Page 670 - Mechanical Engineers' Handbook (Volume 2)

7 Effects of Nonlinearities 661

Figure 37 PI controller added to the system of Fig. 6b—effects of lead break frequency: (a) open-
loop frequency response; (b) root loci ( 2 0.4, K u2 0.35 2 ).

lution is the availability of a limited number of output values and is typical of digital elec-
tronics and many types of transducers, including encoders and wire-wound potentiometers.
Most servoactuators provide output velocities that are force dependent or output forces that
are velocity dependent. The load–velocity curves shown in Fig. 41d are typical of an elec-
trohydraulic servoactuator and are highly nonlinear. Coulomb friction is a constant force that
always opposes motion. Static friction (stiction) is often larger than Coulomb friction but is
very difficult to model.
Mechanical backlash is motion lost when the direction of motion is reversed, as in gear
trains and bearings. Since most servoactuators make use of electromagnetic elements, mag-