Non-Linearities

Abstract

Non-linearities affecting system performance are hysteresis, backlash, dead zone and coulomb friction. In closed loop systems, backlash inside the loop, specifically any backlash associated with feedback transducer drives, should be eliminated to avoid limit cycling. In velocity control systems, a dead zone around the null region is usually deliberately introduced, by using overlapped valves, in order to prevent actuator movement at zero demand signal. In closed loop position control systems employing on—off (or bang-bang) control valves, dead zone has a stabilizing effect, whereas in proportional position control systems, dead zone would give rise to limit cycling. Idealized characteristics of an on—off control system are shown in Figs 26.1 and 26.2. In systems of this type, stability and response are a function of three parameters, i.e. actuator velocity, dead zone and time delay. On—off position control systems can only be used for relatively crude point-to-point positioning.