Instrumentation direct digital control

Since this is a batch system, it might be advisable to use direct digital control. Undoubtedly the throughput could be increased over that with the more traditional analog control system. However, the initial costs and maintenance expenses would also increase. To fully instrument the system would also greatly complicate the equipment required, especially for feeding the reactors (this is discussed later). An economic balance should be run to determine whether this is feasible. I feel it would not be warranted, and have chosen to instrument the plant in the traditional way. 

Automated qualification software is yet another area of CDS development that has been affected by regulatory compliance. When you look at today s CDS solutions, it is typical to see the system providing direct digital control of the instrumentation. Many CDS manufacturers offer software tools that are capable of automating the installation qualification (IQ) and operational qualification (OQ) process. In addition, some manufacturers also offer software that can automate the performance qualification (PQ) for the various forms of instrumentation that it is capable of controlling. These automated software tools are not only time savers for the laboratory, but they also help to properly document the system qualification effort. 

A pilot plant can be computer controlled in one of two ways supervisory or direct digital control as shown in Fig. 10. In the first, a computer supervises another primary control element. In the second, the interfacing element is eliminated and the controller (e.g., a valve) is regulated directly. New computer-based automatic instrumentation is likely to be supervisory because it is faster and less expensive. (See also Refs.P - l.)... 

In the early 1970s, direct digital control of entire processes developed, but the operator had problems. The operator could tell the computer to make a change. It would evaluate many process variables to make the change, which became outside the operator s control. If something went wrong, questions arose as to whether the problem was in the process, field instrumentation, the computer itself, or in the computer software. 

Since a digital instrument functions at fixed points, that is discontinuous, any direct microprocessor control of an experiment has to be in steps. For example, a linear sweep appears as a staircase instead of a continuous ramp. To minimize these effects there are two possibilities ... 

In the past decade there has been an increasing use of digital instrumentation21,22 this involves controlling experiments with a microprocessor inside the instrument or by an external microcomputer. These can also be used for direct analysis of the data obtained. 

Instrument response may be linearly or nonlinearly related to the analyte concentration. Calibration is accomplished by preparing a series of standard solutions of the analyte at known concentrations and me uring the instrument response to each of these (usually after treating them in the same manner as the samples) to prepare an analytical calibration curve of response versus concentration. The concentration of an unknown is then determined from the response, using the calibration curve. With modem computer-controlled instruments, this is often done electronically or digitally, and direct readout of concentration is obtained. 

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