Electrohydraulic Actuator for a Soft Gripper

Abstract

There is a considerable demand to develop robots that can perform sophisticated tasks such as grabbing delicate materials, passing through narrow pathways, and acting as mediators between humans and robots. Soft robots can provide a solution for such applications. In this study, we propose an electrohydraulic gripper, which is based on electrostatic and hydraulic forces. Interestingly, the gripper generates a hydraulic force without an external fluid supply source. In addition, it achieves good compliance, because the gripper is composed of soft materials such as polyethylene film and silicone. We experimentally investigate the characteristics of the actuator of the gripper. In addition, the electrohydraulic gripper demonstrates an ability to grasp delicate materials.

Introduction

Conventional robots consist of rigid parts and materials, limiting the deformation and adaptation of their body to the external environment.^1–3 It is difficult for typical robots composed of stiff materials to perform tasks such as handling delicate materials and passing through narrow pathways, although there are requirements for such functions in many fields of industry.^3–5 This trend has also been observed for the robotic grippers, which are devices that have the ability to pick up and hold items. Traditional robotic grippers are composed of rigid links and joints and have difficulty grasping delicate materials, adapting to the external environment, and acting as mediators between humans and robots.^6–9 Furthermore, robots composed of rigid parts are designed specifically for each target object type, which limits the versatility of such robotic grippers.⁹

Recently, soft grippers, which are constructed entirely of soft elements, have received considerable attention, owing to their capability to deal with deformable and fragile objects and adaptability to uneven shapes.^6,7,10 Soft grippers often exploit new types of actuators to achieve the development of robotic gripper systems that can interact harmlessly with humans and unstructured surroundings and handle unexpected tasks. Pneumatic, dielectric elastomer (DE), and shape memory alloy (SMA) actuators are representative for soft gripper systems.^9,11–13 Pneumatic actuators are prevalent, owing to their large actuation range and force, and they are easily fabricated to suit desired shapes and motions.^14–17 However, pneumatic actuators essentially require an external and massive air pressure source, such as a compressor, which hinders the installation of grippers based on pneumatic actuators in robotic systems.^18–20

DE actuators, operated by electrostatic forces, exhibit relatively high actuation speeds and a wide range of strains.^21–24 Despite these advantages, there are several obstacles to utilizing DE actuators. Specifically, a strong electric field is required to drive a DE actuator. In addition, an elastomer must be prestretched to provide a considerable actuation strain.^25,26 Without prestretching, the actuation strain of a DE actuator becomes noticeably smaller.²⁷ Moreover, the actuation mode of a DE actuator is typically limited in elongation.²⁴ Various actuation modes can be achieved through complicated fabrication methods and processes, such as stacking DE actuators, which leads to an increase in the possibility of electrical failure.²⁸

SMA actuators provide a large actuation force and also achieve a high power density.^29–31 SMA has many advantages over conventional actuators, but there are a series of limitations that can hinder practicable applications. Because SMA is fundamentally activated by thermal source, precise control is relatively difficult and has low efficiency.^29,30

Besides the abovementioned actuators, ionic polymer-metal composites (IPMCs) are an additional class of actuator used for soft robotic systems.^32–35 IPMC actuators have a large bending displacement with low voltage applied on the actuator (typically 1–5 V on the mm scale).^36,37 In addition, they can be utilized in aqueous environments, and they enjoy the advantage of biocompatibility. However, the electrode material used in IPMCs is expensive, and chemical treatment in the fabrication is inevitable.³⁸

In this study, we introduce a soft actuator that is operated by electrohydraulic force and present a robotic gripper using this electrohydraulic actuator. This gripper achieves high compliance because it consists of soft materials and has the characteristics of a fluid actuator. The electrohydraulic actuator is designed to exploit the features of hydraulically amplified self-healing electrostatic (HASEL) actuators.^39,40 A HASEL actuator uses a liquid dielectric as a working fluid, to prevent electrical breakdown and contraction motion of the actuator based on the Peano actuator.⁴¹ We develop a bendable actuator to be utilized as a unit module for grippers while having the electrical characteristics of a HASEL actuator. Furthermore, we experimentally investigate the characteristics of the actuator and gripper. In particular, by varying the fluid volume inside the actuator and the operation frequency, we examine noticeable characteristics of the displacement and blocking force. In addition, the ability and performance of the gripper are evaluated by grasping various objects with different properties to confirm the possibility of usage in industrial fields.

