The miniaturization of actuators for applications that need large displacements, high energy efficiency or output forces is an ongoing challenge . Piezoelectric ultrasonic motors (USM) have proven to be a suitable solution to obtain long motion range, high torque, quick response, high power to weight ratio and high efficiency in comparison to electrostatic, magnetic, and thermal alternatives –. Despite the advantages of USM for linear motion, scaling down to the millimetre range remains a challenge, due to the difficulties in generating standing or travelling waves at high frequencies with enough amplitude .
The monolithic fabrication based on silicon micromachining, in combination with the use of integrated piezoelectric films was successfully applied to the effective size reduction of such positional devices. Furthermore, the addition of 3D-printed legs, for a controlled contact, allowed for a further step into the manufacturing of efficient linear motors. Such hybrid devices have recently demonstrated the conveyance of sliders – surpassing several times the motor weight – with speeds of 1.7 mm/s, while operated at 6 V and 19.3kHz and positional resolution of 70 nm . However, by the optimisation of various aspects of the device such as the vibrational modes and excitation signals, or the legs dimension and distribution, speeds above 30 mm/s could be reached with payloads above 25 times the motor weight.
 Kenji Uchino, MicroMechatronics, Second Edition.
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