Compensation designs, common error

Let us examine a common error in the design of a process control system which we must avoid. Were we to consider the value 0.8 as the true value of the process dead time, we could design a dead-time compensator as in case 2 above. Since there would be no crossover frequency, we could use an arbitrarily large Kc in order to reduce the offset. Let Kc = 100. 

A common source of error arises when sample and standard are not subjected to the same neutron flux as a cause of horizontal or vertical flux gradients. For accurate analyses, they have to be compensated or eliminated (e.g., by rotating the samples). Selfshielding may also introduce significant errors, namely when there is a difference in absorption by the matrices of sample and standard. In general, biological matrices are poor absorbers of neutrons however, the phenomenon must be kept in mind when designing the standard. 

A comprehensive overview of frequency-domain DOT techniques is given in [88]. Particular instraments are described in [166, 347, 410]. It is commonly believed that modulation techniques are less expensive and achieve shorter acquisition times, whereas TCSPC delivers a better absolute accuracy of optical tissue properties. It must be doubted that this general statement is correct for any particular instrument. Certainly, relatively inexpensive frequency-domain instruments can be built by using sine-wave-modulated LEDs, standard avalanche photodiodes, and radio or cellphone receiver chips. Instruments of this type usually have a considerable amplitude-phase crosstalk". Amplitude-phase crosstalk is a dependence of the measured phase on the amplitude of the signal. It results from nonlinearity in the detectors, amplifiers, and mixers, and from synchronous signal pickup [6]. This makes it difficult to obtain absolute optical tissue properties. A carefully designed system [382] reached a systematic phase error of 0.5° at 100 MHz. A system that compensates the amplitude-phase crosstalk via a reference channel reached an RMS phase error of 0.2° at 100 MHz [370]. These phase errors correspond to a time shift of 14 ps and 5.5 ps RMS, respectively. 

The goal of this approach is to maximize efficiency, in terms of both productivity and the utilization of human resources. Table 1 summarizes some of the human resource advantages and disadvantages that have been observed in previous research. Jobs designed according to the mechanistic approach are easier and less expensive to staff. Training times are reduced. Compensation requirements may be less because skill and responsibility are reduced. And because mental demands are less, errors may be less common. 

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