Patent GB-1197662-A (GB)
Title: Improved Tracking Accuracy Control for Analyzers.
IPC: G01J3/42
Filed: 1968-04-11
Abstract: 1,197,662. Spectrophotometers. BECKMAN INSTRUMENTS Inc. April 11, 1968 [May 16, 1967], No. 17441/68. Heading G1A. In a double beam spectrophotometer, tracking accuracy errors, i.e. errors in the recorded spectrum due to the attenuator servo, which controls movement of a compensation attenuator in the reference beam path, lagging behind the monochromator scanning drive, are reduced by reducing the wavelength scan motor speed or reducing the time constant of the filter circuit in the attenuator servo-loop, during the occurrence of high value difference signals in this loop, so as to permit rapid and accurate nulling by the attenuator of the two beam path intensities. The spectrophotometer illustrated utilizes half mirrors 14, 15 rotated in synchronism and mirrors 16, 17 to produce the sample and reference beam paths 18, 19. An attenuator comb 24 is provided to balance the beam intensities. The wavelength of the light reaching detector 13 is continuously varied by monochromator 12 whose prism is rotated by scan motor 29. A programmed change in the scan speed over the spectrum is possible by coupling the motor to dispersing element 27 through a suitably shaped cam. The output of the detector is amplified at 33 and demodulated by circuit 34 operated in synchronism with the mirrors 14, 15 and whose output is a D.C. signal representing the difference between the intensities of the reference and sample beams. It is this signal which is fed through a filter circuit 35, which filters out noise, to the attenuator servoamplifier 36 to control movement of attenuator 24 so as to balance the beam intensities. It is the use of filter (sensitivity, or "period") circuit 35 which, by limiting the response rate of the attenuator loop so as to reduce the sensitivity of the instrument to sharp transients such as those due to noise, introduces the tracking accuracy errors during recording of the spectrum (the output of the attenuator servo representing variation in absorption of the sample 22). To reduce tracking errors the demodulator output is also fed via a filter 42, filtering out any chopping frequency components remaining, but of shorter time constant than filter 35, to a control signal generator 40. This provides conversion to an A.C. signal by modulator 43, level control by potentiometer 44, amplification at 45 and half wave rectification by diode 47. The resultant signal is a negative uni-polar control signal which can be connected by switch 51 to control field effect transistor 52 in the circuit 30 for the wavelength scan motor; or field effect transistor 53, controlling the time constant of filter circuit 35. When connected to transistor 52, the supply to motor 29 is partially diverted to earth when a large difference signal is obtained from demodulator 34 since the proportional control signal causes a consequent reduction in the impedance of transistor 52. The result is a slowing down of the motor allowing the attenuator loop to keep up with the wavelength scan. When connected to transistor 53, large difference signals cause a reduction in the time constant of the circuit 35. A hold-over circuit 48, 49 prevents sudden speeding up of the motor, or return of the time constant of circuit 35 to its normal value at the peak of a sharp absorption band. The use of the field-effect transistor means that no current is drawn from hold-over capacitor 48, obviating the need for isolating transformers in the control signal generating circuit 40.