Development of an Anthropomorphic Soft Manipulator with Rigid-Flexible Coupling for Underwater Adaptive Grasping

Abstract

Inspired by human hands and wrists, an anthropomorphic soft manipulator (ASM) driven by water hydraulics is proposed for underwater operations and exploration. Compared with traditional rigid manipulator, ASM has highly evolved grasping ability with better flexibility and adaptability, while it has better load capacity, grasping ability, and flexibility in comparison with the pneumatic gripper. ASM wrist is composed of rigid-flexible coupling structure with three bellows and a spindle, which generates continuous wrist pitching. The linear elongate characteristics of bellows and pitching performance of ASM wrist are simulated by finite element modeling (FEM) method and tested experimentally. The mathematical model of bending deformation for the water hydraulic soft gripper (WHSG) is established. The bending deformation and contact force of WHSG are simulated by FEM and measured experimentally. The ASM prototype is fabricated, and the grasping experiments in the air and underwater are conducted. It is confirmed that the developed ASM can switch between standard and expanded grasping position to adopt and grasp objects of different shapes and dimensions. And living animals with rough or smooth surfaces such as turtle and carp can also be caught harmlessly. ASM also exhibits preferable adaptability when the objects are out of grasping range or deviating from the grasping center. This study confirms that the developed ASM has enormous application potentials and broader prospects in the field of underwater operation, underwater fishing, underwater sampling, etc.

Introduction

With economy growing and the reduction of terrestrial resources, requirements for the marine resources are in a continuous ascending. Considering the potential risks and physical limitations faced by divers in deep sea operations, using a robotic gripper to replace divers can reduce the health harm to divers and improve the underwater operation efficiency, which makes underwater vehicles-manipulator systems (UVMS) the best solution toward underwater exploration.¹

Hydraulic manipulators with robust power used to be the best solution for subsea applications and are extensively used on ultraheavy work class remote operated vehicles (ROVs) due to their high versatility. These traditional manipulators and grippers usually use rigid actuators driven by electric motors or hydraulic mechanisms. Rigid actuators are usually used to grip hard materials with sound results, but when gripping flexible or brittle materials, it cannot guarantee that these objects will be gripped without being damaged. However, most of these systems are constructed with rigid elements and does not match to natural environments. As the randomicity and diversification keep rising in subsea operations, the defects of traditional single degrees of freedom (DOF) gripper come out that they are hard to achieve accurate positioning and precise operations. This dilemma becomes worse when they are dealing with precious samples, soft materials, or fragile aquatic specimens, and that is why underwater multifingered hands are born.²

To ensure the dexterous grasping of most underwater multifingered hands, three active fingers or more are required, AMADEUS (advanced manipulation for deep underwater sampling),³ See Grip, ⁴ and the hands of Ocean One^2,5 are the typical embodiment under this kind of blueprint. However, even though they all work well in underwater sampling, they are still essentially articulated rigid manipulators that would cause shock and oppressions when the rigid driving factor is taken into account. To make full use of the multifinger hand, the number and variety of sensors and the complexity of the transmission system will be greatly increased, which not only improves the quality and structural complexity but also imposes a burden on the host computer.^6,7

With the proliferation of human–machine interaction, soft actuators used in medical rehabilitation and flexible manufacturing become popular. Relying on the natural flexibility and adaptability of soft material, the soft actuators have the characteristics of high flexibility, complex environment adaptability, and safe human–machine interaction and can easily complete the grasping and collection work. These soft actuators currently are bionic designed and made of elastomer scalable materials such as silicone rubber.⁸ The actuation mechanisms of soft actuators include cable-driven,^9,10 fluid,^11–13 and smart material actuation (e.g., magnetic actuation, thermal, electrical, or chemical reaction).^14–16

