laser displacement sensor technology book · 6 lk-g series technology book 2 stability of received...

LASER DISPLACEMENT SENSORTECHNOLOGY BOOKI n n ova t i v e M e a s u r e m e n t A c c u ra c y a n d S t a b i l i t y

2

BASICS OF LASER DISPLACEMENT SENSORS

LK-G SERIES TECHNOLOGY BOOK

Basic principle of triangulation1

Light-receivingelement

Received light waveform





Measurement at a reference distance Measurement at a longer distanceMeasurement at a shorter distance

Transmitterlens

Transmitterlens

Transmitterlens

Semiconductorlaser

Receiver lens



Semiconductorlaser

Receiver lens



Receiver lens




As the figures above show, a laser beam emitted from the semiconductor laser is applied to the target. The light reflected from the target is collected by the receiver lens and focused on the light-receiving element. When the distance to the target changes, the angle of the reflected light passing through the receiver lens changes, and the light is focused at a different position on the light-receiving element.

The measurement accuracy of the laser sensor utilizing triangulation is greatly affected by the following two factors:

This section explains how these factors affect measurement accuracy.

Optical design Stability of received light intensity and waveform

Optical design (receiver lens aberration)2 - 1

Measurement accuracy influences2

With a single receiver lens, the spot diameter formed on the light-receiving element becomes larger when the measuring distance is shorter or longer than the reference distance, due to the lens aberration. When the spot diameter on the light-receiving element becomes larger, the measurement accuracy factors, such as "resolution", "linearity", and "scan resolution", become poorer than those obtained at the reference distance. Consequently, it is necessary to develop an optical design which ensures a constant spot size regardless of the measuring distance.

Semiconductorlaser

STABLE MEASUREMENT ACCURACY – POINT 1 The high-precision Ernostar lens solves the problem!

It is necessary to develop an optical design that ensures a constant spot size on the light-receiving element.

Check page 4!

3

As the figures above show, the condition of the beam spot formed on the light-receiving element changes depending on the surface condition of the target, which affects the measurement accuracy.

To achieve stable measurement accuracy with laser displacement sensors, it is necessary to obtain optimal received light intensity for the CCD. There are various methods for adjusting received light intensity. For the laser displacement sensors used in actual production lines, not only is the ability to adjust the received light intensity important but the "speed of the adjustment" is also important.

In summary, the measurement accuracy of laser displacement sensors is greatly affected by an optical design (lens aberration) and the stability of the intensity and waveform of the light focused on the light-receiving element. As for the LK-G Series, unprecedented high accuracy is achieved through various techniques.

Adjustment of laser emission power

Adjustment of laser emission time

Adjustment of sensitivity of light-receiving element

Adjustment of light receiving time (exposure time)

❙ Examples of beam spot conditions (received light waveform)

❙ Factors which affect the beam spot condition (received light waveform)

❙ Measures to ensure sufficient received light intensity

The subsequent sections introduce each of the techniques

Color (color irregularity), luster, gloss of a target

Surface condition of a target(roughness, tilt)

Material of the target (such as translucent plastics)

Stability of received light intensity and waveform2 - 2

As described in "1. Basic principle of triangulation", a laser displacement sensor calculates the distance to a target by focusing the light reflected from the target on the light-receiving element.If the light reflected from the target changes due to the color, gloss, surface condition (roughness, tilt) of the target surface, the condition of the beam spot formed on the light-receiving element (received light waveform) also changes. Such a change in the beam spot condition (received light waveform) affects the measurement accuracy of laser displacement sensors.

STABLE MEASUREMENT ACCURACY – POINT 2ABLE control solves the problem!

Check page 6!It is necessary to be able to obtain optimal received light intensity for the CCD. This adjustment should be done quickly.

Received light waveform obtained with a white ceramic target

Received light waveform obtained with a black rubber target

Received light waveform obtained with a mirror-surfaced target

Received lightwaveform



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Optical design1

The lens unit consisting of four lenses minimizes the influence of lens aberrations. The spot formed on the CCD remains the same size regardless of whether a target is at the reference distance or at other distances.

The high-precision Ernostar lens1 - 1

A new receiver lens has been designed to collect the light reflected from a target and focus it on the Li-CCD. A newly developed high-accuracy Ernostar lens system drastically reduces spot distortion caused by aberrations. Moreover, the sensor head-integrated, special die-cast housing provides high rigidity.








Li-CCD

Structure of high-accuracy Ernostar lens

An Ernostar lens is a lens unit which is used as a high-quality camera lens and consists of four lenses. It provides high image-forming ability with minimized aberrations.

What is an Ernostar lens?

