A. Harnoy

Implementation of a friction estimation and compensation technique

A friction estimation and compensation technique was implemented on a laboratory apparatus designed to permit the direct measurement of friction. Experimental results are reported for a friction observer which estimates total friction present assuming it to be a constant times the sign of velocity. A second observer is used to estimate the velocity using the measured position of the rotating shaft in the apparatus, when velocity is not measurable. Experimental results show that the friction estimate is consistent with the measured friction, displaying the theoretical hysteresis phenomenon. Moreover, the performance of the system is substantially improved by the use of the estimated friction to compensate the system, especially at very low velocity.

Modeling and simulation of elastic and friction forces in lubricated bearings for precise motion control

Abstract The study presents a dynamic model for the resistance forces to the rotation of a journal in a lubricated sleeve bearing at low speed. The resistance forces include the sliding friction as well as the presliding Dahl effect, namely, elastic force due to the compliance in the system before the force reaches the break-away magnitude when sliding initiates. Both the stiffness of the surface asperities and the elastic support of the bearing are considered for the presliding rotation of the journal. The model is based on the physical principles of hydrodynamic lubrication as well as the dynamics of the system. The model shows that the instantaneous friction is not only a function of the velocity at that instant, but also depends on the velocity history. The model predicts hysteresis-type friction curves for oscillating velocity, in agreement with previous experiments. The model may be useful to improve the precision of motion in feedback control systems with friction.

Modeling and measuring friction effects

Friction may well be natures most useful phenomenon. In machinery in which it is not the driving force, however, friction is an undesirable parasitic phenomenon, generating heat and wasting energy. Large sums are spent each year on lubricants to eliminate as much friction as possible in mechanical devices. On the other hand, when friction is the source of traction and braking, it is important to keep friction at a high level. To achieve this end, much effort and funding are expended on projects such as improving tires and antilock brakes. The goals of this article are first to review some of the physical characteristics of friction, especially the need for state-space dynamic models; then to describe apparatus for measuring friction effects and to present the results achieved using this apparatus; and, finally, to discuss some of the issues relating to measurement of friction effects.

Dynamic Friction Model of Lubricated Surfaces for Precise Motion Control

A model is developed to describe dynamic friction effects in lubricated surfaces. The model covers the hydrodynamic, mixed and boundary lubrication regions. The dynamic friction model can predict the friction force for time-varying velocity, and is useful in precise motion control. The model presented is for a short journal bearing, but can be extended to other geometries of sliding surfaces, such as point and line contacts or rolling element bearings. The friction is related to a time variable fluid film thickness, resulting from journal vibrations relative to the sleeve. The proposed model agrees qualitatively with experimental results for lubricated line contact. Both show similar hysteresis-type friction curves under oscillating velocity. Presented as a Society of Tribologists and Lubrication Engineers paper at the STLE/ASME Tribology Conference in New Orleans, Louisiana, October 24–27, 1993

Second Order, Elastico-Viscous Lubricants in Dynamically Loaded Bearings

The hydrodynamic theory of lubrication for dynamically loaded journal bearings is extended to elastico-viscous lubricants. The Rivlin-Ericksen equation is assumed for the lubricant, with the second-order terms representing cross-stresses and stress relaxation. The orbits of the journal center deviating from its static equilibrium position were calculated for constant and periodic load conditions. A considerable improvement, compared with the Newtonian solution, is obtained. This is evident from the diminished eccentricity of the orbits and the faster damping of disturbances.

Investigation of Elastico-Viscous Hydrodynamic Lubrication of Sleeve Bearing

An experimental investigation of the pressure distribution in a full film hydrodynamic sleeve bearing was carried out. The pressure profiles of nearly-Newtonian and significantly elastico-viscous fluids were compared. The results verify previous theoretical predictions of a small additional elastico-viscous component, perpendicular to the “Newtonian component.” For the case investigated viscoelasticity does not contribute significantly in steady state to the LCC, which is determined practically by the highest average rate of shear UIC (1-e) Presented as an American Society of Lubrication Engineers paper at the ASLF/ASME Lubrication Conference held in Miami Beach, Florida, October 21–23, 1975

Apparatus for empirical determination of dynamic friction

An apparatus for measuring dynamic friction in a lubricated journal bearing is described. This apparatus has been designed, constructed and used to measure friction in the presence of sinusoidally-varying velocity at various frequencies. Experimental results tend to confirm theoretical predictions which are based on the hydrodynamic lubrication theory.

Feasibility Study of Unconventional Cooling of Electronic Components by Vibrating Plates at Close Proximity

ABSTRACT In this study, an unconventional approach to electronic cooling is described, and its feasibility is investigated. This approach makes use of the transverse oscillations of a short plate placed in close proximity to the electronic component to provide for focused and localized cooling. This method can be applied for intermediate cooling rates, above those of natural convection, in order to extend the no-fan regime. To estimate the cooling potential, a numerical simulation of the temperature field in a channel formed by an oscillating surface and a fixed constant heat flux surface is performed. The results show that transverse oscillations have the potential for significant cooling enhancement compared to pure natural convection. The cooling effect is found to increase with the oscillation amplitude and channel length to mean width ratio and, to a lesser extent, with the oscillation frequency.

Optimization of grain drying — With rest-periods

Abstract A method of drying grain with alternate periods of blowing hot air through the grain bed (bin) followed by rest-periods is considered. An attempt is made to give the basic relationship between the “blowing ratio”, that is, the ratio of the cycle time to the blowing time, and the efficiency of drying for various temperatures and air velocities through the grain. The effect of this drying method on the final quality of the grain is also discussed. The practical application of the above drying system was considered in a multi-bin drier where the number of bins was equal to the value of the blowing ratio. It was established that this system, operating at its optimum blowing ratio, was more economical than the conventional system with continuous blowing.

Modeling of Dynamic Friction in Lubricated Line Contacts for Precise Motion Control

A simple dynamic friction model for an elastohydrodynamic lubrication sliding and rolling line contact has been developed. This model uses the technique introduced earlier by Harnoy and Friedland (1). The model includes low-velocity regions where friction is a combination of contact and elastohydrodynamic friction. The study shows that the time-variable friction is not only a function of instantaneous sliding velocity, but is also a memory function of the velocity history. Simulation of the model for an oscillating velocity exhibits similar hysteresis effects in friction-velocity curves as observed earlier in several experimental studies. The model can be useful for friction compensation to enhance the precision of motion in control systems.