The current soft actuators are not suitable for underwater operations.¹⁷ Pneumatic actuators have poor pressure-holding capacity, low grasping force, and poor anti-interference ability due to the limitation of silicone material and would compress or bend abnormally under external water pressure.¹⁸ Furthermore, gaseous medium leakage inside pneumatic actuator can cause pressure imbalance and bubble spill and seriously affect the vision of the underwater operator. Pneumatic actuators always require external pneumatic sources, which are not adapted to the underwater environment or deep-sea operations. Magnetic fluid-actuated soft manipulator systems are relatively complex, and electrical breakdown failure is unacceptable underwater.^19,20 Thermal electrical actuators totally cannot be used because of the specific heat capacity of water. Considering current manipulators difficult to meet the requirements of high adaptability and strong anti-interference ability in underwater environment,²¹ it is necessary to develop a flexible manipulator driven by water hydraulics to adapt to the complex underwater working conditions.

With the increasing requirements of human society for environmental protection, sustainable development, and safe production, several potential literatures on traditional UVMS have been widely discussed, especially for the leakage problem of the hydraulic system in underwater manipulator; the solution of changing transmission medium has been proposed. Water hydraulic technology has caught attention as a renascent transmission technique and has become one of the research hotspots in the field of fluid transmission and control.^22,23 Water hydraulics can be used in nuclear fusion reactors because of its outstanding characteristics, which include flame retardancy, greenness, cleanliness, and energy saving during the past two or three decades.^24,25 And when it is applied in underwater environment, some auxiliary components such as water tank, heat exchanger, and pressure compensator can technically be dismantled to meet simpler structure, smaller size, and better kinematics characters. Besides, water hydraulics also has several inherent advantages of good driving performance, sound environmental compatibility and adaptability, low maintenance cost, etc.

Compared with pneumatic soft gripper or gripper with other driving forms, the structure of soft grippers powered by water hydraulics can be greatly simplified to achieve miniaturization and lightweight, and the grasping performance of underwater soft manipulator will be greatly improved. Due to these absolute advantages, water hydraulics has a broad application prospect in the fields of ocean exploration and Marine resources exploitation and has been popularized in a small range with soft actuators.^26–28 However, the main problems facing these researches are still the fixed grasping range and low grasping force, which hinder the wide application of underwater soft manipulator.

In this study, inspired by the comprehensive grasping behavior of human hand, an anthropomorphic soft manipulator (ASM) driven by hydraulics was developed actively. The rigid-flexible coupling design was adopted to solve the problem that the existing soft gripper could not be used normally in underwater environment. ASM has similar athletic functions and grasping ability to the human hands and wrists by making good use of the internal space of ASM for bionic design. With the help of dual rigid-flexible coupling designs, namely bellows and spindle coupling, as well as water hydraulic soft gripper (WHSG) and removable palm coupling, ASM has excellent adaptability and ability to grasp objects with multiple shapes and dimensions and living animals. This article mainly focuses on (1) deformation analysis of bellows and soft gripper by finite element analysis, theoretical analysis, and experiment under 1.5 MPa; (2) feasibility of self-adaptive grasp function of ASM; (3) grasping performance of ASM in vertical posture both in the air and underwater; and (4) ASM self-stability and grasping stability in horizontal stance.

Integrated Design and Models for ASM

The taxonomy of human grasp types

The hands and wrists as the most distinctive motor organs in human body perform the most complex and precise tasks in everyday life. According to the analysis carried out by Feix et al.,²⁹ there are 33 different kinds of grasp taxonomies in the human hand with >20 DOF to accomplish movements and functions. Despite the actuation of fingers, the gesture of palm and the position of the wrist are also important factors when defining the grasp taxonomy.

To deal with the increasing diversity and complexity of underwater grasping, it is necessary to enhance same functions for the ASM to act like wrist and palm. By simulating basic movements that human wrist and palm could be achieved, a rigid-flexible coupling design is performed for the underwater manipulator. In addition, the number of artificial actuators of the wrist and fingers used in ASM are reduced so that ASM can imitate the posture of the human hand in a simpler way to achieve self-adaptive grasping.

