Patent ID: 7586303

Claim:
Inductive proximity sensor comprising: an inductive coil defining a front working plane of the sensor, said front working plane forming an active surface or end of the sensor, said inductive coil being associated with a covering plate or a plane part of a housing of said sensor, said plate or part being made of a non magnetic metal with low electrical conductivity and said plate or part being disposed perpendicularly to an axis of the coil and parallel to the front working plane, and thus forming part of an equivalent magnetic circuit; means for supplying the coil repeatedly with current; means for processing signals which correspond to voltages induced in said coil when fed, said voltages being influenced by the presence of an object or body within a given detection area, depending on a distance of said object from the detection area and on a material constituting said object, and said coil being part of a parallel LC circuit; wherein said means for supplying comprises a capacitor of the LC circuit, wherein the LC circuit is repeatedly switched between two states, namely a first state in which the coil is disconnected from the capacitor and said capacitor is charged and a second state in which the coil is connected to the capacitor and said capacitor discharges through said coil, the LC circuit being allowed to freely oscillate while in said second state and until a following change of state, and thus an oscillating voltage signal of said voltages is modified by the object located within the detection area, wherein a voltage oscillating signal amplitude measurement is performed at a certain predefined point of said voltage signal after each switching from the first to the second state and a result of said measurement is computed in connection with reference measurements in order to calculate at least one of a distance to the front working plane, a nature of the material constituting the object, and a mass indicative value of the object or body located within the detection area, and wherein a delay for performing the measurement of a voltage value at coil opposite terminals, after switching of the LC circuit or an LRC circuit from its first state to its second state is computed using time dependency of modulation grade, in order to optimize the delay based on a linear function Modulation ⁢ ⁢ grade ⁢ ⁢ M = f ⁡ ( delay ) , wherein M ⁡ ( % ) = 100 ×  U L ⁢ ⁢ 0 ⁡ ( t ) - U L ⁢ ⁢ 1 ⁡ ( t )  U L ⁢ ⁢ 0 ⁡ ( t ) , U L0 (t) being the coil voltage without any influence, and U L1 (t) being the coil voltage with an object or body within the detection area and influencing said voltage.