Materials and Methods

Design and operation principle of an actuator

Fundamentally, the proposed actuator for a soft gripper utilizes a combination of electrostatic and hydraulic phenomena (as depicted in Fig. 1A). The actuator consists of three major parts as follows: a series of connected pouches that are nonstretchable and flexible, flexible electrodes, and a thin silicone backbone plate. The series of pouches is divided into two parts as follows: a supplying pouch, which is filled with dielectric fluid before activation, and swelling pouches, which are inflated after activation. The flexible electrodes are attached to either side of the supplying pouch. To minimize the possibility of the electrical damage on surrounding environments, the actuator is designed such that the electrodes are attached only from the supplying pouch. A silicone backbone plate is placed on one side of the swelling pouches. An image of the actuator with the trace markers is presented in Figure 1B. The detailed dimensions of the parts are shown in Supplementary Figure S1.

FIG. 1.

Actuation principle and design of actuator. (A) Principle of electrohydraulic actuation and main components of the actuator. (B) Image of the actuator and the positions of trace markers. Color images are available online.

The operation principle is illustrated in Figure 1A. When both electrodes on the supplying pouch are oppositely charged by an external high-voltage input, the electrodes progressively pull each other (zipping motion) through an electrostatic force and push the dielectric fluid from the supplying pouch to the swelling pouches. Then, the pressure in the swelling pouches increases. The pressure in all swelling pouches is equal, because they are connected through a channel. In idealized case of flat electrodes, the hydraulic pressure in the pouches is coupled to the Maxwell pressure ( $P \sim; E^{2}$ ), where ε is the dielectric permittivity of the dielectric fluid, and E is the electric field.⁴² We comment that the hydraulic pressure can be affected by the location and amount of zipping because of the dielectric fluid. The pressure increase in the swelling pouches causes them to inflate, and the cross-section of the pouch changes from a thin ellipse to a thick ellipse. This shape change causes the silicone backbone plate to bend, resulting in angular deflection of the actuator.

Actuator materials

The film used for the actuator module of the gripper was constructed from a 30 μm thick polyethylene (PE) film. The dielectric breakdown strength of the PE film in this study was found to be over 330 Vμm⁻¹. The dielectric fluid in this study was Mictrans A, which was purchased from Michang. Mictrans A is based on uninhibited mineral oil and is an electrical insulating product with a high dielectric breakdown voltage (>50 kV at a 2.5 mm gap) and high resistivity. The oil also has an excellent oxidation stability and is noncorrosive for large amounts of time. Carbon double-sided tape was used for the electrodes, which is purchased from Nisshin EM Co., Ltd. The backbone plate supporting the gripper was constructed from polydimethylsiloxane (PDMS), where the ratio of the hardener to the subject was 1:20.

Fabrication procedure

There are four main components used to fabricate the electrohydraulic actuator: PE film for the pouches, dielectric fluid with a high electric breakdown voltage, silicone backbone plate formed of PDMS, and flexible electrodes (Fig. 2A). Figure 2B illustrates the fabrication procedure. First, two PE films were bonded using a commercial thermal bonding machine (SK-210; SAMBO Tech.) along the pattern illustrated in Supplementary Figure S1, that is, heat sealed. The heating temperature is 72°C, and the heating time is approximately 0.4 s.

FIG. 2.

Fabrication procedure for the electrohydraulic actuator. (A) Major components of the actuator. (B) Bonding of the polyethylene films using a commercial heat-sealing machine. (C) Filling the pouches with dielectric fluid. (D) Covering the supplying pouch by flexible electrodes and attaching a thin silicone plate to one side of the swelling pouches. (E) Completed actuator. Color images are available online.