Layout of ASM wrist part

To endow the ability of bending and pitching like human wrists, ASM is designed with an omnidirectional flexible joint. The spindle is located in the center of three bellows between the upper base and the lower base (as shown in Fig. 1), referring to the functional design of the human ulna and radius. As an important part of maintaining ASM stiffness, the spindle is divided into two parts: the spindle with the ball head at the front and the spindle with the ball pocket at the back. The combined ball joint is located halfway between the upper base and the lower base to constrain the ASM wrist, which is mainly determined by the water pressure inside each bellow. This specific structure has three novel advantages: (1) Achieving spherical range of motion with more grasping orientations in one joint compared with traditional single DOF rigid joints; (2) improving load capacity compared with soft artificial muscles; and (3) better performance in human-machine-environment interaction.

FIG. 1.

Structure composition of ASM. ASM, anthropomorphic soft manipulator.

In this study, three bellows are arranged in a radial circle centering on the ball joint. When one or both bellows are elongated by pressurization, the ASM rotates around the ball joint due to the axial displacement constraints of the bottom base and the top base. By controlling the length of three bellows with different water pressures, the ASM's posture changes to mimic the swing of a human wrist.

Considering the stress and strain of soft materials, the maximum linear deformation of the bellows should not exceed twice of its original.³⁰ The five key parameters affecting the deformation performance of bellows were calculated and calibrated through genetic algorithm in Lingo 18.0, which are listed in Table 1. And the manufacturing process of bellows using DPI 8400 polyurethane resin by vacuum casting 3D printing is shown in Figure 2a.

FIG. 2.

Design and fabrication of bellows. (a) Molding process of tubular bellows. (b) Uniaxial tensile tests of DPI 8400.

Table 1.

Parameters of Anthropomorphic Soft Manipulator Bellows

Paraphrase	Parameter	Value	Unit
Outer diameter	D	29.0	mm
Original trough height	D	5.0	mm
Wall thickness	T	2.5	mm
Original crest height	H₁	6.0	mm
Original crest angle	α₁	18.44	degrees

The accuracy of deformation prediction method for ASM has great influence on the accuracy of simulations. Current models for predicting the elongation of soft materials are mainly Mooney-Rivlin model,³¹ Yeoh third-order model,³² and Hookean hyperelasticity theory.³³ Among the above models, Yeoh third-order model was designed to produce more precise result for elongation.³² Given the hyperelasticity property of DPI 8400 and the long scale of elongate, Yeoh third-order model was used to establish the deformation prediction model.

To confirm the material constants of DPI 8400 in this study, uniaxial tensile tests were proceeded to obtain the material parameters C_i required for finite element analysis using HSM pull gauge—20 kN (China Changchun intelligent instrument equipment Co., Ltd) at a rate of 200 mm/min (Fig. 2b). All experiments were repeated five times, and the results were averaged. The dumbbell-shaped specimens were 3D printed using Type 4 and met the geometric size of ISO 37: 2005/Cor. 1: 2008. The material constants are shown in Table 2. After the comparison between parameters of DPI 8400, it is concluded that considering the service life under maximum water pressure of 1.2 MPa and synergy problems between the soft grippers and bellows, the material of the bellows is chosen as 90A.

Table 2.

Yeoh Third Model Constants for DPI 8400 Super-Elastic Materials

Shore hardness	C₁ (MPa)	C₂ (MPa)	C₃ (MPa)
50A	0.879830	0.000017518	−6.52455E-11
60A	2.790586	−0.00353485	7.3397E-09
70A	4.977106	−0.004515271	2.23428E-08
80A	9.142336	−0.008763834	1.48689E-08
90A	12.05765	−0.007733072	1.33404E-08

When water pours in, a positive pressure is generated inside the bellows, which makes the original crest angle α₁ and original crest height H₁ increase to variant crest angle α₂ and variant crest height H₂ and then produce linear elongate deformation.³⁴ By assuming the walls are unreformable and incompressible, outer diameter D, original trough height d, and wall thickness t can be regarded as approximately invariant (Fig. 3a).³⁵ The linear deformation of bellows was predicted using finite element modeling (FEM) in ANSYS Workbench. Figure 3d displays the deformation schematic diagram of bellows before and after pressurization through simulation. The experimental linear deformation values were also collected by building a testing platform with straight guiding model and using a laser displacement sensor (Fig. 3b, c).

