Page 811 - Sensors and Systems - North American Catalog
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Positioning Systems
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Positioning Systems
Introduction
Distance Measurement using PRT (Pulse Ranging Technology)
PRT sensors are either diffuse, meaning they emit light that is reflected from the object to be sensed back to
the sensor’s receiver, or retroreflective, meaning they emit light that is reflected from a corner-cube reflector
back to the sensor’s receiver. But unlike background suppression and multipixel array technologies, PRT
technology uses only one receiver element. A timer in the sensor determines how long it takes—after it emits
a short burst of light—for the light to make it from the sensor to the object
and then be reflected back to the sensor again.
Calculating this time duration and using the speed of light in air as a constant then determines the distance from the sensor to the object.
PRT is true “time of flight” (TOF) distance measurement. It is critical to note that the term “time of flight” is often misused in industry, as some manufacturers
improperly use it to describe a different method of distance measurement that is more accurately called phase correlation or is chipbased technology. In
phase correlation distance measurement, the reflected light is evaluated at the receiver, not based on the time it took to get from the sensor’s emitter to
the object and be reflected back, but rather by how much the phase angle of the light shifted as it traveled to and from the object. In other words, phase
correlation geometrically calculates the distance rather than directely measuring it, as is the case with PRT.
Phase correlation distance measurement has significant disadvantages compared to PRT distance measurement. Phase correlation sensors have a
weaker LED intensity since they are continuously on, resulting in shorter sensing distances and difficulty detecting dark objects. They are also limited to
short sensing ranges because they detect shifts to the reflected light’s phase angle, but anything greater than 360° can be misinterpreted by the sensor.
This also means they are prone to detecting background objects, especially those that reflect light at the same phase angle as light in the sensing range.
For example, whether reflected light is shifted in phase by 90° or by 450°, there is no way for a phase correlation sensor to differentiate the two.This results
in the detection of “phantom objects” in the background. Other strengths of PRT include its ability to ignore environmental conditions such as ambient light,
temperature, and target color, and measured values don’t drift as they do in phase correlation, even after prolonged use.
Emitted signal
Reflected signal
Time the pulse was sent
Time the pulse was received
S
=
c
•
t
L
2
c = 299.792.458 m/s
t
t
t
L
Pulse ranging technology
VDM Laser Distance Measurement Device
VDM laser sensors provide an analog output (4 to 20 mA) proportional to
how close or far an object or reflector is from the sensor, which makes them
an ideal solution for most types of measurement and inspection applications.
VDM28 and VDM100 series sensors use Pulse Ranging Technology (PRT)
and have long detection ranges, offering completely reliable measurement
results.With the use of intense impulses of laser light, PRT offers a high level
of reliability even in extreme ambient conditions with excessive ambient light
and dust.
Equipped with analog outputs and/or a maximum of two discrete outputs,
the sensors are used for presence checking, threshold monitoring, the
positioning of industrial trucks, measuring distances on monorail conveyors,
manufacturing systems, cranes and gantries, for stack height control, dip
monitoring, and much more.
VDM18, Laser triangulation for short distances
The VDM18 is based on the principle of the triangulation of
laser light and has measuring ranges of 30 mm to 100 mm
and 80 mm to 300 mm. The VDM18 not only offers a
space-saving housing but also optimal measurement
accuracy. Its analog output can be scaled to any distance
in the measuring range. Furthermore, it is fully equipped
with two configurable discrete outputs and is available with
an RS485 interface option.
VDM28, Sensing by ranging
The VDM28 is a universal measuring and monitoring
device. It can be used in a wide range of industries and
applications.With its PRT-based measurement principle, it
is used where sensors with background suppression have
reached their limitations. The VDM28 has a small,
extremely visible red light spot and always provides
accurate, reliable, clear, and reproducible results,
regardless of ambient conditions such as surface texture,
dark color, or ambient light. Thanks to the Teach-in option
or the IO-Link interface for service and process data, the
VDM28 is flexible for your application.
VDM100, Long distance, high accuracy measurement
The VDM100 works with outstanding precision
up to 300 m. It achieves a measuring frequency
of 1000 measurements per second using light
pulses in the range of nanoseconds.
Using noncontact technology that is completely
eye-safe, this modern and robust device is used
for fast and accurate positioning on stock feeders,
moving carriages, and gantry cranes. The Pulse Ranging
Technology (PRT) measuring method certainly demonstrates its strengths in
this area, even at travel speeds of up to 15 m/s.
The VDM100 is the perfect enhancement to the LS680 optical data coupler
that is already popular. Together these form a strong duo to fulfill your
requirements for a flexible automation solution.
