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Design and performance of curved photomultiplier tube - effects of structural configuration on dead time and gain stability
It is found that in the case of 500 image elements and system gains above 3000 only a few per cent of input photons will be lost. Photon loss is not excessive for even lower gains. Aperture variations within a fairly wide range have little effect on the loss. However, losses increase if dead time at the end of a sweep is permitted. A system with continuous sweep at the fastest possible rates is recommended for optimal results.
A system for the recording and processing of telescope data is considered for measurements of EW asymmetry. The information is recorded by 45 channels on a continuously moving 35-mm film. The dead time of the recorder is about 0.1 sec. A sorting electronic circuit is used to reduce the errors when the statistical time distribution of the pulses is recorded. The recorded information is read out by means of photoresistors. The phototransmitter signals are fed either to the mechanical recorder unit for preliminary processing, or to a logical circuit which controls the operation of the punching device. The punched tape is processed by an electronic computer.
The High Resolution Imaging Instrument for the HEAO-B X-ray Telescope satellite is designed to reproduce images of celestial X-ray sources in the range of 0.2 to 4.0 keV. It provides both imaging and temporal data. The basic detector is a two-stage microchannel plate with a crossed-grid charge detector. Preamplifiers, detector bias supplies, and some processing circuits are colocated with the detector. The remaining electronic circuits are located in a remote central electronic assembly. The processing system is designed for single-photon processing. The imaging data rate is limited to 100 data points per second. Non-imaging total-field count rate is acquired with a maximum dead time of 14 microseconds per event.
A voltage to frequency converter (VFC) for general use with photomultipliers is described. For high light levels, when the dead-time corrections for a photon counter would be excessive, the VFC maintains a linear response and allows the recording of data at high time resolution. Results of laboratory tests are given for the signal-to-noise characteristics, linearity, stability, and transient response of the VFC when used in conjunction with EMI 9658 and RCA C31034 photomultipliers.
The characteristics and performance of a Ranicon photon-counting system combined with digital tape recording are described. The most important features are a bialkali photocathode response over 256 X 256 digital pixels, with approximately 100 X 100 resolvable pixels at 50% MTF, a dead time of 16 microsec/count, a maximum recordable count rate of 14,400/sec, and a background of less than 1 count/digital pixel/h. A video cassette recorder serves for the digital recording which retains the temporal sequence of the registered photons. Astronomical applications will include low light level quantitative imaging and speckle imaging.
An experimental study of the fast scintillation counter (FS) system to observe a shower disk structure at Mt. Norikura is described, especially the system performance and a pulse wave-form by a single charge particles. The photomultiplier tube (PT) pulse appears at the leading edge of the main pulse. To remove this PT-pulse from the main pulse, the frame of the scintillator vessel was changed. The fast triggering system was made to decrease the dead time which came from the use of the function of the self triggering of the storage oscilloscope (OSC). To provide a new field on the multi-parameter study of the cosmic ray showers, the system response of the FS system also improved as a result of many considerations.
Random events were simulated by computer and subjected to various statistical methods to extract important parameters. Various forms of curve fitting were explored, such as least squares, least distance from a line, maximum likelihood. Problems considered were dead time, exponential decay, and spectrum extraction from cosmic ray data using binned data and data from individual events. Computer programs, mostly of an iterative nature, were developed to do these simulations and extractions and are partially listed as appendices. The mathematical basis for the compuer programs is given.
Amplifier gain and collimator hole size variations across the field of view, amplifier/filter efficiency, variations in effective collimator hole size and angular response with photon energy, dead-time, and hard X-ray plate transmission, are among the factors for which instrumental corrections have to be incorporated to effect reliable correction and deconvolution of images from the SMM satellite's Hard X-ray Imaging Spectrometer (HXIS). Attention is given to the substantial Poisson noise in these energy bands. The maximum entropy deconvolution/correction routine developed for establishing the spatial structure reliably inferrable from HXIS data is presented, together with the results of the application of this routine to the three impulsive flares reported by Duijemian et al. (1982) from April 10, May 21, and November 5, 1980.
A clock pulse generator for decoding pulse position modulation in an optical communication receiver is synchronized by a delay tracking loop which multiplies impulses of a data pulse by the square wave clock pulses from the generator to produce positive impulses when the clock pulse is of one level, and negative impulses when the clock pulse is of another level. A delay tracking loop integrates the impulses and produces an error signal that adjusts the delay so the clock pulses will be synchronized with data pulses. A dead-time tau sub d is provided between data pulses of an interval tau sub p in the data pulse period tau. When synchronized, the average number of positive impulses integrated will equal the average number of negative impulses over the continuous stream of data pulses.
Propagation of errors and effects of dead time suppressed. System includes precise timing-pulse generator with interval counter and suitably programmed computer determines relative stability or instability of frequency of two signals differing in frequency by about 1 Hz. Designed for use in frequency-standards laboratory.
A correction algorithm for evaluating the particle size distribution measurements of atmospheric aerosols obtained with a forward-scattering spectrometer probe (FSSP) is examined. A model based on Poisson statistics is employed to calculate the average diameter and rms width of the particle size distribution. The dead time and coincidence errors in the measured number density are estimated. The model generated data are compared with a Monte Carlo simulation of the FSSP operation. It is observed that the correlation between the actual and measured size distribution is nonlinear. It is noted that the algorithm permits more accurate calculation of the average diameter and rms width of the distribution compared to uncorrected measured quantities.
