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Backup System Overview
By Dave Otten, Sr. Applications EngineerAdvanced Microcontroller Architectures Division-and-
Chris Appleton, Far East Marketing ManagerAutomotive Products Group
Microchip Technology Inc.
System Overview
The number of electronic control modules within the vehicle continues to increase. Convenience and safety are continuing to drive the growth of body-control electronics within the vehicle. Systems to aid the driver in handling the blind spots that exist on vehicles, especially the larger SUVs, are increasing. Plus, today’s aerodynamically designed cars are not driver friendly for parallel parking or other tight parking situations. Sensor technology to enable electronic control modules that offer near-range obstacle detection for backup warning and parking assistance is cost effective and can be used for a wide range of vehicle models. Ultrasonic-based sensors allow the driver to safely backup, using ultrasonic sound waves to warn of any obstacles detected in the path of (or near) the vehicle. The systems can be designed to provide a combination of audible and visual warnings to alert the driver about the proximity and direction of a detected obstacle that might be outside of the line of sight. So, whether it’s parking or maneuvering in tight spaces, reverse-warning systems allow the driver to perform the necessary actions with less stress and more comfort.
Transmitter
The transmitter provides the pulse train to drive the ultrasonic transducer. Ultrasonic frequencies vary, although 40 kHz is a common transmission frequency. Since the voltage amplitude of the pulse train is higher than the system voltage, a boost circuit can be incorporated within the block. A simple way to implement a robust voltage boost is to use a step-up DC/DC controller, such as Microchip Technology’s MCP1650 which requires only one capacitor, one inductor and two resistors to select the desired output voltage. To generate the transmitted pulse train, a driver such as the TC1428 MOSFET Driver can be implemented. In microcontroller-based designs, a PWM signal is used to modulate the boost’s output voltage.
Receiver
The receiver block consists of an ultrasonic receiving transducer, filter, amplifier and a comparator. The output of the receiving transducer is a low amplitude sine wave of the same frequency as the transmitted pulse train. To amplify the incoming signal, Microchip’s MCP6293 Op Amp can be used. This op amp features a wide 10 MHz bandwidth and pin-selectable low-power modes in a small package. The output can be fed into an amplitude-detection circuit to provide another level of hardware filtering. This circuit converts the received pulses into a smooth, integrated slope. High-frequency noise is filtered and a more stable, easily detected signal is produced. The signal is then ready to be fed to the comparator, where it is compared to a reference voltage. By comparing to a decaying voltage, which is generated by a simple resistor-capacitor circuit, the decreasing reference voltage characterizes the decaying amplitude of the received signal over time.
Display
Backup sensor displays can be comprised of audible and visual indicators to alert the driver of a detected obstacle. Since multiple sensors are needed to provide adequate spatial coverage, the closest measured distance is determined and displayed.
Microcontroller
The microcontroller is at the core of the system. Using the time lag between the transmitted pulse and received echo, the microcontroller calculates the distance between the vehicle and obstacle. When the distance is less than a predetermined value, an acoustic or optical signal is generated. Microchip’s PIC® Microcontrollers are well suited for this type of body-control application within the vehicle. The PIC18F8490 features an LCD controller, two PWMs, two comparators and four timers, providing a highly integrated solution for backup ultrasonic sensor applications. Additionally, the power-management features enabled by nanoWatt Technology increase power efficiency and system robustness.
Depending on the required accuracy, distance and system cost, various implementation options are available for consideration. Accuracy improves with higher transducer frequency and power. Higher-frequency transducers are smaller than their low-frequency counterparts and allow the system to be more inconspicuously installed on the automobile. Lower-frequency transducers provide better range and are more capable of detecting objects to the sides of the transducer. An inexpensive technique to reduce interference is to add a 3 cm tube around the receiving transducer to focus the received signal and increase directivity.
System accuracy can be improved via a number of methods. Since the speed of sound is affected by temperature, a temperature sensor such as Microchip’s TC1047A can be used to measure air temperature. Improving the filtering of the receiver increases the signal fidelity and improves system accuracy. Additional gain stages on the reflected signal also increase the range and accuracy.
A significant factor that diminishes system performance is crosstalk between the transmitter and speaker. The received signal can not be detected until the effects of the transmitted pulses have dissipated in the receiving transducer. It is important to minimize the mechanical coupling between these two components. Techniques to consider are to mount each transducer on different PCBs. If they share a substrate, put a thin piece of foam behind the transducers. If a single-transducer solution is used to transmit and receive, provide an adequate software delay after the transmission and before enabling the receiver block. Any type of protective coating that covers the transducers should be avoided. All of these techniques improve the performance of the ultrasonic system.
Summary
This type of electronic control unit is another example of the role that semiconductor and sensor technologies are playing in enhancing the driving experience for today’s vehicle. Since the system recognizes obstacles outside of the driver’s field of view, the driver enjoys significant benefits and a more comfortable driving experience.
Note: The Microchip name and logo, and PIC are registered trademarks of Microchip Technology Inc. in the USA and other countries. All other trademarks mentioned herein are property of their respective companies.