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Pepperl+Fuchs Blog

Photoelectric Sensors Use Time-of-Flight Measurement

Posted by Tom Corbett on Tue, Feb 04, 2014

Not just distance measurement, but customized detection!

You may already know that pulse ranging technology (PRT) is Pepperl+Fuchs’ advanced time-of-flight measurement used in photoelectric sensors like the popular VDM28 distance sensor. PRT provides continuous measurement of a target’s distance by using the speed of light as a constant, sending out extremely short but very powerful pulses of light, and calculating the duration between the time a pulse is sent and the time it is received.

Distance measurement sensors

The measured value is available as a 4 mA … 20 mA analog output from the VDM28 and can be easily scaled in a PLC to relate to specific distances. But what you may not know about PRT is how customizable it can make the VDM28’s discrete outputs.

With PRT and the VDM28, you can teach the discrete output to turn on or off at a certain distance, or turn on or off throughout a specific sensing range. Not only does this provide you with simple presence detection, but also advanced sensing capabilities like background and foreground suppression along with window detection and window suppression.

Let’s first look at the ability to use the VDM28 as a background suppression sensor. With this mode, you teach the VDM28 to detect targets up to a user-defined cut-off point, while objects beyond the cut-off are ignored. When taught this way, the VDM28 will ignore conveyor sidewalls, shiny background machine surfaces, and even people or vehicles beyond the cut-off point. The advantage of using the VDM28 as a background suppression sensor is the much longer range it offers as compared to traditional triangulation sensors.

Background suppression

Next let’s look at using the VDM28 as a foreground suppression sensor. This mode allows you to teach a defined deadband from the VDM28’s sensing face out to a user-defined point. Objects in the deadband area are ignored while targets beyond are detected normally. Like background suppression, the advantage of using the VDM28 as a foreground suppression sensor is the much longer sensing ranges that you can achieve. These long sensing ranges allow the sensor to be mounted out of harm’s way and ignore objects in the foreground, like rotating or moving machine parts and even people who may pass between the sensor and the target.

Foreground suppression

Now let’s look at using the VDM28 in window detection, which is a combination of background suppression and foreground suppression. Window detection allows you to program a user-defined range (or window) anywhere within the VDM28’s sensing range. Targets within the window area are detected while objects in the background and foreground are ignored. Window detection can be used for ensuring consistent web tension and monitoring web stock diameters. The advantage of the VDM28 over non-photoelectric devices (e.g., ultrasonic sensors) is the small, defined light spot of the laser light source.

Window detection

And last, let’s look at using the VDM28 in window suppression, which is the opposite of window detection. In window suppression, objects within a user-defined window area are ignored while targets in the background and foreground are detected. In the foreground, even targets as close as the sensing face are detected, thus eliminating the unusable area of the sensor. The advantage with window suppression is that targets with irregular or uneven surfaces are accurately detected even if no light is reflected back to the VDM28.

Window suppressionDownload: Selecting Distance-based Photoelectric Sensors

Topics: Photoelectric Sensors, Distance Sensors

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