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System Design Theory
5
TIDUDO4–May 2018
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Automotive Ultrasonic Sensing Module Reference Design for Park Assist
4 System Design Theory
A PAS solution can be divided in 3 subsections – Ultra sound sensor (also known as Piezo sensor),
sensor driver and receiver circuit (PGA460-Q1), and communication interface for multiple sensors with
ECU. This section covers details of each system (or sensor?) here.
4.1 Ultrasound Sensor and Characteristics
PAS controls steering, acceleration, and braking automatically, based on the parking zone and location
information gained from the ultrasonic sensor, to achieve parallel parking and garage parking. In the case
of the rear sonar, two to four ultrasonic sensors are mounted on the rear bumper to detect an obstacle
from 2 to 2.5 m away. The main characteristics of ultrasonic sensors for rear sonar are directivity, ringing
time, sensitivity and sound pressure. Directivity of an ultrasonic sensor corresponds to the size and shape
of the vibrating surface (that is emitting the ultrasound) and the frequency at which it vibrates.
Consider the MA58MF14-7N device as an example sensor. This device has notches to identify the
directivity of the sensor. As Figure 4 shows, the MA58MF14-7N device has a wider view in the horizontal
direction than in the vertical direction which means it is an “Asymmetric” ultrasonic sensor. We will be
using “Horizontal” placement, which can provide wider coverage with fewer sensors, while narrower
vertical directivity improves sensor usability by limiting the effect of reflection from the ground.
4.2 Sensor Circuit Implementation
The transformer-driven configuration uses a center-tap transformer to boost the DC VPWR voltage to a
high-voltage sinusoidal driving signal at the secondary. The transformer installed on the daughter card is a
fixed-type EPCOS B78416A2232A003. The transformer-driven configuration is typically reserved for
closed-top transducers, which require higher driving voltages than open-top transducers. For this reason,
the closed-top 58.5-kHz Murata MA58MF14-7N is paired with the transformer. As can be seen in the
schematic above (which image?). As Figure 3 shows, overall there are very few components on the
sensor PCB and the major space-consuming components are the transformer, 100-µF capacitor (C11),
and the PG460-Q1 device.
For automotive applications, the Iso pole is a 75-mm diameter pole which needs to be detected within the
entire range from minimum to maximum distance. In this section, register settings and associated device
response are illustrated which can be used as a starting point for device configuration. These settings can
be further optimized based on algorithm and range requirements.
The PGA460-Q1 EVM GUI was used to optimize the device parameters. There are two threshold sets –
Preset 1 (P1) and Preset 2 (P2). P1 is optimized to detect objects in the range of 20 cm to 50 cm with less
number of pulses, lower current and shorter record time. P2 is optimized to detect objects in the range of
40 cm to 2.7 m, with a larger number of pulses, higher current, and a longer record time. With the settings
shown in Figure 5, the sensor was able to detect an ISO pole in the range as shown in Figure 4.
