Optical Distance Measurement
Optical distance measurement systems are
used in touchless measurement. The sensors have different range, spot
size and resolution. The customer may select the best of many different
models for his application
Features
- Adjustable integration time
- Analog output ±10 V,
4 ... 20 mA; RS 232
- Resolution / accuracy sdown to 0,1 µm
- Insensitive to surface variations and colour
- Automatic adjustment on black surface
- Insensitive against ambient light up to 20.000 LUX
- Thresholds far / OK / near adjustable
- Analog output for intensity of reflected light
- Error output for too much / not enough light
- Thickness measurement also on transparent objects
- Range from 0.5 mm to 400 mm and up to 30 (100) m
Typical Applications
- Check of dimensions
- Detection of position of small parts
- Detection of presence of parts on a conveyor
- Detection of material overlap
- Position control of robot arm (tool center point)
- Detection of presence for parts
- Detection of position of parts through small holes
- Level monitoring
- Vibration analysis
- Crash testing
- Car handling test
Comparison :LED- / Laser
Sensors
Benefits of LED sensors
- No Laser safety regulations
- Bigger spot improves performance of rough targets
- Temperature range up to 70°C
Benfits of Laser
sensors
- Fast reaction
- Small spot captures tiny structures
- Higher accuracy on striped surface
- No problem with shiny surfaces near to object
- Visible spot
Function principle
Optical distance sensors
measure position and presence of objects. The sensors use the triangulation
priciple (fig.1). The spot is focussed onto the objects surface. The reflected
light is mapped on a position sensor (PSD or CCD).
For the measurement
only the diffuse reflection is relevant. For good results, the sensor needs
approximately 10% of surface reflection. When the reflected light is too weak,
and the regulation circuit can not compensate, the error LED comes on. For
additional information the intensity output gives an analog output voltage
proportional to the amount of reflected light.
Two comparators can be set to
detect individual threshold limits of the objects position for MIN, OK and MAX.
The LEDs MIN, OK and MAX show the actual status. With the objects position
(distance), the linear analog output voltage varies.
With permanent
monitoring the object position and light intensity are checked continuously.
The sensor system uses pulsed light. Therefore ambient light has barely
no influence on the measurement.
Digital / analog sensors
Analog sensors use a
fast PSD sensing element for detection, digital sensors use a CCD or CMOS line
sensor element.
Analog sensors capture fast transients (crash
testing, vibration analysis), reaction time is up to 2 µs. Compensation of
colour difference on surfaces works fast with analog sensors. Digital
sensors were much slower.
Digital sensors have their advantage at edges,
when reflections form nearby objects radiate into the sensor. High precision
digital sensors achieve good absolute precision independent from surface
quality.
Phase shift measurement
M10L100m sensor uses light travelling time measurement. A laser beam is sent
out, and in the axis of the beam, the reflection on the target is measured by
comparison of the returning wave.
Ambient light
Ambient light of up to 5.000 LUX
will be compensated without error, up to 20.000 LUX with a small error. This
corresponds to average sun-light on a white surface.
Noise
The resolution of the sensor depends on the
objects surface reflectivity. A high diffuse reflection (mat white) minimises
noise.
Repeatability
Other than mechanical measurement
systems, optical sensors do not have hysteresis or problem with repeatability.
Limiting factors for repeatability are noise, linearity and rough
surfaces.
For measurement in the µm-range, accuracy and expansion due to
temperature changes should be considered. On machine treated objects, for
example with drilling, grinding or milling machines, micro-prisms and
micromirrors in the object surface cause erroneous measurement results.
Basically, the sensor head should be oriented in the axis of the optical system
to the direction of the surface structure
Possible disturbances
Optical
disturbances
- Sparks from electro welding devices avoid direct radiation of arc light into
the sensor
- Sunlight reflecting on the object surface may influence the sensors output
signal
- Direct sunlight radiation into the sensor may reduce the accuracy
considerably
- Light from normal light bulbs (electrical and fluorescent) does not have any
impact on the sensors function, except for the M500.
Electrical disturbances
- Power lines with high current and big sparks parallel to the cable from the
sensor head to the electronic unit
- Big surges and high ripple on the +24 V supply voltage lines, for example
only a rectifier and no buffer capacitor.
- Sensor head captures transients, while not mounted isolated
Cleaning
The protecting glass should be cleaned
with alcohol, using a soft and clean cloth, optic cleaning pads or similar. Do
not wipe dry, this will cause scratches in the glass surface..
Security
M7 Laser systems are classified in
protection class 2 or 3R. The laser safety label (fig. 8) shall be mounted in a
place where the label is clearly visible, before somebody can look into the
beam. Laser radiation is so weak, that the human eye will not be hurt, while the
closing reflex will prevent further damage to the retina.
Looking to the diffuse reflected laser light is not dangerous. Follow the safety
instructions:
- Do not stare into the beam
- Do not stare into beam reflections on shiny surfaces
- Only authorised personal may open the sensor
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