Designing an ignition control for automotive applications
05 July 2013
This article discusses the design of an ignition control used in the electrical power steering (EPS) system of automobiles by using a microcontroller plus ASIC or a microcontroller supported by a programmable logic SoC. Such a system receives the ignition input from the user and 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 systems used in automotive applications.
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 the torque sensor and vehicle inputs signals 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 vary over time and be controlled via acceleration and 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 sensors 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-lights, headlights and aiming directional lights.
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 analogue-to-digital conversion capabilities, and commonly used fixed-function peripherals. Its 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 motor. The 16-bit PWM is used to drive the FET-based gate driver circuitry. The duty cycle of the PWM is varied based upon the speed required as set by the system and driver.
The internal PGA, comparators, and 12-bit 1MSPS SAR ADC with sample-and-hold (S/H) capabilities are used to control the speed of the motor by varying the PWM duty cycle. They are also used to measure different sensor inputs like battery monitoring, low-cost temperature sensing using devices like thermistors or RTDs, implementing obstacle sensors, and fuel sensors. Because these capabilities are integrated into the MCU, no external amplifies or comparators are required.
In addition to the EPS system, the MCU running the ignition subsystem can directly drive the relay for the horn, brake light/ headlight, and directional lights as well as directly drive the LCD display to display temperature readings, battery status, the vehicle speed, and distance and error/ warning messages. PSoC has an operating rage of 1.71V to 5.5V so it can be easily interfaced 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 longer battery life.
Using the PSoC Creator IDE tool, 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 analogue 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 customers to tap into an entire tools ecosystem with integrated compiler tool chains, RTOS solutions, and production programmers. They can also create and share user-defined, custom peripherals using hierarchical schematic design. Customers can automatically place and route 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 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, 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 require Hall sensors, Instead, it uses a back-EMF zero crossing detection technique to control the motor movement. When the motor rotates, each winding generates a voltage known as 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.
In 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 works as a closed loop system.
A high-performance integrated microcontroller with a higher MIPS CPU core, faster ADC (>=500Ksps @ 10-bit), internal Flash, SRAM memory, Internal EEPROM, and integrated analogue and digital peripherals is required to perform key functions. These can include 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 a three-phase automotive motor. Designing the board with high power MOSFET driver circuitry that can handle the high on-board current from the battery input can be a design challenge for the board designer.
As this system involves electro-mechanical components, designing a compact and cost effective electro-mechanical solution for the ignition control in the EPS system is the job for an experienced system designer. Certifying this electro-mechanical design with EMI/EMC standards is also a 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 the EPS 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.
PSoC MCUs also support CapSense technology, which replaces mechanical buttons with a CapSense based keypad. It also reduces failure due to mechanical buttons and provides better product reliability. Implementation of a touch screen-based design on the front panel instead of an LCD display and keypad will provide a better user interface and flexibility in automobiles.
Implementing interfaces for external devices like iPods and iPhones enables communication to these devices through the UART or USB connections. Users can control the devices and charge them 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 access the system. This is one of the single largest limitations of this embedded system.
Ignition control in EPS systems as used in automotive applications are currently implemented using a microcontroller plus ASIC based solutions. PSoC is a combination of microcontroller and ASIC. Using a PSoC based ignition control, one can reduce the complete product cost (by reducing the BOM cost) and project cost (implementation in PSoC creator).
Figure 1 Block diagram of an ignition control in an EPS system
Figure 2 PSoC based sensorless motor control
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