A capacitive sensing system for detecting the angular position of a rotatable member of a motor. The position detection system includes a cylindrical excitation pin attached to the housing of the motor and a tubular signal plate, having a plurality of sensing surfaces symmetrically arranged about the axis of rotation of the rotatable member, which surrounds the cylindrical excitation pin and, which is also attached to the motor housing. A lobed dielectric is attached to the rotatable member of the motor and inserted between the cylindrical excitation pin and the tubular signal plate. An electric circuit connected to the sensing surfaces of the tubular signal plate calculates the relative angular position of the rotatable member from the amount of energy detected at each of the sensing surfaces.

A capacitive position sensor includes a capacitor arrangement assembled of a stator (5) and a rotor (6). The stator supports at least three coplanar electrodes (21, 22, 23) having the shape of circle sectors 120.degree.. The rotor supports at least a counterelectrode (25) having the shape of a circle sector of 240.degree.. An oscillator (30) applies two a.c. voltage (u1, u2) of equal amplitude and opposite phase to two of the stator electrode (21, 22). The range of rotation of the rotor is such that the remaining third electrode on the stator is always overlapped by the counterelectrode. Voltage induced on the counterelectrode by means of the a.c. voltages on the stator electrodes is inverted and fed back as a neutralizing voltage (uR) to the third stator electrode (23) until the induced voltage on the counterelectrode reaches zero value. The neutralizing voltage is rectified and integrated into a d.c. voltage (UA) which serves as a measure of the angular position of the rotor (6).

The circuit arrangement is used for capacitance or admittance measurement by the principle of apparent current measurement in that to the capacitance or admittance to be measured an AC voltage of fixed magnitude and fixed frequency is applied and the AC current flowing through the capacitance or admittance is used as a measure of the measured value. The test object whose capacitance or admittance is to be measured lies in the emitter circuit of a transistor connected as voltage follower, to the base electrode of which the AC voltage is applied. The emitter terminal of the transistor connected to the test object is connected to the corresponding pole of the operating voltage source via an adjustable current source. The collector connection of the transistor is connected via an impedance of high resistance for alternating current to the other pole of the operating voltage source. Connected to the collector is a rectifier circuit which converts the alternating current applied thereto to a measuring DC voltage proportional thereto. The direct current flowing via the adjustable current source is set in dependence upon the measuring DC voltage such that it is always slightly greater than the peak value of the alternating current flowing through the test object. Instead of a transistor, another amplifier element with controllable current path may also be used, for example a field-effect transistor. The impedance of high resistance to alternating current is preferably a second adjustable current source which is so constructed that it is controlled automatically to synchronize with the first adjustable current source.

A transducer for generating a feedback signal representative of the position of a movable member includes a signal generating mechanism for providing an electrical feedback signal representative of the position of the movable member. The signal generating mechanism includes a first shaft portion having a first longitudinal centerline and a drive mechanism having a shaft member coupled in driving engagement to the first shaft portion. The drive mechanism is adapted to be cable connected to one of a plurality of movable members having maximum travel spans different one from the other. The shaft member has a shaft longitudinal centerline which is concentric with the first centerline, the centerlines defining a first axis. An alignment mechanism is coupled to the drive mechanism and the signal generating mechanism and permits limited movement of the first axis with respect to a reference axis. The alignment mechanism thereby isolates the signal generating mechanism from misalignment reaction forces.

An analog optical position transducer for detecting angular position of a rotatable member of a motor. The transducer includes a plurality of sensing surfaces placed in one frame of reference and a modulator located in another frame of reference, one of the frames of reference being stationary and the other moving with the rotatable member. The relative angular position of the two frames of reference is determined from the amount of light varied by the modulator and detected by the sensing surfaces. The modulator employs a radiation source which is located on a diffusive surface or which irradiates the diffusive surface of the modulator. Radiation emitted from the diffusive surface is modulated by the modulator depending on the relative position of the modulator and the sensing surfaces. A lens focuses the emitted radiation onto the sensing surfaces. The sensing surfaces are symmetrically arranged about an axis passing through the center of rotation of the rotor, and their relative distribution with respect to the surfaces of the modulator enables differential detection. An analog circuit connected to the sensing surfaces continuously determines the angular position of the rotatable member from the amount of light detected at each of the sensing surfaces. The system is effective to accurately position a mirror in a high speed laser scanner.