DESIGNING AN IGNITION CONTROL FOR AUTOMOTIVE APPLICATIONS By Ronak Desai, System Design Engineer, Cypress Semiconductor Corp. This article will discuss the design of an ignition control used in Electrical Power Steering system of automobiles by using a microcontroller plus ASIC or microcontroller plus programmable logic SoC. Such a system receives the ignition input from the user and vehicle inputs through a CAN transceiver and drives the three-phase brushless automotive 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 article discusses design techniques as well as design challenges for Ignition control in Electrical Power Steering system used in automotive application. The Ignition control in Electrical Power Steering system used in an automotive application includes below blocks: Backlight LCD Temperature Reading Speedometer Battery Status Fuel Status Distance Optional EEPROM Temperature Sensor Hall Effect Relay Drivers Vehicle Inputs CAN Transceiver Microcontroller Plus ASIC Signal Conditioning Inputs OR Obstacle sensor PSoC Gate Drivers Fuel sensor Battery Input DC/DC Converter Battery Charger Fuel Gauge Battery Protection Temperature Sesnsor Battery Management DESIGNING AN IGNITION CONTROL FOR AUTOMOTIVE APPLICATIONS Page 1 of 6 Published in EE Times Automotive (http://www.eetimes.com) Month 2011
Figure 1. Ignition control in Electrical Power Steering system Block diagram Microcontroller: An ultra-low power microcontroller is required for the operation as it is battery operated system. In addition to handling the ignition system, motor, and other system features, it can also be used for the central locking system as well as for 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 sensored (Hall Effect based) or sensorless. It needs to be reliable and efficient for an automotive application. Rechargeable Lead Acid/ Lithium 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, and 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 a lead-acid battery. It also provides safety and critical information about battery to the user. Ignition Systems Ignition systems used in the automotive industry are commonly 16- or 32-bit microcontrollers with ASIC-based circuitry for Ignition control. The PSoC 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 automotive motor. The microcontroller also receives the vehicle steering angle, monitors torque sensor and vehicle inputs signals from the user through the CAN transceiver, and moves the vehicle. PSoC MCUs implement driver circuitry in programmable logic to drive the three-phase brushless automotive motor at the speed required by the driver. The speed of the motor will be varying over time and be controlled as via acceleration brake sensor input from the driver. The microcontroller uses either internal or external serial EEPROM (I2C/SPI based) for storing data like 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 Electrical Power Steering system, the MCU can use obstacle sensor to get information about nearby vehicles while parking, In addition, a fuel sensor provides information about how much fuel is in the engine. The MCU also monitors the battery input and displays its status on the LCD display. Relay driver circuitry is used to switch ON/OFF Brake light/ Head lights and aiming directional lights. DESIGNING AN IGNITION CONTROL FOR AUTOMOTIVE APPLICATIONS Page 2 of 6
The power supply subsystem consists of a rechargeable Lead Acid/ Lithium 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. Use of the ignition key enables and disables on board regulators. The power supply subsystem also implements protections mechanisms such as over-current, over-heating, and start-up fail condition. Power is also provided for charging external devices like cell phones. Implementation of an Ignition 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. It s ARM Cortex M3 microprocessor core offers Flash memory up to 256KB, SRAM up to 64KB, and internal EEPROM up to 2KB. The Ignition control system uses 6 on-board 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 speed required as set by the system and driver. An internal PGA, Comparators, and 12-bit 1MSPS SAR ADC with sample-and-hold (S/H) 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, low-cost temperature sensing using temperature sensing devices like thermistor or RTD, implementing an obstacle sensor, and fuel sensor. Because these capabilities are integrated into the MCU, no external amplifies o or comparators are required. In addition to the Electrical Power Steering system, the MCU running the ignition subsystem can directly drive the relay for the horn, Brake light/ Headlight, and Directional lights as well as direct drive the LCD display to displaying temperature readings, battery status, the vehicle speed, and distance and Error/ Warning messages. PSoC has operating rage of 1.71V to 5.5V so it can be easily interface with external peripherals for other applications. When using a rechargeable Lead Acid/ Lithium Battery as the power source, the input voltage is down converted by an onboard board step-down regulator. MCUs like PSoC support low operating voltages down to 1.71V and ultra low power operation achieves larger 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 SARADC and PGA for analog sensors and other inputs, as well as components like PWMs, CLK, MUX, 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, customer can create and share user-defined, custom peripherals using hierarchical schematic design. Customer can automatically place and route select components and integrate simple glue logic, normally located in discrete multiplexers. Overcurrent protection in an ignition 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 cutoff 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 designer of the Ignition control system. Sensorless motor control A sensorless motor control system does not requires Hall sensors, instead, it uses ba ack-emf zero crossing detection technique to control the motor movement. When the motor rotates, each winding generates a voltage known as the back Electromotive Force (Back EMF) 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. DESIGNING AN IGNITION CONTROL FOR AUTOMOTIVE APPLICATIONS Page 3 of 6