After constructing the pouches by bonding the films, they are filled with a dielectric fluid. Then, the silicone backbone plate was attached to one of the surfaces of the swelling pouches. Finally, the flexible electrodes were attached to both sides of the supplying pouch.

Experimental setup and method

Our experimental study consisted of four measurements: pressure, voltage, optical, and force measurements. To quantify the pressure in the actuator, we connected the pouch and a pressure gauge (7MF4433; Siemens AG) using urethane tube. The power supply (MK-200002B; MK POWER, Inc.) to apply the voltage to the electrodes was controlled by a waveform generator (33500B; Keysight Technologies). The electrodes on the supplying pouch work as a capacitor. To release the electric charge between the electrodes, two different resistors (50 kΩ, 100 MΩ) were installed between the power supply and electrodes in parallel. The input voltage was measured using the resistor of 50 kΩ. We measured the resistance value of the resistors using a precise multimeter, for accuracy (DMM7510; Keithley Instruments). The uncertainties of the instrument are 0.0065% for 50 kΩ and 0.26% for 100 MΩ, respectively. We measured the voltage across the smaller resistor and calculated the voltage applied to the actuator using the ratio of the two resistors. All of the data were acquired by a data acquisition board (NI 9229; National Instruments Co.) at a frequency of 2000 Hz. The schematics of the experimental setup are depicted in Supplementary Figure S2. In addition, a camera (DSC-RX10M3; Sony Co.) was utilized to obtain the optical data (Supplementary Fig. S2A). The camera captured images at a rate of 120 frames per second. A load cell (GS0-500; Transducer Techniques) was installed to measure the blocking force of the actuator (Supplementary Fig. S2B). To obtain the displacement data, the optical data were processed by a color tracking code (Matlab2018b; Mathworks).

Results

Displacements of electrohydraulic actuator

Figure 3 depicts the two-dimensional coordinate system of the actuator, where x is the lateral axis of the trace markers on the actuator and y is the vertical axis. Each coordinate is normalized by dividing through by the length of the swelling pouches L (Fig. 3A). Specifically, in the actuator the fluid volume ( $V_{p}$ ) in the pouches was half of the volume of the perfect circle ( $V_{c}$ ) that had the same geometry as the swelling pouches ( $V_{p}$ / $V_{c}$ = 0.5) (Supplementary Fig. S3). The frequency of the input signal was f = 1/T = 0.1 Hz. The linear strain of the pouch at the end of the actuator is 4.6%. The normalized coordinate for the four trace markers on the actuator is illustrated in Figure 3B and C, where Figure 3B shows the activation process, and Figure 3C shows the recovery process. The driving voltage signal is sinusoidal with amplitude of DC 5.5 kV ± AC 5.5 kV and the frequency of 0.1 Hz. In particular, the first marker M₁ is the origin, which is fixed to the floor. As shown in Figure 3B and C, the points M₃ and M₄ have a large range in their motion while range of point M₂ is rather small. When a sine wave voltage input is applied, the actuator begins to move after a momentary inactive period (0–3/T), where T is the period of the driving signal. Meanwhile, as the actuator moves downward, it does not undergo an inactive period.

FIG. 3.

Displacement characteristics of the electrohydraulic actuator. (A) Time-lapse photograph according to the actuator movement and the parameters and coordinate system for the experiments. (B) Normalized displacement of four trace markers on the actuator as the actuator is moving upward. (C) Normalized displacement of four trace markers on the actuator as the actuator is moving downward. (D) Normalized displacement at the tip of the actuator overlapped with the voltage applied to the electrodes for one period. Color images are available online.

Figure 3D shows the input voltage (V) and normalized displacement difference (Δy/L) of the marker M₄ on the actuator over one cycle. The time (t) is nondimensionalized by the period of the input sine wave signal (T). Furthermore, Δy is defined as the difference between the position of the actuator in the actuating process and the vertical position in the initial stage (Fig. 3A). After the voltage is applied to the actuator, we also observe that a plateau in the normalized displacement occurs for 0.0 < t/T < 0.3. Subsequently, the normalized displacement increases to 0.33 at 0.54 t/T and then decreases monotonically.

