LASER DISPLACEMENT SENSOR
TECHNOLOGY BOOK
Innovative Measurement Accurac y and Stabilit y
LK-G SE R I E S T EC HN O LO G Y B O O K
BASICS OF LASER DISPLACEMENT SENSORS
1
Basic principle of triangulation
Measurement at a reference distance
Measurement at a shorter distance
Light-receiving
element
Light-receiving
element
Light-receiving
element
Semiconductor
laser
Semiconductor
laser
Transmitter
lens
Measurement at a longer distance
Transmitter
lens
Receiver lens
Semiconductor
laser
Transmitter
lens
Receiver lens
Receiver lens
Light-receiving
element
Light-receiving
element
Light-receiving
element
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.
2
Measurement accuracy influences
The measurement accuracy of the laser sensor utilizing triangulation is greatly affected by the following two factors:
Optical design
Stability of received light intensity and waveform
This section explains how these factors affect measurement accuracy.
2-1
Optical design (receiver lens aberration)
Measurement at a reference distance
Light-receiving
element
Measurement at a shorter distance
Received light
waveform
Light-receiving
element
Measurement at a longer distance
Received light
waveform
Light-receiving
element
Received light
waveform
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.
STABLE MEASUREMENT ACCURACY – POINT 1
It is necessary to develop an optical design that ensures
a constant spot size on the light-receiving element.
2
The high-precision
Ernostar lens
solves the problem!
Check page 4!
2-2
Stability of received light intensity and waveform
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.
❙ Examples of beam spot conditions (received light waveform)
Received light intensity
Received light intensity
Received light
waveform
Received light
waveform
Received light waveform obtained
with a mirror-surfaced target
Received light intensity
Received light waveform obtained
with a black rubber target
Received light waveform obtained
with a white ceramic target
Received light
waveform
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.
❙ Factors which affect the beam spot condition (received light waveform)
Color (color irregularity),
luster, gloss of a target
Surface condition of a target
(roughness, tilt)
Material of the target
(such as translucent plastics)
❙ Measures to ensure sufficient received light intensity
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.
Adjustment of
laser emission power
Adjustment of
laser emission time
Adjustment of sensitivity of
light-receiving element
Adjustment of light
receiving time (exposure time)
STABLE MEASUREMENT ACCURACY – POINT 2
It is necessary to be able to obtain optimal received light
intensity for the CCD. This adjustment should be done
quickly.
ABLE control
solves the problem!
Check page 6!
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.
The subsequent sections introduce each of the techniques
3
LK-G SE R I E S T EC HN O LO G Y B O O K
TECHNOLOGIES FOR IMPROVING
MEASUREMENT CAPABILITIES
The 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.
1
Optical design
1-1
The high-precision Ernostar lens
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.
Measurement at a reference distance
Light-receiving
element
Received light
waveform
Measurement at a shorter distance
Light-receiving
element
Measurement at a longer distance
Received light
waveform
Light-receiving
element
Received light
waveform
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.
What is an 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.
Structure of high-accuracy Ernostar lens
Li-CCD
4
1-2
Newly developed delta cut technology!
❙ Comparison with conventional model (10 times higher sensitivity achieved)
When delta cut is used
When delta cut is not used
CCD
CCD
Out of focus
Sharp focus
High-accuracy
Ernostar lens
Attenuation caused
by surface reflection
Filter glass
Refraction caused by the angle between
the filter glass and optical axis
* LK-G155/G405/G505 (Series)
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.
1-3
Newly developed Li-CCD (Linearized CCD)!
❙ Output comparison with a conventional CCD
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
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.
Li-CCD
output
The output of the adjacent pixel changes according to the position of the
reflected light in a pixel, providing more linear characteristics.
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.
❙ Measurement data sample
Linearity obtained with a white ceramic target
Linearity data obtained with a white ceramic gauge (Typical)
Linearity of ±0.05%, which is
two times higher than
conventional models,
has been achieved!
Linearity range of ±0.1%
(conventional models)
Measurement position (μm)
5
LK-G SE R I E S T EC HN O LO G Y B O O K
2
Stability of received light intensity and waveform
With the "ABLE technology", KEYENCE's LK-G Series laser displacement sensor has achieved extremely high accuracy.