Figure 2. PSoC based Sensorless Motor control In the figure 2, back EMF signals from three phases terminate and the DC bus is scaled and routed to the MCU. The MCU switches the terminate input to the comparator using the MUX, and then compares it with the DC bus voltage. Cascaded digital logic filters out the PWM signal to get the real zero-crossing signal. The microcontroller will decide the commutation according to this information. An optional current control 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 A sensor-based brushless motor control uses a Hall sensor input to detect rotor position and thus control the motor movement. It provides HALL sensor inputs to the microcontroller and work as a closed loop system. Design Challenges A high-performance integrated microcontroller with a higher MIPS CPU core, faster ADC (>= 500Ksps @ 10-bit), internal Flash, SRAM memory, Internal EEPROM, and integrated analog and digital peripherals is required to perform key functions like high-performance analog measurements, operate a CAN interface, drive the three-phase Motor control, LCD drive, Low power operation, RTC, Interfaces with different external protocols. A power MOSFET with Low Ron and Low gate capacitance is required for driving three phase automotive motor. Designing the board with high power MOSFET driver circuitry and handling high on-board current from 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 control in Electrical Power Steering system is a design challenge for system designer. DESIGNING AN IGNITION CONTROL FOR AUTOMOTIVE APPLICATIONS Page 4 of 6
Certifying this electro-mechanical design with EMI/EMC standards is a design challenge for system designers. Fault detection and recovery mechanism is required for automotive applications. Power supply design with Battery protection, over-current, overheating, start-up fail condition is required for Ignition control in Electrical Power Steering system used in automotive application. It is advantageous to choose a microcontroller with OTP features to prevent reverse engineering of firmware by competitors and hackers. System Limitations PSoC MCUs also support CapSense technology which replaces mechanical button with CapSense based keypad. It also reduces failure due to mechanical buttons and provides better product reliability. Implementation of touch screen-based design on the front panel instead of LCD display and keypad will provide better user interface and flexibility in the automobiles. Implementing interfaces for external devices like ipod / iphone enables communicate to these devices through UART or USB. User can control ipod / iphone devices and charge the devices in the vehicle. Failure Analysis and Returned Materials: 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 control in Electrical Power Steering system used in automotive application is currently implemented using Microcontroller plus ASIC based solutions. PSoC is a combination of Microcontroller and ASIC. Using PSoC Based Ignition control one can reduce the complete product cost (by reducing the BOM cost) and project cost (Implementation in PSoC Creator) in Automotive industry. Cypress Semiconductor 198 Champion Court San Jose, CA 95134-1709 Phone: 408-943-2600 Fax: 408-943-4730 http://www.cypress.com Cypress Semiconductor Corporation, 2007. The information contained herein is subject to change without notice. Cypress Semiconductor Corporation assumes no responsibility for the use of any circuitry other than circuitry embodied in a Cypress product. Nor does it convey or imply any license under patent or other rights. Cypress products are not warranted nor intended to be used for medical, life support, life saving, critical control or safety applications, unless pursuant to an express written agreement with Cypress. Furthermore, Cypress does not authorize its products for use as critical components in life-support systems where a malfunction or failure may reasonably be expected to result in significant injury to the user. The inclusion of Cypress products in life-support systems application implies that the manufacturer assumes all risk of such use and in doing so indemnifies Cypress against all charges. PSoC Designer, Programmable System-on-Chip, and PSoC Express are trademarks and PSoC is a registered trademark of Cypress Semiconductor Corp. All other trademarks or registered trademarks referenced herein are property of the respective corporations. This Source Code (software and/or firmware) is owned by Cypress Semiconductor Corporation (Cypress) and is protected by and subject to worldwide patent protection (United States and foreign), United States copyright laws and international treaty provisions. Cypress hereby grants to licensee a personal, non-exclusive, non-transferable license to copy, use, modify, create derivative works of, and compile the Cypress Source Code and derivative works for the sole purpose of creating custom software and or firmware in support of licensee product to be used only in conjunction with a Cypress integrated circuit as specified in the applicable agreement. Any reproduction, modification, translation, compilation, or representation of this Source Code except as specified above is prohibited without the express written permission of Cypress. Disclaimer: CYPRESS MAKES NO WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, WITH REGARD TO THIS MATERIAL, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE. Cypress reserves the right to make changes without further notice to the materials described herein. Cypress does not assume any liability arising out of the application or use of any product or circuit described herein. Cypress does not authorize its products for use as critical components in life-support systems where a malfunction or failure may reasonably be expected to result in significant injury to the user. The inclusion of Cypress product in a life-support systems application implies that the manufacturer assumes all risk of such use and in doing so indemnifies Cypress against all charges. DESIGNING AN IGNITION CONTROL FOR AUTOMOTIVE APPLICATIONS Page 5 of 6
Use may be limited by and subject to the applicable Cypress software license agreement. DESIGNING AN IGNITION CONTROL FOR AUTOMOTIVE APPLICATIONS Page 6 of 6