A New 10-mg SMA-Based Fast Bimorph Actuator for Microrobotics

TL;DR

The paper presents a 10 mg SMA-based bimorph actuator enabling bidirectional, high-bandwidth actuation for microrobotics and demonstrates its viability with the FRISSHBot swimmer. The authors detail a three-layer Cu-FR4/CF fabrication approach, a small actuation angle to prevent SMA collision, and integration into a 30 mg, 34 mm microswimmer featuring a rigid head and soft tail. Characterization shows operation up to 20 Hz, with low-frequency displacements around 3.5 mm unimorph and 7 mm bimorph, and a PWM-driven power profile suitable for onboard power systems. Locomotion experiments reveal forward speeds up to 3.06 mm/s at 4 Hz, evidencing promising applicability of the bimorph actuator for autonomous aquatic microrobots and motivating further optimization of geometry and high-temperature SMA variants.

Abstract

We present a new millimeter-scale bimorph actuator for microrobotic applications, driven by feedforward controlled shape-memory alloy (SMA) wires. The device weighs 10 mg, measures 14 mm in length, and occupies a volume of 4.8 mm3, which makes it the lightest and smallest fully functional SMA-based bimorph actuator for microrobotics developed to date. The experimentally measured operational bandwidth is on the order of 20 Hz, and the unimorph and bimorph maximum low-frequency displacement outputs are on the order of 3.5 and 7 mm, respectively. To test and demonstrate the functionality and suitability of the actuator for microrobotics, we developed the Fish-&-Ribbon-Inspired Small Swimming Harmonic roBot (FRISSHBot). Loosely inspired by carangiformes, the FRISSHBot leverages fluid-structure interaction (FSI) phenomena to propel itself forward, weighs 30 mg, measures 34 mm in length, operates at frequencies of up to 4 Hz, and swims at speeds of up to 3.06 mm/s (0.09 Bl/s). This robot is the lightest and smallest swimmer with onboard actuation developed to date.

Figures (8)

• Figure 1: Photograph of the Fish-&-Ribbon--Inspired Small Swimming Harmonic roBot (FRISSHBot). The FRISSHBot is a $30$-mg microswimmer driven by the new $10$-mg SMA-based fast bimorph actuator introduced in this paper.
• Figure 2: Robotic design, actuation functionality, and locomotion. (a) The FRISSHBot is composed of four main components: (i) an SMA-based bimorph actuator, installed with integrated fabrication features, that drives the swimmer's propulsor; (ii) two pads made of CF that support the weight of the swimmer on water using surface tension; (iii) a rigid head that, interacting with water, generates high lateral drag relative to that produced by the swimmer's tail; and, (iv) a soft tail made of $25$-µm-thick fluoropolymer film~(AirTech A4000R14417) and with a shape bioinspired by forked-caudal fins. We hypothesize that the rigid head, by producing high lateral drag, anchors the body of the swimmer while the tail undulates in the surrounding fluid. The SMA wires that drive each subsystem of the bimorph actuator are installed symmetrically with respect to the device's longitudinal axis with an angle $\alpha$ (see inset). This design feature prevents the relaxed SMA wires at one side of the actuator from colliding with the central CF beam of the device's structure when the SMA wires at the other side contract due to Joule heating. (b) Functionality of the proposed SMA-based bimorph actuator and microswimmer. The actuator was conceived to continuously transition between the two possible directions of deformation---up and down quantized states---corresponding to the sequential contraction and relaxation of the SMA subsystems at each side of the device. During locomotion, the bimorph actuator oscillates the swimmer's soft tail with large amplitudes relative to those excited for the rigid head. We hypothesize that hydrodynamic interactions between the flows generated by the FRISSHBot's head and tail produce the thrust required for forward propulsion.
• Figure 3: Fabrication of the proposed SMA-based bimorph actuator and FRISSHBot.(a) The actuator is fabricated in three steps. In Step 1, a multi-material stack composed of three premachined layers is aligned and fastened using four pins according to the technique described in BeePlus_2019, and then secured with CA glue; the top and bottom layers are made of Cu-FR4 material, and the central layer is made of CF. In Step 2, using the technique described in SMALLBug_2020, SMA NiTi wires are tied in tension, and secured using simple knots and drops of CA glue, to two opposite sides of the assembled stack over the CF structures that become the central beam-springs of the actuators after fabrication; the diameter and nominal transition temperature of the SMA wires are $38.1$ µm and $90\,^{\circ}\text{C}$, respectively. In Step 3, the actuators are released from the assembled multi-material stack using a $3$-W $355$-nm DPSS laser~(Photonics Industries DCH-$355$-$3$). (b) The FRISSHBot is fabricated according to the following procedure. First, the soft tail is cut from fluoropolymer film (AirTech A4000R14417) and mechanically connected to the bimorph actuator using integrated assembly tabs. Then, the support pads, head, and connecting tabs are cut out of CF layers and assembled to create the swimmer. Last, we connect $52$-AWG tether wires to the SMA subsystems at each side of the bimorph actuator, using conductive silver epoxy (MG Chemicals $8331$D).
• Figure 4: Experimental setup used to characterize the proposed $\boldsymbol{10}$-mg SMA-based bimorph actuator for microrobotics. To assess the functionality, performance, and operational range of the actuator, we employed a laser displacement sensor (Keyence LK-G$32$), a Mathworks Simulink Real-Time host--target system equipped with an AD/DA board (National Instruments PCI-$6229$), and a MOSFET circuit board (four-channel YYNMOS-$4$). In this case, the host--target configuration generates the PWM signals that control two independent channels of the MOSFET circuit board used to excite the actuator. As seen, the power required by the MOSFET circuits to amplify the PWM signals is provided by an external power-supply unit. Essentially, the two PWM voltages outputted by the AD/DA board open and close the two channels of the MOSFET circuit board that function as switches for the power provided by the external power-supply unit that excite the SMA wires of the actuator. In this scheme, the SMA materials of the subsystems at both sides of the actuator are periodically Joule heated and allowed to cool down to induce transitions from the detwinned martensite phase to the austenite phase, and vice versa. As seen in the inset, during the performance of the characterization tests, one distal end of the bimorph actuator is held precisely aligned under the laser sensor, which outputs a voltage proportional to the actuator displacement. Also, we place the tested actuator at a distance from the sensor such that the zero output corresponds to the center of the measurement range. Last, the output displacement measurement is sampled and recorded by the host--target system using the AD/DA board of the target computer. All signals transmitted between the components of the experimental setup are generated and sampled at a frequency of $2$ kHz.
• Figure 5: Dynamic responses of the tested SMA-based bimorph actuator.(a) Ten cycles of the steady-state actuator response to a PWM excitation with frequency of $1$ Hz and DC of $10\,\%$. In this test, we measured an AMADO value of $7.08$ mm. (b) Ten cycles of the steady-state actuator response to a PWM excitation with frequency of $5$ Hz and DC of $10\,\%$. In this test, we measured an AMADO value of $1.83$ mm. (c) Ten cycles of the steady-state actuator response to a PWM excitation with frequency of $10$ Hz and DC of $10\,\%$. In this test, we measured an AMADO value of $0.56$ mm. (d) Ten cycles of the steady-state actuator response to a PWM excitation with frequency of $15$ Hz and DC of $10\,\%$. In this test, we measured an AMADO value of $0.28$ mm. (e) One-second photographic sequence of actuator response to a PWM excitation with frequency of $1$ Hz and DC of $10\,\%$. Video footage of these experiments can be viewed in the accompanying supplementary movie.