Programmable logic, SoC simplify power steering, accessory control
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
"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 MCU’s 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.
Implementation of an "ignition"/drive control system
PSoC is a combination of a 32-bit microcontroller with programmable logic, high-performance analog-to-digital conversion capabilities, and commonly used fixed-function peripherals. Its ARM Cortex M3 microprocessor core offers flash memory up to 256 kB, SRAM up to 64 kB, and internal EEPROM up to 2 kB.
The "ignition"/drive control system uses six onboard N-channel MOSFETs and gate driver circuitry to drive the three-phase brushless motor. An internal PWM, clock, multiplexer, and comparators drive and control the three-phase brushless motor. The 16-bit PWM is used to drive the FET-based gate driver circuitry to control the motor. The duty cycle of the PWM is varied, based upon the quickness required as set by the system and driver inputs.
An internal PGA (programmable gain amplifier), comparators, and 12-bit, 1 MSPS successive approximation (SAR) ADC with sample-and-hold capabilities is used to control the speed of the motor by varying the PWM duty cycle. It is also used to measure different sensor inputs like battery monitoring, temperature sensing using a thermistor or RTD, and implementing an obstacle sensor, and fuel sensor. Because these capabilities are integrated into the MCU, no external amplifiers or comparators are required.
In addition to the electric power steering system, the MCU can directly drive the relay for the horn, brake lights, headlights, and directional signals as well as direct drive the LCD display to displaying temperature readings, battery status, vehicle speed, distance and error/warning messages. PSoC has operating rage of 1.71 to 5.5V so it easily interfaces with external peripherals for other applications.
When using a rechargeable battery as the power source, the input voltage is down converted by an onboard board step-down regulator. MCUs such as the PSoC support low operating voltages down to 1.71V, and ultra low-power operation achieves longer battery life.
Using the PSoC Creator IDE tool, all the interface and logic can be designed within a single development environment. PSoC Creator provides a readily-available library of component blocks for designing interfaces and logic like SAR ADC and PGA for analog sensors and other inputs, as well as components like PWMs, clocks, MUXes, and comparators for the motor drive application. Components are also available for directly driving character and segment LCDs, operating a CAN protocol interface, a RTC component for real-time measurements, and an internal system clock that does not requires external clock/oscillator circuitry.
PSoC Creator also enables customer to tap into an entire tools ecosystem with integrated compiler tool chains, RTOS solutions, and production programmers. With PSoC Creator, the user can create and share user-defined, custom peripherals using hierarchical schematic design, and automatically place and route select components, and integrate simple glue logic, which is normally located in discrete multiplexers.
Overcurrent protection in an "ignition"/drive control system is used to turn off the motor driving PWMs and thus stop the motor operation. PSoC has comparator-based triggering of PWM "kill" signals to quickly and reliably terminate motor-driving when an overcurrent condition is detected. The input to this block is from the bus current. The cut-off reference to this block is the maximum amount of the current that can be drawn by the motor. The bus current input is given to the comparator and the cut-off reference is configurable and set by the DAC.
The comparator output is set high if the bus current is less than the reference threshold. The comparator output is connected to the kill signal input of the PWM. When this kill input is high, the PWM output is turned off, thus preventing the motor from being damaged. The implementation of this complete block using PSoC creator components does not require any addition firmware to be written by the control system designer.
Sensorless motor control
A sensorless motor control system does not require Hall sensors but rather uses a back-EMF zero crossing detection technique to control the motor movement. When the motor rotates, each winding generates a back electromotive force (back EMF, i.e. voltage) which opposes the main voltage supplied to the windings. Back EMF polarity is in the opposite direction of the voltage used for winding excitation and directly proportional to the motor speed.
Figure 2 PSoC based sensorless motor control.
In Figure 2, above, back EMF signals from the three-phase terminals and the DC bus (at upper left) are scaled and routed to the MCU. The MCU switches the terminal input to the comparator using the MUX, and then compares it with the DC bus voltage. Cascaded digital logic filters the PWM signal to get the real zero-crossing signal. The microcontroller will decide the commutation according to this information.
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 12-bit DAC. Changing the DAC output will modify the output current value.
Sensor-based (Hall-effect ) 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 @ 10-bit), internal flash, SRAM memory, internal EEPROM, and integrated analog and digital peripherals are required to perform key functions such as high-performance analog 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.
Failure analysis and returned materials: 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).
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. You can reach Ronak at rkad@cypress.com.
If you enjoyed this article, you will like the following ones: don't miss them by subscribing to :
