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Control Engineering - A guide for beginners - Chapter 6

Tham khảo tài liệu 'control engineering - a guide for beginners - chapter 6', kỹ thuật - công nghệ, tự động hoá phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 6 Improved control quality through special controls So far we have only considered single-loop control circuits where controller and process form a closed signal loop. However when using such single-loop control circuits there are limits to the control quality which can be achieved in certain control processes. It is possible to go beyond the control quality limits imposed by the single-loop control circuit by using multi-loop control circuits or by switching auxiliary variables on and off. To some extent relatively simple solutions can lead to considerable improvements in control quality. 6.1 Base load With a base load setting the controller only influences part of the total manipulating variable and a fixed proportion is continuously supplied to the process combination of control and operation . It could then be the case that for example in an electrically heated furnace one section of the heating elements is controlled by the controller whereas another section is supplied at full supply voltage see Fig. 65 . Fig. 65 Base load setting Essentially base load setting offers the following advantages - The actuator e.g. a thyristor controller can be more compact and less expensive as it only needs to control low power. - Load fluctuations on the supply network as a result of power consumption in bursts caused by the switching controller are similarly reduced. - If the controller fails the process can still be operated with the base load component of the total power. JUMO FAS 525 Edition 02.04 101 6 Improved control quality through special controls Against the advantages shown there are also a number of disadvantages - The dynamic control action is impaired especially with regard to disturbances. As the controller now no longer provides the full manipulating variable the cooling curve for instance is not only shifted by the amount of the base load setting but is also clearly flatter see Fig. 65 . If for any reason the power requirement suddenly becomes less than the .