2-1
ABLE control (outline)
ABLE
Mirror-surfaced plate
(ABLE=Active Balanced Laser control Engine)
Black rubber
Reflectance
High
Low
Emission power
Laser
power:
Low
Laser
power:
High
Emission time: Short
Emission time: Long
❙ Range of light intensity adjustment
❙ High-speed real-time control
(90 times max. compared with conventional models)
LK-G Series
Conventional model
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).
(120 times compared with conventional models)
Laser power
Emission time
Adjustment range
8 times
1662 times (0.6 to 997 μs)
13296 times
-
150 times (3.2 to 480 μs)
150 times
Sampling time
Adjustment time
LK-G Series
20μs
0.06ms
Conventional model
512μs
7ms
❙ Effect of ABLE control <Received light waveform obtained with a black rubber target>
Received light
waveform
2-2
STEP
1
When ABLE is used
Received light intensity
Received light intensity
When ABLE is not used
Received light
waveform
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.
ABLE control mechanism
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.)
W: Received
light intensity
P
Difference from
the optimal intensity
Ws: Optimal
intensity
T
"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.
6
STEP
2
The result of P x T x G and the received light intensity is stored for each sampling and compared with the optimal intensity.
Ws: Optimal
intensity
W’: Received
light intensity
P
T’
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.
2-3
Effects of ABLE control
Emission time
adjustment resolution
The laser emission time is adjusted in units of "100 ns".
Such fine adjustment ensures stable measurement of every kind of target.
Real-time control by a
high-speed CPU
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.
Original algorithm for
optimal intensity adjustment
2-4
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.
More effects of ABLE control
❙ Angle characteristics
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.
<Shape measurement of a 10 mm diameter pin gauge>
Conventional model
Target shape
When ABLE is used [Ultimate laser displacement sensor]
Measured value [mm]
Measured value [mm]
When ABLE is not used [Conventional model]
Ultimate laser displacement sensor
Position [mm]
Position [mm]
10 mm diameter pin gauge
❙ High-speed
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.
<Measurement of IC pins warpage>
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.
When ABLE is used [Ultimate laser displacement sensor]
The high-speed
response of the
light intensity
control allows
proper
measurement of
all IC pins without
lack of data.
Moving distance [μm]
Measured value [mm]
Each IC pin has a
different width.
Not a single pin
was detected
properly.
Measured value [mm]
When ABLE is not used [Conventional model]
Moving distance [μm]
IC pins
7
LK-G SE R I E S T EC HN O LO G Y B O O K
TECHNOLOGIES FOR IMPROVING ABILITIES
FOR VARIOUS TARGETS
Conventional 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.
1
Technology for improving ability for transparent target measurement (Multi-ABLE control)
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.
1-1
Sensor head position
When the sensor head is positioned perpendicular to the target
The receiver cannot receive the reflected light.
Received light intensity
Light-receiving
element
Semiconductor
laser
Transmitter
lens
Receiver lens
No waveform because
the receiver cannot receive
the reflected light.
Specular reflection
Transparent target
Light-receiving position on the CCD
Front
surface
Back
surface
Light-receiving
element
Light passing through the target
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.
When the laser application angle is adjusted
Measurement is possible because the receiver can receive the reflected light.
Light reflected from the
front surface of the
glass target
Receiver lens
Specular reflection
Transparent target
Light passing through the target
8
Front
surface
Back
surface Light-receiving
element
Received light intensity
Light-receiving
element
Light reflected from
the back surface of the
glass target
The receiver receives the
light reflected from both
the front and back surfaces.
Light-receiving position on the CCD
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.
1-2
Received light waveform of transparent target measurement
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.
When a metal object exists in the background
The receiver receives a light reflected from the surface of the metal object.
Light-receiving
element
Receiver lens
Specular reflection
Transparent
target
metal objec
Front
surface
Back
surface
Light reflected from
the metal object
Received light intensity
Light reflected from
the front surface of the
glass target
Light-receiving position on the CCD
Light-receiving
element
When a tinted glass exists in the background
The intensity of the light reflected from the back surface decreases.
