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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 

The power supply sub-system consists of a rechargeable battery as a power source. The sub-system 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 sub-system 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
PSoC is a combination of a 32bit 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 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 16bit 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 12bit, 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 analogue 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 over-current 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, 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.

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