Patent GB-1194295-A (GB)
Title: Stepper Motor Control System.
IPC: G05B19/40
Filed: 1968-08-05
Abstract: 1,194,295. Control of stepping motors. FAIRCHILD CAMERA & INSTRUMENT CORP. 5 Aug., 1968 [22 Sept., 1967], No. 37317/68. Heading H2J. [Also in Division G4] A stepper motor 10, e.g. for driving a type font drum 13 of a photocomposing machine as described in U.S. Specification 3,434,402, has a control system for operating the motor 10 to a preselected position in response to an encoded digital signal, binary-coded signals 14 representative of the desired position of the motor and the actual shaft position signals 19 being applied to a digital comparator and direction logic circuit 23 which develops output pulses at terminal 24 until the motor advances to bring the shaft position signals 19 and the input signals 14 into coincidence. The comparator 23 also develops CW or CCW signals at terminals 37, 38, indicating the direction of rotation of the motor in order to reach the desired position in the shortest direction. On electromagnetic detent 54, 55, 56, Figs. 2, 3 (not shown), on the motor shaft 12 is controlled by the comparator 23 to lock the motor shaft in the final desired position. To prevent overshoot of the motor past a desired position n and subsequent oscillation about that position, an auxiliary comparator 43 responsive to the signals 14, 19, CW and CCW develops a simulated coincidence signal at terminal 44 when the motor reaches the (n + 1) or (n-1) position, depending upon whether its rotation is CCW or CW, and this signal is used to excite the motor momentarily in the reverse direction to check its momentum. Shortly before the motor reaches the final desired position, a forward direction pulse is applied to the motor to drive it to that position. The shaft position signals 19 are provided from a drum 15 on shaft 12 having a plurality of encoded position-representative holes 15b arranged in angularly spaced axial rows of code groups and a peripheral ring of equally spaced timing or sprocket holes 15a, a photo-electric hole-sensing apparatus 16, 17, 18, developing signals 19 from light passing through holes 15b and a signal 20 from light passing through holes 15a. A computer clock 21 provides timing signals in the form of pulses of waveform Cc, and delay circuits 22, 22b, provide delayed pulses C1, C2. Output pulses from comparator 23 at terminal 24 enable a coincidence gate 25 to which the clock pulses Cc are also applied. Gate 25, triggered by the clock pulses Cc, times the pulses from terminal 24 and applies them to trigger or time the sprocket pulses supplied via terminal 20, buffer amplifier 27 and inverter 28 to JK flip-flop 26, which generates motor driving pulses and supplies them to a coincidence gate 29. Gate 29 also receives output pulses from comparator 24 to enable it to pass the timed sprocket pulses to JK flip-flop 30, which is triggered by pulses C1. Pulses from JK unit 30 pass through OR gate 33 and are "anded" with CCW pulses or CW pulses in one of the AND gates 35, 36, the outputs of which are applied, in turn, to one of the NOR gates 49, 52 and inverters 50, 53, to supply CCW or CW pulses to the stepper motor unit 51, so that the motor rotates step-by-step, block pulses Cc from gate 25 are applied to terminal 32 of comparator 23 to supply timing pulses to this unit except upon the occurrence of a signal (absence of a pulse) at terminal 24 indicating coincidence between signals 14, 19, when the motor driving pulses are interrupted and motor 10 stepped. Simultaneously, a pulse width generator 58 (which is disabled by output signals from terminal 24) is caused to develop pulses which are applied to unit 51 at terminal 60, Fig. 4, and thence via SCR gate 71 to terminal 73 of electromagnetic brake winding 56 to lock the type font drum 13 in the desired position. The + 1/-1 comparator 43, Fig. 7, includes a timing pulse generator 120 receiving a sprocket pulse from terminal 20 and the clock pulses Cc, C1, C2, from the computer clock 21. Generator 120 develops four timing pulses and applies them to an up-down computer register 121 to which are also applied signals 19. Unit 121 develops the simulated shaft position signals and applies them to a coincidence comparator 125, to which are also supplied signals 14, and unit 125 develops the simulated coincidence signal at terminal 44 at the (n-1) or (n+1) position.