FIG. 3.

Deformation analysis of bellows. (a) Dimensions and the cross-sectional view of bellows before and after being pressurized. D: 29 mm, d: 5 mm, t: 2.5 mm, α₁: 18.44°, α₂: 89.54, H₁: 6 mm, H₂: 10 mm (each cell of the bellows has same dimensions). (b) Comparison of the deformation of bellows before and after pressurization. (c) Testing platform for deformation measurement. (d) Analytical model of displacement of bellows. (e) Comparison of experiment and simulation deformation of bellows.

The experimental and FEM displacement results are shown in Figure 3e. The elongation experiment value is smaller in comparison with FEM value when the water pressure is <0.5 MPa and is larger when the water pressure is larger than 0.5 MPa. The discrepancy is attributed to the presence of printing overlap on the inner corner and the compactness problem. Due to the complex structure of the bellows, the molding and demolding process are difficult. A zigzag notch is found on the outer edge of the 3D printing mold (1), which makes the inner corners of the bellows change and stick together. When the water pressure is not high enough, the initial linear displacement of the bellows decreases, making them stick together. Meanwhile, the real density and compactness of bellows are definitely lower than the ideal model in the simulation, which will cause higher ascending rate of elongation than simulation value, especially when the water pressure is higher.

The experiment reveals that the maximum pitching angle β of ASM wrist can reach 34° under the maximum pressure of 1.2 MPa (Fig. 4a, b). When three bellows are identically pressurized at the same time, the ASM wrist will become more rigid in all directions, which can bear extra heavy loads at any attitude.

FIG. 4.

Analysis of wrist pitch characteristic. (a) Analytical model of wrist pitch angle. (b) Testing platform of wrist pitch angle in vertical attitude. (c) Comparison of experimental and simulated wrist pitch angle with single bellow pressurized. (d) Comparison of experimental and simulated wrist pitch angle with double bellows pressurized.

The comparison between experimental and simulated pitching angle with single and double bellows pressurized is shown in Figure 4c and d, respectively. The experimental pitching angle versus water pressure curve of single pressurized bellows matches its simulation curve perfectly with an average error of only 4.3% and the minimum error of 0.24%. However, the experimental pitching angle of double pressurized bellows is smaller than the simulation data, possibly due to the insufficiently accurate prediction of forced movement of bellows during wrist pitching.

Due to the incompressible characteristics of DPI 8400, same bellows have different deformation trend under different compression displacements. When two bellows are pressurized, bellows on the opposite side have larger compression distance, and the forced bending arc is shorter, which leads to an increase in the rigidity of the bellows after compression, making it less likely to be bent. This reduces the pitch angle of ASM.

In FEM simulation, the wrist pitching can be divided into two sections: pressurized bellows elongating actively and unpressurized bellows bending synergistically. This prediction is proved when single bellows are pressurized. But when double bellows are pressurized in experiment, the unpressured bellows compress instead of bending, which causes less pitching angle, more stress on the pressurized bellows, and less ability of holding pressure owing to the bad compression performance of bellows structure and the inaccurate prediction on compression of Yeoh third model.³²

In practical tasks, this characteristic obtained from experiment can also help for better adjusting the attitude of ASM to counter different external disturbances. Since the stiffness of ASM wrist is adjustable at different angles, when grasping heavy objects, ASM can adjust single bellows on one side to the direction under the compression force for greater stability. However, just as the muscle bundles of the human body are not all arranged along the same growth direction as the bones, we will try to regulate the installation inclination angle of the bellows in the follow-up research, so that ASM wrist can realize rotation around the spindle under the condition of three pressurized bellows.

