A PSD (semiconductor image position sensitive device) has been used widely for various target position determining apparatus because of its simplicity of instrumentation and high speed operation of a bright spot target position sensing. The image position of a bright spot target can be calculated as its gravitational center with analogue circuit at high speed. In this apparatus, relative resolution of position sensing is mainly limited by the stability and accuracy of the signal processing circuit and A/D converter. In order to overcome this limitation, we had invented a new type of PSD which was named as R-HPSD (Riken Hybrid Position Sensitive Device) and image position of a bright spot was calculated in combination of analogue mode and digital mode. In addition, in order to fabricate the dividing resister accurately and stably, we proposed the separate photosensitive element type PSD in which a dividing register was fabricated separately from photosensitive area and photosensitive area were divided into many photosensitive elements which were connected to the dividing register at the corresponding position.
When the background light were existing surround the target image, they also produce the photo current which contribute to the image position calculation, then the sensing errors were produced and could not be neglected. One of the techniques for avoiding the position sensing error caused by the background light, target spot modulating method has been used widely. The brightness of the target were modulated and the subtractions of the signal currents were performed between light on state and light off state. Thus the effect of current produced by background light could be cancelled. However, this method could not be applied such as when the background light were produced by the target light itself or target brightness could not be modulated.
In this study, we have considered these situation and proposed a new type of PSD by which these kind of background light problem could be reduced. In this new type of PSD, the current corresponding to the background light were subtracted from the photo current produced in the detecting area of PSD so that these portion do not flow into the dividing resister and do not affect to the image position calculation.
In the case of separate photosensitive element type PSD, this method could be applied easily and effectively because each separated photosensitive element could be assumed as a current source whose current was corresponding to the incident light. The substructions of background light current were made from individual photosensitive element through the ideal diode and the interaction of subtracting current between photosensitive current could be prevented. Which means that when the subtracting current were bigger than the photo current produced in each separated photosensitive element, subtraction would be made until the current flow into dividing resister become to 0; then the current flow into dividing resister would be 0 where the photo current were smaller than that of subtracting current.
We have been performed the computer simulations and it was confirmed that the proposed method will be useful mean for reducing the position sensing error caused by the background light current. In addition, in this simulations, it was shown that the possibility of the position sensitive device which can detect the black target image on the light back ground by adopting the similar method to the proposed method.
We believe that the proposed method would be promising to realize the PSD which can reduce the noise error caused by background light and make possible to fabricate the PSD accurately; then they are expected to be used widely as a position sensor in Robotics and an automation system.