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Texas Instruments Ultrasonic Floor-Type and Cliff Detection on Automated Vacuum Robots Application notes
Ultrasonic Floor-Type and Cliff Detection on Automated
Vacuum Robots
Esteban Garcia, Current & Position Sensing (CSPS)
Introduction
Hardware Configuration
Autonomous vacuum robots today use sensors for
obstacle detection, and there is a drive to integrate
other additional sensing technologies for floor-type and
cliff detection. Ultrasonic sensing can be used in these
applications due to its ability to distinguish between
different material types and its immunity to smoke and
dust while providing a reliable response.
The BOOSTXL-PGA460 was used in all the
experiments listed in this document. The PGA460 is
an ultrasonic transducer driver and signal conditioner.
Currently, separate cleaning robots are required to
vacuum and mop, which doubles the cost of the
solution. To integrate these functions into one system,
it is important for the cleaning robot to know what type
of floor it is on. Ultrasonic technology is able to
determine the hardness of a target, based on the
strength of the received echo amplitude observed by
the sensor. Harder surfaces like tile reflect a stronger
echo than softer surfaces like carpet.
Physical barriers are needed to prevent service robots
from falling down a flight of stairs as these drops can
lead to costly malfunctions. One solution is to
implement cliff detection sensors to avoid these
dangers. The immunity of ultrasonic sensors against
dust makes it a reliable solution for cliff detection
applications, considering that the sensor will be
mounted underneath the robot.
Ultrasonic Theory of Operation
Ultrasonic sensors emit high-frequency sound waves
above the upper limit of human hearing. The time it
takes for the emitted sound wave to bounce back to
the transducer determines the distance between the
object and the sensor as shown in Figure 1. The
distance is calculated by multiplying the time of flight
by the speed of sound and dividing that by two.
To optimize the performance of the system, it is
important to consider the frequency and topology of
the transducer. As the frequency increases, the
resolution, directivity, and attenuation increase while
the measurable distance decreases. Topology is
whether the transducer has the emitter and receiver
combined (monostatic) or are independent as a
dedicated pair (bistatic).
Table 1 shows the advantages and disadvantages of
each transducer type. Table 2 shows how topology
affects the effectiveness of your transducer selection.
Table 1. Transducer Type
TRANSDUCER
TYPE
PREFERRED
DRIVE MODE
ENVIRONMENTAL
CONDITIONS
COST
Closed-Top
Transformer
Drive
Applicable in harsh
environments
$$
Open-Top
Direct Drive
Limited to protected
environments
$
Table 2. Transducer Topology
TOPOLOGY
RANGE
COST
Monostatic
Mid to Long Range
$
Bistatic
No blind-zone allows for
near 0cm detection
$$
Figure 2 shows how the BOOSTXL-PGA460 was set
up to simulate these applications.
Figure 1. Ultrasonic Sensor Theory of Operation
SPACER
SLAA909 – July 2019
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Figure 2. BOOSTXL-PGA460 Test Setup
Ultrasonic Floor-Type and Cliff Detection on Automated Vacuum Robots
Copyright © 2019, Texas Instruments Incorporated
Esteban Garcia, Current & Position Sensing (CSPS)
1
www.ti.com
Floor-Type Detection
Cliff Detection
Figure 3 shows a high level representation of floortype detection by using ultrasonic sensing. For hard
surfaces, the echo response saturates whereas soft
surfaces have a weak echo response. The 40 kHz bistatic PUI Audio UTR-1440K-TT-R transducer pair was
used for this experiment.
It is imperative for the vacuum robot to detect when it
is approaching a cliff so that the system can change
directions and prevent a sudden fall.
The transducer is mounted 1 cm above the floor,
therefore the robot will halt and change directions if a
distance of more than 1 cm is detected. Figure 5
shows the echo response of hardwood at a distance of
1 cm from the transducer pair, along with different floor
samples that are each 20 cm away from the
transducer pair, which is the standard stair height in an
American household. Under a bistatic configuration
with a dedicated transmitter and receiver, there is no
observable transducer ring time.
Because of the transducer's wide field of view, it was
unable to detect a cliff until the sensor passed over the
hardwood edge by 2 cm, therefore a transducer with a
narrower field of view is recommended for faster cliff
detection.
Figure 3. Floor-Type Detection
Figure 4 shows the echo responses across hardwood,
tile and, carpet types that are 1 cm away from the
surface (due to the limited spacing between where the
sensor is mounted and the floor). The graph depicts
how the echo response across each surface varies
slightly. For example, carpet is an extremely porous
surface, therefore more sound is absorbed and not
reflected back. Tile is a hard material that allows
sound waves to reflect off more effectively, producing
a higher amplitude response. With this information, the
vacuum robot is able to determine the floor type it is
on. For this experiment, the MuRata MA40S4S
transmitter and MuRata MA40S4R receiver were used
in a bi-static configuration to allow a detection range
less than 5 cm because there is no blind-zone.
Figure 5. Cliff Detection at 20 cm
Device Recommendations
Ultrasonic sensing benefits vacuum robot applications
because it enables both cliff and floor-type detection.
TI recommends that the designer use a closed-top
transducer in a bistatic configuration in these
applications, because the transducer will be exposed
to dust and dirt and will require short range detection.
Table 3 has a list of collateral resources to help ensure
proper usage of the BOOSTXL-PGA460, along with
different transducers that are available. For more
information, visit ti.com/ultrasonic.
Table 3. Recommended Collateral
Figure 4. Echo Response Across Different
Surfaces
2
COLLATERAL
DESCRIPTION
Application Note
PGA460 Ultrasonic Module Hardware and Software
Optimization
Quick Start Guide
PGA460-Q1 EVM Quick Start Guide
Excel Spreadsheet
PGA460: Air-Coupled Ultrasonic Transducers &
Transformers Listing
Ultrasonic Floor-Type and Cliff Detection on Automated Vacuum Robots
Esteban Garcia, Current & Position Sensing (CSPS)
Copyright © 2019, Texas Instruments Incorporated
SLAA909 – July 2019
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