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Texas Instruments Accurate distance measurements with a highly integrated ToF position sensor Application notes
Accurate distance measurements with a highly integrated
ToF position sensor
In many applications, it is not possible to measure the
distance to a target by establishing a physical contact.
Typical examples include measuring the presence of
objects on a conveyor belt in logistic centers, ensuring
safe distances around moving robot arms, and
establishing the position of assets from humans or
robots in a warehouse. Time of Flight (ToF) position
sensors help you measure the distance by using the
time it takes the light to travel back and forth to the
object. The OPT3101 is a good example of a highspeed, high-resolution, AFE for continuous-wave, fullyintegrated ToF-based position sensor. The sensor has
16-bit distance output at 15-m unambiguous range.
For more information on the OPT3101, see OPT3101
ToF-Based Long-Range Proximity and Distance
Sensor AFE. The most important considerations for
position sensors are:
• Distance
• Accuracy
• Repeatability
• Response time
The measured distance range is from 0.43 m to 2.5 m
with a white wall as a target. The OPT3101's
maximum sample rate is 4 kHz, resulting in the fastest
response time of 1/4 kfps = 0.25 ms. This application
uses 32 samples, where you achieve a 125 fps output
data rate. The response time based on the data rate of
125 fps is 1/125 fps = 8 ms. Use the above set-up to
measure the OPT3101's performance. Figure 1 shows
the measured versus actual distance plot is linear.
Figure 1. Measured Distance
2500
2000
Meas. Distance (mm)
Introduction
0
SPECIFICATIONS
0.43 to 2.5 m
Accuracy
0.78% @ 1 m
Repeatability
1.02 mm @ 2 m
Response Time
0.25 ms minimum
0
500
1000
1500
Actual Distance (mm)
2000
2500
D004
The measurement accuracy is a significant parameter
for the position sensor. Figure 2 shows that the
OPT3101's distance measurement error is 0.1 mm at a
distance of 2.5 m.
Table 1. Key System Specifications
Distance
1000
500
For this application, the dedicated distance range is up
to 2.5 m.
PARAMETER
1500
Figure 2. Distance Measurement Error
30
In this application, the OPT3101 EVM measures the
distance to a flat 1.5m x 2 m white wall. The wall is
effective under a small (±3 degrees) Field of View
(FoV). To prevent the FoV from falling to the ground,
mount the OPT3101 EVM on the edge of a movable
desk. Use a laser distance measurement to get the
actual distance data. The OPT3101 has an internal
ambient cancellation block that allows for great
performance in different ambient conditions. This
module uses two components to transmit and receive
information, which is illumination driven by an
OPT3101 internal LED driver and photodiode. The
OPT3101 EVM takes the test data and the TI Latte
records the distance, accuracy, and amplitude.
Test Results
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Distance Error (mm)
Implementation
20
10
0
-10
0
500
1000
1500
Actual Distance (mm)
2000
2500
D002
Figure 3 shows the distance measurement error
percentage. The tests were completed in indoor
conditions. Within 2.5 m, the distance measurement
error is within 1.6% of the actual distance.
Accurate distance measurements with a highly integrated ToF position
sensor
Copyright © 2019, Texas Instruments Incorporated
1
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Figure 3. Distance Measurement Error %
Figure 5. Received Signal
2%
0
Received Signal (-dBFS)
-5
Distance Error (%)
1%
0
-1%
-10
-15
-20
-25
-30
-2%
-35
0
500
1000
1500
Actual Distance (mm)
2000
2500
Figure 4 shows the standard deviation of the distance
measurement for the 32 frames. The OPT3101 gives
the flexibility to design a system with various frame
options, so you can easily customize the application
and the use case. An increase in average frames
decreases the standard deviation in measurements
while a decrease in average frames increases the data
collection rate.
500
1000
1500
Actual Distance (mm)
2000
2500
D005
14
ToF technologies independently determine phase and
amplitude, resulting in color independency. Color
independency measures the distance independent of
object color. Repeatability is the degree of consistency
between measurement results by measuring the same
object multiple times under the same conditions in a
short time interval. To test repeatability, the OPT3101
EVM and the desk are moved back and forth to the
same position in a short time interval. The results are
shown in Table 2. For example, the repeatability is
1.02 mm at 2 m.
12
Table 2. Repeatability
Figure 4. Distance Standard Deviation
Distance STD (mm)
0
D001
10
8
6
4
2
Actual
Distance
(mm)
1 (mm)
2 (mm)
500
496.7
800
800.7
1200
3 mm)
4 (mm)
5 (mm)
Repeatability
(mm)
495.6
497
495.7
496.2
0.61
800.2
800.2
800.6
800
0.3
1216.2
1216.3
1215.7
1216.7
1215.8
0.4
1500
1523.3
1523.3
1522.2
1523.9
1521.5
0.97
2000
2020.7
2020.8
2022.6
2020.4
2019.9
1.02
2500
2498
2497.6
2499.8
2498.7
2500.2
1.12
0
0
500
1000
1500
Actual Distance (mm)
2000
2500
D003
The received signal is shown in Figure 5. A longer
distance results in a lower received signal and more
noise.
2
Accurate distance measurements with a highly integrated ToF position
sensor
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SBOA342 – February 2019
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