Backlash nonlinearity

Figure 22.11 Backlash nonlinearity, with backlash width, 2b. = 0.1.

Although we do not include backlash nonlinearity in our analysis of friction-induced vibration, it is worthwhile presenting a slightly modified version of the above model that enables one to include the effect of backlash, approximately. 

Chapter 22 provides equations for typical process controllers and control valve dynamics. The controllers considered are the proportional controller, the proportional plus integral (PI) controller and the proportional plus integral plus derivative (PID) controller. Integral desaturation is an important feature of PI controllers, and mathematical mc els are produced for three different types in industrial use. The control valve is almost always the final actuator in process plan. A simple model for the transient response of the control valve is given, which makes allowance for limitations on the maximum velocity of movement. In addition, backlash and velocity deadband methods are presented to model the nonlinear effect of static friction on the valve. 

Using nonlinearity blocks the backlash description of valve static friction... 

Nonlinear blocks can be very useful aids to programming controller functions, and their use will be discussed in this section. The backlash function will... 

Control Valve Dynamics. Control valve dynamics tend to be relatively fast compared to the dynamics of the process itself. However, the overall behavior of pneumatic control valves can include nonlinear phenomena such as dead band, stick-slip phenomena, backlash and hysteresis (Blevins et al., 2003 Edgar et al., 2008). Dead band and hysteresis are illustrated in Fig. 9.9. Fortunately, their effects can be reduced significantly by employing valve positioners. 

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