Operating Principle
Semiconductor laser displacement sensors and meters
Semiconductor laser displacement sensors and meters are comprised of a light-emitting element and a position sensitive detector (PSD) and detect targets using triangulation. A semiconductor laser is used as the light emitting element. A lens focuses the beam on the target. The target reflects the beam back through the lens where it is focused on the position-sensitive detector (PSD), forming a beam spot. The beam spot moves as the target moves. Displacement can be determined by detecting the movement of the beam spot.
Surface Reflection and Sensor Head Structure
The surface-reflected light from a target object consists of a diffuse reflection and a regular reflection. The quality of reflected light differs depending on material and surface conditions of the target object, and a regular reflection is dominant for a specular body while a diffuse reflection is dominant for objects with a normal surface. Because specular bodies or close-to-specular bodies are usually measured with a high accuracy displacement meter, Keyence adopted the sensor structure which accepts regular reflections for this type of displacement meter. On the other hand, for the displacement meters that measure normal objects, Keyence adopted the sensor structure which accepts a diffuse reflection component in order to expand the measuring distance.
| Sensor head configuration |
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| Surface conditions and reflections |
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| Diffuse reflective surface |
Specular surface |
| Sensor head design and operating principle |
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| Diffuse-reflective receiver(Long distance, high-resolution types) |
Specular-reflective receiver(Ultra-high-resolution type) |
Effective Use of Laser Displacement Sensor
Mounting
Measuring displacement of target with height differences
When measuring the displacement of a target with height differences, mount the sensor as shown to minimize interference caused by target edges. |
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When mounting the sensor head to a wall surface
Mount the sensor head so that the light reflected from the wall does not enter the receiver lens. When the wall is shiny, painting the surface matte black is effective. |
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Measurement of targets with sharp variations in color or material
KEYENCE's displacement sensors are designed to measure regardless of target color or material. However, when measuring a target with sharp changes in color or material mount the sensor head at the edge of the change, as shown, to minimize error. |
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Displacement measurement of mirror-surfaced targets
When measuring shiny metal or mirror-surfaced targets, the specular-reflective component of a light beam either blocks all light beams, or prevents the sensor from receiving the level required for stable operation. This can be corrected by tilting the light receiving portion of the sensor head. If the sensor head cannot be moved, or if higher measurement accuracy is required, use the LC Series specular reflective sensor. |
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Displacement measurement of black targets
When the target is a low-reflectance object such as a black object, light quantity reflected from the target is small, and therefore the signal from the position sensitive detector (PSD) is also small. As a result, the signal-to-noise ratio will drop, decreasing the resolution. Mounting the sensor head as shown below will increase received light quantity, increasing the resolution.
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As shown on the left, tilt the sensor head in the direction of specular reflection. The light quantity will increase, thus increasing the resolution.
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Place the sensor head closer to the target within the measuring range. The light quantity will increase, increasing the resolution.
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| Displacement measurement of a target in a hole |
| When measuring the displacement of a target in a small diameter hole, mount the sensor head so that the emitted and reflected light are not blocked by the wall surface of the hole. |
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| Eccentricity measurement |
| When measuring the eccentricity of a revolving target, mount the sensor as shown to minimize the effect of vertical oscillation or displacement. |
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