Actuators hysteresis

If actuator hysteresis effects cause excessive pH fluctuation, consider reducing valve size, using parallel valves, or using specialized reagent addition equipment. It is not likely that such effects will require an increase in backmixed volume as the 1% hysteresis associated with valves using positioners is unlikely to be the worst disturbance to exit pH with sensibly sized equipment (see Section V.A.2). 

Open loop operations with high accuracy remain possible if the behaviour of the piezo actuator (hysteresis, drift effect) is well known, and if the command applied to the piezo is known [16]. Two examples have been recently investigated in active optics ... 

Static performance measurements related to positioner/actuator operation include the conformity, measured accuracy, hysteresis, dead band, repeatability, and locked stem pressure gain. Definitions and standardized test procedures for determining these measurements can be found in ISA-S75.13, "Method of Evaluating the Performance of Positioners with Analog Input Signals and Pneumatic Output. ... 

Trip Valves The trip valve is part of a system used where a specific valve action (i.e., fail up, fail down, or lock in last position) is required when pneumatic supply pressure to the control valve falls below a preset level. Trip systems are used primarily on springless piston actuators requiring fail-open or fail-closed action. An air storage or "volume tank and a check valve are used with the trip valve to provide power to stroke the valve when supply pressure is lost. Trip valves are designed with hysteresis around the trip point to avoid instability when the trip pressure and the reset pressure settings are too close to the same value. 

The nonlinearity and hysteresis have a profound influence on application of piezoelectric sensors and actuators, particularly in high precision devices. Details and additional references can be found in [4],... 

Electrostrictive materials offer important advantages over piezoelectric ceramics in actuator applications. They do not contain domains (of the usual ferroelectric type), and so return to their original dimensions immediately a field is reduced to zero, and they do not age. Figure 6.24(a) shows the strain-electric field characteristic for a PLZT (7/62/38) piezoelectric and Fig. 6.24(b) the absence of significant hysteresis in a PMN (0.9Pb(Mg1/3Nb2/303-0.1 PbTi03) electrostrictive ceramic. 

Crytal chemitry. The effect of solid solution on the transition behavior of perovskite (ABX3) structures has been intensively scrutinized for more than 50 years. These materials have merited continuous attention because of their enormous technological versatility. As multilayer capacitors, piezoelectric transducers, and positive temperature coefficient (PTC) thermistors they generate a market of over 3 billion every year (Newnham 1989, 1997). In addition to ease of fabrication, these compounds exhibit a number of attributes required of ideal actuators (1) They display very large field-induced strains (2) They offer quick response times and (3) Their strain-field hysteresis can be chemically controlled to be very large or negligibly small, depending on the application. Details of their technical applications can be found in Jaffe et al. (1971) and Cross (1993). 

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