Light-receiving
element
Received light intensity
Light reflected from the
front surface of the
tinted glass target
Receiver lens
Specular reflection
Transparent
target
Light passing through the target
POINT
Front
surface
Back
surface
Light-receiving
element
Light reflected from the
back surface of the
tinted glass target
Light-receiving position on the CCD
To ensure stability of the measurement accuracy for a transparent target:
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.
Multi-ABLE
control solves
these problems!
9
LK-G SE R I E S T EC HN O LO G Y B O O K
1-3
Multi-ABLE control
❙ Waveform synthesized by the
multi-ABLE control
Saturation level
Received light intensity
Received light intensity
❙ Waveform obtained by optimizing the peak of the
first surface with the ABLE control
Light-receiving position on the CCD
Received light intensity
❙ Waveform obtained by optimizing
the peak of the second surface
with the ABLE control
1-4
Light-receiving position on the CCD
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.
Light-receiving position on the CCD
Effects of the multi-ABLE control
Individual light intensity correction
for each peak of the received light
waveform
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.
Up to four peaks of a waveform
can be corrected.
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.
1st
2nd
3rd
4th
surface surface surface surface
Received light intensity
❙ Received light waveform obtained
with a transparent target
Gap
Gap
1st surface
2nd surface
3rd surface
4th surface
Light-receiving position on the CCD
More effects of the multi-ABLE control
Light-receiving position on the CCD
Measurement data
Measurement is impossible
because no light is received
from the 2nd surface.
Position (mm)
10
Optimal waveform
Light reflected from
the back surface of
the tinted glass target
Measurement data
Measurement (µm)
Measuring the thickness of a
tinted glass target
Light reflected from the front
surface of the tinted glass
target
❙ When multi-ABLE is used
Received light intensity
Optimal waveform
Received light intensity
❙ When multi-ABLE is not used
Light reflected from the front
surface of the tinted glass
target
Light reflected from
the back surface of
the tinted glass
target
Light-receiving position on the CCD
Measurement (µm)
1-5
Position (mm)
Technology for improving the measurement ability of translucent targets (RPD algorithm)
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.
2-1
Waveforms obtained with translucent targets
When an opaque target is measured
When a translucent target is measured
Light reflected
inside the target
Opaque target
Transparent target
Light-receiving
element
Received light waveform
Light reflected
inside the target
Light-receiving
element
Rreceived light waveform
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.
RPD algorithm (RPD: Real Peak Detect)
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.
Peak value
Received light intensity
2-2
Barycentric value
Common received
light waveform
Received
light intensity t
hreshold value
Received light waveform of a
translucent target
Light-receiving position on the CCD
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.
Peak value
Received light intensity
2
Light-receiving position on the CCD
11
LK-G SE R I E S T EC HN O LO G Y B O O K
3
Technology to improve measurement ability for targets generating multiple reflection (MRC algorithm)
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.
3-1
What are multiple reflections?
When a target surface is flat
When a target surface has a V-shaped groove
Reflected light
with multiple
reflections
Reflected light
with multiple
reflections
Target with a
V-shaped groove
Opaque target
Light-receiving
element
Light-receiving
element
Received light
waveform
Received light
waveform
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.
3-2
MRC algorithm (MRC: Multiple Reflection Cancel)
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.
Received light intensity
Canceling multiple reflection
Received light intensity
Light-receiving position on the CCD
Dissimilar received light waveform
(multiple reflection)
Similar received
light waveform
Light-receiving position on the CCD
12
Previous waveform
(dotted line)
4
Technology for improving measurement stability for rough-surfaced targets
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.
❙ Samples of the surface condition of targets
Bearing outer ring (300x)
4-1
Ceramic plate (1500x)
Principle of the wide spot laser
Measurement with small spot laser
Measurement with wide spot laser
Semiconductor
laser
Semiconductor
laser
Cylindrical lens
The small spot erroneously traces the surface roughness of
a target as shown above.
4-2
The broad laser spot generated with a special cylindrical
lens measures a target by averaging the surface roughness.
Effect of the wide spot laser
❙ Measurement data sample
Brushed meta
Abrasive pad
Wide spot
Wide spot
Small spot
Small spot
13
Specifications are subject to change without notice.
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