The Delta Motor—A High-Speed, Multi-Degree-of-Freedom, Flexure-Based Piezoelectric Motor

As size scales are reduced to the millimeter and sub-gram regime, electromagnetic rotary motors have reduced performance due to physical scaling effects that affect friction, current density, and heat dissipation. In this regime, piezoelectric actuators are a useful alternative due to their high energy density (a few J/kg), high efficiency, and increasing power density at small scales. However, converting their linear motion into rotary motion without significant energy losses remains challenging, and while such devices can provide high torque, bandwidth, and precision, they typically have low rotational rates. When multi-degree-of-freedom (DOF) motions are needed, these challenges are exacerbated, with few examples of multi-DOF motors at the millimeter-scale.<br/>Here we present our recently published work on ‘The Delta Motor’, a novel 130-mg multi-modal piezoelectric motor that can produce both rotational and translational motion of its output axle. In rotational mode, rotational rates up to 30,000 RPM (much faster than typical piezoelectric motors) and torques of 2.5 µNm have been demonstrated, while in translational mode, free displacements of ±0.6 mm and blocked forces of ±100 mN have been achieved at frequencies up to 500 Hz. The motor operates by using a flexure-based delta mechanism, which implements universal joints via closely-spaced laminated in-plane and out-of-plane joints, to convert the oscillatory motion of three piezoelectric bending actuators into the motion of a variable-transmission crank-shaft in 3D space. This crank is then used to rotate the main output axle. In contrast to most piezoelectric motors, which utilize some form of frequency-leveraging such as stick-slip or traveling-wave based approaches, this means that the output axle of the Delta Motor is rotated at the same frequency as the oscillation frequency of its internal bending actuators, which allows the motor to reach very high rotational rates without requiring high-frequency operation of the internal actuators (e.g., 30,000 RPM is achieved at 500 Hz). In addition, because the output is connected to the input actuators, the rotational rate is independent of the load torque up until blocked torque is reached. Since the crank can be commanded to move along arbitrary trajectories within a small 3D volume, the output axle can be translated as well as rotated. This multi-modal capability enables many possibilities for interesting new applications in microsurgery and microrobotics. For example, the Delta Motor could be used as a miniature drill press by providing a translational force while rotating the output axle at high speed. Switching between different output shafts could be enabled by translating the output axle to different positions, then activating the rotational mode. Rotation has been demonstrated together with linear translations up to ±280 µm. Activating the translational mode and the rotational mode simultaneously could be useful for microrobot limb actuation, such as 'figure-eight' wing flapping and comparisons between flapping wings and rotary propellers at the insect scale.