Chopsticks are a common and simple tool used for picking up food in oriental culture [1
]. At least 1.5 billion people use chopsticks daily [3
]. Chopsticks enable the user to pick up a variety of food pieces in comparison to a fork. Experienced people manipulate their fingers to properly move food with chopsticks, regardless of the size and hardness of the pieces [4
There are several ways to handle chopsticks. Normally, when people use chopsticks, they pass two chopsticks through the space between the thumb and index finger (Figure 1
]. One stick is caught in the angle between the thumb and the index finger and is supported by the ring finger. The other stick is held between the middle and index fingers and is supported by the thumb. By the movement of the latter stick, the tips of the chopsticks are opened and closed.
Several studies related to chopsticks have been conducted. Specifically, in regards to the effective use of chopsticks, their mechanism, optimum size, and shape have been studied [2
]. Furthermore, virtual chopsticks driven by captured movement of a user’s hand have been demonstrated in a virtual reality system [8
]. In addition, by applying the advantages of chopsticks to surgery and other research areas, new devices and techniques have been developed for grasp and transport [9
Although chopsticks constitute a useful tool and have many advantages, they need accurate muscle manipulation [14
]. Chopsticks require fine-tuning and adjustment of intrinsic and extrinsic muscles in the hands. Thus, auxiliary devices for chopstick operation have also been developed to improve user performance [15
]. Moreover, chopstick robots have been developed for the elderly and disabled people who have weak hand muscles.
Chopstick robots are evolving into various driving forms. For instance, there is a way to attach chopsticks to a robot arm. The robot provides meal assistance through a 5-axis horizontal-type robot system and a camera [17
]. These robots can deliver food with various actions including mixing, stabbing, cutting, separating, and spooning food with chopsticks. Another chopstick robot utilizes a gripper based on the concept of under-actuation and a planar mechanism with 2 degree of freedom (DOF) composed of a combination of 2 four-bar mechanisms [4
]. A chopstick robot operated by the fingers of a human-inspired robot hand has also been developed [19
]. It has bones, joints, ligaments, and tendons. Thus, it is able to use chopsticks through precise control of the fingers. The robot is also capable of handling chopsticks for grasping various objects.
In this study, we propose a soft chopstick robot that works by directly using a soft actuator to manipulate the chopsticks with minimum effort. Soft robots are used for a variety of applications, such as wearable devices, artificial muscles, and grippers [20
]. Typically, robots consist either of rigid and electromechanical parts (e.g., magnet, copper, and steel bearings) or internal combustion engines made of steel and aluminum alloys [24
]. Owing to their composition, conventional robots are fast, accurate, and very powerful. However, they are also substantially heavier and cumbersome. Soft robots, on the other hand, have soft body structures and are composed of flexible materials which can be operated smoothly [24
]. Soft robots can also be adjusted in size and are lightweight. Moreover, they have agile mobility, and they show great potential for future applications. For example, soft robot hands can reach any point in a 3D workspace, using a variety of shapes and configurations. They also have a low resistance to compression and carry fragile objects without damaging them [29
]. Therefore, soft robots can be used in various forms, such as service robots that interact with humans, exploration robots, and medical robots used in surgery and rehabilitation [24
Soft actuators are key for soft robots. Electroactive polymers (EAPs) are a novel class of soft actuators. EAPs that activated by electrostatic force can show fast reaction speeds and are compatible with various manufacturing technologies [35
]. In general, flexible actuators can be divided into two main categories: electroactive polymer (EAP) actuators driven by electric fields and actuators driven by other stimuli, including optical, thermal and chemical stimuli [36
]. In addition, EAP actuators generally include two actuators: dielectric and ionic actuators [37
]. In dielectric elastomer actuators, electric field-induced activation is generated by electrostatic attraction between two charged conductive layers applied to the surface of a polymer film [38
]. When a voltage potential difference is applied between the two compliant electrodes, it causes compression of thickness and elongation in the region of the polymer film. On the other hand, ionic EAP actuators are driven by the movement of mobile ions within the polymer [36
]. Small changes in external variables such as electric and magnetic fields, temperature, solvent quality, and pH cause discontinuous changes. Examples of ionic EAPs include polymer electrolyte gels, ionic polymer metal composites (IPMCs), conductive polymers, and bucky gel actuators [36
Recently, studies are being conducted to improve the thermal stability and mechanical performance of the material for actuators [40
]. These studies suggest new process methods for long life, high repeatability, and fast actuators compared to pneumatic and hydraulic actuators [41
]. In particular, multilayer dielectric elastomer actuators based on silicone materials and elastomeric electrodes have many advantages in terms of thickness and manufacturing [42
]. Further advantages include light weight, easy fabrication, and miniaturization [43
Many different kinds of polymeric materials have been used for soft actuators—among them, silicone has been widely used for a lot of soft robot parts including actuators [44
]. Silicone has a high flexibility with an average Young’s modulus of 68.9 kPa, tensile strength at 100% strain, and Poisson’s ratio of 0.499 [49
]. Due to its high elasticity, the actuator does not require additional force when it is restored to its original state [50
]. Further, since the silicone is harmless to the human body, silicone is widely used for surgical material and in daily necessities [51
]. Thus, we use an EAP actuator driven by an electric field.
By using this material, we make X-shaped soft actuators for a chopstick robot. Specifically, the X shape is designed by combining two semi-ellipses with curved surfaces. To control the actuator using an electric field, conductive tapes are attached to the surfaces of the X-shaped silicone. In addition, the X-shaped silicone structure can be recovered after activation due to its elasticity. This paper is organized as follows. Section 2
describes the fabrication method of the X-shaped actuator and the experimental setup. Section 3
shows the operation of the chopstick robot with the X-shaped actuator. We also analyze the force required to move the chopsticks and the force of the X-shaped actuator. The conclusions are summarized in the final chapter.
In this study, we designed and manufactured a chopstick robot that uses an X-shaped actuator and is driven by an electrostatic force and elasticity. In order to close the tips of the chopsticks, we utilized an electrostatic force. In order to open the tips of the chopsticks, the elasticity of the X-shaped actuator was utilized. The balance between the opening force and the closing force at the tips is very important for the chopstick robot. If the elasticity of the soft actuator is too strong, the electrostatic force cannot overcome the elastic force and the chopsticks cannot be closed. On the other hand, if the elastic force is too small, the chopsticks cannot return to the original shape immediately. Both the electrostatic force and elastic force were calculated through simple theoretical analysis. We also investigated the effects of the parameters using theoretical calculations. Finally, we demonstrated that the chopstick robot could successfully pick up various objects. The theoretical modeling and simulation to study the dynamics of the X-shaped actuator will be our future work.
The X-shaped actuator can be used to control the distance of two parallel panels. Typically, it can be applied to a gripper that has a mechanism similar to a chopstick. Recently, much research has been conducted aimed at grasping and moving fine and fragile materials without damage in various fields including in semiconductor fabrication. The grasping force of the X-shaped actuator can be controlled depending on the size or shape of the target sample. In particular, if all the gripper parts are replaced with soft materials, they will be able to specialize in carrying small and fragile objects. In addition, the X-shaped actuator can be used as a valve that controls the flow of fluids because it can manage the distance and angle of two parallel panels.