May 2004System Integration & Control Systems Design
Baumer Electric, the suppliers and manufacturers of a large variety of sensors, has offered the following useful information on how to use sheet glass, acrylic (or any other transparent sheeting) as a cover in the optical path of a laser distance sensor, because there are some precautions that must be taken.
Firstly, the optical path must be covered - both the laser outputs as well as the receiver optics. The red front glass of the sensor defines the position of the receiver optics. The cover must be kept outside the measuring range. If the cover is inside the measuring range, then any dust or dirt that accumulates on the cover could be detected as a valid object within the range, and will lead to a constant, but wrong output.
The cover must be parallel to the front sensor (±5° is tolerable). The reason for this is because the cover might cause disturbing reflections. Larger angles cause linearity errors. Even direct reflections back into the receiver are possible and spoil the results.
The cover must always be flat in an optical sense. If the cover shows 'waves' or curvature, then it will affect the laser beam and the optical path of the detection. The linearity will be affected in a strong and unpredictable way. Normal sheet glass or acrylic glass or macrolon sheets are plane. A simple test can be done to show uneven plane surfaces, by looking with one eye through the sheet and looking at a regular pattern of lines. Then turn the sheet. If the pattern is warped or deformed, the sheet has some optical effect this affects the sensor. This sheet must NOT be used. The pattern must remain stable through rotation.
There is a constant error in absolute distance. Every cover will affect the measured distance! The measured value will always be smaller than the real distance. The following rule is a good approximation for the error:
Change in distance = (thickness of material)/(constant for the material)
Constants:
Glass - 2,5
Acrylic - 2,8
Water - 3,5
Example:
3 mm acrylic leads to a constant error throughout the measuring range of 3/2,8 = 1,07 mm
This affect can be used to indirectly measure the height of transparent and undisturbed fluids by measuring the distance through the liquid down to the bottom of the container.
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