System Dead Time Research Articles

Characterization of the count rate performance of modern gamma cameras

Evaluation of count rate performance (CRP) is an integral component of gamma camera quality assurance and system deadtime (τ) may be utilized for image correction in quantitative studies. This work characterizes the CRP of three modern gamma cameras and estimates τ using two established methods (decay and dual source) under a variety of experimental conditions. For the decay method, uncollimated detectors were exposed to a Tc-99m source of relatively high activity and count rates were sampled regularly over 48 h. Input count rate at each time point was based on the lowest observed count rate data point. The input count rate was plotted against the observed count rate and fit via least-squares to the paralyzable detector model (PDM) to estimate τ (rates method). A novel expression for observed counts as a function of measurement time interval was derived, taking into account the PDM and the presence of background but making no assumption regarding input count rate. The observed counts were fit via least-squares to this novel expression to estimate τ (counts method). Correlation and Bland-Altman analyses were performed to assess agreement in estimates of τ between the rates and counts methods. The dependence of τ on energy window definition and incident energy spectrum were characterized. The dual source method was also used to estimate τ and its agreement with estimates from the decay method under identical conditions was also investigated. The dependences of τ on the total activity and the ratio of source activities were characterized. The observed CRP curves for each gamma camera agreed with the PDM at low count rates but deviated substantially from it at high count rates. The estimates of τ determined from the paralyzable portion of the CPR curve using the rates method and the counts method were found to be highly correlated (r = 0.999) but with a small (~6%) difference. No statistically significant difference was observed between the estimates of τ using the decay or dual source methods under identical experimental conditions (p = 0.13). Estimates of τ increased as a power-law function with decreasing ratio of counts in the photopeak to the total counts. Also, estimates of τ increased linearly as spectral effective energy decreased. No significant difference was observed between the dependences of τ on energy window definition or incident spectrum between the decay and dual source methods. Estimates of τ using the dual source method varied as a quadratic on the ratio of the single source to combined source activities and linearly with total activity. The CRP curves for three modern gamma camera models have been characterized, demonstrating unexpected behavior that necessitates the determination of both τ and maximum count rate to fully characterize the CRP curve. τ was estimated under a variety of experimental conditions, based on which guidelines for the performance of CRP testing in a clinical setting have been proposed.

Development of FPGA-based digital signal processing system for radiation spectroscopy

Abstract We have developed an FPGA-based digital signal processing system that performs both online digital signal filtering and pulse-shape analysis for both particle and gamma-ray spectroscopy. Such functionalities were made possible by a state-of-the-art programmable logic device and system architectures employed. The system performance as measured, for example, in the system dead time and accuracy for pulse-height and rise-time determination, was evaluated with standard alpha- and gamma-ray sources using a CsI(Tl) scintillation detector. It is resulted that the present system has shown its potential application to various radiation-related fields such as particle identification, radiography, and radiation imaging.

Verification and validation of the MCNPX-PoliMi code for simulations of neutron multiplicity counting systems

Neutron coincidence counting is widely used in nuclear safeguards. Simulations of these systems can be performed using Monte Carlo codes such as MCNPX to aid in calibration or measurement design. However, the MCNPX coincidence-counting routines treat particle histories individually, therefore the dead time of the acquisition electronics is not treated. The MCNPX-PoliMi code provides the ability to model detailed effects such as data-acquisition electronics and system dead times. A specialized post-processing code has been developed to interpret the collision-log file and determine the response of a 3He multiplicity counter. The MCNPX-PoliMi simulation provides the full neutron multiplicity distribution measured by the 3He tubes. This distribution is used to compute the singles, doubles, and triples rates which are the quantities used to determine 235U mass. MCNPX-PoliMi has previously been validated with passive multiplicity measurements. In this study, a detailed analysis of the measurement system operating in active mode is presented for uranium-oxide standards ranging from 0.5 to 4.0kg with a Canberra JCC-51 active well coincidence counter. MCNPX-PoliMi calculations are also compared with MCNPX. The two codes agree to within 1% for the cases with negligible dead times. The simulations are validated with measurements performed at the Y-12 National Security Complex.