Water hydraulic soft gripper

As shown in Figure 5a and b, soft gripper of ASM adopts an interconnected multichamber structure. When water flows in, the deformation layer expands while the restriction layer constrict the axial elongation, whereby WHSG undergoes radial bending deformation.³⁶ The bending angle of WHSG can be precisely controlled by adjusting the water pressure.

FIG. 5.

Design and fabrication of WHSG. (a) Cross-sectional view of the model of WHSG. (b) Model structure of WHSG. (c) Manufacturing process of WHSG. (d) Assembly of WHSG. WHSG, water hydraulic soft gripper.

To meet the requirements of WHSG, the manufacturing material needs to have high compressive resistance, high resilience, and good fatigue resistance. Considering that the WHSG works in an underwater environment, it also needs to have good corrosion resistance, low temperature resistance characteristics, etc. In this study, thermoplastic urethane (TPU-95A) materials are selected as the material for fabricating WHSG (TPU; Shenzhen eSUN Industrial Co., Ltd., China) (Fig. 5c, d).

Yeoh third-order model is used to establish the bending deformation prediction model. The model constants of TPU-95A are obtained by uniaxial tensile tests (shown in Table 3). The mathematical model of WHSG bending angle is established by combining the intrinsic model of hyperelasticity material and the principle of virtual work.

Table 3.

Yeoh Third Model Constants for TPU-95A

Material	C₁ (MPa)	C₂ (MPa)	C₃ (MPa)
TPU-95A	12.3375	0.163302	−0.000505747

TPU, thermoplastic urethane.

Assuming that each chamber of WHSG produces the same scale of deformation during the bending process, the total bending angle can be considered as the sum of single bending angle from multiple chambers: $φ = k \cdot θ$ (1)

where k is the number of chambers, ϕ is total bending angle, and θ is bending angle of single chamber.

The bending schematic of WHSG is shown in Figure 6a, and the arc length radius R can be calculated as follows:

FIG. 6.

Analysis of bending deformation and contact force of WHSG. (a) Bending deformation curves for predicting WHSG deformation under compression. (b) Structural parameter diagrams of WHSG. (c) Testing platform of deformation. (d) Comparison of experimental and simulated bending deformation of WHSG. (e) Testing platform of contact force. (f) Comparison of experimental and simulated contact force of WHSG.

R = \frac{d}{2 sin (θ ∕ 2)}

(2)

where d is the chord length corresponding to the arc of a single chamber after deformation.

Assuming that TPU is isotropic and incompressible, its intrinsic structure is established based on the stress-strain theory and expressed as a strain energy density function W:

$\{\begin{matrix} I_{1} = λ_{1}^{2} + λ_{2}^{2} + λ_{3}^{2} \\ I_{2} = λ_{1}^{2} λ_{2}^{2} + λ_{2}^{2} λ_{3}^{2} + λ_{1}^{2} λ_{3}^{2} \\ I_{3} = λ_{1}^{2} λ_{2}^{2} λ_{3}^{2} \equiv 1 \end{matrix}$ (4)

where I₁, I₂, I₃ are the deformation tensor invariant, λ₁, λ₂, λ₃ are the principal stretch ratio in the length, width, and height directions of the chamber, respectively.

Considering the incompressibility of material, it is assumed that the WHSG does not deform in the width direction, that is, λ₃ = 1, I₃ = 1. Thus, according to the Yeoh third order model, the strain energy density function model can be expressed as:

where C₁₀, C₂₀, and C₃₀ are material constants.

Neglecting the effect of gravity, the work done by the driving water pressure p is completely transformed into the energy stored by the soft gripper after deformation: $p d V_{a}^{} = V_{r}^{} d W$ (6)

where V_a is the volume of the chamber after deformation, and V_r is the volume of the thermoplastic material after deformation.

