To measure a target using a laser displacement sensor, the receiver must be able to obtain the light reflected from the object. With measurement systems that use the triangulation method, and depending on the surface conditions of an object, sensor heads are installed at an angle so that reflected light can be received properly.
A) With opaque targets
The target and sensor head are installed parallel to one another.
A portion of diffuse light is received by the receiver.
B) With transparent targets
The sensor is installed so that the incident angle and reflection angle are equal.
Specular reflection light is received by the receiver.
Reference distance
The reference distance is the sensor heads default zero point. This is commonly represented as the distance from the bottom of the sensor head to the center of the measuring range.
A
Reference distance
Measurement range
The measurement range is the range that a sensor can perform measurement. Measurement ranges are generally written as ±xx mm based on the reference distance.
[Example]
Reference distance: 30 mm
Measurement range: ±5 mm
A laser displacement sensor as described above can measure targets in the following range.
Light source
In non-contact measurement systems, the transmitter irradiates the target with light, and the reflected light is captured by the receiver. A wide variety of light sources can be used for this illumination, including red semiconductor lasers, blue semiconductor lasers, white light, SLDs, and green LEDs. The type of light source used is determined by the principle of the measurement system. Using the appropriate lens or light receiving element for a light source, makes high-accuracy measurement possible.
Spot diameter
With non-contact measurement systems, there are generally two types of beam spots, elliptical and circular. Elliptical spots are used to measure average heights within an area and are less susceptible to influence from the target's surface roughness. However, as the spot size increases, the less suitable it becomes for profiling shapes or measuring small targets. On the other hand, because the size of a circular spot is smaller, it is effective at performing these inspections.
Elliptical Spot
Circular Spot
Repeatability
Repeatability represents the overall difference in a measurement value taken at the same location on a target.
Linearity
Linearity is an indicator of a measurement system's capability. The value represents the maximum error value between an ideal value and the actual measurement result. For example, when a target is moved 1 mm using a measurement system with a linearity of ±5 µm, the displayed value is said to possibly include an error margin of ±5 µm (e.g., 9.995 µm to 1.005 µm). Linearity specifications are defined as ±○○% of F.S., where F.S. represents the measurement range. Calculation is performed as follows. It is reasonable to say that measurement systems with smaller linearity are better.
For example, with a measurement system that has a linearity of 0.02% of F.S. and a measurement range of ±3 mm (F.S. = 6 mm), the linearity will be 0.02% × 6 mm = ±1.2 µm.
Linearity graph
X
Distance
Y
Voltage
A
Linearity
B
Actual value
C
Ideal line
Temperature characteristics
Temperature characteristics represent the maximum measurement error value that occurs when the temperature of the sensor head changes one degree. Inside the sensor head is a lens and a CMOS sensor, and jigs to secure these items. Temperature changes cause these components to expand and contract, changing the position of the imaging location on the CMOS and causing errors. Temperature characteristics are defined as ±○○% of F.S./℃, where F.S. represents the measurement range. Calculation is performed as follows. It is reasonable to say that measurement systems with smaller temperature characteristics are better. For example, with a measurement system that has a temperature characteristic of 0.01% of F.S./℃ and a measurement range of ±3 mm (F.S. = 6 mm), the linearity will be 0.01% × 6 mm = ±0.6 µm.
Ambient light
Ambient light refers to the maximum illumination intensity of an external light source at which the measurement system can measure without being affected.
Ambient temperature
Ambient temperature refers to the temperature environment at which operation of the measurement system can be guaranteed.
Ambient humidity
Ambient humidity refers to the humidity environment at which operation of the measurement system can be guaranteed.
Vibration resistance
Vibration resistance is an index that shows how much of an influence vibration will have on a measurement system. The displayed values indicate the evaluation performed. For example, the general description of “10 to 55 Hz, 1.5 mm double amplitude, 2 hours each in X, Y, and Z directions” specifically implies that the following test has been performed. Vibration performed for 2 hours at a frequency of 10 to 55 Hz at an amplitude of ±0.75 mm in the X direction ⇓ Vibration performed for 2 hours at a frequency of 10 to 55 Hz at an amplitude of ±0.75 mm in the Y direction ⇓ Vibration performed for 2 hours at a frequency of 10 to 55 Hz at an amplitude of ±0.75 mm in the Z direction
Sampling frequency / Sampling speed
The sampling frequency / sampling speed represents the number of data points the measurement system can measure per second. A measurement system with a sampling frequency of 100 Hz can perform 100 measurements in 1 second. Measurement systems with faster sampling frequencies are capable of providing more accurate target measurements with inline measurement, and because multiple averaging processes can be performed at once, measurements will be stable.
Received-light wave pattern
The received-light wave pattern represents the state of light received by the light-receiving element. The vertical axis represents the strength of the light, and horizontal axis shows the position on the light-receiving element. Checking the shape of the received-light wave pattern makes it possible to determine whether the current measurement is being performed accurately.
(1) Ideal received-light wave pattern
This received-light wave pattern allows for stable measurement.
(2) Short received-light wave pattern height
Measurement cannot be performed because the amount of reflected light obtained is insufficient.
The reflected light is highly saturated. In such cases, significant variation will occur in the measured values.
(4) The received-light wave pattern is not horizontally symmetrical
When measuring resins and the like, the laser beam sinks into the object and the received-light wave pattern becomes horizontally asymmetrical. In such cases, shifting is performed on measured values to compensate for the amount of sinking from the true value.
(5) Multiple received-light wave patterns
More than one peak occurs when measuring transparent objects such as glass. When measuring glass, two peaks can be obtained: the peak reflected from the top surface and the peak reflected from the back surface.
Optical axis, optical axis region
The central axis of the light emitted from the measurement system's transmitter is called the optical axis. The optical axis region diagram shows the path of light from the transmitter to the receiver. Because no light will reach the target or the receiver if a jig or other object enters this region, measurement will not be possible.
Optical axis region diagramx
Power supply voltage
The power supply voltage is the necessary voltage required to operate the device. With a specification of 24 VDC ±10%, a DC power supply of 24 V with fluctuations of no more than ±2.4 V is required.
Maximum current consumption
The maximum current consumption represents the amount of current that is consumed when operating the device. It is necessary to select a device that has a power supply with a larger capacity than the maximum current consumption.
