Patent GB-685733-A (GB)
Title: Line tracker control electronic drive
IPC: B23Q35/128
Filed: 1950-11-13
Abstract: 685,733. Photo-electric automatic control systems for contour reproduction. LINDE AIR PRODUCTS CO. Nov. 13, 1950 [Nov. 25, 1949], No. 27721/50. Class 40 (i) [Also in Group XL(b)] Apparatus for automatically reproducing a contour in a workpiece from a representation of the contour as described in Specifications 561,171, [Group XL], and 583,520. is modified in respect of the circuits controlling the drive motors. In the figure, the tracer phototube T<SP>1</SP> responds to the light pulse transmitted through template 2 by synchronously rotating shutter 4 and controls trigger stage T' through amplifier T<SP>2</SP>. The output from the trigger stage is applied via leads 16 and 18 to four discriminator tubes T<SP>4</SP>-T<SP>7</SP> and is such as to bring the tube grids from cut-off to zero grid voltage when light falls on the phototube. The anodes of the four discriminator tubes are connected to a four phase supply provided by Scott connected transformers Tr<SP>1</SP>, Tr<SP>2</SP>and any one of the tubes passes current if its grid voltage is brought up to zero at any time during the 180‹ in which its anode voltage is positive. The tubes are connected in pairs corresponding to the north and south, and east and west directions, the anode voltages in a pair being 180‹ out of phase. Each tube has a correspondingly numbered resistancecapacity network in its cathode circuit across which a control voltage is developed when the tube conducts. The voltages from each pair are connected in opposition and to the grids of associated pairs of thyratron tubes Th<SP>4</SP>-Th<SP>7</SP> which supply the armatures of N-S and E-W traverse motors. The anodes of the thyratrons are supplied by Scott connected transformers Tr', Tr<SP>4</SP>. In the grid of each thyratron is a transformer Tr<SP>5</SP>-Tr<SP>8</SP> which applies a voltage lagging the plate voltage by 90 degrees. Bias sources 60 set the operating points so that the thyratrons will not fire with zero control voltage. When a control voltage develops from a pair of discriminator tubes, one thyratron is driven positive and fires whilst the other is driven negative and blocked, the resulting cathode current causing the armature of the associated motor to rotate in the proper direction. As the motor comes up to speed a counter e.m.f. proportional to speed appears in the grid circuit and offsets the control voltage. The motor speed, therefore, stabilizes at a value proportional to the amplitude of the control voltage. Any tendency for the motor speed to vary is cancelled by the change in counter e.m.f. which advances or retards the thyratron firing point and so alters the armature current. Pronounced overspeeding of the motor, such as might occur due to a sudden change in tracing direction, causes the fired thyratron to be cut off and the opposite thyratron to fire and supply a braking current. If the motor tends to rotate due to inertia after the control voltage has fallen to zero,.the counter em.f. applies a positive grid voltage on the appropriate thyratron to provide a braking current. To prevent the thyratrons from firing incorrectly from the current impulses flowing in the CR combinations of the discriminator tubes instead of the average level, the alternating voltage on the anode of any thyratron is arranged to be 180‹ out of phase with the anode voltage of the corresponding discriminator tube. The traverse speed of the apparatus controlled by adjusting the anode voltage supply for the discriminator tubes. Switches S<SP>4</SP>-S<SP>7</SP> allow any distributer tube to be rendered conducting and the corresponding thyratron fired for the purpose of adjusting the machine to a suitable tracing position.