TECHNOLOGIES FOR IMPROVING MEASUREMENT CAPABILITIESThe LK-G Series has achieved unprecedented high accuracy through KEYENCE's techniques optimizing "optical design" and "stable received light intensity and waveform", which are two major factors affecting measurement accuracy.This section introduces these technologies.

5

Linearity range of ±0.1%(conventional models)

The new housing design reduces the reflection on the filter glass surface in the receiver unit to allow the CCD to reliably receive the light reflected even from a distant target.

Since a CCD has digital output characteristics for each pixel, the errors caused by gradation outputs generated at the edge of the pixel were the barrier to higher accuracy. As a countermeasure, KEYENCE has developed an Li-CCD that can detect the position of reflected light in a pixel, achieving excellent accuracy that is two times higher than conventional models. In addition, the dedicated sensor design has achieved a speed that is 25 times faster and sensitivity 10 times better than conventional models.

When delta cut is used

❙ Comparison with conventional model (10 times higher sensitivity achieved)

CCD

Filter glass

High-accuracy Ernostar lens

Sharp focus

When delta cut is not used

Out of focus

CCD

Refraction caused by the angle between the filter glass and optical axis

Attenuation caused by surface reflection

Light received at the center of a pixel

Light received near the edge of a pixel

Light received by the adjacent pixel

Reflected light

CCD

A conventional CCD output

Li-CCD output

The position of the reflected light in a pixel cannot be detected. As a result, gradation changes are generated near the edge of the pixel, resulting in measurement errors.

The output of the adjacent pixel changes according to the position of the reflected light in a pixel, providing more linear characteristics.

❙ Output comparison with a conventional CCD

* LK-G155/G405/G505 (Series)

Linearity of ±0.05%, which is two times higher than conventional models, has been achieved!

Linearity obtained with a white ceramic target

❙ Measurement data sample

Newly developed delta cut technology!1 - 2

Newly developed Li-CCD (Linearized CCD)!1 - 3

Linearity data obtained with a white ceramic gauge (Typical)

Measurement position (μm)

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Stability of received light intensity and waveform2

With the "ABLE technology", KEYENCE's LK-G Series laser displacement sensor has achieved extremely high accuracy.

The received light intensity which offers optimal sensitivity for the CCD is stored in the controller as "optimal light intensity". (This is necessary because accuracy deteriorates not only when the light intensity received by the CCD is low but also when it is too high, due to the saturation of the received light waveform which makes the spot on the CCD larger.)

"P x T x G" is calculated, where P is the laser power, T is the emission time, and G is the amplification factor, and the received light intensity (W) for the case is measured. Then the received light intensity (W) is compared with the optimal intensity (Ws) to calculate the multiplication factor.

P

T

Ws: Optimal intensity

W: Received light intensity

Difference from the optimal intensity

1STEP

ABLE control (outline)2 - 1

Even with a black rubber target, which reflects almost no light, the ABLE control provides the same level of received light intensity as a white ceramic target.

❙ Effect of ABLE control <Received light waveform obtained with a black rubber target>

ReflectanceEmission power

High Low

Laserpower: Low

Laser power:

High

Emission time: Short Emission time: Long

Mirror-surfaced plate Black rubber

LK-G SeriesConventional model

Laser power

8 times

-

Emission time

1662 times (0.6 to 997 μs)

150 times (3.2 to 480 μs)

Adjustment range

13296 times

150 times

❙ Range of light intensity adjustment (90 times max. compared with conventional models)

LK-G SeriesConventional model

Sampling time

20μs

512μs

Adjustment time

0.06ms

7ms

❙ High-speed real-time control (120 times compared with conventional models)

ABLE(ABLE=Active Balanced Laser control Engine)

This is a function that senses the surface of a target and adjusts the intensity of laser light to an optimal level. The high performance CPU allows the real time control of the three elements, laser emission time, laser power, and gain (CCD amplification factor).

When ABLE is not used When ABLE is used

ABLE control mechanism2 - 2



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Effects of ABLE control2 - 3

Not all targets have a flat and level surface. When a target with a curved or tilted surface is measured, the received light intensity decreases. Excessively low intensity may disable measurement. The ABLE control is effective in this case.

❙ Angle characteristics

With a moving target, the received light intensity changes affecting scanning resolution. The laser reflection from connector pins or patterned glass boards, moving at high speeds, can change to the point where the measurement may not be performed properly. The ABLE control corrects these problems.

❙ High-speed

When ABLE is not used [Conventional model] When ABLE is used [Ultimate laser displacement sensor]

When ABLE is not used [Conventional model] When ABLE is used [Ultimate laser displacement sensor]

10 mm diameter pin gauge

IC pins

<Shape measurement of a 10 mm diameter pin gauge>

<Measurement of IC pins warpage>

The laser emission time is adjusted in units of "100 ns".Such fine adjustment ensures stable measurement of every kind of target.

