Texas Instruments | Ultrasonic Terrain-Type and Obstacle Detection for Robotic Lawn Mowers (Rev. A) | Application notes | Texas Instruments Ultrasonic Terrain-Type and Obstacle Detection for Robotic Lawn Mowers (Rev. A) Application notes

Texas Instruments Ultrasonic Terrain-Type and Obstacle Detection for Robotic Lawn Mowers (Rev. A) Application notes
Ultrasonic Terrain-Type and Obstacle Detection for
Robotic Lawn Mowers
Esteban Garcia, Current and Position Sensing (CSPS)
Introduction
For robotic lawn mowers to be fully autonomous, they
must be equipped with sensors that can detect the
surrounding environment to bypass human operation.
Foreign objects on lawns must be avoided, as
collisions could cause expensive malfunctions.
Sensing technology must be able to perform reliably
outdoors and be able to detect thin objects such as
chair legs. Contactless ultrasonic sensing can be
considered for obstacle detection, as it can perform
efficiently while being exposed to harsh environmental
conditions.
Current robotic lawn mowers require a perimeter wire
around the desired area of operation, which can be
costly, to maintain a directed path. Without a system in
place to monitor where the robot is on the lawn, a
robot can potentially go into unwanted areas and
cause extensive damage. Ultrasonic sensing can
detect the type of terrain the robot is on, therefore
allowing the robot to efficiently traverse through grass
without the need of external mechanisms.
The PGA460, which is a fully-integrated ultrasonic
transducer driver and conditioner, was used to gather
accurate results for these experiments. It is compatible
with ultrasonic transducers that operate at a center
frequency between 30–80 kHz and 180–480 kHz.
Proper Insulation and Mounting
Robotic lawn mower applications require that
transducers are exposed to harsh environments,
therefore a closed-top transducer is recommended. It
is important to surround the transducer in a soft
material to allow vibration, followed by a firm material
to keep the transducer in place and negate any
exterior vibrations from affecting the device.
Figure 2 shows how a transducer must be insulated to
optimize the signal observed on the BOOSTXLPGA460 Graphical User Interface (GUI).
Ultrasonic Theory of Operation
Ultrasonic sensors use high-frequency sound waves
that are above the upper limit of human hearing and
can transverse through different mediums (which affect
the speed of sound) to determine where an object is
relative to the point of the transducer. The time it takes
for the emitted sound wave to reflect back to the
transducer is multiplied by the speed of sound and
divided by two for an accurate distance measurement.
It is important to note that, as frequency increases, the
resolution, directivity, and attenuation increase, but the
measurable distance decreases. Figure 1 shows how
an ultrasonic sensor uses sound waves to operate.
Figure 2. Proper Insulation
If the transducer is not properly mounted, the chances
of false positives increase, which decreases the
reliability of the system.
For obstacle detection, TI recommends to mount the
transducer on the front of the lawn mower, facing
outwards and slightly above the ground, to avoid
reflections from the ground that could produce false
positives. For terrain-type detection, TI recommends to
mount the transducer underneath the lawn mower with
the transducer pointing down. Figure 3 shows the
recommended transducer mountings for obstacle and
terrain-type detection on a robotic lawn mower.
Figure 1. Ultrasonic Sensor Theory of Operation
Figure 3. Transducer Mounting on Lawn Mower
SLAA910A – August 2019 – Revised October 2019
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Ultrasonic Terrain-Type and Obstacle Detection for Robotic Lawn Mowers
Copyright © 2019, Texas Instruments Incorporated
Esteban Garcia, Current and Position Sensing (CSPS)
1
www.ti.com
Obstacle Detection
Terrain-Type Detection
An object that comes within 15 cm of the mower
should be recognized as a threat, and the system
should instruct the robotic lawn mower to change
directions and find an unblocked path. For obstacle
detection, the 220-kHz Massa E188/220 closed-top
transducer is recommended because the device is
highly reliable within the 20-cm range.
Figure 6 shows how the 400-kHz Steminc
SMATR400H99XDA closed-top transducer performed
in terrain-type detection when the sensor was 5 cm
above the ground. A higher-frequency transducer is
used for this application to reduce ring time, which
allows for a shorter minimum detection range. Cement
had a large echo response because of the hardness of
the surface, whereas grass had close to no echo
response due to the diffuse reflection from the random
pattern surface type.
Figure 4 shows the echo response of the transducer
when an obstacle is not obstructing the device. The
initial peak observed is the ring time produced by
monostatic transducers. Ring time can be shortened
either by external hardware configuration or by
implementing a bi-static topology. Refer to the
PGA460 Ultrasonic Module Hardware and Software
Optimization for more information.
Figure 6. Terrain-Type Detection
Figure 4. Open Field Echo Response
Figure 5 shows the detection of objects commonly
found on lawns using the Massa E188/220. When an
object is within the 15-cm threshold, the transducer
receives an echo response. Unlike other sensing
technologies, ultrasonic sensing is able to reliably
detect targets in outdoor conditions, including thin
objects like chair legs and poles.
Device Recommendations
Because the robotic lawn mower is exposed to harsh
environments, including water and dirt, a closed-top
transducer must be implemented. For reliable short
range detection, the transducer must be able to emit
high-frequency sound waves.
For terrain-type detection, the minimum measurable
distance was 5 cm in these experiments. Therefore, a
400-kHz transducer was used to accommodate for low
range detection. Because obstacle detection requires
low to mid-range detection, a 220-kHz transducer was
used.
The PGA460 device is a highly-integrated system with
an on-chip ultrasonic transducer driver and signal
conditioner with an advanced DSP core. Table 1
includes a list of collateral resources to help ensure
proper usage of the BOOSTXL-PGA460, along with
different transducers that are available. For more
information, go to ti.com/ultrasonic.
Figure 5. Obstacle Detection Under 15 cm
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Table 1. Recommended Collateral
COLLATERAL
DESCRIPTION
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Application Note
PGA460 Ultrasonic Module Hardware
and Software Optimization
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Quick Start Guide
PGA460-Q1 EVM Quick Start Guide
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Excel Spreadsheet
PGA460: Air-Coupled Ultrasonic
Transducers & Transformers Listing
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Ultrasonic Terrain-Type and Obstacle Detection for Robotic Lawn Mowers
Esteban Garcia, Current and Position Sensing (CSPS)
SLAA910A – August 2019 – Revised October 2019
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