Capacitive focus gauge for LED printer

摘要

According to a feature of the present invention, an imaging apparatus has a support with a cylindrical inner surface for receiving a recording media about an axis of the support. A rotor forms a gap with the inner surface such that the gap varies as the rotor rotates and as an inverse function of the concentricity of the support and the rotor. An electrode is carried at the portion of the rotor forming the gap with the inner surface such that a capacitance between the electrode and the inner surface varies as the rotor rotates and as an inverse function of the concentricity of the support and the rotor. A detector, including a power source adapted to produce an electrical charge across the gap and an impedance, measures the change in the dimension of the gap as the rotor rotates so that the concentricity of the support and the rotor can be measured while the rotor is rotating.