of battery types are used from lead-acid to lithium batteries. A rechargeable lead-acid battery is commonly used in automotive applications.
- Display: Typically an LCD display with backlight is used for showing temperature, battery input, speed value, distance, and error/warning messages.
- Keypad: Automotive applications typically use a mechanical button-based keypad.
- Power management: This subsystem provides power to run functional blocks and oversees battery activity. The host microcontroller with comparators and discrete logic or internal programmable logic can be used to manage the lead-acid battery and provide safety and critical information about battery to the user.
"Ignition"/drive systems used in the automotive industry are commonly 16- or 32-bit microcontrollers with ASIC-based circuitry. The PSoC (Programmable SoC)
family from Cypress
, for example, provides an MCU plus programmable logic to control and manage the many functions and features within the automobile. Once the driver uses the ignition key to start the automobile, an input is sent to the microcontroller to start the three-phase brushless steering-motor control circuitry. The microcontroller receives the vehicle steering angle and also monitors a torque sensor and vehicle inputs signals from the user through the CAN transceiver, and controls the vehicle throttle. PSoC MCUs implement driver circuitry in programmable logic to drive the three-phase brushless motor.
The microcontroller uses either internal or external serial EEPROM (I2C/SPI based) for storing data like odometer distance readings. The MCUís RTC provides accurate time to be shown on the display. Temperature monitoring is done using an on-board RTD or thermistor-based temperature sensing device.
Apart from electric power steering, the MCU can use an obstacle sensor to obtain information about nearby vehicles while parking, In addition, a fuel sensor provides how much fuel is in the tank. The MCU also monitors the battery input and displays its status on the LCD display. Relay driver circuitry is used to switch brake lights, head lights, and directional signals on/off.
The power supply subsystem consists of a rechargeable battery as a power source. The subsystem also implements the battery charger. The battery input is down converted to a DC voltage for the microcontroller and other circuitry. Ignition key position enables and disables on board regulators. The power supply subsystem also implements protection mechanisms such as over-current, over-heating, and start-up fail condition. Power is also provided for charging external devices such as consumer electronics.