Patent GB-976287-A (GB)
Title: Apparatus for detecting radiant energy
IPC: G01J5/0805
Filed: 1961-01-09
Abstract: 976, 287. Detecting hot bearings. WESTINGHOUSE AIR BRAKE CO. Jan. 9, 1961 [Jan. 25, 1960], No. 812/61. Addition to 966,676. Heading G1A. Relates to apparatus of the type disclosed in the parent Specification for detecting radiant energy by employing detector means locate adjacent a railway track for producing electrical signals in response to radiant energy from the wheel bearings of a railway vehicle passing over said track, said apparatus including a rotating chopper wheel for modulating radiant energy impinging on the detector means and means for deriving an output signal of amplitude which is a function of the detected radiant energy. In such an arrangement it is required that the amplitude of the output signal is representative of the temperature of the bearings relative to the external ambient temperature but ambiguities may arise due to reception by the detector of radiation from the chopper whose temperature may differ from the ambient temperature and according to the present invention such ambiguities are eliminated by providing electrical clamping means for holding one amplitude extreme of the derived output signal to a given reference level so that the output from the clamping circuit is proportional to the difference between the amplitude of the derived output signal produced when a vehicle which is passing the detector and the corresponding amplitude produced in the absence of a wheel. As shown heat from a portion of a wheel 19 near an axle box 23 passes through an opening 15a in a casing 15 and a silicon disc filter 18 to a parabolic mirror 17 from which it is reflected to pass through holes in the rotating chopper wheel, 25 to a heat-detector phototransistor 13 having its output fed to a preamplifier 35 and a phase-sensitive demodulator 37. The chopper wheel also interrupts a beam from a light source 34 to a photo-electric cell 39 to provide an alternating reference wave form also applied to the demodulator, the arrangement being such that the positive half-waves of the reference voltage coincide with the passage of the reflected radiation beam to the cell 13 through holes in the chopper wheel. A shutter 15b, normally closing the aperture 15a of the casing 15, is operated to open the aperture when a train enters the section of track in which the detecting device is located as described in Specification 952,996. The signals from the demodulator 37 are passed via a clamping circuit comprising capacitor 41 and resistor 47 of relatively long time period and a diode 43 to an amplifier formed by transistors 49, 61 to an output line 69. If the chopper is at a temperature above the ambient temperature, a negative D.C. signal is derived from the demodulator 37 and applied to capacitor 41 during the periods when there is no vehicle wheel opposite the opening 15a, but this signal is rapidly discharged to earth by the diode 43 and the capacitor assumes a zero reference level. If, however, the chopper wheel is colder than the ambient temperature, a positive signal appears across resistor 47 and forms a reference level for the signal derived from demodulator 37 when the temperature of the wheel is detected. A normally energized relay TR releases when a train enters the track section, and on occurrence of a pulse from a wheel detector circuit, a relay 45 operates, locks up over its a contact and releases contact a of relay TR, and at contacts b opens a short circuit of the diode 43 in the clamping circuit which is rendered operative whilst the train is in the section to detect the ambient radiation during the periods between the passage of successive wheels of the train past the detecting device. In modified arrangements, Fig. 1a (not shown) the demodulator 37 and the associated source of reference signals are omitted and heat from an auxiliary source is projected on to the mirror 17, or alternatively the chopper wheel 25 and cell 13 are enclosed in a refrigerated compartment. In the first alternative arrangement the mean value of the output from the cell 13 in the absence of a vehicle wheel is proportional to X + Y/2 where X and Y are the intensities of radiation received respectively from the auxiliary source and the chopper; in the presence of a vehicle wheel the mean output is proportional to B+Y/2 where B is the intensity of the radiation from the wheel and the difference between these outputs as detected by the clamping circuit is B-X/2. In the second alternative arrangement the radiation from the chopper is negligible. In a further modification, Fig. 1b (not shown), the diode 43 is replaced by a transistor arranged to operate as a diode by voltage applied from a bias source controlled by the wheel detector circuit.