tailieunhanh - Control Engineering - A guide for beginners - Chapter 3

Tham khảo tài liệu 'control engineering - a guide for beginners - chapter 3', kỹ thuật - công nghệ, tự động hoá phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 3 Continuous controllers Introduction After discussing processes in Chapter 2 we now turn to the second important element of the control loop the controller. The controller has already been described as the element which makes the comparison between process variable PV and setpoint SP and which depending on the control deviation produces the manipulating variable MV. The output of a continuous controller carries a continuous or analog signal either a voltage or a current which can take up all intermediate values between a start value and an end value. The other form of controller is the discontinuous or quasi-continuous controller in which the manipulating variable can only be switched on or off. Continuous controllers offer advantages for certain control systems since their action on the process can be continuously modified to meet demands imposed by process events. Common industry standard output signals for continuous controllers are 0 10V 0 20mA 4 20mA. On a continuous controller with a 0 20 mA output 10 manipulating variable corresponds to an output of 2 mA 80 corresponds to 16 mA and 100 equals 20 mA. As discussed in Chapter 1 continuous controllers are used to operate actuators such as thyristor units regulating valves etc. which need a continuous signal. P controller In a P controller the control deviation is produced by forming the difference between the process variable PV and the selected setpoint SP this is then amplified to give the manipulating variable MV which operates a suitable actuator see Fig. 29 . The control deviation signal has to be amplified since it is too small and cannot be used directly as the manipulating variable. The gain Kp of a P controller must be adjustable so that the controller can be matched to the process. The continuous output signal is directly proportional to the control deviation and follows the same course it is merely amplified by a certain factor. A step change in the deviation e caused for example by a sudden