An electronic substitute for the feedback synchro in a synchro control system is described. The three-wire output from a conventional synchro device is delivered to electronic circuitry which generates a rectangular output waveform, the phase of which varies linearly with respect to the vector angle of the synchro rotor shaft. This variable phase, when synchronized, provides a linear output to a feedback control system when deviations from a reference synchro angle setting occur.

An improved numerically-controlled machine tool including means for adding incremental set values, which depend on a programmed input, and incremental actual values, generated by a self-balancing distance measuring system, at defined time intervals to a set distance value and an actual distance value, and means for forming a set speed value for a controller of an input drive by the difference between the set distance value and the actual distance value. The difference between the values is proportional to a lag distance, and the self-balancing distance measuring system comprises an inductive measuring transducer which is fed from a primary side thereof and which generates a secondary error voltage on the secondary side thereof, the magnitude and polarity of which depends on the position of a flux vector generated on the primary side relative to the position of a secondary winding of the transducer. The transducer also includes means for generating pulse signals which are proportional to the secondary error voltage for servoing the flux vector in the direction toward balance and measuring the actual value of the distance travelled by the secondary winding. The improvement of the invention comprises the measuring transducer including means for briefly rotating the flux vector through defined angles in sensing intervals of an incremental, actual-value interrogation, and means for detecting polarity changes in the secondary error voltage.

A converter for converting resolver sine and cosine output to a d.c. signal which is a linear function of resolver shaft angle and including means for comparing the resolver sine output with a reference signal generated from the sine and cosine outputs, and which reference signal has an amplitude equal to the maximum excursion of said outputs so as to be independent of shaft angle change for providing a more accurate conversion.

Phase information in a multiple-coil indictive displacement sensor is retained by summing the excitation signal with each of the secondary signals to provide augmented secondary signals which may then be converted to a digital format. A signal processor may then be employed to extract both the magnitude and phase information from the augmented secondary signals by subtracting the magnitude of the primary excitation signal from each of the augmented secondary signals. The resulting signals retain both phase and magnitude information.

The differences between two pairs of the three windings of a synchro stator are individually scaled so as to provide a function of the rotor angle plus a 45.degree. phase lead. The scaled values are applied to a CR/RC bridge, the output of which bears a phase relationship relating directly to the synchro rotor angle, but is identical for shaft angles from 0.degree. to 180.degree. and from 180.degree. to 360.degree.. These signals are shaped and applied to a pulsewidth modulator; the pulsewidth modulation ambiguity is resolved by generation of a sign bit indicative of angles between 180.degree. and 360.degree.. The pulsewidth modulated output is converted to an analog level, as is the reference supply voltage, and an analog to digital converter provides a digital output as a function of the ratio of the angle voltage level to the reference voltage level. The sign bit is provided as an additional digital bit thereby providing a digital output indication of shaft angle. Multiplexing of synchro inputs to the apparatus may be provided if desired.