Digital motor control system for web registration

Claim 1

A web control system for maintaining a register condition between successive repeat lengths of a moving web and a cyclically operating work applying means which acts on the web at a work location along the path of movement of the web, so as to tend to cause the work applying means to act on the web at a predetermined work-receiving region of each succesive repeat length of the web, said web control system comprising an encoder for generating encoder pulses as a function of successive increments of web movement, an error counter connected with said encoder and operable for counting encoder pulses. a counter control circuit connectd with said error counter for controlling the counting of encoder pulses thereby, and responsive to a timing error in the arrival of a work-receiving region of the web at the work location in a cycle of operation of the work applying means to cause the error counter to count encoder pulses for the duration of the timing error, thereby to generate an error count in said error counter for such cycle of operation. a digital to analog converter connected to the error counter and operable for generating an analog error signal in accordance with the error count in said error counter, a blanking circuit operable for generating a blanking signal during the time that the error counter is counting encoder pulses in each cycle of operation, an error correcting motor having an armature circuit producing a back voltage as a function of the speed thereof, a rEgenerative control circuit for the motor including bidirectional thyristor means for responding to the back voltage of the armature and having an alternating current supply for applying energy to the motor armature circuit under the control of said thyristor means, drive circuit means connected to said digital to analog converter for receiving the analog error signal therefrom and operable to control the turn-on angle of the thyristor means in accordance therewith so as to tend to reduce the timing error and thus to restore the register condition, said drive circuit means connected with said blanking circuit and being responsive to said blanking signal to prevent operation of said error correcting motor for the duration of said blanking signal, and to enable the drive means at the termination of said blanking signal in each cyle of operation, and coupling means for coupling to the thyristor alternating current supply for activating said drive circuit means in each cycle at a predetermined point of the supply waveform such that the motor will be operated by said drive circuit means in proportion to the analog error signal while avoiding any error in firing of the thyristor means as the drive circuit means is activated in each cycle of operation under the control of said blanking circuit, the thyrister means responding to excessive back voltage of the motor to reverse energize said motor.