Patent GB-1214225-A (GB)
Title: Vehicle velocity sensor
IPC: G01P3/68
Filed: 1968-05-09
Abstract: 1,214,225. Electric selective signalling. INTERNATIONAL STANDARD ELECTRIC CORP. 9 May, 1968 [16 May, 1967], No. 21998/68. Heading G4H. [Also in Division G1] The velocity of an aircraft is measured by photo-electrically sensing the speed of rotation of the drum of the aircraft's D.M.E. (distance measuring equipment). The drum 1 is accurately rotated at, e.g., one revolution per ten miles of vehicle ground travel. In order to sense this rotation without exerting a load on the drum, a photo-electricsensing arrangement, comprising light source 3 and two pairs PC1 and PC3, PC2 and PC4, of photo-resistors, is used. The drum is provided with a window of at least 90 degrees arc so that the photo-cells are sequentially illuminated, causing operation of triggers 4, 5. Triggering pulses are thus produced alternately on output lines of triggers 4, 5, each pulse resulting from the sudden illumination of a photo-cell when the leading edge 18 of the window passes it. A trigger pulse from circuit 4 due to, e.g. photo-cell PC1, causes switching of multivibrator 6 to its 1 state and initiates a timing cycle by timer 8, there being no effect on multivibrator 10, which is already in its 1 state, thereby causing the clamping circuit 13 to prevent operation of servo amplifier 9. A trigger pulse caused by PC2 from circuit 5 causes multivibrator 6 to be reset and the timer to stop. The output from multivibrator 6 causes multivibrator 10 to switch to 0, release clamp 13 and enable the servo amplifier. The voltage held in timer 8 is thus compared with the feedback voltage from potentiometer 14 to cause operation of motor 16 to balance the servo inputs. The motor also controls movement of an indicator 50, giving an indication depending on the time as timed by device 8 for the drum to rotate a quarter-cycle, representative of aircraft travel of 2¢ miles. Multivibrator 10 also switches on a time delay circuit which has a delay sufficient to allow the servo to balance before a pulse is fed to multivibrator 7 to reset the timer and back to multivibrator 10 to reset it and reclamp the servo. The timer 8 of Fig. 2 may be analogue or digital. An analogue timer, Fig. 3 (not shown), includes a capacitor (33) which is normally charged. On receipt of a pulse from multivibrator 6 to initiate timing, a transistor (30) is switched on to allow the capacitor to discharge to earth until the multivibrator output again disappears. The voltage retained on capacitor 33 is measured by the servo-system. A reset input (17) allows recharging of the capacitor (33). A digital timer, Fig. 5 (not shown), comprises a gated clock the output of which is recorded by a counter which feeds a D/A converter. Either of these timers feed the servo indicator of Fig. 2 which includes an indicator 50 calibrated in ground speed, of the type in which a negator spring, carrying a scale, and wound on two reels is driven by the motor, Figs. 8A, 8B, 9 (not shown). Alternatively, the analogue timer of Fig. 3 is connected to an A/D converter and digital indicator, Fig. 4 (not shown). As a further alternative the digital timer of Fig. 5 may be connected to a digital indicator instead of the A/D converter and servo-indicator. The indicator scales may be logarithmic. Means may be provided (44 in Fig. 2) to prevent operation of triggers 4, 5 once the aircraft is within a certain distance, e.g. 20 miles of a beacon, when inaccuracies in the D.M.E. arise.