Design for low friction and hysteresis
Minimization of friction and virtual play in high-precision structures, connections and guideways
Hysteresis is the dependence of the state of a system on its history. This is detrimental for precision positioning because the position-force relation is not unique, causing position uncertainty which is generally difficult to compensate for.
Effects that contribute to hysteresis are (1) backlash, (2) friction, (3) micro slip, and (4) material hysteresis. Viscous damping does not contribute to hysteresis because the forces are only velocity dependent and history independent. Statically it does not influence positioning, and in motion the forces can be obtained without history.
This chapter will illustrate a number of techniques to either avoid or to reduce hysteresis. Backlash results in large position uncertainty and therefore it should be avoided in precision applications. A number of principles will be shown. Friction in positioning causes virtual play which should be minimized. Virtual play in precision applications is typically experienced in positioning against end stops and in positioning using kinematic couplings. If a load is transferred by means of friction, a certain area with distributed normal force is required to generate enough normal force to transfer the entire load without slip. If the load is not directly acting on the entire area but is gradually transferred by friction between two surfaces a portion of the area in contact may initially slip if the local stiffness of the parts in contact are unequal. This is called micro slip, which is typically experienced in the clamping and bolting of parts and specifically leafsprings. Material hysteresis is the primary cause of rolling resistance when using rolling elements. This is attributed to the viscoelastic characteristics of the material of the rolling element. The hysteresis in positioning systems can be mitigated by using air bearings, or minimized by using flexures shown in chapter 3.