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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