The direct digital control system has three features: online real-time control, time-sharing control, flexibility and versatility.
Online real-time control
The direct digital control system is an online real-time control system. Online control means that all operations of the controlled object (feedback information detection and control information output) are carried out with the direct participation of the computer, without the intervention of system administrators, also known as online control. Real-time control refers to the processing speed of the computer for external information, which is sufficient to ensure the detection and processing of the motion state of the controlled object within the allowed time interval, and to form and implement the corresponding control. The size of this allowable time interval is determined by the dynamic characteristics of the controlled process. For a fast controlled process, the allowable time interval is small; for a slow controlled process, the allowable time interval is large. The computer should also be equipped with a real-time clock and a complete interrupt system, and should have a fairly high reliability to meet real-time requirements. An online system is not necessarily a real-time system, but a real-time system must be an online system.
The direct digital control system is controlled in a time-sharing manner, that is, all controlled loops are sampled one by one according to a fixed sampling cycle time, and control outputs are sequentially calculated and formed to realize control of a plurality of controlled loops by one computer. The operation of each loop is divided into three steps: sampling, calculation and output. In order to increase the control loop (sampling time constant) or shorten the sampling period (the number of control loops is constant) to meet real-time requirements, the three steps are usually arranged in a staggered manner in time. For example, when the output control of the first loop is performed, the second loop can be simultaneously calculated and the third loop can be sampled and input. This can improve the utilization of the computer and shorten the operation time of each loop.