GB942874A - Tracking accuracy control for analysers - Google Patents

Abstract

942,874. Photo-electric spectrometers. BECKMAN INSTRUMENTS Inc. March 17,1961 [March 21,1960], No. 9766/61. Heading G1A. The invention relates to double beam spectrometers of the type in which light is alternately passed through the substance to be investigated and through an adjustable attenuator. The two beams are recombined and directed through a variable monochromator to a thermocouple or like detector whose output is fed back to adjust the attenuator. In such a spectrometer it is possible to use an integrator to smooth the signal controlling the attenuator in order to eliminate the spurious high-frequency noise signals. This arrangement, however, results in an error in instantaneous reading which becomes more serious the greater the rate of variation of the detector signal. According to the invention this error is minimized by either reducing the time constant of the integrating circuit or reducing the rate of change of frequency in the monochromator (or both) when large changes in the detector output occur. As shown rotating frames 14,15 containing half mirrors direct radiation alternately through cell 22 containing the test example and through comb attenuator 23. The recombined beam passes through monochromator 12, swept through a range of frequencies by motor 29, and on to thermocouple 13. The output of the couple is amplified and fed demodulator 34 and control circuit 40. The demodulator output is fed to filter-type integrator 35 whose output controls attenuator 23 in a manner reducing the alternating signal from the thermocouple. The control circuit, as shown in more detail in Fig. 2, includes an amplifier tuned to the switching frequency, and a rectifier and filter stage driving a lamp 45. The intensity of the light from the lamp is sensed by photo-diode 46 and serves, according to the position of switch 42, to reduce the time constant of integrating circuit 35 or to reduce the speed of motor 29. In another arrangement (Fig. 3, not shown) two photo-diodes view lamp 45, one controlling the integrator and the other the motor speed. It may be arranged that one of these controls only becomes effective when the range of adjustment of the other is exhausted.