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Automotive ultrasonic ranging: Increasing gain may not improve detection distance

As ultrasonic-based distance ranging becomes more prevalent in applications such as blind spot detection, objects at distances greater than six meters (20 ft) have to be detected. The amplitude of the echo signal reflected by objects at far distances is very small. So there is a temptation to increase the amplifier gain, K, in order to detect objects at such distances. In this article, we show that increasing the amplifier gain, K, may not always result in the ability to detect objects at farther distances.

Background
One application for advanced driver assistance systems (ADAS) in a passenger car is ultrasonic-based distance ranging. Ultrasonic sound wave time-of-flight (TOF) is used to calculate distances to objects to assist the driver in parking the car, identifying parking spots, or detecting objects in the driver’s blind spot.

In ultrasonic-based ADAS, piezoelectric transducers typically are used to convert the ultrasonic waves into electrical signals. The receiver sensitivity of piezoelectric ultrasonic transducers usually is small, resulting in very small voltages. Figure 1, below, shows a typical signal chain used to process the echo voltage. (For an example of an integrated automotive ultrasonic signal conditioner for automotive park assist systems, see TI’s PGA450-Q1)

Figure 1: Using ultrasonic-based echo processing to detect objects deals with noise—both external (shown) and internal.

This echo signal, which is an AM signal, is corrupted with noise. The noise in Figure 1 is input-referred noise and is the sum of noise from external environment and from all components in the signal chain. This corrupted signal is then amplified by an amplifier with gain K. The amplified signal is digitized using an analog-to-digital converter (ADC). The digitized AM signal is bandpass-filtered.

The bandpass filter (BPF) primarily is used to improve the signal’s signal-to-noise ratio (SNR). The filtered signal level is compared against a threshold, L, to detect the presence of an object. Bandpass filters typically are followed by an amplitude demodulator. However, for the purpose of this article, the demodulator is not relevant.

Threshold analysis

Observations
Equation 8 can be used to analyze the smallest detectable distance by considering noise from components upstream and downstream from the amplifier. The components upstream from the amplifier include environment noise, transducer noise, noise from any current limiting resistors, and noise the amplifier itself. Downstream components include the ADC quantization noise and filter calculations errors.

Here are three different scenarios that illustrate distance detection.

Figure 2: Effect of increasing the gain does not produce a change in the smallest possible threshold value or largest measurable distance.

Conclusions
Figure 2 shows that the effect of increasing the amplifier gain really depends on the relative noise magnitudes of the components both upstream and downstream from the amplifier. If noise from upstream components dominates the overall noise, then increasing the gain will not help detect objects at farther distances.

Reference:
Datasheet for the PGA450-Q1

About the author:
Arun Tej Vemuri is a systems architect with the Mixed-Signal Automotive group at Texas Instruments where he is responsible for product definition of automotive sensor signal conditioners. Arun received his Ph.D. in Electrical Engineering from the University of Cincinnati, his MS in Systems Science from IISc Bangalore, India, and BSEE in Electrical Engineering from IIT Roorkee, India. Arun can be reached at ti_arunvemuri@list.ti.com.


This story appeared first on Automotive Desingline courtesy of EE Times USA

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