R91 Lidar Annex Technology Description
Scanning Lidar Speedmeter
1.0
Technology overview
A scanning light detection and ranging (lidar) speedmeter is a system which is
operated from a stationary location on the side of a roadway. The scanning lidar system
determines the distance to a target vehicle by measuring the time it takes a pulse of laser light
to travel to, and reflect back from, the target. Taking successive distance measurements over
time, the scanning lidar can then calculate the speed of the vehicle. At the same time, the
emitted laser energy is scanned horizontally over multiple lanes of a roadway. The resultant
fan of laser pulses created by the scanning action allows for the concurrent profiling and
tracking of multiple target vehicles on the roadway.
2.0
Technology details
2.1
Range measurement
A solid-state laser diode is electrically pulsed causing short emissions of infrared laser
light. These pulses of laser light travel to a target which ultimately disperses the laser energy
and reflects a small portion of the energy back to the emitting device. A receiving
photodiode in conjunction with the laser emitter detects that portion of laser light reflected
back from the target. Since light travels at a known and constant velocity, the time it takes
the laser energy to travel to, and return from, the target is proportional to two times the
distance to the target. Thus distance to the target can be calculated as:
𝒅=
𝒕×𝒗
𝟐
Where:
𝑑 = 𝑑𝑖𝑠𝑡𝑎𝑛𝑐𝑒
𝑡 = 𝑡𝑖𝑚𝑒 𝑜𝑓 𝑓𝑙𝑖𝑔ℎ𝑡 of a pulse of laser light
𝑣 = 𝑣𝑒𝑙𝑜𝑐𝑖𝑡𝑦 𝑎𝑡 𝑡ℎ𝑒 𝑠𝑝𝑒𝑒𝑑 𝑜𝑓 𝑙𝑖𝑔ℎ𝑡 (~2998 × 108 𝑚𝑒𝑡𝑒𝑟𝑠 𝑝𝑒𝑟 𝑠𝑒𝑐𝑜𝑛𝑑)
The infrared laser energy emitted is invisible to the human eye and does not cause
driver distraction. The emitted laser light is defined as IEC 60825 Class 1, which is
considered eye-safe under all conditions.
Documentation
Unit
Laser
Photo-receiver
laser pulse
back
reflected
laser pulse
distance
target
Time-of-flight distance measurement
1
2.2
Laser scanning
The discrete pulses of laser energy emitted by the laser diode are focussed onto an
oscillatory rotating mirror within the scanning lidar system which redirects the emitted laser
pulses out across a wide scanning angle. The laser pulses then travel to a target and a portion
of the energy is reflected back to the mirror and the receiving photodiode. The time-of-flight
calculation of the laser pulses provides the distance to the target while the position of the
rotating mirror provides an angular position on the target. Subsequently emitted laser pulses,
together with the differing mirror positions, provides the target dimensions. This scanning
process allows for the profiling and tracking of multiple targets and is only limited by the
system’s operational measurement distance and by the rotating mirror’s scanning angle.
de /
r dio
Lase diode
o
Ph o t
Laser energy and scanning angle created by the rotating mirror
Typical scanning angles and ranges
2
2.3
System function
The laser pulses which are reflected back to the photodiode in the scanning lidar
system are filtered and processed by a special detection algorithm. The velocity ( v ) of the
moving target is calculated from several laser reflections detected within the measurement
area.
(𝑥1 × 𝑐𝑜𝑠(𝑎1 )) − (𝑥2 × 𝑐𝑜𝑠(𝑎2 ))
𝑣=
𝑡1 − 𝑡2
The lidar speedmeter, or measurement unit, is capable of tracking multiple targets at
the same time. However if a speed violation is detected, clear substantiation of the violation
is ultimately required. For this reason a documentation unit is operationally combined with
the measurement unit. The documentation unit monitors the optical characteristics of the
scanner (i.e. mirror position) and can accurately identify the offending vehicle.
field viewing angle
laser scanning angle
Measurement area
Measurement area
Multiple targets within the scan area
3
Multiple targets with vehicles changing lanes
2.4
Definitions and descriptions
2.4.1
Eye-safety:
The scanning lidar system must meet laser class certification testing which ensures eye safety
for use in public.
2.4.2
Full scanning angle:
The full scanning angle is limited by the rotation angle of the mirror.
2.4.3
Identification marking:
The documentation unit takes a picture of the violation scene and labels the offending
vehicle with a clear marking.
2.4.4
Installation setup:
Due to the relatively small scanning area, the scanning lidar system must be properly
aligned with the level of the road. If misaligned, the scanning lidar system will not be
able to detect passing vehicles. Thus, proper site setup includes:
•
the installation height of the scanning lidar system;
•
the number of lanes of traffic to be monitored;
•
the width of each lane on the roadway; and
•
the distance offset between the scanning lidar system and the first traffic lane
2.4.5
Measurement area:
The scanning fan is a limited area which is adjusted to the road level. This limited
area assures that the laser pulses remain within the area of interest where the target
vehicles are located on the roadway.
2.4.6
Multi-target-tracking algorithm:
All laser pulses reflected back to the scanning lidar system are processed with an
algorithm which filters the returns and is capable of calculating an unlimited number
of objects. The length, width, and position of all moving objects within the target area
are continually monitored.
4
2.4.7
Operation distance of the scanner:
The operation distance is set by the energy level of each laser pulse from the laser
diode. The laser energy level must not exceed eye safety limits.
2.4.8
Scanning resolution:
Resolution of the scanner is obtained from the mirror rotation step size (angle of
increment) and the divergence of the laser beam. The divergence angle is an inherent
characteristic of a laser beam in which the beam increases in width with the distance
travelled.
3.0
Measurement validation
The following steps are used to validate the scanning lidar system measurement
accuracy:
•
Adjustment of the scanning plane (angle and height)
•
Adjustment of zero level of the scanning plane (rectangular to the scanning
plane)
•
Equality of the scanning angle (degree and equidistance)
•
Divergence of the laser beam
•
Accuracy of the distance measurement
•
Power output of the laser diode
•
Time trigger and clocking of the movement of the mirror
•
Compensation for different reflecting capacities
•
Alignment of the laser scanner to the associated documentation unit
5