FIGURE 9.19
• HARDWARE SIMPLIFICATION
• FLEXIBLE ALGORITHM
• NO DRIFT OR PARAMETER VARIATION PROBLEMS
• NO EMI OR NOISE PROBLEMS
• COMPATIBILITY WITH HIGHER SYSTEM LEVEL DIGITAL
COMMUNICATION
• POSSIBILITY OF UNIVERSAL HARDWARE
• COMPLEX CONTROL, ESTIMATION, AND DECISION MAKING
• MONITORING, WARNING, AND DATA ACQUISITION FOR
STORAGE AND POSTPROCESSING
• POWERFUL DIAGNOSTICS
Digital control normally refers to control that uses a microcomputer or DSP (digital signal processor) software,
dedicated digital hardware, or a combination of both, although the term is commonly used for microcomputer control. In the pre-microcomputer era, power electronic systems were controlled by hardwired analog devices and digital circuits. However, the majority of systems today is controlled digitally because digital control has a number of advantages. The simplification of control hardware and the corresponding reductions in cost are the principal advantages of microcomputer control. Microcomputer/ DSP technology is advancing very fast with higher processing speeds and more functional integration. Modern VLSI ASIC chips with integration of total control hardware for a specific application in large-volume production can be very economical. Miniature size, reliability, and reduced power consumption can be added advantages. With the same hardware, the software-based control algorithm can be flexible, i.e., it can be easily modified or additional functions can be added. In fact, universal hardware can be designed for a wide range of products, where the algorithms may be different. Digital processing eliminates the signal drift and parameter variation effects that are prevalent in analog systems. Digital computation is 100% accurate with control of signal overflow and underflow by appropriate scaling. Large hardware integration permits decoupling of large voltage and current transients in power electronic systems by nominal shielding. Conduction of EMI can be avoided by adequate filtering of the power supply and I/O signals. Microcomputer control of local power electronic subsystems permits easy compatibility with the higher level host computer in the large integrated industrial environment, where frequent information exchange may be needed. Complex control functions, feedback signal estimations, and decision making in modern power electronic systems are impossible without the help of software. It is interesting to note that although modern vector control was invented in the early 1970s using analog/digital hardware, it could not be commercialized until the 1980s because of the nonavailability of microcomputers. Digital control permits easy signal monitoring, warning, and data acquisition with storage for postprocessing. Powerful off-line or on-line diagnostic software can be developed that can be used for fault identification and fault-tolerant control. A few disadvantages of digital control are sluggish sequential computation, particularly with multitasking, quantization error due to peripheral A/D and D/A converters, lack of easy access of software signals, and, of course, software development may be time consuming.