Method and apparatus for generating a control signal

A method for generating a control signal for controlling movement of a first object, the control signal corresponding to movement of a second object in contact with a first surface of a platen, the platen being characterized by a critical angle beyond which light incident upon the first surface is not transmitted, the method comprising the steps of:

• transmitting electromagnetic radiation through the platen which is diffused by the second object;
• transmitting a portion of the electromagnetic radiation beyond the critical angle using a lens;
• receiving the portion of the electromagnetic radiation with a single detector having a plurality of adjacent sectors, the sectors of the detector generating charge in response to the portion of the electromagnetic radiation;
• and
• converting the charge generated in the sectors of the detector to the control signal, the control signal representing a speed and a direction of movement of the first object, the speed and direction of movement corresponding to a relationship between charge generated in each of the plurality of sectors.

A method of manufacturing a control device comprising a plastic housing having a detector mounting surface with an image plane associated therewith and a platen surface with an object plane associated therewith, the object and image planes being fixed with respect to each other, the control device also comprising an optical element, there being an optical path between the object plane and the image plane which is defined in part by the optical element, the method comprising the steps of:

• affixing a detector to the detector mounting surface;
• affixing the optical element to the plastic housing with an epoxy;
• before the epoxy hardens, moving the optical element, thereby adjusting the optical path to cause an image of a test pattern at the object plane to come into focus at the image plane;
• and
• hardening the epoxy to fix the optical element with respect to the plastic housing.