This paper presents a novel structure-controlled variable stiffness robotic gripper that enables adaptive gripping of soft and rigid objects with a wide range of compliance. With the structure-controllable principle, the stiffness is controlled by the mechanical structure configurations rather than by material properties or electronic means. The principle is realized by changing the effective second moment of area of the gripper finger through rotating a built-in flexure hinge shaft. Based on this principle, the states of the stiffness can be continuously, instead of discretely, studied and assessed over the intermediate states from compliant to almost completely rigid. A variable stiffness mechanism has been developed to demonstrate the validity of the proposed principle. It enables that the finger stiffness and gripping position are independently controlled. With the introduction of flexure hinges, the undesired lateral buckling resulted from the rotation of a normal leaf spring is eliminated. In addition, a two-finger parallel gripper with this variable stiffness mechanism is developed which can provide the grasping stiffness according to the grasping task requirements. The effectiveness of the gripper has been demonstrated to handle the objects range from light, fragile to heavy, rigid without using any feedback loop or soft pads. Note to Practitioners—This work was inspired by the fact that the capability of variable stiffness in the robot actuator can reduce the undesired impacts to the robot arms. It can also allow a safer interaction between robot and human. Gripping of objects with uncertainties in shapes and position, as well as large variations in fragility and weight can also benefit from the concept of the variable stiffness. However, existing designs of variable stiffness grippers either have limited stiffness range or bulky configurations. They compromised the practical applications. This paper introduces a design in that a rotating flexure hinge shaft is embedded inside the robotic gripper finger. The mechanical stiffness of such fingers can be varied by changing the rotation angle of the flexure hinges. We present the working principle supported by mathematical models in the design and development. We also show an example design of a two-finger parallel gripper equipped with the VSFs. Extensive experiments demonstrated that the gripper is effective in gripping objects with wide range of uncertainties. Such gripper design avoids the use of soft pads as well as closed-loop control and high-precision sensors. In the future work, we shall implement such grippers for actual industrial applications.

中文翻译：

---

基于结构控制原理的变刚度机械手

本文介绍了一种新颖的结构控制的可变刚度机器人抓手，该抓手能够以广泛的适应性来自适应抓握柔软和坚硬的物体。根据结构可控的原理，刚度是由机械结构配置控制的，而不是由材料特性或电子方式控制的。该原理是通过旋转内置的挠性铰链轴来更改抓手手指的有效第二面积矩来实现的。基于此原理，可以连续地而不是离散地对刚度的状态进行研究，并评估从柔度到几乎完全刚度的中间状态。已经开发了可变刚度机制来证明所提出原理的有效性。这样可以独立控制手指的刚度和抓握位置。通过引入挠性铰链，消除了由于正常板簧的旋转而导致的不希望的横向屈曲。另外，开发了具有这种可变刚度机构的两指平行抓取器，其可以根据抓握任务要求提供抓握刚度。已经证明，抓手的有效性可在不使用任何反馈环或软垫的情况下处理从轻，易碎到重，刚性的物体。给从业者的注意-这项工作的灵感来自于以下事实：机器人执行器具有可变的刚度，可以减少对机器人手臂的不良影响。它还可以使机器人与人之间进行更安全的交互。形状和位置不确定的物体的抓握以及脆弱性和重量的较大变化也可以从可变刚度的概念中受益。但是，可变刚度抓爪的现有设计具有有限的刚度范围或笨重的构造。他们损害了实际应用。本文介绍了一种设计，其中将旋转的挠性铰链轴嵌入到机械手夹爪的内部。可以通过改变挠性铰链的旋转角度来改变这种指状件的机械刚度。我们介绍了在设计和开发中数学模型所支持的工作原理。我们还展示了配备VSF的两指平行抓爪的示例设计。大量的实验表明，该夹具可以有效地夹持不确定性范围较大的物体。这种抓手设计避免了使用软垫以及闭环控制和高精度传感器。在将来的工作中，我们将在实际的工业应用中使用此类夹持器。