Displacement Sensors / Measurement Sensors
What Is a Displacement Sensor?
A Displacement Sensor is a device that measures the distance between the sensor and an object by detecting the amount of displacement through a variety of elements and converting it into a distance. Depending on what element is used, there are several types of sensors, such as optical displacement sensors, linear proximity sensors, and ultrasonic displacement sensors.
What Is a Measurement Sensor?
A Measurement Sensor is a device that measures the dimensions of an object by converting changes in amount of light into electrical signals when the object interrupts a wide laser beam.
Features
1. A physical quantity of an object can be measured.
A Displacement Sensor measures and detects changes (displacement) in a physical quantity.
The Sensor can measure the height, width, and thickness of an object by determining the amount of displacement of that object.
A Measurement Sensor measures the position and dimensions of an object.
2. Physical quantity output is also possible in addition to ON/OFF signal output.
Analog output of physical quantities (current output or voltage output) can also be performed (excluding some models).
Some models also support digital (serial) communications.
Operating Principles and Classification
Displacement Sensors
1. Optical Displacement Sensors
Triangulation Measurement Method
These sensors use a triangulation measurement system.
Some sensors employ a PSD, and others employ a Imaging Device (CCD and CMOS) as the light receiving element.
• PSD Method
Light from the light source is condensed by the lens and directed onto the object.
Light reflected from the object is condensed onto a one-dimensional position sensing device (PSD)* by the receiving lens. If the position of the object (the distance to the measuring device) changes, the image formation positions on the PSD will differ and the balance of the two PSD outputs will change.
If the two outputs are A and B, calculate A/(A + B) and use appropriate values for the span coefficient “k” and the offset “C” as shown below.
* PSD: Position Sensitive Device
• Imaging Device Method (CCD Method and CMOS Method)
Compared with a sensor that employs the PSD method, a sensor that employs a CMOS (CCD) as the light receiving element provides a more accurate measurement of displacement without being affected by surface color and texture of objects.
The sensor detects the amounts of light on individual pixels in the CMOS (CCD) and converts them into a distance when a spot beam that reflects off of the surface of the object is projected onto the light receiving element.
Differences between CMOS and CCD
CCD stands for Charge Coupled Device, and CMOS stands for Complementary Metal Oxide Semiconductor
Regular Reflection Model and Diffuse Reflection Model
Regular reflection
A specular reflection is produced, such as from a mirror surfaced or glossy object.
Diffuse reflection
A beam is reflected in all directions from an object with a standard surface.
Regular reflection model
Light from the object is directly received by regular reflection, and stable measurement is possible of metal and other objects with a glossy surface.
Diffuse reflection model
A light beam is projected perpendicularly onto the surface of the object, and the diffuse light that is reflected back is received for a wide measurement area.
Regular-reflective Sensor Heads receive direct light from regular reflections off the object. Stable measurements can be achieved for objects made of metal or other materials with a glossy surface, but there is a narrower measurement range than Diffuse-reflective Sensors.
Diffuse-reflective Sensors use a Sensor Head tilted at an angle to receive regular-reflection light. This allows the Sensor Head to be placed at a distance away from the object.
Line Beams and Spot Beams
Line Beam Model
This model measures the average displacement within a line beam. Depending on the measurement conditions, this model provides stable measurements without being affected by bumps or unevenness on the object surface.
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