Friction Reduction through Ultrasonic Vibration Part 2: Experimental Evaluation of Intermittent Contact and Squeeze Film Levitation

Abstract

In part 1 of the current study of haptic displays, a finite element (FE) model of a finger exploring a plate vibrating out-of-plane at ultrasonic frequencies was developed as well as a spring-frictional slider model. It was concluded that the reduction in friction induced by the vibrations could be ascribed to ratchet mechanism as a result of intermittent contact. The relative reduction in friction calculated using the FE model could be superimposed onto an exponential function of a dimensionless group defined from relevant parameters. The current paper presents measurements of the reduction in friction, involving real and artificial fingertips, as a function of the vibrational amplitude and frequency, the applied normal force and the exploration velocity. The results are reasonably similar to the calculated FE values and also could be superimposed using the exponential function provided that the intermittent contact was sufficiently well developed, which for the frequencies examined correspond to a minimum vibrational amplitude of ∼ 1 µm P-P. It was observed that the reduction in friction depends on the exploration velocity and is independent of the applied normal force and ambient air pressure, which is not consistent with the squeeze film mechanism. However, the modelling did not incorporate the influence of air and the effect of ambient pressure was measured under a limited range of conditions, Thus squeeze film levitation may be synergistic with the mechanical interaction.

Why pens have rubbery grips

“…The illusion of a ridge (33) may be created by rapidly increasing friction during the exploration by a finger as a result of decreasing the amplitude of vibration or increasing the electrostatic field (3). The strength and stability of this effect are dependent on the contrast in friction that can be induced, which has critical implications for power requirements (34,35).…”

Tactile Perception of Virtual Edges and Gratings Displayed by Friction Modulation via Ultrasonic Actuation

“…Two different mechanisms have been suggested to explain the cause of friction reduction in ultrasonic tactile displays; Watanabe et al [2] proposed that the friction reduction is due to the formation of a squeezed film of air between finger and the surface. Alternatively, it has been suggested that, when a surface vibrates at an ultrasonic frequency, an intermittent mechanical contact develops between finger and the surface such that the finger bounces on the surface while sliding [3]- [5]. A recent study conducted by Wiertlewski et al [6] using a stroboscope revealed that both mechanisms indeed contribute to the friction reduction.…”

“…Hence, in order to modulate friction on a touch surface properly, it is important to understand our tactile perception of change in friction and the contact mechanics behind it. Although, the earlier studies have investigated the contact mechanics of friction [5], [10]- [17] and our perceptual ability to discriminate two different surfaces based on friction [18]- [20], the number of studies investigating the intermittent contact mechanics during a change in friction on a touch surface is very limited [21]- [24]. Furthermore, the factors affecting our tactile perception of this change have not been investigated in depth yet [22]- [26].…”

“…Their results also showed that the finger velocity affects the friction coefficient at lower normal loads. On the other hand, Sednaoui et al [5] reported that reduction in friction coefficient using ultrasonic vibrations depends on exploration velocity (tested at 20, 40, and 80 mm/s) but independent of normal force (tested at 0.2, 0.5, and 0.75 N). In [21], Cornuault et al have investigated the friction contrast on real and ultrasonically actuated surfaces.…”

The Perception of Ultrasonic Square Reductions of Friction With Variable Sharpness and Duration

“…One such technique is electrovibration, which creates an electrostatic attraction between the finger and the actuated surface and increases the finger-surface friction [1], [2]. Another technique, ultrasonic lubrication [3] is able to reduce the finger-surface friction by generating a squeeze film effect and an intermittent contact with the surface [4]- [6] either through a stationary wave on the whole surface or through evanescent waves within a precise location [7]. These technologies have very different means of action but they nevertheless induce a change in the dynamic friction between the finger and the tactile plate, which is similarly perceived [8].…”