Assuming TPU material is incompressible, the volume of the material is identical before and after deformation, which can be obtained as: $V_{r}^{} = l h w + (l_{1}^{} + l_{3}^{}) (h_{3}^{} + h_{4}^{}) w - l_{2}^{} h_{2}^{} w_{2}^{}$ (7) $l = 2 l_{1}^{} + l_{2}^{}$ (8)

h = h_{1}^{} + h_{2}^{} + h_{3}^{}

(9)

w = 2 w_{1}^{} + w_{2}^{}

(10)

where l₁, h₁, and w₁ are the chamber wall thickness in three directions, l₂, h₂, and w₂ are the chamber length in three directions, h₃ is the thickness of the top surface of the chamber, l₃ is the chamber spacing. h₄ is the height of the chamber connection section, h₅ is the height of the passage inside the chamber. The details of structural parameters are shown in Figure 6b and Table 4.

Table 4.

Parameters of Water Hydraulic Soft Gripper

Paraphrase	Parameter	Value	Unit
Chamber wall thickness in length directions	l₁	4.0	mm
Chamber's length	l₂	4.0	mm
chamber spacing	l₃	4.0	mm
Chamber wall thickness in width directions	w₁	4.0	mm
Chamber's width	w₂	32.0	mm
Chamber wall thickness in height directions	h₁	4.0	mm
Chamber's height	h₂	22.0	mm
Thickness of the top surface of the chamber	h₃	4.0	mm
Height of the chamber connection section	h₄	6.0	mm
Height of the passage inside the chamber	h₅	2.0	mm

The volume of the chamber after deformation can be expressed as: $\begin{matrix} V_{a}^{} = V - V_{r}^{} = \frac{(1 + λ) [l h + l_{3}^{} (h_{1}^{} + h_{4}^{})] w}{2} - V_{r}^{} \end{matrix}$ (11)

where V is the volume of single unit (including volume of deformed chamber and the volume of thermoplastic material) after deformation.

The elongation ratio in the length direction for a single chamber can be expressed as: $λ = \frac{θ}{sin θ}$ (12)

Thus, the mathematical model between the input water pressure and the structural parameters can be combined as: $p = \frac{2 {sin}^{2} θ [l h w + (l_{1}^{} + l_{3}^{}) (h_{3}^{} + h_{4}^{}) w - l_{2}^{} h_{2}^{} w_{2}^{}]}{[l h + l_{3}^{} (h_{3}^{} + h_{4}^{})] w (sin θ - θ cos θ)} \cdot \frac{d W}{d θ}$ (13)

The bending deformation and contact force of WHSG were also predicted using FEM in ANSYS Workbench. The experimental bending deformation and contact force were tested using laser displacement sensor and force sensor to verify the validity of the FEM data. Since each WHSG works in the same working state, the force and deformation of only one WHSG are discussed to reflect the force and deformation of the whole manipulator. As shown in Figure 6c and e, WHSG is mounted on a rail to restrict the slippage so that more precise results of directional deformation and contact force could be recorded.

The comparison of experimental and simulated bending deformation of WHSG is shown in Figure 6d. In the deformation experiment, WHSG bends as pressure goes up and performs well with a maximum pressure of 1.5 MPa and the relevant deformation is 141.2 mm. The experiment curve reveals the same trend as the simulation curve, but quite fluctuates and is higher than the simulation results. This difference between simulation and experiment results may be attributed to the following reasons. During the experiment, when the WHSG deformation exceeds 180°, the bottom fingertip as location point will be blocked by the top fingertip on the laser path of the displacement sensor, resulting in a decrease in the deformation value when the water pressure exceeds 1.1 MPa. At the same time, since the pressure and flow regulation ability of the piston pump are relatively poor in the low-pressure area, this causes the deformation value of WHSG to fluctuate when the water pressure is lower than 0.5 MPa.