An expert system which tunes a Proportional-Integral-Derivative (PID) controller on-line for a single-input-single-output multiple-lag process with dead time is described. The expert system examines features of the previous transient responses and their corresponding sets of controller parameters. It determines a new set of controller gains to obtain a more desirable time response. This technique can be used to determine and implement a different set of PID gains for each operating regime and, once in steady state, the system can be used to find optimal parameters for load disturbance rejection. The expert system can be applied to any system of the specified form (aerospace, industrial, etc.) and can be expanded to include additional process models.
An expert system which tunes a proportional-integral-derivative (PID) controller online for a single-input-single-output multiple-lag process with dead time is described. The expert system examines features of the previous transient responses and their corresponding sets of controller parameters. It determines a new set of controller gains to obtain a more desirable time response. This technique can be used to determine and implement a different set of PID gains for each operating regime and, once in steady state, the system can be used to find optimal parameters for load disturbance rejection. The expert system can be applied to any system of the specified form (aerospace, industrial, etc.) and can be expanded to include additional process models.
Partial discharges cause progressive deterioration of insulating materials working in high voltage conditions and may lead ultimately to insulator failure. Experimental findings indicate that deterioration increases with the number of discharges and is consequently proportional to the magnitude and frequency of the applied voltage. In order to obtain a better understanding of the mechanisms of deterioration produced by partial discharges, instrumentation capable of individual pulse resolution is required. A new computer-based partial discharge detection system was designed and constructed to conduct long duration tests on sample capacitors. This system is capable of recording large number of pulses without dead time and producing valuable information related to amplitude, polarity, and charge content of the discharges. The operation of the system is automatic and no human supervision is required during the testing stage. Ceramic capacitors were tested at high voltage in long duration tests. The obtained results indicated that the charge content of partial discharges shift towards high levels of charge as the level of deterioration in the capacitor increases.
Engine dynamics and controls data are presented for a ram-jet engine which was operated in a free-jet facility at a Mach number of 2.76 and altitudes from 68,000 to 82,000 feet. The predominant engine dynamic characteristics was dead time, with values ranging from 0.018 to 0.053 sec. The control systems were designed to hold a constant ratio of a diffuser static pressure to an inlet-cone reference static pressure. Response times and percent overshoot to fuel-flow disturbances that would have caused subcritical operation without control are included for a wide range of control settings. For all settings, the control response was fast enough (0.08 to 0.5 sec) to prevent subcritical operation.
Self-tuning control algorithms are potential successors to manually tuned PID controllers traditionally used in process control applications. A very attractive design method for self-tuning controllers, which has been developed over recent years, is the long-range predictive control (LRPC). The success of LRPC is due to its effectiveness with plants of unknown order and dead-time which may be simultaneously nonminimum phase and unstable or have multiple lightly damped poles (as in the case of flexible structures or flexible robot arms). LRPC is a receding horizon strategy and can be, in general terms, summarized as follows. Using assumed long-range (or multi-step) cost function the optimal control law is found in terms of unknown parameters of the predictor model of the process, current input-output sequence, and future reference signal sequence. The common approach is to assume that the input-output process model is known or separately identified and then to find the parameters of the predictor model. Once these are known, the optimal control law determines control signal at the current time t which is applied at the process input and the whole procedure is repeated at the next time instant. Most of the recent research in this field is apparently centered around the LRPC formulation developed by Clarke et al., known as generalized predictive control (GPC). GPC uses ARIMAX/CARIMA model of the process in its input-output formulation. In this paper, the GPC formulation is used but the process predictor model is derived from the state space formulation of the ARIMAX model and is directly identified over the receding horizon, i.e., using current input-output sequence. The underlying technique in the design of identified predictive control (IPC) algorithm is the identification algorithm of observer/Kalman filter Markov parameters developed by Juang et al. at NASA Langley Research Center and successfully applied to identification of flexible structures.
We consider the optimum design of photon-counting microlaser altimeters operating from airborne and spaceborne platforms under both day and night conditions. Extremely compact Q-switched microlaser transmitters produce trains of low energy pulses at multi-kHz rates and can easily generate subnanosecond pulse-widths for precise ranging. To guide the design, we have modeled the solar noise background and developed simple algorithms, based on Post-Detection Poisson Filtering (PDPF), to optimally extract the weak altimeter signal from a high noise background during daytime operations. Practical technology issues, such as detector and/or receiver dead times, have also been considered in the analysis. We describe an airborne prototype, being developed under NASA's instrument Incubator Program, which is designed to operate at a 10 kHz rate from aircraft cruise altitudes up to 12 km with laser pulse energies on the order of a few microjoules. We also analyze a compact and power efficient system designed to operate from Mars orbit at an altitude of 300 km and sample the Martian surface at rates up to 4.3 kHz using a 1 watt laser transmitter and an 18 cm telescope. This yields a Power-Aperture Product of 0.24 W-square meter, corresponding to a value almost 4 times smaller than the Mars Orbiting Laser Altimeter (0. 88W-square meter), yet the sampling rate is roughly 400 times greater (4 kHz vs 10 Hz) Relative to conventional high power laser altimeters, advantages of photon-counting laser altimeters include: (1) a more efficient use of available laser photons providing up to two orders of magnitude greater surface sampling rates for a given laser power-telescope aperture product; (2) a simultaneous two order of magnitude reduction in the volume, cost and weight of the telescope system; (3) the unique ability to spatially resolve the source of the surface return in a photon counting mode through the use of pixellated or imaging detectors; and (4) improved vertical and transverse spatial resolution resulting from both (1) and (3). Furthermore, because of significantly lower laser pulse energies, the microaltimeter is inherently more eyesafe to observers on the ground and less prone to internal optical damage, which can terminate a space mission prematurely.