Abstract
A compliant constant-force mechanism is a passive force regulation device that can generate a nearly constant output force over a range of input or output displacements while without the use of sensors and feedback control. In topology synthesis of a compliant constant-force mechanism for a given input displacement range, the constant output force can be achieved by maintaining the output displacement to be nearly a same value while the input displacement increases. In order to further control the desired output displacement for the compliant constant-force mechanism before contacting the object, this article introduces a new composite objective function that can consider both the output force (with contact) and the output displacement (without contact) of the synthesized compliant mechanism. The sensitivity for the proposed objective function with respect to the element density is derived while considering the effect of nonlinearity in the large deformation condition. The proposed topology optimization method is used to design an innovative constant-force compliant finger, and its prototype is manufactured by 3-D printing using a flexible thermoplastic elastomer. The experimental results show the developed constant-force compliant finger can provide a nearly constant output force of 41.9 N over the input displacement ranging from 15 to 30 mm while the maximum and average force variations within the constant-force range are 2.2% and 0.9%, respectively. In addition, the developed constant-force compliant finger is used to design a three-fingered constant-force compliant gripper that can be used in robotic grasping of fragile objects.
Highlights
T HE development of constant-force mechanisms has drawn considerable attention over the years
Chen and Lan [12] presented a constant-force mechanism to passively regulate the contact force; the design combines a linear spring with positive stiffness and a bistable mechanism with negative stiffness to provide a nearly constant output force
As shown in (1), (3), and (4), the objective function is a combination of the error ratios h1, h2, h3, and h4 according to the output displacement and the output force values
Summary
T HE development of constant-force mechanisms has drawn considerable attention over the years. Chen and Lan [12] presented a constant-force mechanism to passively regulate the contact force; the design combines a linear spring with positive stiffness and a bistable mechanism with negative stiffness to provide a nearly constant output force. There are only two journal articles [10], [22] presented their design of compliant constant-force mechanisms using topology optimization. The proposed method is used to design a constant-force compliant finger which can provide a nearly constant gripping force at the fingertip over a range of the input displacements.
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