Emission time adjustment resolution

All of three parameters of laser power, emission time, and amplification factor, are digitized. A high-speed CPU processes the digital data for real-time calculation and correction to control the optimal setting instantly. This high-speed processing has achieved higher accuracy for all targets.

Real-time control by a high-speed CPU

KEYENCE original algorithm provides feedback of light intensity on the non-linear light-receiving characteristic of the CCD.This ensures stable measurement of every target.

Original algorithm for optimal intensity adjustment

More effects of ABLE control2 - 4

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[mm

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Position [mm]

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[mm

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Position [mm]

Conventional modelTarget shape

Ultimate laser displacement sensor

Moving distance [μm] Moving distance [μm]

Each IC pin has a different width.Not a single pin was detected properly.

Since the sensor cannot follow the quick changes in the received light intensity, it cannot detect the first edge of the IC pin, resulting in lack of data.

The high-speed response of the light intensity control allows proper measurement of all IC pins without lack of data.

W’: Received light intensity

The result of P x T x G and the received light intensity is stored for each sampling and compared with the optimal intensity.

The multiplication factor calculated in STEP 1 is fed back to decide the emission time of the next sampling, and the resulted received light intensity (W') is compared with the optimal intensity. Finally, fine adjustment is made during the next sampling.

2STEP

P

T’

Ws: Optimal intensity

8


When the sensor head is positioned perpendicular to the target

When the laser application angle is adjusted

Transparent target

Frontsurface

Backsurface

Light passing through the target

Specular reflection

Transparent target

Frontsurface

Backsurface


Receiver lens

Specular reflection


Technology for improving ability for transparent target measurement (Multi-ABLE control)1

TECHNOLOGIES FOR IMPROVING ABILITIES FOR VARIOUS TARGETSConventional laser displacement sensors may fail to measure targets which do not cause diffuse surface reflections, such as transparent glass or translucent plastic targets. The LK-G Series can offer accurate measurement even for these targets using its special measurement algorithms.

Sensor head position1 - 1

When a transparent target is measured from a normal position, the laser beam passes through the target, resulting in failed measurement. The measurement may also fail if there is another object in the background of the target, due to the difference in the intensity of the reflection from the front surface and back surface. This section introduces techniques and measurement algorithms effective for these targets.

When the laser beam is applied perpendicular to the transparent glass target as shown on the left, no light reflects from the glass surface to the receiver, resulting in failed measurement.

The receiver cannot receive the reflected light.

Light-receiving position on the CCD

No waveform because the receiver cannot receive the reflected light.

When the laser application angle to the transparent glass target is adjusted as shown on the left, the light reflects from the glass surface to the receiver, resulting in a successful measurement.

Measurement is possible because the receiver can receive the reflected light.

The receiver receives the light reflected from both the front and back surfaces.

Light reflected from the front surface of the glass target

Receiver lens

Semiconductorlaser



Transmitterlens

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Light reflected from the back surface of the glass target

9

Transparent target

Frontsurface

Backsurface

1. It is necessary to correct light intensity individually for each peak of the received light waveform.

2. It is necessary to be able to specify the peak(s) used for measurement in the received light waveform.

To ensure stability of the measurement accuracy for a transparent target:

When a metal object exists in the background

When a tinted glass exists in the background

Transparent target

Frontsurface

Backsurface


Receiver lens

Specular reflection


Receiver lens

Specular reflection

metal objec Light-receivingelement



Received light waveform of transparent target measurement1 - 2

As described in the previous section, a transparent glass target generates two types of reflection, the reflection from the front surface and the one from the back surface, resulting in two peaks of light intensity on the light-receiving element. The position of the glass surface can be detected by measuring just one of these peaks, and the thickness of the target can be measured by measuring the difference between the peaks. During actual measurement, however, the reflected light intensity may be different between the front and back surfaces, because that a metal or other glossy object exist in the background of the glass, or that the target is tinted glass. In these cases, the received light waveform becomes similar to the shape in the figure below, resulting in an unstable peak condition and a failed measurement.

Multi-ABLEcontrol solves these problems!

The intensity of the light reflected from the back surface decreases.

Light reflected from the back surface of the tinted glass target

Light reflected from the front surface of the tinted glass target

The receiver receives a light reflected from the surface of the metal object.

Light reflected from the metal object

Light reflected from the front surface of the glass target


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POINT

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The multi-ABLE control is effective for the cases described in the previous sections, where the peak conditions of the light focused on the light-receiving element vary, because a metal object exists in the background of a glass target, or because the target is tinted glass. This function individually corrects the peaks of the received light waveform to create an optimal waveform (synthesized waveform) as shown above.

