Development of a miniature quadrupedal piezoelectric robot combining fast speed and nano-resolution

Highlights

• A miniature quadruped piezoelectric robot combining two locomotion is proposed.

• The robot can realize a resolution of 8.8 nm in the quasi-static locomotion.

• The robot achieves high speeds of 393.5 mm/s and 246.5 °/s in resonant locomotion.

• The cooperation of two locomotion is realized by resonant locomotion in pulse mode.

• The robot can carry a load of 100 g, which is about 5 times its own weight.

Abstract

Miniature piezoelectric robots exhibit superior performances and have been favored by many researchers, however, there is an intractable contradiction between nano-resolution and fast speed. Inspired by motion of quadrupeds, we proposed a miniature quadruped piezoelectric robot (MQPR), which combined quasi-static motion and resonant motion to realize nano-resolution and high speed, respectively. Two theoretical models were established to determine the parameters of the proposed piezo-leg, and they were verified by experiments. A prototype with size of 35 × 44 × 12 mm³ and weight of 21.0 g was manufactured and tested. The measured results showed that the MQPR realized multi-DOF motions with resolution of less than 8.8 nm and range of about 2 μm in quasi-static motion, and achieved a linear speed of 393.5 mm/s (11.2 BL/s) and a rotational speed of 246.5 °/s in resonant motion. Moreover, a resolution of 0.31 μm was obtained by resonant motion in pulse mode, which could achieve effective cooperation of the above two motion mechanisms. In addition, the load capacity of the MQPR was 100 g, which was about 5 times its own weight. These characteristics help the MQPR have great potential to efficiently perform precision manipulations at multiple target locations over a large area, such as detection of circuit board in large integrated circuits or wafers.

Introduction

Miniature ambulatory robots have been a popular research direction in the field of robotics in recent years [1], [2], [3], [4], [5], [6]. Benefiting from their small sizes, flexible motions and lower power cost compared to middle or large robots [7], [8], [9], they have been widely developed for applications in fault inspection, search and rescue, micro-manipulation, micro-manufacturing and so on [10], [11], [12], [13]. Traditional miniature ambulatory robots are usually actuated by electromagnetic motors [14], [15], [16], [17]. For example, Su et al. [18] proposed a quadruped robot with a size of 72 × 76 × 35 mm³, which were driven by vibrations of its cantilever legs, and could trot at speeds up to 206 mm/s. Besides, Birkmeyer et al. [19] designed a dynamic autonomous sprawled hexapod (DASH) with a length of 100 mm; the DASH, actuated by a single motor, realized a maximum speed of 1500 mm/s. The miniature ambulatory robots driven by electromagnetic motors exhibited impressive performances, especially for high speed. However, with the rapid development in the field of biology, nano-manufacturing, etc., the miniature robots are required to exhibit superior performances, such as nano-resolution, large working range, multi-DOF flexible motions and further miniaturization [20], [21], [22]. The robots, driven by electromagnetic motors, cannot satisfy above strict requirements duo to the restrictions of their driving principle and transmission mechanism.

For further improving the performance of the miniature ambulatory robots, much of the interest in new driving methods has been motivated. Recent advances include the miniature robots actuated by shape memory alloys [23], [24], [25], dielectric elastomers [26], [27], [28], magnetostrictive materials [29], [30], [31], artificial muscles [32], [33], [34], piezoelectric ceramics [35], [36], [37] and so on. Among them, the miniature piezoelectric robots, developed by the inverse piezoelectric effect of piezoelectric ceramics, exhibit many advantages such as fast response, high resolution, compact structure and immunity to electromagnetic interference [38], [39], [40], [41], [42], and have been favored by many researchers in recent years. Wu et al. [43] proposed an insect-scale piezoelectric robot with a size of 30 × 10 × 5 mm³ and a weight of 0.024 g, the robot was actuated by a PVDF (Polyvinylidene difluoride film) and achieved a high speed of 200 mm/s and a load of 0.406 g. Zhong et al. [44] designed a cubic centimeter robot based on inertial stick-slip driving, the robot was limited in 10 × 10 × 10 mm³, the realized maximum speed and resolution were 13.1 mm/s and 4.8 μm, respectively. Some other miniature piezoelectric robots with impressive performances are successfully developed, such as LPMRs [45], [46], [47], HAMRs [48], [49], [50] Miniwalkers [51], [52], [53], Min-RARs [54,55].