The relevant experimental procedures will be further improved in follow-up research. Meanwhile, since the sensor is not fixed but moveable on a rail, this may lead to problems such as inaccurate repositioning during the movement. The relevant experimental procedures will be further improved in follow-up research. When WHSG is under lower water pressure, there will be a forced movement on the opposite of bending direction, which is generated in the middle of WHSG (Fig. 6f). This forced movement reduces the contact force due to the counteraction of bending deformation. Meanwhile, during the deformation process of WHSG, the constitution of measured contact force changes as well. When the bending angle of WHSG exceeds θ/2, the measured contact force can be divided into the bending deformation force and thrust force of contact surface. This thrust force is perpendicular to the force sensor and cannot be measured under lower pressure.³⁷

Self-adaptive grasp functions of ASM palm

Generally, most soft grippers are fixed on one base, which not only increase the weight and complexity but also seriously limit the grasp performance. The separate end of the soft gripper is used only once and has no auxiliary function, which is why the current soft gripper is only suitable for its design purpose and cannot bear more function.³⁸ In this study, a movable palm of ASM is proposed (illustrated in Fig. 7a) by imitating the function of human palms in a grasping activity.

FIG. 7.

Standard and expanded grasping position. (a) Change of grasping position. (b) Specifications of standard grasping position (standard gripper opening ø₁ = 76 mm, standard gripper length a₁ = 125 mm). (c) Specifications of expanded grasping position (expanded gripper opening ø₂ = 250 mm, expanded gripper deflection θ₂ = 88°).

To accomplish the function of human palms with a simpler way, a small sized water hydraulic cylinder is equipped to drive the moveable palm opening and closing actively, comprising of a 3D-printed linkage mechanism used to deflect WHSG. The piston rod is connected with the center of the moveable palm, and the cylinder is integrated with the front part of spindle. The opening and closing angle and the expanded diameter of palm are determined by the flow rate of the hydraulic cylinder, where the maximum expanded gripper deflection θ₂ is about bilateral 88° and the expanded gripper opening ø₂ = 250 mm as shown in Figure 7b and c. Obviously, by adjusting the opening and closing angle of the movable disc, ASM can actively suit the shape and volume of the grasping object (Fig. 7a).

Grasping Experiments

Experimental setup

As shown in Figure 8c, ASM is driven by a water hydraulic power source. Water hydraulic high speed on-off valves are used to control soft grippers of ASM system with the maximum pressure of 5.0 MPa and the response time of 4.0 ms.³⁹ STM 32 is programmed as the system controller. In this study, ASM is mounted on an experimental platform with three DOF, which can perform linear action in three directions (Fig. 8a). The bottom base of ASM will be screwed to the forearm of water hydraulic manipulator to provide a higher driving torque and larger range of motion (Fig. 8b).

FIG. 8.

Experimental platform and ASM system. (a) Experimental platform of ASM. (b) Modeling of WHM. (c) Schematic drawing of ASM system. WHM, water hydraulic manipulator.

Grasping experiment in the air

The grasping performance and self-adaptive grasp function of ASM were first tested by grasping objects with various shapes and dimensions in the vertical stance (Fig. 9a and Supplementary Video S1). At this stage, those objects were separated into six different groups by their shapes and characters: spherical (ordinary and fragile), slice, slender, conical (over-sized and small), columnar, and irregular objects. All fragile objects such as egg and beaker were not broken during grasping experiment. For slice such as mobile phone and folder, ASM could grasp in two different directions: from top and bottom or from sides. Irregular objects could also be adapted and grasped by ASM directly to firmly move. Objects like bowl that exceed the standard grasping range (210 mm in diameter) can be grasped by controlling the expanded gripper deflection θ₂. If objects are out of grasping range or deviating from the grasping center, ASM could perform a wrist pitching to retarget the objects (Fig. 9b).

FIG. 9.

Grasping experiment in the air. (a) Vertical grasp experiment for different shapes and dimensions. (b) Wrist pitching experiment to grasp objects out of range. (c) Horizontal grasp experiment for different weights and volumes. (d) Pressure contrast for bellows and WHSG in horizontal grasp experiment.

