Tham khảo tài liệu 'essentials of process control phần 9', ngoại ngữ, ngữ pháp tiếng anh phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | CHAPTER 14 Sampling z Transforms and Stability 479 . the output of the hold is maintained at a constant value over the sampling period. The hold converts the discrete signal which is a series of pulses into a continuous signal that is a stairstep function. The equivalent block diagram of this system is shown at the bottom of Fig. . The transfer function of the hold is Hịs . The transfer function of the computer controller is Figure shows a digital control computer. The process output variables yi are sensed and converted to continuous analog signals by transmitters T T1 . Tn- These data signals enter the digital computer through a multiplexed analog-to-digital A D converter. The feedback control calculations are done in the FIGURE Computer control. 480 PART five Sampled-Data Systems computer using some algorithm and the calculated controller output signals are sent to holds associated with each control valve through a multiplexed digital-to-analog D A converter. A block diagram of one loop is shown in the bottom of Fig. . The sampling rate of these digital control computers can vary from several times a second to only several times an dynamics of the process dictate the pling time required. The faster the process the smaller the sampling period Ts must be for good control. One of the important questions that we explore in this chapter and the following one is what the sampling rate should be for a given process. For a given number of loops the smaller the value of Ts specified the faster the computer and the input output equipment must be. This increases the cost of the digital hardware. IMPULSE SAMPLER A real sampler Fig. is closed for a finite period of time. This time of closure is usually small compared with the sampling period Therefore the real sampler can be closely approximated by an impulse sampler An impulse sampler is a device that converts a continuous input signal to a sequence of impulses or delta functions. Remember .