Proposing of a new fitting and iteration method (FIM) to correct measured reactor core reactivity

China experimental fast reactor (CEFR) achieved its first criticality in 21st July 2010. In CEFR physics startup test, an external neutron source (252Cf) with initial intense of 109n/s has been placed in the core center. When using period method or inverse kinetic method to measure reactivity, the relatively strong neutron source will lead to over-estimation of reactivity measured, meanwhile, dead-time of neutron detection system (pulse mode) will lead to under-estimation of reactivity measured. To solve these problems that actually encountered in the measurement of reactivity during CEFR physics startup test stage, a new fitting and iteration method (FIM) has been proposed to correct measured reactivity, and to determine dead-time of detection system. The basic algorithm and the calculation flow are described in the paper, and based on this method a computing code is developed. Two categories of data are used to verify this method, including dummy analytic data and actually measured data. The calculation results indicate that, according to recorded neutron count sequence, the FIM method can successfully calculate accurate reactivity with consideration on the impact of external neutron source effect and can obtain relatively accurate dead-time value of detection system; both of the results show the proposed fitting and iteration method is effective and practicable.

Trigger-less readout system with pulse pile-up recovery for the PANDA electromagnetic calorimeter

A simple, efficient, and robust on-line data-processing scheme was developed for the digital front-end electronics of the electromagnetic calorimeter of the PANDA spectrometer at FAIR, Darmstadt. The implementation of the processing algorithm in FPGA enables the construction of an almost dead-time free data acquisition system. The prototype of a complete trigger-less readout chain has been developed and evaluated. The precision of time synchronisation commands has been verified. A pile-up recovery algorithm was developed and evaluated over a large dynamic range of signal amplitudes.

Design of Reset Control Systems: The PI + CI Compensator

Reset compensation has been used to overcome limitations of linear and time invariant (LTI) compensation. In this work, a new reset compensator, referred to as proportional and integral (PI) + CI (Clegg integrator), is introduced. It basically consists of adding a Clegg integrator to a PI compensator, with the goal of improving the closed loop response by using the nonlinear characteristic of this element. It turns out that by resetting a percentage of the integral term in a PI compensator, a significant improvement can be obtained over a well-tuned PI compensator in some relevant practical cases, such as systems with dominant lag and integrating systems. The work is devoted to the development of PI + CI tuning rules for basic dynamic systems in a wide range of applications, including first and higher order plus dead time systems.

Control of dead-time systems using derivative free local search guided population based incremental learning algorithms

This paper introduces two improved forms of population based incremental learning (PBIL) algorithm applied to proportional integral derivative (PID) controller and Smith predictor design. Derivative free optimization methods, namely simplex derivative pattern search (SDPS) and implicit filtering (IMF) are used to intensify search mechanism in PBIL algorithm with improved convergence than that of the original PBIL. Although the idea of combining local methods and global methods is not new, this paper focuses application of hybrid heuristics to the vast field of control design especially, control of systems having dead-time. The effectiveness of the controller schemes arrived using the developed algorithms namely simplex derivative pattern search guided population based incremental learning (SDPS-PBIL) and implicit filtering guided population based incremental learning (IMF-PBIL) are demonstrated using unit step set point response for a class of dead-time systems. The results are compared with some existing methods of controller tuning.

Root locus for SISO dead-time systems: A continuation based approach

We present a numerical method to plot the root locus of Single-Input–Single-Output (SISO) dead-time systems with respect to the controller gain or the system delay. We compute the trajectories of characteristic roots of the closed-loop system on a prescribed complex right half-plane. We calculate the starting, branch and boundary crossing roots of root-locus branches inside the region. We compute the root locus of each characteristic root based on a predictor–corrector type continuation method. To avoid the high sensitivity of roots with respect to the locus parameter in the neighborhood of branch points, the continuation method relies on a natural parameterization of the root-locus trajectory in terms of a distance in the (characteristic root, locus parameter)-space. The method is numerically stable for high order SISO dead-time systems.