Effect of fluid volume variation

The amount of internal fluid can affect the operational performance, because the actuator is activated using limited internal working fluid, unlike conventional hydraulic actuators where the working fluid is supplied by external instruments. Herein, we conducted several experiments to determine the characteristics of the electrohydraulic actuator depending on the fluid volume in the actuator. In this experiment, the actuator performance was compared when $V_{p}$ / $V_{c}$ was 0.5, 0.6, and 0.7. The input signals applied to the actuators with $V_{p}$ / $V_{c}$ = 0.5, 0.6, and 0.7 were 0.1 Hz sine waves with amplitudes of DC 5.5 kV ± AC 5.5 kV, DC 6 kV ± AC 6 kV, and DC 6.5 kV ± AC 6.5 kV, respectively (Supplementary Movie S1).

Figure 4A shows the maximum displacement difference (Δy_m) of the point M₄ on the actuator and the maximum pressure difference (ΔP_m) in the pouch at 0.1 Hz. We utilized the maximum displacement difference divided by square of the adjusted voltage, which is obtained by subtracting 5 kV from the applied peak-to-peak voltage amplitude, because the displacement is proportional to the square of the applied voltage after 5 kV (Supplementary Fig. S4). We normalized the maximum pressure difference in the actuator using the applied peak-to-peak voltage amplitude, owing to the quadratic relationship between the pressure and the applied voltage.⁴² As the amount of dielectric fluid in the pouch increases, the maximum pressure difference per unit voltage squared (ΔP_m/V²) tends to decrease, and this tendency appears at the maximum displacement difference as per adjusted voltage squared (Δy_m/V_adj²). When a 10 kV voltage is applied to the actuator, the normalized displacement difference at $V_{p}$ / $V_{c}$ = 0.5 is higher, 53% and 38%, than $V_{p}$ / $V_{c}$ = 0.6 and 0.7, respectively. This shows that the actuator with $V_{p}$ / $V_{c}$ = $0.5$ is more efficient than in the other cases. This phenomenon can be observed in more detail in Figure 4B, which depicts the normalized displacement difference (Δy/L) of the endpoint (M₄) on the actuator against the input voltage at 0.1 Hz.

FIG. 4.

Displacement characteristics with fluid volume change in the actuator. (A) Maximum displacement per adjusted input voltage squared and pressure difference per input voltage squared for $V_{p}$ / $V_{c}$ = 0.5, 0.6, and 0.7 in the pouches. (B) Variation of displacement difference over voltage for $V_{p}$ / $V_{c}$ = 0.5, 0.6, and 0.7 in the pouches. Color images are available online.

We observe a hysteresis effect of the displacement, which could be due to the nonlinearity from the fluid in the pouch and flexibility of the electrodes. The electric field and electrostatic force between the electrodes can be changed by the location and amount of the dielectric fluid remained in the supplying pouch; the electrodes of the actuator act as a capacitor. The viscous effect of the dielectric fluid obstructs the flow of the dielectric fluid, which affects the position and quantity of the fluid remained in the supplying pouch at the recovery state.

Because the electrodes are deformable, the zipping area can be different at the same applied voltage. Specifically, when the actuator is in the actuation process, the electrostatic force in the contacting area of the electrodes is enough to enforce the dielectric fluid to flow into the swelling pouch, but the fluid in the area where the electrodes are detached stays in the supplying pouch due to the low electrostatic force. However, when the actuator is in the recovery cycle, the area where the electrodes are already attached is not separated from each other even if the applied voltage decreases because the electrostatic force is high enough to maintain zipping due to the close distance between the electrodes. It may be one of the causes of the hysteresis.

