Introducing POLYPUS: A novel adaptive vacuum gripper

Highlights

• A novel gripper that features underactuated and vacuum grasping.

• Uneven and even objects made of different materials can be handled.

• Light and heavy objects manipulated in contrast to standard grippers.

• Results from an extensive set of simulations to evaluate the grasping performance.

Abstract

Underactuated grippers represent an appealing solution that allows complex objects to be grasped and manipulated via passive adaptation of the hand with objects via simple control inputs. The grasping ability can be further improved in combination with vacuum gripping, i.e., by outfitting the gripper phalanges with suction cups, that remains a largely underinvestigated solution. This paper presents a novel gripper, referred to as POLYPUS, that features underactuation and vacuum grasping to handle uneven and even objects made of different materials, including cardboard, glass, sheet metal, and plastic. Being characterized by a solid frame, POLYPUS does not fall in the soft gripper category, while preserving similar adaptability. It provides unique load lifting capacity that ranges from light to heavy objects, whereas most of the existing grippers are tailored for a specific target payload. Being modular in design, POLYPUS can be easily reconfigured for a wide range of object sizes and applications. Results obtained from an extensive set of simulations are included to evaluate the grasping performance expressed in terms of the minimum suction force to handle objects of varying shape, material, and pose.

Introduction

Grippers are widely employed in many fields, e.g., medical as prosthesis, robotic as end-effectors, industrial for several tasks like assembly or pick and place, and so on. Based on the scope, different skills are required, aiming for simplicity, reliability, and cost-effectiveness. Although the most common architecture for industrial applications is the parallel gripper driven by pneumatic energy [1], also vacuum grippers have been adopted for their versatility. Indeed, in the 2015 Amazon Picking Challenge, where teams had to grasp different objects in tight and crowded spaces, 9 out of 26 groups chose to use vacuum grasping [2]. Among the 13 devices able to pick at least one object, 8 employed suction strategy including the winning team [3]. While further automation and mechanization are raising interest in dexterous manipulation, using the well-established rigid robotics is a complex task both in manufacture and control [4], [5]. Increasing the number of degrees of freedom (DoF) and degrees of actuation (DoA) requires more motors, thus heavier and more complex gripper. As a result, these high-DoF grippers hardly reach the market failing to reach a great trade-off between cost and advantages for industrial applications.

One of the motivations of this research is to extend the potential of vacuum grasping via suction cups to highly adaptive grippers for unstructured objects, i.e., without predefined shape, geometry, and weight. To achieve a high level of compliance with the grasping object as well as to reduce the actuation complexity, underactuated systems are a widespread solution [6], [7], [8].

A large body of research has been devoted to soft robotics, which presents continuous deformation, thus soft robots have an infinite number of DoF and only a few DoA [9]. Progress in this branch has been significant, indeed, examples of soft grippers are numerous. Shintake et al. [10] provided a recent review, classifying soft grippers by grasping strategy i.e., actuation, controlled stiffness, and controlled adhesion. Jiand et al. [11] proposed a gripper with three soft fingers pneumatically actuated, whereas Zhou et al. [12] combined the pneumatic actuation with a rigid frame. Following a topological optimization, Liu et al. [13] realized a soft gripper actuated by the displacement of a central pivot. To exploit the compliance several examples adopt cables as actuation, like the origami-inspired in [14], the soft hand in [15] or mixing soft and rigid materials [16], [17], [18]. Interestingly, a single suction cup located in the palm of the hand was also adopted to stabilize the grasping [19], [20]. Many octopus-inspired grippers able to work in different environments have been proposed [21], [22], [23], [24], [25], [26], other examples of controlled adhesion are the gecko-inspired like in [27], even combined with the electrostatic effect [28]. Although many notable examples have been demonstrated, some drawbacks remain in the adoption of soft devices, especially in industrial applications, one over all the maximum payload.

In this paper, a novel underactuated vacuum gripper, referred to as POLYPUS, an acronym of POLYthecnic of Bari’s octoPUS, is studied that features high adaptability to the shape, size, and weight of grasped objects. By preserving a rigid structure, POLYPUS does not belong to the soft gripper category. The simplicity of the architecture makes the system easily controllable and cost-effective in terms of components and manufacturing. The gripper, shown in Fig. 1, has a rigid modular backbone with $N$ fingers constrained to a central body. Each finger is articulated in $M$ phalanxes that are outfitted with a suction cup. POLYPUS fingers are tendon actuated and the presence of rotary springs as antagonists in the phalanx hinges facilitates the approaching stage of the gripper to the target object. Besides, blocking devices are foreseen to lock the phalanx hinges after the object wrapping is over, resulting in an improvement of the payload, as shown later in the paper.

As a result of the design, the gripper has $N\times M$ DoF but only $N$ DoA resulting in $N\times (M-1)$ degrees of underactuation. This feature is the trending direction of the grippers to improve compliance with the object with a simple control approach [29], without needing of motion planning [30].

The novel aspects of the proposed system are:

• the adoption of rigid phalanxes outfitted with suction cups to fulfill challenging, e.g. unilateral and asymmetrical, grasping tasks in the manipulation of objects with medium and large weight ($\ge$ 5 kg). In contrast, soft vacuum-based solutions have shown similar adaptability but for objects with lower weight ($\le$ 1 kg);

• in contrast to most of the existing cable-driven grippers [15], [16], [17], [18], [19], tendon actuation is necessary only during the initial approaching stage where the underactuated fingers conform to the particular geometry. Once the suction cups adhere with the object surface, no further action is required by the actuation system. This is not the case for most of the above-mentioned devices where the tendons remain under tension during the whole manipulation stage as well;

• the modular underactuated multi-finger architecture that can be easily reconfigured to adapt to objects with different shapes and geometry, ranging from everyday household to industrial items;

• The model-based optimization framework proposed to predict the minimum suction force required for the manipulation of a given object. The analytical model of a multi-contact envelop vacuum grasping is seldom described in the literature.

The paper continues as follows: a description of the architecture and grasping strategy is proposed in Section 2. Section 3 explains the analytical model that serves as a basis to set the optimization problem that is solved for the required suction force. In Section 4, simulations and results are provided for objects with varying shapes and operating conditions. Conclusions wrap up the research findings and lessons learned, in the last section.