What Is Direct Digital Control?

Direct Digit Control, referred to as DDC system, uses a computer to detect the controlled parameters, then performs calculations based on the set value and control algorithm, and then outputs to the actuator to control the production to make the The parameter is stable at the given value. A multifunctional digital control system that uses the computer's time-sharing processing function to directly implement multiple forms of control on multiple control loops. In this type of system, the output of the computer directly affects the control object, so it is called direct digital control, English abbreviation DDC.

The form used by the computer control system is closely related to the complexity of the production process it controls. Different controlled objects and different requirements should have different control schemes. According to the functions and structural characteristics of the computer control system, the computer control system can be divided into:

(1) Operation guidance control system;

(2) Direct digital control system;

(3) Computer supervision and control system;

(4) Distributed control system;

(5) Fieldbus control system. ^[1]

The input and output of the computer of the direct digital control system are digital. First, the controlled quantity signal (4 ~ 20mA) from the sensor or transmitter is converted into a digital quantity by the AI channel and sent to the computer. Then the software implements the PID algorithm. The digital control value is then converted into an analog signal via the AO channel and sent to the actuator (electric or

The direct digital control system consists of the controlled object,

The hardware structure of the direct digital control system includes template and modular . The installation methods can be divided into the following types:

Box type It integrates the main control unit, input and output unit and operation display unit. The front of the box is an LCD display and a membrane keyboard. The box structure is small in size and light in weight, and can be directly installed on production equipment, which is convenient for on-site operation monitoring and is suitable for small-scale data acquisition and control systems. ^[3]

Direct digital control system is a

The computer in the DDC system participates in the closed-loop control process. It not only completely replaces the analog regulator, and realizes the PID (proportional-integral-derivative) adjustment of multiple circuits, but also does not need to change the hardware. It can achieve a variety of more complex by changing the program Control, such as cascade control, feedforward control, nonlinear control, adaptive control, most worry control, fuzzy control, etc. ^[1]

Because the computer in the DDC system directly assumes the control task, it is required that the (system) has good real-time performance, high reliability and strong adaptability. In order to make full use of the computer's utilization rate, a computer usually needs to control several or dozens of circuits, so it is necessary to design the application software reasonably so that it can complete all functions without losing time. DDC system is the most typical type of computer used in industrial process control. It has been widely used in materials thermal processing, chemical industry, machinery, metallurgy and other departments. In the DDC system, the computer is used as a digital controller. ^[1]