A fully digital control system for the speed of the arm shaft of a sewing machine provides separate counters for counting a counting frequency during the pulses and pulse intervals of a lower frequency increment generator measuring the actual value of the shaft speed. The inverse of the last complete count thus registered is compared with a set point value in a microcomputer preprogrammed with data blocks representing these set point values and provided with parameters for the respective keying ratios associated therewith. The control bus and data bus connecting the counters with the microcomputer also connect the latter to another unit including a bit-rate generator and a programmable retriggerable multivibrator from which the digital output signal is emitted to effect direct control over a DC motor or to drive a clutch connected to the shaft.
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