A combined series-elastic actuator & parallel-elastic leg no-latch bio-inspired jumping robot

Highlights

• A design concept of frog-inspired no-latch jumping strategy is proposed.

• A symmetrical high-stroke one-DOF robotic leg mechanism is designed and analyzed.

• The 100.3 g prototype jumps to a height of 1.3 m with slight body rotation in the air.

• Simulation results of jumping performance are highly consistent with the experiments.

• The energy-storing capacity of the robot exceeds all the existing no-latch jumping robots.

Abstract

Compared with the catapult mechanism widely employed by small jumping robots, recently proposed jumping strategies based on series-elastic actuators (SEA) without latch mechanisms perform better in terms of agility, structural robustness and maneuverability. However, in some practical applications, they have difficulty in effectively storing energy before the push-off. This paper presents a novel no-latch jumping strategy inspired by frogs, achieving highly effective energy storage. The jumping strategy combines an SEA with a parallel-elastic linkage, which allows one motor to rotate in one direction to store the elastic energy and automatically trigger its release. Combined with this strategy, a frog-inspired robotic leg mechanism is designed. The jumping process is analysed in detail and the kinematic and dynamic models are derived. Bars’ dimensions and springs’ parameters are determined by the optimization to maximize the energy-storing capacity. The simulation is performed to predict the jumping performance. A 100.7 g prototype is fabricated and jumps to a height of 1.3 m with slight aerial body rotation. The energy-storing capacity of the robot is 18.1 J/Kg.

Introduction

Jumping as a mobile strategy is widely used by many animals, from small insects to large mammals. Jumping enables these animals to quickly catch prey, escape predators and overcome obstacles that are several or even dozens of times their size [1]. Differing from traditional mobile methods, such as walking, running, flying, and swimming, jumping animals push off the ground using their limbs and take off in a short amount of time, which requires producing an instantaneous high power output that far exceeds the maximum power their muscles can provide [2], [3]. Various jumping strategies are accepted by animals to enhance the power output by using elastic elements, such as catapult jumps in fleas [4], locusts [5], and froghoppers [6], dynamic catch jumps in frogs [7], and countermovement jumps in most mammals [8]. Jumping robots have shown great potential for some practical applications [9]. They can better adapt to complex terrains in post-disaster rescue [10] and surveillance [11], [12] than some traditional mobile strategies [13] due to their discrete standing points and stronger abilities to overcome obstacles. Additionally, they are especially suitable for low-gravity planetary exploration [14], [15] since they can jump higher and farther than on the earth. To attain the required take-off speed in a short amount of time, and limited by the power density of conventional actuators, jumping robots widely employ animals jumping strategies to enhance their jumping performance.

Catapult mechanisms are most frequently accepted by jumping robots, which allow them to slowly store energy in elastic elements with actuators, and then suddenly release the elastic energy via latch or catch mechanisms to trigger the push-off. The frogbot [15] uses a geared six-bar mechanism paralleled with a linear spring, and the elastic energy storage or release is achieved by a motor with one-way clutches. The Jollbot [16] has a spherical skeletal structure that comes from steel-wire springs, utilizing a slider roller moving along the path of the face cam to store energy and trigger the push-off. The miniature 7 g EPFL jumper [17] uses a torsional spring as the elastic element and a four-bar mechanism to thrust against the ground. A cam mechanism driven by one motor is employed to load and release the spring. Cams are also utilized by Grillo [18], Bio-inspired jumping robot [19], and the Locust-inspired robot [20] to slowly store elastic energy and abruptly release it. Strategies for energy storage and release using cables and clutches are used in Multimo-Bat [21], [22] and Jump-flapper [23]. The JumpRoACH uses both torsional springs and linear springs to enhance its energy density, and the height-adjustable trigger is achieved by an active clutch [24]. The TAUB [25], [26] has a pair of two-bar legs jointed by torsional springs. The motor first rotates in a forward direction to convolve the tendon-like wire to fold the legs and load springs, and then changes the direction of rotation to release energy. The Seoul National University has proposed a series of jumping robots based on torque reversal [27], [28], [29]. The 1.1 g Flea-inspired Robot [30] has a four-bar leg mechanism. The elastic energy is stored in SMA springs and can be suddenly released via torque reversal triggering.

