Computer control data errors

A computer-controlled data acquisition system allows many data points to be taken at designated positions. For a typical profile measurement, it may take an hour to measure the naphthalene sublimation depth at several thousand measured locations. Hie extra sublimation loss during the profile measurements should be taken into account in order to reduce the measurement errors. 

Reliable process data are the key to the efficient operation of chemical plants. With the increasing use of on-line digital computers, numerous data are acquired and used for on-line optimization and control. Frequently these activities are based on small improvements in process performance, but it must be noted that errors in process data, or inaccurate and unreliable methods of resolving these errors, can easily exceed or mask actual changes in process performance. 

Many of these methods are based on the rate law shown in Eq. (8.6), which is not a general rate law because it can not be put in a form to describe diffusion control or Avrami rate laws (see Chapter 7). In 1983, Reich and Stivala removed the constraint imposed by Eq. (8.6) by developing a kinetic analysis procedure that tests most of the common types of rate laws including Avrami, diffusion control, and others not covered by Eq. (8.6). The method is based on a computer program that fits the (a,T) data to the rate laws and computes the standard error of estimate (SEE) for each so that the rate law that provides the best fit to the data can be identified. It is stiU true that when data from a large number of runs are considered, it is rare that a given rate law fits the data from all the runs. It is still necessary to make a large number of runs and examine the results to determine the rate law that fits the data from most of the runs. 

Humans can play a variety of roles in a control system. In the simplest cases, they create the control commands and apply them directly to the controlled process. For a variety of reasons, particularly speed and efficiency, the system may be designed with a computer between the human controller and the system. The computer may exist only in the feedback loop to process and present data to the human operator. In other systems, the computer actually issues the control instructions with the human operator either providing high-level supervision of the computer or simply monitoring the computer to detect errors or problems. 

The control path is defined as the decision logic of a processor. It is responsible for calculating the next instmction to be fetched and setting the internal flags, such as to command the ALU to sum or subtract, and a branch to be taken or not. The control path is mostly combinational, but since it has to cross the pipeline stages, also has sequential logic. The main difference between the control path and the data path is that an error in the control path will most likely lead to control flow errors, such as a branch being taken, when it should not have. Such control flow errors may cause an erroneous result in the end of the computation or even an infinite loop. 

It looks as if this procedure would be complicated, but it is not when a computer-controlled densitometer is used. The analyst applies samples and standards with an applicator, scans the undeveloped plate, and then develops it and rescans it. All calculations are performed automatically. While running, the program suggests the steps necessary to improve the quality of the data and their evaluation. When the times comes when digital cameras will be used, these procedures will become even simpler. After the application, the basic image of a spotted plate will be taken, and after the development, the plate will be scanned again. The data from both pictures will be compared and processed, and, because of the large volume of collected information, it is expected that most of errors will be detected and reduced. 

In order to illustrate the effectiveness of the proposed method three individual industrial case studies are presented in this section. AU necessary input data measurements were performed by means of a direct computer controlled CMM (Mistral, Brown Sharpe-DEA) with ISO 10360-2 max. permissible error 3.5pm and Fd-DMIS measurement software. A Renishaw PHIOM head with TP200 probe and a 10mm length tip with diameter of 2mm were used. The number and distribution of sampling points conformed with the recommendations of BS7172 1989, (Flad 2001), (9 points for planes and 15 for cylinders). 

Whereas the microprocessor controls an individual basic operation, the central computer, which has all the analytical procedures held in its memory, controls the particular analytical procedure required. At the appropriate time, the central computer transmits the relevant set of parameters to the corresponding units and provides the schedule for the sample-transport operation. All units are monitored to ensure proper functioning. If one of the units signals an error, a predetermined action, such as disposing of the sample, is taken. The basic results from the units are transferred to the central computer, the final results are calculated, and the report is passed to the output terminal. These results can also be transmitted to other data processing equipment for administrative or management purposes. The central control is, therefore, the leading element in a hierarchy of... 

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