❙ Waveform obtained by optimizing the peak of the first surface with the ABLE control

❙ Waveform obtained by optimizing the peak of the second surface with the ABLE control

❙ Waveform synthesized by the multi-ABLE controlSaturation level

Multi-ABLE control1 - 3

Effects of the multi-ABLE control1 - 4

More effects of the multi-ABLE control1 - 5

❙ Received light waveform obtained with a transparent target

1stsurface

2ndsurface

3rdsurface

4thsurface

1st surface

2nd surface

3rd surface

4th surface

GapGap

❙ When multi-ABLE is not used

Measuring the thickness of a tinted glass target



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Since the light intensity of each peak of the received light waveform can be corrected to optimal levels individually, the measurement is stable even when the reflectance of the target varies.

Individual light intensity correction for each peak of the received light waveform

New applications can be handled such as the measurement of a gap in a layered glass plate (for example, between the second and third surfaces) by increasing the number of peaks to be corrected.

Up to four peaks of a waveform can be corrected.


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Position (mm)



❙ When multi-ABLE is used

Optimal waveform

Measurement data


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Optimal waveform

Measurement data

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t (µm

)

Position (mm)

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t (µm

)

Measurement is impossible because no light is received from the 2nd surface.

11

Waveforms obtained with translucent targets2 - 1

As shown in the figure above, with an opaque target, the diffuse reflected light of from the target surface forms a spot on the light-receiving element. With a translucent target, however, the light reflected inside the target also forms a spot on the light-receiving element, resulting in a broader and more gradual waveform. This leads to a larger spot diameter on the light-receiving element, which affects the measurement accuracy.

Technology for improving the measurement ability of translucent targets (RPD algorithm)2

With a translucent target, the measurement is sometimes not stable because the laser beam penetrates the target.This section introduces a measurement algorithm effective for these targets.

RPD algorithm (RPD: Real Peak Detect)2 - 2

With a translucent target, the received light waveform becomes broader as shown in the figure. Consequently, using a barycentric (center of mass) value of the received light intensity higher than the threshold value as a measurement point may result in an error in the measured value.

In order to not generate the measurement error above, the RPD algorithm detects the true peak of the received light waveform and uses it as the measured value. Consequently, it eliminates measurement errors even for translucent targets.

Light-receiving element


Opaque target


Rreceived light waveform

Transparent target

Light reflected inside the target

Light reflected inside the target

When an opaque target is measured When a translucent target is measured

Barycentric valuePeak value

Common received light waveform

Received light intensity threshold value

Received light waveform of a translucent target

Peak value

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Canceling multiple reflection

What are multiple reflections?3 - 1

Technology to improve measurement ability for targets generating multiple reflection (MRC algorithm)3

If a target has microscopic projections or a V-shaped groove, multiple reflections occur which make measurements unstable unstable. This section introduces a measurement algorithm effective for these targets.

Target with a V-shaped groove

Reflected light with multiple reflections

Reflected light with multiple reflections

When a target surface is flat When a target surface has a V-shaped groove

If a metal or other glossy target has a V-shaped groove or a sharp rise, the laser beam may cause irregular reflection on the target surface, and the reflection may be received by the CCD as shown above. When the CCD receives the reflection from the area other than the point where the laser beam reflects (multiple reflection), the measurement accuracy deteriorates.

MRC algorithm (MRC: Multiple Reflection Cancel)3 - 2

Before measurement, the detected received light waveform is compared with the previous waveform and the peak which has the most similar shape to the previous waveform is recognized as a "correct received light waveform". Consequently, the errors caused by the multiple reflections can be eliminated.



Opaque target




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Dissimilar received light waveform(multiple reflection)

Previous waveform (dotted line)

Similar received light waveform


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Technology for improving measurement stability for rough-surfaced targets4

A target that appears to have a flat surface may actually have an unexpectedly rough surface when magnified. If the surface condition of a target greatly varies due to roughness, the measured value may fluctuate because the laser traces the surface roughness while the target moves. This section introduces wide spot laser effective for these targets.

Principle of the wide spot laser4 - 1

❙ Samples of the surface condition of targets

Cylindrical lens

Semiconductorlaser

Semiconductorlaser

The small spot erroneously traces the surface roughness of a target as shown above.

The broad laser spot generated with a special cylindrical lens measures a target by averaging the surface roughness.

Measurement with small spot laser Measurement with wide spot laser

Brushed meta Abrasive pad

❙ Measurement data sample

Effect of the wide spot laser4 - 2

Bearing outer ring (300x) Ceramic plate (1500x)

Wide spot

Small spot

Wide spot

Small spot

© KEYENCE CORPORATION, 2008 LKG-TechBook-KA-L-E 0058-1 600308 Printed in JapanKA1-0038

Specifications are subject to change without notice.

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laser displacement sensor technology book · 6 lk-g series technology book 2 stability of received...

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