In the inactive state, where the input voltage is increasing until 7.5 kV, tiny variations in the normalized displacement difference are observed. After this state, a sharp increase in the normalized displacement difference occurs along with the voltage increase. In the region where voltage is decreasing, the normalized displacement difference decreases with a gentle curve compared with when the voltage increases. The trend of the normalized displacement difference slightly differs between the $V_{p}$ / $V_{c}$ = $0.5$ and $V_{p}$ / $V_{c}$ > $0.5$ cases. As the voltage increases, all cases exhibit the same trend in the normalized displacement difference except $V_{p}$ / $V_{c}$ = $0.5$ case. Meanwhile, as the voltage decreases, a plateau occurs in the normalized displacement difference (7.7 kV < V < 10.6 kV) in the case with $V_{p}$ / $V_{c}$ = $0.5$ . Following the plateau region, the normalized displacement difference decreases monotonically. In the case with $V_{p}$ / $V_{c}$ = $0.5$ , when the input voltage is over 10 kV the electrodes on the supply pouch completely attach to each other, and all the dielectric fluid in the supplying pouch flows into the swelling pouches. The plateau region might be linked to this perfect zipping, as well as the adhesive force owing to a thin liquid film.⁴³

Figure 5 illustrates the phase difference between the input signal and the tip displacement depending on the internal volume of the actuator. The phase difference between the two signals is determined using an algorithm based on zero-crossing detection. The phase delay derived from this method contains information of initial phase lag.⁴⁴ The phase difference (φ_y) tends to increase along with the fluid volume in the pouches. In the cases where the relative fluid volume ( $V_{p}$ / $V_{c}$ ) is 0.6 and 0.7, a larger phase difference is observed compared to when $V_{p}$ / $V_{c}$ = $0.5$ . This increase in phase difference may be ascribed to the low initial electrostatic force. As the volume in the actuator increases, the gap between the electrodes attached on either side of the supplying pouch increases in the initial state, resulting in the electrostatic force between the electrodes decreasing. We observe that the phase lag also occurs at low driving signal frequency. The phase lag can be attributed to insufficient starting electrostatic force pushing fluid between the electrodes.

FIG. 5.

Feature of the phase difference. Phase difference between the input voltage and the displacement of the endpoint on the actuator depending on the fluid volume in the actuator. Color images are available online.

In summary, actuators with lower fluid volumes exert higher displacements than the cases with higher fluid volumes. This means that the case with $V_{p}$ / $V_{c}$ = $0.5$ has a higher efficiency than the other cases. In addition, the phase delay for the case with a low fluid volume ( $V_{p}$ / $V_{c}$ = $0.5$ ) is shorter than for the cases with higher fluid volumes in the pouch.

Blocking force of the actuator

The force delivered by this actuator can be inferred from the blocking force measured at the edge of the actuator. We conducted experiments to measure the blocking force of the actuator. The input signal is a sine wave with a frequency of 0.1 Hz and amplitude DC 5kV ± AC 5kV. Figure 6 illustrates the blocking forces and pressure in the pouch according to the fluid volume ratio. The blocking force and pressure in the pouch are highest at $V_{p}$ / $V_{c}$ = $0.5$ and decrease as the fluid volume contained in the pouches increases. As the internal volume increases, the blocking forces in the cases with $V_{p}$ / $V_{c}$ = 0.6 and 0.7 decrease to 73% and 68% of that in the case with $V_{p}$ / $V_{c}$ = $0.5$ , respectively. The force per mass of the actuator is 8.57 N/kg in the case of 0.5 relative fluid volume.

FIG. 6.

Characteristics of the blocking force. Blocking force and pressure inside the pouches of the electrohydraulic actuator for three different fluid volumes in the pouches. Color images are available online.

