Direct digital control

Digital control A control loop in which a microprocessor-based controller directly controls equipment by means of sensors. Its operation depends on a series of on-off pulses arranged to convey information. 

The Larmor frequency rf reference signal is generated using a DDS (direct digital synthesizer) board (FSW01, DST Inc., Asaka, Japan). This board can generate coherent and spectrally pure rf signals from 5 to 200 MHz, of which the frequency is controlled via the ISA bus. This board has an essential role in the NMR lock process. 

Digital computers, however, can do some things the analog equipment cannot. They offer the opportunity of using more advanced control concepts. These are not necessary for 90% of the processes, but they can increase production and reduce waste for some hard-to-manage processes. For multiproduct plants, direct digital... 

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. 

Baker, W., Weber, J.C. Direct Digital Control of Batch Processes Pays Off, Chemical Engineering ... 

Direct current closed furnace technology, in silicon production, 22 506 Direct digital control (DDC), 20 668 Direct dissolution processes, for regenerated cellulose fibers,... 

Direct digital control systems appeared in the mid-1980s and displaced older analog closed-loop schemes for temperature control. These digital systems improved both accuracy and reliability. The earlier systems were modeled after existing system architectures and did not contain intelligent, standalone field devices. There were numerous interfaces to the various building systems and the major decisions were made at a central computer. 

Next in the control evolution was the utilization of Direct Digital Con-... 

One industrial facility that has done exhaust system retrofit is the Eldec Corporation, an aerospace electronic manufacturer. With the help of the local utility, Eldec implemented a control project to reduce exhaust air by up to 30% for the first shift and 60% for the rest of the time and achieved great savings with one year simple payback. The project closed the exhaust inlets with dampers and controlled the exhaust fan speeds with variable frequency drives (VFD). The exhaust fans are now monitored and controlled by the building direct digital controls (DDC) system to ensure proper operation and save energy. 

Figure 18.3h shows a DDC (direct digital control) system. All the control calculations are done in the digital computer. The computer outputs go, through holds, directly to the control valves. 

For our second sampled-data closedloop system, let us consider the direct digital control (DDC) loop sketched in Fig. 18.12b. The equations describing the system are... 

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. 

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