Experimental Investigation of an Ultrasonic Motor for the Rotary

Action of an Ultrasonic/Sonic Planetary Drill

Dimitris Drazinos Introduction

It has been suggested that lightweight rotary-hammer drills, that require low preload and power input, are ideal for remote planetary

drilling operations. The hammering action of the drill is provided by an ultrasonic/sonic drill that incorporates a free-mass between the

horn tip and the drill bit. The free-mass converts ultrasonic to sonic energy through continuous impacts with the horn tip and drill bit’s

head, generating a stress at the bit’s rock interface [1]. When that stress exceeds the rock’s strength, the rock fractures. Ultrasonic

motors could be potential rotary actuators for planetary hammer drills, as they require low power input, they are able to operate in

extreme conditions and they weigh much less than conventional electric and electromagnetic motors [2].

The Ultrasonic Motor

It has been proposed that ultrasonic motors with longitudinal-

torsional converter horns generate higher torque compared to

other types [3]. Such a horn has the same shape as a standard

½ wavelength step horn, tuned to a resonant longitudinal

frequency, with the only difference being the 45̊ angle diagonal

slits machined on the nodal point. The diagonal slits generate

vibrations of elliptical trajectory on the surface of the step, and

when a hollow cylinder (rotor) is in contact with it, it rotates. The

horn’s tip vibrates longitudinally.

Ultrasonic motor with longitudinal-torsional converter horn

Test Rig and Methods

A test rig was designed and manufactured in order to mount the

transducer and motor assembly as well as a force transducer so

that the driving force could be measured under various loads.

The design included two arms bolted to the rotor, that provided a

flat surface to contact the force transducer as well as a uniform

load.

CAD model and set-up of the test rig

Once a steady state driving force was measured by the force

transducer, the torque, τ, was calculated.

τ = F ∗ r

Then rpm of the rotor was measured by using a laser tachometer

while the resultant power output and efficiency, η, of the motor

were calculated.

Power Output = τ ∗ rpm ∗2π

60

η =Power Output

Power Input∗ 100%

The average power input was 53.5W (52-55W) and the

excitation input amplitude was 2μm, while the loads varied from

0N to 9N. The motor was driven at resonance, approximately

20kHz.

Results

The maximum rpm measured was 890rpm under 9N of load on

the rotor, at a torque of 0.058Nm and efficiency of 10.15%.

Torque against various loads on rotor

RPM against various loads on rotor.

It was observed that the rpm, torque and efficiency were

increasing while the load was increasing.

Efficiency against torque

When the loads were placed on the arms, the rpm measured

was significantly higher, 1455rpm under 4N of load on each arm

(8N in total). Due to experimental limitations though, it was not

possible to measure the driving force and calculate the torque.

Rpm response under loads on arms.

References [1] P. Harkness, M. Lucas, A.Cardoni, ’’Maximization of the effective impulse

by a High-Frequency/Low Frequency Planetary Drill Tool,’’ IEEE Transactions

on Ultrasonics, Ferroelectrics and Frequency Control, Vol. 58, No. 11, 2011.

[2] S. Sherrit, L. Domn, X. Bao, Y. Bar-Cohen, Z. Chang, M. Badescu, ‘’Single

Piezo-Actuator Rotary-Hammering (SPARH) Drill,’’ Proceedings of SPIE

Smart Structures and Materials, San Diego, paper # 8345-79, 2012.

[3] J. Tsujino, R. Suzuki, M. Takeuchi, ‘’Load characteristics of ultrasonic

rotary motor using a longitudinal-torsional vibration converter with diagonal

slits. Large torque ultrasonic rotary motor,’’ Ultrasonics 34, 265-269, 1996.

University of Glasgow, charity number SC004401

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