Effect of actuation frequency

We investigated the actuation characteristics according to the frequency of the input voltage signal. For the electrohydraulic actuator, the local displacement and the pressure in the pouch were measured at three actuation frequencies (f = 0.1, 0.3, and 0.5 Hz). Based on the results in the previous section, we selected the fluid volume of $V_{p}$ / $V_{c}$ = $0.5$ . The nominal peak-to-peak voltage amplitude applied to the actuator is 11 kV, with a 10% variance (Supplementary Movie S2). The maximum displacement difference per adjusted voltage squared (Δy_m/V_adj²) and maximum pressure difference per voltage squared (ΔP_m/V²) according to the actuation frequency are illustrated in Figure 7A. In this study, both values decrease monotonically as the frequency increases. Figure 7B shows the normalized displacement difference (Δy/L) of the point M₄ on the actuator for one cycle. In all cases, the marker M₄ before the voltage is applied is placed at same point. Interestingly, in the cases with f = 0.3 and 0.5 Hz, the actuators do not return to the initial position. This may be explained by the fluid transport. In the recovery cycle, the fluid in the swelling pouch should flow back to the supplying pouch, owing to the elasticity of the silicone backbone plate. However, the fluid does not fully return, owing to the low-speed fluid transport. Therefore, the actuator at a high frequency exhibits a low displacement and does not recover to the initial position.

FIG. 7.

Actuation performance according to the frequency of the input voltage signal. (A) The maximum displacement difference per adjusted input voltage squared and the maximum pressure difference per input voltage squared. (B) Variation of the displacement over the voltage for f = 0.1, 0.3, and 0.5 Hz. Color images are available online.

Soft gripper using the electrohydraulic actuator

We demonstrate a soft gripper based on the electrohydraulic actuator. The gripper consists of two electrohydraulic actuators and an acrylic body to support the actuators (Fig. 8; Supplementary Fig. S5; Supplementary Movie S3). The gripper can handle fragile objects or delicate materials without complicated control. Figure 8 demonstrates the grasping ability of the electrohydraulic gripper. The gripper can pick up a sphere ball (Supplementary Movie S4). In addition, the gripper can handle delicate materials, such as aluminum foil, a marshmallow, and a balloon in the vertical (Fig. 8A) and horizontal positions (Fig. 8B; Supplementary Movies S5–S7). The results demonstrate a notable ability to grasp objects with various material properties.

FIG. 8.

Grasping performance. (A) Grasping of a table tennis ball, aluminum foil, a marshmallow, and a balloon in the vertical position. (B) Grasping of a table tennis ball, aluminum foil, a marshmallow, and a balloon in the horizontal position. The voltage applied to the actuator is 10 kV. Color images are available online.

Conclusion

In this study, we have introduced an actuator harnessing a combination of electrostatic and hydraulic forces without rigid parts. The actuator is a small isolated system, which can easily be installed in a commercial robotic system. The actuator can operate without an external fluid supply source, such as an oil pump. We conducted a series of experiments to determine the characteristics of the unit module of the gripper. The performance of the actuator was altered by changing the volume inside the pouches. In addition, we reported that the actuator with $V_{p}$ / $V_{c}$ = $0.5$ was more efficient than in other cases. Furthermore, we investigated the actuation characteristics of the actuator under frequency variations. Finally, we constructed a novel gripper using the electrohydraulic actuator and confirmed the utility and performance of the electrohydraulic gripper. The results demonstrate the feasibility of an electrohydraulic based gripper.

The result of this study calls attention to several topics in need of further investigation. We need a more thorough understanding of the hysteresis in the actuation to improve the performance and control of the actuator. The proposed electrohydraulic actuator shows low actuation force and speed than electrostatic actuator published by Kellaris et al.⁴⁰ For better performance in the actuation force, we can use a dielectric fluid with a higher permittivity. Exploring upgraded design of the actuator can also improve the actuation force. The actuator should achieve the same actuation performance at higher frequency by improvement of the design of the actuator and exploring new materials, such as dielectric fluid having low viscosity.

Footnotes

Author Disclosure Statement

No competing financial interests exist.

Funding Information

This work was supported, in part, by the Convergence Technology Development Program for Bionic Arm through the National Research Foundation of Korea funded by the Ministry of Science, ICT and Future Planning under grant number 2014M3C1B2048419, and, in part, by Korea Institute of Science and Technology Institutional Program under grant number 2E29460.

Supplementary Material

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Supplementary Material

Please find the following supplemental material available below.

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