Scan interval

The converter output signal was monitored continuously. When no sample was in the system the signal was zero and the scan frequency was 60 second. When a sample was injected, the signal increased to a previously selected baseline value, the scan interval changed to 5 seconds, and the contents of the current value table, when filled, were stored on... 

Fig. 7. Projection plot of the X-ray scattering from a solution of oscillating microtubules, showing the X-ray intensity (S I(S), z-axis) as a function of scattering vector (S = 2 sin 9/lambda, x-axis) and time (y-axis, 3 sec scan interval). Microtubule protein, 32 mg/ml. The central scatter (left) indicates overall assembly, the subsidiary maximum arises from microtubules. The temperature jump is at time zero. The periodicity of the fluctuations is about 2 min. The final state (after disappearance of the oscillations) is dominated by the scattering from oligomers. The scattering curves here and in Fig. 8 have been smoothed by cubic splines. From [16]...

Figure I. C NMR spectra of a, double-stranded and b, single-stranded 120 bp DNA, obtedned at 100.6 MHz (scales approximately identical) a, 80 mg/mL, 32°C, 8000 scans, 20 Hz digital broadening, and scan interval 1 s b, 80 mg/mL, 85°C, 3500 scans, 20 Hz digital broadening, and scan interval 1.5 s. (Reproduced, with permission, from Ref. 20. Copyright 1981, Adenine Press.)...

Extended services package This element is used to store gusts of data from scan devices, including scan interval, scan lists, and scan period. 

The XRD scans of the parent beta zeolite support as a function of temperature/ scan interval are compiled in Fig. 5.1. The initial increase of the intensity from room temperature to 253 °C (Scan numbers = 1-6 in Hg. 5.1) for the peak at 20-7.5° can be attributed to dehydration of water from the zeolite [38]. The peak maintains its intensity until 415 °C (Scan number = 9 in Fig. 5.1). Above this temperature, a slow decline in the peak intensity can be seen, suggesting a gradual loss of crystallinity. The trend is slightly different for the diffraction at 20-22.8° and other peaks. Their intensity remains relatively constant until the temperature reaches 600 °C (Scan number = 13 in Fig. 5.1). Thereafter, a slow decrease in the intensity becomes noticeable. All these reflection peaks (including the one at 20-7.5°) however, remain even after 1 h dwell at 970 °C. This indicates that the material still maintains its framework structure. Indeed, a final XRD scan of the sample when it is cooled to room temperature reveals that the zeolite structure remains relatively intact, although it does show about a 40 % loss in intensity compared to the XRD pattern before the heat treatment. Nevertheless, the beta zeolite sample used in this study has good thermal stability. 

Converting Equation (3.10) to its discrete form (where ts is the controller scan interval) gives... 

Assuming the process deadtime is longer than the controller scan interval then, at the next scan, the PV will not yet have responded to this change and so both E and E will now have the value E. The derivative action will then be a change of the same magnitude but opposite in direction, i.e. 

Figure 3.24 shows the lAE. The area between the PV and the SP comprises a series of rectangles of width ts (the scan interval) and height l l (the absolute value of the error). The sum of the areas of these rectangles is the lAE. 

It is designed for digital rather than analog control. If the scan interval is used in the tuning calculations then this is likely to be the case. But this only becomes an issue if the process dynamics are very fast and approach the scan interval. 

Figures 3.31 to 3.33 give the recommended tuning for the preferred algorithm (noninter-active, proportional-on-PV, integral-on-Zin, derivative-on-PV and no derivative filtering). It is assumed that the scan interval is small compared to the process dynamics. The mning is designed to minimise ITAE subject to a maximum MV overshoot of 15 % on a self-...

Figure 3.47 shows the impact of increasing the scan interval from zero (analog control) to a value equal to double the process lag (2t) and retuning the controller to take account of the... 

Figure 3.44 Effect of scan interval on controller gain...

Figure 3.45 Effect of scan interval on integral time...

Finally we need the level controller scan interval (ts). 

Let us assume that before the flow disturbance, the level is at steady state and at SP, i.e. i will be zero. Since the flow imbalance (f) will have existed for one controller scan interval (ts), the current error (in dimensionless form) is given by... 

The tightest possible control would be to take this corrective action in the shortest possible time, i.e. the scan interval (ts). By combining Equations (4.9) to (4.11) we can derive the largest possible controller gain (Aimax)-... 

Care should again be taken with the choice of engineering units. Controller scan interval ts) in most DCS is measured in seconds. So, if the flow range (F) is measured in m /hr, the result of this calculation should be multiplied by 3600 to ensure is dimensionless. If the... 

Examination of Equation (4.12) shows Fmax is independent off. This means that, no matter what size the flow disturbance, the controller will set the SP of the manipulated flow equal to the variable flow within one scan interval. Of course control valve dynamics and the tuning of the secondary flow controller (if present) will mean the change in actual flow will lag a little, but nevertheless the controller should be effective. 

Similar examination of the result shows that Fmax is dependent on ts. Unlike most controllers a small change in scan interval (e.g. from 1 to 2 seconds) will have a dramatic effect on the required tuning. 

Comparing this to the controller described by Equation (4.20), the previous value of the error (E i) is now multiple by IE il rather than IE I. Since the two values are measured only one scan interval apart, they will be almost identical and one would think this would have little impact on controller tuning. 

Care needs to be taken when calculating F from x or vice-versa, to work in consistent units of time. The relationship between P and Ty depends on the controller scan interval (ts seconds), as shown in Figure 5.14. While it is unusual to change the scan interval of the DCS it is common for controllers to be moved from one system to another that may have a different scanning frequency. The filter will then perform differently both in terms of noise reduction and the effect it has on the apparent process dynamics. Hence the performance of the controller may degrade. 

The algorithm was originally developed for use when the controller scan interval (ts) is signihcant compared to the process dynamics. This makes it suitable for use if the PV is discontinuous, such as that from some types of on-stream analysers. Analysers are a major source of deadtime. They may located well downstream of the MV and their sample systems and analytical sequence can introduce a delay. An optimally tuned PID controller would then have a great deal of derivative action. However this wUl produce the spiking shown in Figure 7.6. 

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