The catapult mechanisms can only be identified in insects [1]. Some small jumping vertebrates, such as kangaroo rats [31], frogs [32], and galagos [33] take off at considerable speeds without latch or catch mechanisms involved. There has been little research on no-latch jumping robots. A galago-inspired robotic prototype, Salto, was built to attain a vertical jump of 1 m and achieved unprecedented agility using a novel jumping strategy called power modulation [34]. This jumping strategy combines a series-elastic actuator (SEA) with a variable mechanical advantage leg mechanism [35], [36]. Early low mechanical advantage enables the rotating motor to charge the elastic element with a slight leg extension, and then it increases as the leg stretches to allow an instant release of elastic energy without any latch.The energy storage in a variable mechanical advantage jump is strongly related to the ground reaction force in the leg extension direction. Hence, charging a series-elastic element via the actuator working is less effective when the robot jumps in low gravity planets or in a tilted attitude due to the reduced maximum loading torque on the SEA. The MSU jumper [12] mounts a rotation link on a one-way bearing and uses a cable to pull the parallel-elastic leg mechanism. Energy is stored as the link rotates counterclockwise and its release is triggered when the link is parallel to the cable. Another no-latch design is RHop which enables fast energy storage [37]. RHop uses an actuation strategy combining a motor with a clockwork spring and employs a symmetrical ten-bar leg mechanism, which allows the robot to realize continuous hopping with the motor’s unidirectional rotation and isolate the motor from impact when it lands. But the jumping height of RHop is only 0.145 m for its insufficient energy-storing capacity.

The comparison of jumping robots is listed in Table 1, where the energy-storing capacity is the stored elastic energy before the push-off. While the energy-storing capacity limits the use of existing no-latch designs in some practical applications, jumping without a latch has unique advantages over the catapult jump. First, robots can achieve high agility when using high power density actuators [35]. Second, the transition from the energy storage to the push-off is seamless and smooth, so there is no sudden change in the dynamic parameters, such as the ground reaction force and joint torques, which avoid instantly impacting the robot and improves its structural robustness. Third, it allows more maneuverability in the jumping speed adjustment and active landing buffering [38]. We thus focus on studying a new no-latch jumping strategy to store the elastic energy more effectively before the push-off: that is, to enhance the energy-storing capacity and store energy without being affected by the body attitude or environmental gravity.

The main contribution of this paper is to present a novel no-latch frog-inspired jumping strategy achieving highly effective energy storage. In a dynamic catch mechanism of frogs, the gravity serves to create a dynamic loading force on the muscle-tendon complex, which results in early energy storage and subsequent energy release. We identify a more effective dynamic loading torque produced by a linear spring with a variable moment and combine it with an SEA to form a new jumping strategy. Additionally, the linear spring can power the push-off when the loading torque is reversed, which enhances the energy-storing capacity. Combined with this strategy, a frog-inspired symmetrical high-stroke geared five-bar leg mechanism is designed. The robot can store at least 45.6% more elastic energy than the variable mechanical advantage jumping strategy that uses the same SEA. The fabricated prototype jumps to a height of 1.3 m with slight body rotation in the air.

The remainder of the paper is arranged as follows: In Section 2, we present the design ideas and principles, and then introduce the designed jumping mechanism from the jumping strategy, the leg mechanism, and the detailed jumping process. In Section 3, we describe the mathematical model of the robot. In Section 4, we determine the optimal parameters of the robot. In Section 5, we discuss the fabrication of the prototype and the setup for the experiments. In Section 6, we present the simulated results and the experimental results and evaluate the jumping performance of the prototype.