Haptic Light-Emitting Diodes: Miniature, Luminous Tactile Actuators

Abstract

We present Haptic Light-Emitting Diodes (HLEDs), luminous thermopneumatic actuators that directly convert pulsed light into mechanical forces and displacements. Each device packages a miniature surface-mount LED in a gas-filled cavity that contains a low-inertia graphite photoabsorber. The cavity is sealed by an elastic membrane, which functions as a working diaphragm. Brief optical pulses heat the photoabsorber, which heats the gas. The resulting rapid pressure increases generate forces and displacements at the working diaphragm. Millimeter-scale HLEDs produce forces exceeding 0.4 N and displacements of 1 mm at low voltages, with 5 to 100 ms response times, making them attractive as actuators providing tactile feedback in human-machine interfaces. Perceptual testing revealed that the strength of tactile feedback increased linearly with optical power. HLEDs devices are mechanically simple and efficient to fabricate. Unusually, these actuators are also light-emitting, as a fraction of optical energy is transmitted through the membrane. These opto-mechanical actuators have many potential applications in tactile displays, human interface engineering, wearable computing, and other areas.

Figures (5)

• Figure 1: Design and operating principle of the Haptic Light-Emitting Diode. (a) Light from the LED is converted into heat by the photoabsorber. Heat is transferred to air in the cavity, raising gas temperature $T$ and pressure $P$. Gas expansion drives the deflection of the elastic membrane. (b) The HLED is an assembly of patterned layers, supporting manufacturability. From top: Elastic membrane (PDMS), cavity walls (Aluminum), thin photoabsorber (pyrolytic graphite sheet suspended on nichrome wires), cavity walls (PS), PCB with an LED. (c) Photo of a 3x3 array of HLEDs with the elastic membrane partially removed. (d) Photo of PGS threaded on NiCr wires.
• Figure 2: (a) Experimental configurations: Finite element analysis (FEA), isometric force measurement, and free-displacement measurement. (b) Photoabsorber temperature $T_\mathrm{abs}(t)$, cavity air temperature $T_\mathrm{air}(t)$, isometric force $F(t)$, and free displacement $z(t)$, for a 100-ms, 2.5-W optical pulse. Solid black lines are laboratory measurements, except for air temperature, which is obtained after application of the ideal gas law. Dashed line and shaded region: numerical (FEA) results, mean response and variation envelope obtained by sweeping geometric and material parameters across uncertainty ranges. (c) Force, $F(t)$, for pulse durations $t_p = 25, 50, 75$ ms at optical power $P_L=2.5$ W. Laboratory measurements. (d) Peak measured force as a function of LED optical power $P_L$ (Error bar: standard deviation, $n=4$).
• Figure 3: (a) Isometric force responses to pulse trains at pulse rates $f = 5$, $10$, and $25$ Hz (top to bottom) at duty cycle $t_p/T=0.3$. (b) Peak-to-peak force component $F_{pp}$ as a function of pulse rate $f$ with duty cycle $t_p/T=0.2$. Regression fit: $\log_{10}(F_{pp})=\alpha\log_{10}(f)+\beta$, $\alpha=-1.08$, $\beta=2.83$, $r^2=0.99$. (c) Mean free displacement is consistent over 13,000 actuations. Each dot corresponds to the mean of 100 peaks.
• Figure 4: (a) Transient surface temperature rise for duty cycles of 0.1, 0.2, and 0.3 during a 2.5-s actuation window, followed by passive cooling. (b) Adding an additional PGS heat-spreading layer beneath the PDMS membrane suppresses the temperature rise across all duty cycles.
• Figure 5: The perceived intensity of a pulse train from a single HLED increased linearly with optical power $P_L$. Regression fit: $I = \alpha P_L+\beta$, $\alpha = 0.0197$, $\beta = -0.2693$, $r^2 = 0.99$.