Positioning detector

A positioning detector for detecting whether a workpiece is positioned on a reference surface, by detecting a back pressure of a nozzle from which a pressurized fluid is jetted toward the surface of said workpiece, said positioning detector comprising:

• a positioning table having said reference surface and said nozzle with the tip thereof disposed on and perpendicular to said reference surface;
• a first fluid passage communicating with said nozzle through a tube;
• a second fluid passage communicating with atmospheric pressure via a variable restriction;
• a third fluid passage, said third fluid passage having one end in communication with a fluid pressure source for supplying said pressurized fluid, said third fluid passage being disposed in a first casing having at least one fluid port connected with said fluid pressure source;
• a pair of fixed restrictions for respectively supplying the pressurized fluid to said first and second fluid passages, said fixed restrictions being defined as respective orifices in a unitary restriction member, said unitary restriction member having a central fluid passage therein, wherein said orifices are disposed on diametrically opposite sides of said central fluid passage;
• a semiconductor pressure sensor having an electrical bridge circuit for detecting a differential pressure between the pressures in said first and second fluid passages, wherein said electrical bridge circuit is driven by a fixed DC voltage power source;
• a second casing coupled to said first casing, wherein said unitary restriction member, said variable restriction, and said pressure sensor are housed in said second casing, said unitary restriction member being positioned in said second casing such that an end of said central fluid passage is held in communication with another end of said third fluid passage;
• a threshold generating circuit for generating a plurality of different threshold values, said threshold generating circuit comprising a plurality of resistors connected in series for dividing a fixed voltage of said fixed DC voltage power source, wherein said fixed DC voltage is simultaneously supplied to said bridge circuit and to said plurality of resistors;
• comparing means for comparing said differential pressure with respective different threshold values, said comparing means comprising a plurality of comparators, wherein each of said comparators are respectively given the different threshold values as respective reference signals to be compared with said differential pressure, each of said threshold values corresponding to a different distance at which the workpiece has approached said reference surface;
• each of said comparators comprising a pair of input terminals, one of said input terminals being supplied with an output signal from said electrical bridge circuit, and the other of said terminals being connected to a respective junction of the series-connected resistors;
• a plurality of indicating means each connected respectively to output terminals of said comparators for indicating respective results of comparison by said comparing means, said comparators energizing said respective indicating means depending on the distance at which the workpiece has approached said reference surface.