Tuning Method of PI Controller with Desired Damping Coefficient for a First-order Lag Plus Deadtime System

This paper presents an entirely new tuning method of PI controller with a desired damping coefficient. We consider a plant model described by a first-order lag plus deadtime system in the process control. The ultimate sensitivity method presented by Ziegler and Nichols has been still widely used, but it has a disadvantage that gives an oscillatory response. In recent years a less oscillatory response has been judged to be more appropriate for process controls. In tuning PI controller it is often convenient to determine the damping coefficient to obtain the desired control performance. The deadtime element can be approximated by the Padé equation to determine the damping coefficient for the equivalent second-order lag system. In this paper the integral time is normally chosen equal to the time constant of a plant to degenerate the order of the closed-loop transfer function so that pole-zero is cancelled. As a result, the relations among the gain constant, the time constant, the deadtime of the plant and the proportional gain of the controller are clarified when the desired damping coefficient is provided.

Design of PID controllers for dead-time systems using hybrid biogeography based optimisation

This paper introduces the application of BBO algorithms to Proportional Integral Derivative (PID) controller tuning. Derivative free optimisation methods, namely simplex derivative pattern search and implicit filtering are used to hybridize BBO algorithm resulting in improved convergence than original BBO. The effectiveness of the proposed algorithms namely SDPS–BBO, IMF–BBO are demonstrated using PID and Smith predictor controller schemes applied to unit step set point response for a class of dead–time systems and the results are compared with earlier controller tunings proposed by Kookos and Syrcos, Chidambaram, Luyben, Kanthaswamy and Jovitha Jerome.

On the explicit dead-time compensation for robust model predictive control

Computational simplicity is one of the most important aspects to take into account in robust model predictive control (MPC). In dead-time processes, it is common to use an augmented state-space representation in order to apply robust MPC strategies but, this procedure may affect computational aspects. In this paper, explicit dead-time compensation will be used to avoid augmented representation. This technique will be analyzed in terms of robust stability and constraint satisfaction for discrete-time linear systems. The results of this discussion will be applied to a robust tube-based MPC strategy which is able to guarantee robust stability and constraint satisfaction of a dead-time system by considering a prediction model without dead-time. Moreover, taking advantage of the proposed scheme, the robust MPC will be particularized for first-order plus dead-time models which simplifies significantly controller synthesis. The proposed dead-time compensation method will be applied to different robust MPC strategies in two case studies: (i) a simulated quadruple-tank system, and (ii) an experimental scaled laboratory heater process.

CONTROL OF DEAD-TIME SYSTEMS USING HYBRID ANT COLONY OPTIMIZATION

This paper introduces two improved forms of the ant colony optimization (ACO) algorithm applied to a proportional integral derivative (PID) controller and Smith predictor design. Derivative free optimization methods, namely simplex derivative based pattern search (SDPS) and implicit filtering (IMF), are used to intensify the search mechanism in the ACO algorithm with improved convergence over the original ACO. The effectiveness of the controller schemes using the proposed algorithms, namely SDPS-ACO, and IMF-ACO, is demonstrated using unit step set point response for a class of dead-time systems, and the results are compared with some existing methods of controller tuning.

Parameter estimation of integrating and time delay processes using single relay feedback test

Autotuning using relay feedback is widely used to identify low order integrating plus dead time (IPDT) systems as the method is simple and is operated in closed-loop without interrupting the production process. Oscillatory responses from the process due to ideal relay input are collected to calculate ultimate properties of the system that in turn are used to model the responses as functions of system model parameters. These theoretical models of relay response are validated. After adjusting the phase shift, input and output responses are used to find land mark points that are used to formulate algorithms for parameter estimation of the process model. The method is even applicable to distorted relay responses due to load disturbance or measurement noise. Closed-loop simulations are carried out using model based control strategy and performances are calculated.

