Speed-regulated sewing machine

Claim 1

In a sewing machine having a main drive shaft, an improved rpm-regulated drive system, the drive system being of the type which comprises a coupling motor unit which includes an output drive member drivingly coupled to the sewing-machine drive shaft, an electric motor having a constantly rotating output shaft, a braking structure, and electrically controlled electromagnet means for causing the output drive member to engage and become disengaged from the output shaft and the braking structure in order to increase and decrease the rotary speed of the output drive member and thereby of the main drive shaft of the sewing machine; regulating means having an output connected to and controlling the electromagnet means and having a first input for receipt of a desired-rpm command signal and a second input for receipt of an actual-rpm feedback signal; and actual-rpm feedback transducer means, including a pole wheel rotating synchronously with the sewing-machine drive shaft, the pole wheel having a circumferential succession of transducer-detectable induction-modifying elements, a plurality of inductive pick-up transducers arranged along the periphery of the pole wheel and each responding to passage of the succession of induction-modifying elements by producing an A.C. output voltage whose amplitude is proportional to the rpm of the pole wheel, and connecting means connecting together the outputs of the plurality of inductive pick-up transducers to form a voltage constituting said actual-rpm feedback signal, the improvement wherein the inductive pick-up transducers are so spaced relative to the circumferential spacing of the induction-modifying elements of the pole wheel that the individual A.C. voltages induced in the individual pick-up transducers are phase-shifted relative to one another, said connecting means comprising a plurality of rectifiers, one for each inductive pick-up transducer, each rectifying the output voltage of the respective transducer to form a respective pulsating rectifier output signal and means connecting together the outputs of the plurality of rectifiers and connecting them to said second input of the regulating means and operative for applying to the latter a sum signal which constitutes said actual-rpm feedback signal and is derived from the pulsating rectifier output signals by summation alone.