DDC(ID:8188/)for Direct Digital Control Williams' digital control system for processes previously requiring analog computers T Williams, Monsanto 1963 References: The Dow experience has shown that a DDC system works, is readily accepted by plant personnel, is versatile, and adaptable to advanced control techniques. The comment is made that system failures occur due to programming errors. Economic advantages are not clear. which computers might provide benefits for the user. One early effort wast he application of computers tot he monitoring and control of industrial processes such as those used in oil refinery units, steel plants, and paper machines. Over time, these early efforts were generalized to a broader class of applications in which the computer was connected directly into an external process which placed time response requirements on the computer system. These systems have become known as real-time systems. In this paper, the evolution of ZBM small real-time systems is traced from the late 1950s to the present. Emphasis is placed on a few featuresa nd requirements which characterize these systems. Extract: DDC Of the approaches, Direct Digital Control (DDC), championed by Dr. T. J. Williams of Monsanto (currently with Purdue University), probably was the most influential in guiding the technical evolution of process control computers [6]. The then-current (and still used) analog control method utilized a specialized analog computer, known as a controller, which continuously solved the linear equation Output = K,e + K,de/dt + KJedt, where e , the error, is the difference between the actual process parameter value and the desired value (the setpoint), and the Ks are constants. A dedicated analog controller was used for each process ?loop? consisting of a sensor (possibly more than one), which provided the current value, and a process actuator (e.g., a valve) driven by the controller output. The limitations of using the linear analog controller to control a nonlinear process were recognized by Williams and others. In the DDC concept, the computer samples the loop input, calculates the digital equivalent of the analog controller equation, and drives the process actuator directly-thus the name ?Direct Digital Control.? At computer speeds, a single computer can handle hundreds of loops on a time-shared basis. In addition, the computational capabilities of the computer simplified the implementation of advanced control algorithms, such as nonlinear and adaptive control, without extensive process hardware changes. It initially was estimated that DDC could be justified if the amortized computer cost was less than $1000 per loop. This target price provided an elusive goal for the designers of industrial control computers for most of the decade. The technology was not yet ripe for this low cost and the broad acceptance of DDC has only recently been realized. Although DDC was a persuasive force, other factors were important in shaping the systems of the 1960s. Prices were continuously decreasing but process control computers still were expensive and proved economically attractive only on rather large processes. But these large processes were complex and economic justification often required that the total process, or even several small processes, be controlled by the same computer. This led to the early development of executive programming systems which incorporated multiprogramming concepts, and to the early use of high-level languages and application packages such as FORTRAN and PROSPRO for process use. |