The sensorsystem COVIDIS is working on a basis of the tried-and-tested spatial filtering method which is working by observing a moving object through a grating. For high precision a further development of this method has been introduced. A superposition of the moving image and a grating structure results in a variation of the detector’s output signal at a frequency proportional to the velocity of the object. This effect is nicely illustrated by a simple model: on sunny days the trees of an avenue create a regular pattern of dark and bright zones. The amount of light reflected by a single car driving on this road depends on whether the car is passing through a shady or bright area. The faster the car moves, the shorter the intervals where light flashes back from the car.In a practical measurement situation light reflected from the illuminated object passes a grating and is then detected by a high-speed imaging sensor. The spacings represent the distances between the trees. The sensor can measure the frequency the object flashes back to determine the time of the displacement. Normally many objects are moving at the same time through the complete measuring area. This could be demonstrated by a lot of different cars reflecting the light.