Then, the grasping ability of ASM was tested in vertical and horizontal postures in the air with the increased weights. Standard weights and some common objects were grasped, respectively, in this phase. When grasped in vertical, weights of grasping objects did not affect the stability of ASM (Fig. 9c). Considering that in the weight experiment, when ASM grabs the same object in two different postures, the water pressure of WHSG is almost the same, so the analysis will focus on grasping in horizontal postures. The results show that the bellows do not need pressurization to maintain stability when the weight of grasping object is under 1.5 kg. When the weight exceeds 1.5 kg, the bellows only need lower water pressure to hold ASM in comparison with WHSG. In conclusion, posture of ASM has little effect on its grasping ability. The relationship between the weight of grasped objects and water pressure of the bellows and WHSG are, respectively, shown in Figure 9d.

Grasping experiment underwater

The underwater grasping ability of ASM was significantly essential and was tested by grasping objects with different surfaces and volumes (Fig. 10a). In addition to grasping normal objects, live grasping experiments were also performed due to the flexibility of WHSG. Turtle and carp with different physiological structure and skin surface roughness were selected as experimental subjects (Fig. 10d, e and Supplementary Video S2). The experiment displays that ASM can catch the turtle on the back and belly by producing limited deformation. For carp, ASM can grasp it by contacting the body side with higher water pressure and hold it tightly. It is worth noting that ASM can realize the nondestructive grasping of the objects for turtles and crap can swim freely in the water after opening the WHSG. All the experiment involving subjects in this article were approved by the Research Ethics Committee of Beijing University of Technology, China.

FIG. 10.

Grasping experiment underwater. (a) Experiment overview. (b) Grasping of cup. (c) Grasping of aquatic plant. (d) Grasping of moving turtle. (e) Grasping of living carp.

The normal objects and living body grasping experiment demonstrated the performance of the ASM's catching robustness and practicality.

Conclusion

This article presented a novel water hydraulic ASM with rigid-flexible coupling design motivated by human hands and wrists, which can effectively target and grasp different objects with multiple dimensions and shapes. The linear elongate characteristic of bellows, pitching performance of ASM wrist, and bending deformation of WHSG are simulated and measured experimentally, respectively.

To verify the flexibility, adaptability, and biocompatibility of the ASM, the grasping experiments both in the air and underwater are conducted successfully. The grasping experiment shows that the maximum grasping diameter of ASM is 210 mm and the maximum grasping weight is 3.0 kg. ASM has good flexible adaptability and grasping effect on fragile irregular objects and aquatic organism. The proposed ASM exhibits good imitation of humanoid action and presents a broad application prospect both on land grasping and underwater exploration. In the future, a flexible force sensor will be installed on the bottom of WHSG to realize the precise grasping weight and monitor the behavior in service.

This study confirms that the developed ASM has enormous application potentials and broader prospects in the field of underwater operation, underwater fishing, underwater sampling, etc.

Footnotes

Acknowledgments

The authors thank Dr. Zhonghai Ma, Mrs. ZiWei Wu, and Pengwang Gao for their help in simulation and experiment. The authors are very grateful to the editors and the anonymous reviewers for their insightful comments and suggestions.

Authors' Contributions

H.J.: Investigation, Methodology (lead); writing—review and editing (lead). Y.L.: Writing—original draft (lead); Software (lead); Data curation (lead). S.N.: Conceptualization; Project administration; Supervision. L.H.: Methodology (equal); Data curation (equal). F.Y.: Validation; writing—review and editing (equal). R.H.: Investigation; Validation.

Author Disclosure Statement

No competing financial interests exist.

Funding Information

This study was funded by the National Natural Science Foundation of China (Grant Nos. 51905011, 51975010, 52075007, and 52005013) and Beijing Postdoctoral Research Foundation (Grant No. 2022-ZZ-045).

Supplementary Material

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