Variable stiffness actuation and energy harvesting have been important yet separate challenges in robotics. Both functions are needed, however, for mobile robots on extended missions when actuators and generators must be used together. In this paper, we present a unique piezoelectric cellular system that combines motion generation and energy harvesting capabilities into a single, scalable device. Each of the discrete cellular units provides linear, contractile motion at 10% strain using the converse piezoelectric effect. These units may also be back-driven from environmental loading and thereby generate energy using the direct piezoelectric effect. Furthermore, each cell has the capability to toggle between a low stiffness ON state and a high stiffness OFF state, which allows an assembly of individual cells to tune both their static stiffness and structural resonant frequencies online. We demonstrate the effectiveness of our device for tuning both locomotion speed and the harvested power of an underwater flapping fin system.

A new approach to a variable stiffness actuator with tunable resonant frequencies is presented in this paper. Variable stiffness actuators have become increasingly important to meet safety requirements and achieve adaptive manipulation or locomotion. For cyclic motion, exploiting dynamic resonance can lead to high power transmission, high energy efficiency, and large motion amplitude. Resonance and variable stiffness characteristics have yet to be incorporated into a single actuator design. In this paper, a cellular artificial muscle actuator that achieves both variable stiffness and variable resonance capabilities is presented. The design is based on piezoelectric stack actuators. First, the principle of variable stiffness and variable resonant frequencies is described. The static and dynamic performance are then quantified with theoretical models. Theoretical analysis reveals that the proposed actuator can be tuned over a broad range of resonant frequencies by selectively turning specific units on or off. Initial prototypes are tested experimentally and exhibit 15% static strain, over 300% static stiffness tunability, and over 100% dynamic resonance tunability.

A simple and efficient approach for varying the inherent stiffness and impedance of a muscle-like actuator is presented. The basic architecture of PZT cellular actuators has already achieved a large effective strain (10-20%). This architecture is modified and extended so that each cellular unit can be switched between a zero compliance state and constant compliance state. The effective stiffness of the cellular actuator is varied by changing the distribution of cellular units in the rigid versus compliant state. Furthermore, by placing a multitude of these cellular units in series or parallel, the stiffness can vary within a large set of discrete values. This paper also demonstrates the viability of the variable stiffness cellular actuator for cyclic tasks such as running and flapping. The basic principle and design concept for the actuator is described, followed by force-displacement analysis. A dynamic model is then constructed to demonstrate the variable resonance properties of the actuator under load.

This paper presents the dynamic analysis of an artificial muscle actuator designed for high-bandwidth, power-law strain amplification. The actuator is based on a nested cellular architecture of PZT stack actuators. Most smart material actuators have seen limited use in mobile robotic applications because of their small strain, low stress capacity, low bandwidth, and stringent input requirements. The proposed actuator design overcomes these limitations and can serve as a high-bandwidth multifunctional artificial muscle. The dynamic characteristics of the actuator design are derived analytically and validated experimentally. A test system mimicking flapping flight is then used to illustrate the actuator dynamics.

A static lumped parameter model is proposed for the design and analysis of nested piezoelectric cellular actuators with exponential strain amplification mechanisms. Piezoelectric ceramic material, such as Lead Zirconate Titanate (PZT), has large stress and bandwidth, but its extremely small strain, i.e. only 0.1%, has been a major bottleneck for broad applications. We have proposed a "nested rhombus" multi-layer mechanism for PZT actuators, which increases strain exponentially through its hierarchical cellular structure, for over 20% effective strain. To drive a large load, however, care must be taken in the design of the strain amplification structure. Through kinematic and static analysis this paper addresses how the output force and displacement are attenuated by the joint stiffness and beam compliance involved in the strain amplification mechanism. An insightful lumped parameter model is developed to quantify the performance degradation and facilitate design trade-offs. A prototype nested PZT cellular actuator that weighs only 15 g has produced 21% effective strain (2.49 mm displacement from 12 mm actuator length) and 1.7 N blocking force.

This paper describes the design and analysis of a humanoid foot constructed using polypyrrole (PPy) conducting polymer (CP) actuators. The compliance and damping of natural muscles plays an important role in natural human gait. Conducting polymers actuators and other smart structure actuators can store energy by means of inherent mechanical compliance that traditional DC motor actuators do not possess. This paper presents a method for optimizing the inherent compliance and damping of the actuators in order to minimize the active control effort required to generate a natural human gait. A simplified kinematic model of the design is evaluated using biomechanical joint angle and ground reaction force (GRF) data to yield the desired force versus displacement characteristics of the posterior and dorsal actuators. Numerical simulations illustrate the multifunctional nature of the PPy actuators and the overall power requirements of the system during the stance phase of walking gait.

The demand for high-force and compact actuators with large strain is increasing in robotics. PZT is known as one of the promising materials for this purpose with respect to bandwidth, stress, and reliability. However, the most critical drawback of PZT is its extremely small strain, i.e. only 0.1 %. This paper presents a nested rhombus structure for strain amplification of PZT stack actuators. This structure provides a "power-law" strain amplification, resulting in over 20 % strain, which is particularly useful for gaining a large strain in a compact body, appropriate for many robotic applications. The notational convention and fundamental force-displacement analysis will be presented. In addition, the feasibility of the design concept will be addressed through the design of a proof- of-concept prototype.