Optical position monitoring using spatial filters for improved magnet-inductive prospection of metal pieces
S. Bergeler, Hartmut Ewald, H. Krambeer, Eriko Kubota · Proceedings of SPIE, the International Society for Optical Engineering/Proceedings of SPIE · 2005
Conventional humanitarian mine detectors based on magnetic and magneto-inductive procedures are able to detect very small metal pieces in the ground. These evaluation methods however result in a high rate of false alarm; the presence of metallic parts detected which are not to be assigned as mines. If you want to classify the metal piece in the ground (e.g. the shape) you have to measure the electro-magnetic field at different positions. Therefore the actual position must be known for each measuring point. By use of the optical spatial filtering method we are able to measure the velocity vector. With the sample time we get the required x-y-position. In our approach we use structured photo detectors as a filter grating and as a detector too. This technique for position determination possesses some interesting advantages such as the use of incoherent light and simplicity of the optical and mechanical set up. New two-dimensional CMOS sensor arrays with direct pixel access allow a fast read out of sub frames. A disadvantage is the slow signal to noise ratio and the price of industrial CMOS cameras that facilitate frame grabbing. The use of simple CCD web cameras limit the maximum measurable velocity, having a read out time of 60 Hz (max), but the price decrease extreme. Early tests using structured photo detectors and spatial filtering methods for position determination show very good results for velocities from 0 to 250 mm/s. A local resolution of 1 mm can be achieved. Tests have also been performed using an ordinary optical mouse as the position determination system.