Sewing machine speed control circuit

Claim 1

A control circuit for use with electric motors and the like, particularly with electric motors in sewing machines, the circuit allowing a user to select a desired operating speed from a plurality of available operating speeds, and to vary such selected speed at will, the control circuit automatically causing the motor to maintain such desired operating speed until a user varies it comprising: a manually operable programmer producing a digital code signal which represents a desired operating speed, the programmer operating in a manner that the code signal produced can be varied to provide a plurality of codes corresponding to a plurality of available operating speeds and any code provided will be maintained until changed by a user; a clock pulse generator producing clock pulses of constant frequency; a clocked and programmable counter, the counter being connected to the programmer in order to be programmed thereby and being connected to the clock pulse generator in order to be clocked thereby and operating in a manner that when the counter is programmed with a code signal it will count one count away from the code signal each time the counter receives a clock pulse and will cease to count after counting to a predetermined number, the counter generating a digital drive signal, the drive signal having a first value when and only when the counter is counting and having a second value when and only when the counter has ceased to count, the counter further operating in a manner that it can always be reprogrammed with a code signal to begin counting again; a buffer responsive to the drive signal and producing an analog control signal, the control signal increasing when the drive signal has its second value and decreasing when the drive signal has its first value; a switch circuit in series with the motor, whereby the motor is turned on when the switch circuit is closed and whereby the motor is turned off when the switch circuit is opened; a switch control responsive to the analog signal and cooperating with the switch circuit in a manner that the switch circuit is closed and opened in response to changes of the analog signal; and a reprogram pulse generator responsive to position of the motor and cooperating with the counter, the reprogram pulse generator generating a reprogram pulse every time the motor rotates through a predetermined angle, whereby the counter is periodically reprogrammed with the code maintained by the programmer when such reprogramming occurs.