PI and PID auto-tuning procedure based on simplified single parameter optimization

Abstract In this paper a new auto-tuning algorithm for PI and PID controllers based on relay experiments is proposed to minimize the load disturbance integral error (IE) by maximizing the integral gain, subject to a desired phase margin, and a minimum required gain margin constraint. The main advantage of the proposed auto-tuning algorithm with respect to previous works is that it leads, for most of the processes, to PID tuning with close loop performance similar to PID designed using off-line numerical optimization. Moreover the algorithm is applicable to any linear model structure, including dead time and non-minimum phase systems.

一次遅れ＋むだ時間系の限界感度と減衰度との関係（第2報）

一次遅れ＋むだ時間系の限界感度と減衰度との関係（第2報）

Relative stability analysis of closed-loop SISO dead-time systems: non-imaginary axis case

We present a numerical method to analyse the relative stability of closed-loop single-input–single-output (SISO) dead-time systems on a given left complex half-plane for all positive delays. The well-known boundary crossing method for the imaginary axis is extended to a given vertical line stability boundary in the complex plane for these types of systems. The method allows us to compute the characteristic roots crossing the relative stability boundary and their corresponding delays up to a maximum predefined delay. Based on this method, we analyse the relative stability of the closed-loop system for all positive delays. Both numerical methods are effective for high-order SISO dead-time systems.

Control of dead-time systems using derivative free particle swarm optimisation

Particle swarm optimisation (PSO), a population-based nature inspired algorithm has mostly been used for solving continuous optimisation problems, discrete variants also exist. It finds application in most of the engineering design problems. This paper introduces two improved forms of PSO algorithm applied to PID controller and Smith predictor design for a class of time delay systems. In this paper, derivative free optimisation methods, namely simplex derivative pattern search and implicit filtering are used to hybridise PSO algorithm with improved convergence than original PSO. The effectiveness of the proposed algorithms namely SDPS-PSO, IMF-PSO are demonstrated using unit step set point response for a class of dead-time systems using PID controller and Smith predictor designed using the proposed hybrid PSO algorithms. The results are compared with earlier controller tunings proposed by Kookos, Syrcos, Chidambaram, Kanthaswamy and Luyben.

Timing performance comparison of digital methods in positron emission tomography

Accurate timing information is essential in positron emission tomography (PET). Recent improvements in high speed electronics made digital methods more attractive to find alternative solutions to create a time mark for an event. Two new digital methods (mean PMT pulse model, MPPM, and median filtered zero crossing method, MFZCM) were introduced in this work and compared to traditional methods such as digital leading edge (LE) and digital constant fraction discrimination (CFD). In addition, the performances of all four digital methods were compared to analog based LE and CFD. The time resolution values for MPPM and MFZCM were measured below 300 ps at 1.6 GS/s and above that was similar to the analog based coincidence timing results. In addition, the two digital methods were insensitive to the changes in threshold setting that might give some improvement in system dead time.

Tuning of PID controllers based on simplified single parameter optimisation

A simple method to design PID controllers in the frequency domain based on a simplified constrained optimisation is proposed. The method is based on the use of a single tuning parameter, defined as the quotient between the final crossover frequency and the zero of the controller. The tuning procedure is based on the maximisation of the controller gain subject to an equality constraint in the phase margin and an inequality constraint in the gain margin. The main advantage of the proposed method is that, even though the maximisation of the controller gain is straightforward, since there is only one parameter to be tuned, the solution is close to the optimal tuning obtained with direct numerical optimisation methods. Moreover the method is applicable to any linear model structure, including dead time and non-minimum phase systems.

Deadtime calibration in high-rate microstrip gas counters

The advent of the microstrip gas chamber (MSGC) has permitted the development of position-sensitive gas avalanche detectors, which combine sub-millimeter spatial resolution with multi-MHz global rate capabilities not attainable with traditional types of gas counters. Based on massively parallel electronic readout, the need to keep down electronic channel costs imposes a simple front-end design without in-built deadtime compensation. In this report we show that insitu calibration of the deadtime of the detector system is straight-forward and reliable. Methods are described for applications to X-ray and neutron detection.