Patent GB-799933-A (GB)
Title: Improvements relating to profile tracing apparatus
Inventor: OHLSEN GEOFFREY TURNER
IPC: B23Q35/123
Filed: 1955-04-21
Abstract: 799,933. Copying machines. BRITISH THOMSON-HOUSTON CO., Ltd., and CRAVEN BROS. (MANCHESTER), Ltd. April 20, 1956 [April 21,1955], No. 11574/55. Class 40 (1). In profile tracing apparatus having a stylus 2 deflectable by engagement with a template or other model 9 and which provides an electrical signal substantially proportional to the magnitude of the deflection at least up to a predetermined value, means are provided for controlling the relation motion between a tracing head 1 which supports the stylus and the model such that for a first range of signal magnitudes relative motion between the tracing head and model is produced in a first direction, for a second range of signal magnitudes above the first range such relative motion is at right angles to the said direction and that for a third range of signal magnitudes above the second range such relative motion is in the opposite direction to said first direction. As shown in Figs. 1 and 2 the stylus 2 is pivoted at 3 in the tracing head 1 and moves against the restoring force provided by spring 4. On each of two axes a1 a2 the tracing head carries a pair of coils 5, 5 and 6, 6 which are wound on cores 7 disposed on opposite sides of the longitudinal axis of the stylus 2. The stylus carries at its inner end four magnetic parts 8 which cooperate with the core 7 to vary the inductances of the coils 5 and 6 as the air gap between the coils and the magnetic parts 8 are varied; the inductances of the coils of each pair are thus varied differentially. One pair of coils is connected in a bridge circuit B1, Fig. 5, and the other pair is connected in a bridge circuit B2. The bridge B1 is fed from one phase of a pair of Scott-connected transformers T1, T2 and the bridge B2 is fed from the other phase. The output of the bridge B 1 is fed to the primary of a transformer T3 through an amplifier A1 and the output of the bridge B2 is fed to the primary of transformer T4 through an amplifier A2. The secondaries of the transformers T3 and T4 are connected in series so that the signals from the bridges B1 and B2 are vectorially added. The resultant signal which is proportional to the deflection of the stylus is applied through a resistor R6 and a rectifier bridge RfB to a resistance R1 and the voltage built-up across the resistor R1 is applied between the control grids and cathodes of four valves V6a, V7a, V8a and V9a. Each value is half of a double triode, the two halves of which are connected as a long tail pair by cathode coupling the two halves through a common resistance R. The other halves of the pairs V6b, V7b, V8b and V9b are biased from potentiometers P1 to P4. Initially the valves V6a to V9a are all cut off and the valves V6b to V9b are all conductive; a thyratron V1 having its grid connected to the anode of V6a is then conductive and a thyratron V5 having its grid connected to the anode of V8a through a diode D1 is also conductive. V1 energizes a clutch coil WC and V5 energizes a brake coil NSB. The clutch operated by coil WC connects a shaft to a motor to move the tracing head in the first direction so that the stylus approaches the surface of the model; the brake energized by coil NSB prevents the motion of a second motor driven shaft which is provided for moving the tracing head in the second direction at right angles to the first direction. The two shafts also drive a copying tool or their relative motion may be recorded for the subsequent control of a copying tool. When the stylus contacts the model surface it is deflected along one of the axes a1, a2 and an increasing signal is developed across the resistor R1. Eventually the signal is large enough to cause the valve V8a to become conductive and the valve V8b then becomes non-conductive since its grid is connected to the anode of the valve V8a through a feedback circuit; the transfer of conduction from one valve to the other is controlled by the potentiometer P3. The negativegoing voltage at the anode of valve V8a extinguishes the thyratron V5 and the brake coil NSB is de-energized; at the same time the positive-going voltage at the anode of valve V8b causes a thyratron V2 to fire and this energizes a clutch coil NC to connect one of the motors to give the tracing head a component of movement in a direction at right angles to the first direction. The oblique motion of the tracing head deflects the stylus further and at a predetermined magnitude of deflection signal, valve V6a is cut off and valve V6b becomes conductive ; the thyratron V is then extinguished, the clutch coil WC is disengaged and a thyratron V4 connected to the valve V6b fires and energizes a brake coil EWB; the component of motion of the tracing heading in the approach direction then ceases and the tracing head moves only at right angles to the approach direction so that the stylus moves along the surface of the model. If the surface of the model slopes away from the approach direction the deflection of the stylus decreases and the decreasing signal across the resistor R1 causes first the firing of thyratrons V1 and V4 so that the tracing head moves in an oblique direction and subsequently if necessary the firing of thyratron V5 and the extinction of thyratron V2 so that the tracing head moves in the approach direction only; when the stylus is again deflected sufficiently the first sequence takes place followed by the reverse sequence so that the slope is traversed in a series of steps. If the model surface slopes towards the approach direction the deflection of the stylus increases and the increasing signal across the resistor R1 causes the valve V7a to become conductive and the valve V7b to be cut off; this causes a thyratron V3 to fire and the thyratron V4 to be extinguished. The thyratron V3 energizes a clutch coil EC and the thyratron V4 de-energizes the brake coil EWB and the tracing head is then given a component of relative motion in the direction opposite to the approach direction so that it moves obliquely. A further increase of signal across R1 causes the valve V9a to become conductive and the valve V9b to be cut off; this causes the extinction of the thyratron V2 to deenergize clutch coil NC and the firing of thyratron V5 through a diode D2 to energize the brake coil NSB; motion of the tracing head is then in the direction opposite to the approach direction only. When the deflection of the stylus decreases, oblique motion and motion at right angles to the approach direction are re-instituted in turn and hence a slope facing towards the approach direction is also traversed in a series of steps. When the stylus is traversing a surface of the model which faces away from the approach direction, to prevent unwanted operation of the clutch coil EC, if the stylus deflection should at any time reach the level which would cause the valve V7a to conduct, means are provided to limit the signal; these means comprise a transformer T5 which has its primary winding energized from the same phase of the Scott-connected transformer as that which energizes the bridge which provides the deflecting signal for the approach direction. The output from a centre-tapped secondary T51 of this transformer is rectified by rectifiers Rf and applied as bias to a bridge B3 connected in series with a further secondary T52 of the transformer across the secondary of transformer T3 and the resistor R6. The connections are such that for deflection of the stylus when the tracing head is moving in the approach direction the phase of the voltage across the secondary of transformer T3 adds to the voltage across the secondary T52 and if the net value of the voltages exceeds the bias applied to the bridge B3 from the secondary T51, current flows through the rectifier arms of the bridge and the increased voltage drop across the resistor R6 limits the effective amplitude across the secondary of transformer T3. Switches are provided for selecting the direction of approach motion and the direction of transverse motion and also for moving the tracing head independently of the model.