The miniature piezoelectric robots can be divided into the resonant piezoelectric robots [56], [57], [58] and the non-resonant piezoelectric robots [59], [60], [61] according to whether the piezoelectric actuators vibrate at their resonant frequencies. The resonant ones, usually actuated by unimorph or bimorph actuators, are easier to realize high speed (hundreds of mm/s) due to their high working frequency and relative larger vibration amplitudes [62], [63], [64]. For instance, Oldham et al. [65] presented a six-legged resonant piezoelectric robot with a size of 21 × 31.8 × 3 mm³, it could operate with a high speed of 170 mm/s and a jumping height of 1 mm, through the vibrations of their cantilever-like legs. Peng et al. [66] proposed a miniature four-legged resonant piezoelectric robot, which was limited in a size of 35 × 30 × 9 mm³ and a weight of 9 g; the robot could trot at a high speed of 330 mm/s and carry a load of 55 g. To improve the resolution of the resonant piezoelectric robot, Liu et al. [67] designed an exciting signal in pulse mode to help their resonant robot achieve a resolution of 0.44 μm; meanwhile, the robot was limited in 58 × 44 × 12 mm³, and could trot at a speed of 516 mm/s. Although the resonant piezoelectric robots can realize resolutions of submicron level, reaching the nanometer resolution is very difficult for them.

On the other hand, the non-resonant piezoelectric robots, usually driven by piezoelectric stacks, can achieve nano-resolution by the direct deformation of piezoelectric stacks, and realize large range motion by accumulating tiny steps [68], [69], [70], [71]. For example, Ma et al. [72] presented a pole-climbing non-resonant piezoelectric robot with a size of 50 × 54 × 86 mm³, which exhibited a resolution of 85 nm and a maximum speed of 100.9 μm/s. Fuchiwaki et al. [73] designed an inchworm-actuated non-resonant piezoelectric robot with a size of 32 × 32 × 20 mm³ and a weight of 35 g, the robot realized a high resolution of 25 nm and a maximum speed of about 1.4 mm/s. To increase the speed of the non-resonant piezoelectric robot, Deng et al. [74] proposed a fast impact mechanism, which made their non-resonant piezoelectric robot realize a speed of 3.65 mm/s. In addition, the robot was limited in 200 × 200 × 40 mm³, and achieved a high resolution of 16 nm, however, the fast impact made the robot be unstable during its motions.

To sum up, there is an intractable contradiction between the nano-resolution and high speed of the miniature piezoelectric robots, which limit their efficient applications. In other words, although the nano-resolution is enough for the applications in large scale precision detection, nano-manufacturing, etc., the slow speed could hinder the working efficiency. A common strategy is to combine different actuators in one robot. For example, Sun et al. [75] proposed a miniature robot integrating two driving methods (brushless DC motor and piezoelectric driver) to achieve a speed of 50 mm/s and a resolution of 17.4 μrad, respectively, but combining multiple driving methods caused the complex structure and control system. Therefore, a solution integrating the nano-resolution and high speed without increasing combining other actuators is valuable to the applications of the miniature piezoelectric robots, and even has much potential to revolutionize current technologies of miniature robots.

Contribution of this work: Interestingly, we noticed that the quadrupeds, such as dogs, can not only swing freely in place, but also trot fast in a wide range by their legs. Inspired by the multi-mode motions of quadrupeds, a miniature quadruped piezoelectric robot (MQPR) with quasi-static motion and resonant motion was proposed, which can not only operate in place with nano-resolution, but also move fast in large range. These performances give the MQPR great potential to efficiently perform precision manipulations at multiple target locations over a large area, such as detection of circuit board in large integrated circuit or wafers. Specifically, the MQPR achieves several key advancements as follows:

• The proposed MQPR, with size of 35 × 44 × 12 mm³ and weight of 21.0 g, could operate in quasi-static motion and resonant motion to combine the nano-resolution and fast speed.

• With the quasi-static motion, the MQPR could realize multi-DOF motions with resolution of higher than 8.8 nm and range of about 2 μm.

• With the resonant motion, the proposed robot could achieve a linear speed of 393.5 mm/s (11.2 BL/s) and a rotational speed of 246.5 °/s.

• In order to the effective cooperation of the above two motion mechanisms, the resonant motion in pulse mode was designed and a resolution of 0.31 μm was obtained.

• The robot could trot smoothly carrying a load of 100 g, which is about 5 times its own weight.

The structure of the proposed MQPR and the two motion mechanisms are indicated in Section 2. Then, Section 3 establishes the theoretical models of the piezo-leg and determines its parameters. The experimental set-ups are established in Section 4, and the characteristics of the piezo-leg and the prototype MQPR are tested in Section 5, and a conclusion is followed in Section 6.