Apparatus and method for detecting the thickness of a sheet document

A two dimensional sheet document thickness detection assembly, for detecting the thickness of a sheet document on a document transport path, the assembly comprising a plurality of sheet document thickness detection apparatuses, each apparatus comprising:

• a first detector unit having a sheet document contact point adjacent the document transport path, the detector unit comprising a piezoelectric element, a first portion of the piezoelectric element being fixed relative to the document transport path, and the piezoelectric element being arranged such that contact between a sheet document and the sheet document contact point as a sheet document passes the detector unit along the document transport path causes flexing of a second portion of the piezoelectric element relative to its first portion;
• and
• a sensing circuit connected to the piezoelectric element for outputting an electric signal based on a charge generated by the piezoelectric element upon flexing, the outputted signal being related to the thickness of the sheet document;
• wherein the respective sheet document contact points of each apparatus are spaced laterally from one another in a direction substantially perpendicular to the direction of passage of the sheet document, such that the outputted signal from each respective sensing circuit relates to the thickness of corresponding laterally spaced regions of the sheet document;
• the assembly further comprising a processor adapted to generate a two-dimensional spatial map of the thickness of at least a portion of the sheet document, based on at least some of the outputted signals, the relative positions of the corresponding detector units and knowledge of the speed at which the sheet document passes between the reference surface and the detector unit.

A method for detecting the thickness of a sheet document on a document transport path, comprising:

• conveying a sheet document along the document transport path past a past a plurality of first detector units, each first detector unit having a sheet document contact point adjacent the document transport path, the detector unit comprising a piezoelectric element, a first portion of the piezoelectric element being fixed relative to the document transport path, contact between the sheet document and the sheet document contact point causing flexing of a second portion of the piezoelectric element relative to its first portion, the plurality of first detection units being spaced laterally from one another;
• and
• outputting via respective sensing circuits electric signals based on the charges generated by the piezoelectric elements upon flexing, the outputted signals being related to the thickness of corresponding laterally spaced regions of the sheet document;
• the method further comprising generating a two-dimensional spatial map of the thickness of at least a portion of the sheet document, based on at least some of the outputted signals, the relative positions of the corresponding detector units and knowledge of the speed at which the sheet document passes between the reference surface and the detector unit.