Monolithic Rapid Prototype Flexured Ultrasonic Horns - IEEE Xplore

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Stewart Sherrit, Xiaoqi Bao, Mircea Badescu, Yoseph Bar-Cohen, Phillip Allen. Jet Propulsion Laboratory, California Institute of Technology. 4800 Oak Grove ...
10.1109/ULTSYM.2010.0224

Monolithic Rapid Prototype Flexured Ultrasonic Horns Stewart Sherrit, Xiaoqi Bao, Mircea Badescu, Yoseph Bar-Cohen, Phillip Allen Jet Propulsion Laboratory, California Institute of Technology 4800 Oak Grove, Pasadena, CA, 91109 ABSTRACT − Piezoelectric ultrasonic horn actuators are used in high power medical/surgical, automotive, food preparation, textile cutting, and material joining applications. Typically, these horn actuators are assembled by pre-stressing piezoelectric rings between the horn and backing layer using a pre-stress bolt. In the ultrasonic horn actuators presented in this paper the bolt was removed and the number of overall parts was reduced significantly allowing for easy fabrication and integration of the actuators into other structures. The elimination of the need for the conventional stress bolt internal to the piezoelectric stack and the reduction of the related complexity were achieved by using external flexures. The actuator structure was produced by an electron beam melting rapid prototype manufacturing process. This manufacturing process allows for horn structures with internal cavities if required. This design allows for using solid piezoelectric stacks (with no central hole) to produce a highly effective actuator. This paper presents the results of a novel design of a monolithic ultrasonic horn.

The horn length primarily determines the resonance frequency. For a 22 kHz resonance frequency a stepped horn of titanium has a length of approximately 8 cm. In previous publications we described more complicated horns[5,6,7,8]. This paper describes a new method to fabricate horns based on an external flexure to apply presstress. It should be noted that we are using flexures to produce an axial stress rather than for a transverse amplification as is found in flextensional actuators. The removal of the pre-stress bolt removes a potential internal electric discharge points in the actuator. In addition, it reduces the chances of mechanical failure in the piezoelectric stacks that results from the free surface in the hole of conventional ring stacks. Other advantages of this approach are that the actuator area is not reduced to accommodate the stress bolt which allows for an increase in the energy density. The producing of piezoelectric plates without a hole has a higher production yield and is thus less expensive. Also, by reducing the stiffness and increasing the displacement of the external flexure one can increase the electromechanical coupling of the actuator. An additional benefit is the increased thermal preload stability due to the compliant spring. Finally, it is noted that at high pre-stress and high frequency one does not have to resort to exotic alloys to accommodate the stress and fatigue in the stress bolt. The results of this investigation suggest that the use of flexures and rapid prototyping can effectively be applied to the manufacture of power ultrasonic horns. It also opens up the potential to easily integrate high power horns into 2D or 3D structures. Although we investigated horns fabricated with electron beam melting (EBM), the design is also amenable to investment casting and other low cost high production techniques. The approach of using flexures can also be scaled to miniaturized horns for other specialized applications[9].

Keywords: piezoelectric ultrasonic horns, flexures, rapid prototyping, pre-stress

I.

INTRODUCTION

A variety of industrial applications exist where power ultrasonic actuators such as the ultrasonic horn are used to produce large amplitude vibrations. These include the medical/surgical, automotive, food preparation, textile cutting, as well as fabrication industries[1,2,3,4]. The actuators are typically assembled with a horn, piezoelectric rings, backing and a stress bolt. The ring elements are connected electrically in parallel and placed between the horn and the backing ring. A prestress bolt is inserted through a backing ring and the piezoelectric rings and screwed into the horn until a prestress of > 20 MPa. is reached. The horn can produced in a variety of configurations including constant, linear, exponential and stepped cross sections. These names refer to the degree in which the area changes from the base to the tip. A magnification in the strain occurs in the stepped horn that in general is a function of the ratio of diameters. In addition, the device is generally driven at resonance to further amplify the strain. The resonance amplification is determined by the mechanical Q (attenuation) of the horn material and radiation damping.

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II.

EXPERIMENTAL

In order to determine appropriate flexure geometry the monolithic horn was modeled in Solidworks. A solid model of the flexure is shown in Figure 1. along with a static Von Mises stress profile for a given gap separation. In order to design a flexure which acts like a spring, which remains in the materials elastic

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region, the flexure was designed to operate at stresses lower than the material’s yield strength. It was design such that a compressive pressure of 20 MPa was needed to ensure that the PZT never experiences tensile stress under maximal drive voltages. The 25.4mm diameter PZT cylinder had an area of 5.1 cm2 and in order to experience 20 MPa of stress, the PZT required a compressive force of 10.1 kN.

strengths to as cast and wrought materials[10]. The part quality is such that they are now used in both the aerospace and in medical implant fields. The EBM manufacturing approach is useful for small production runs. If larger production and cheaper cost per part is required one could use investment casting tree approach[11] where it is also possible to co-cast stainless steel and titanium. The piezoelectric stacks were purchased from Filament Anode Powder Supply Building Table

Focus Coil

Deflection Coil

Vacuum Electron Beam Product

Figure 1. SolidWorks-COSMOS Finite Element Analysis. Maximum von Mises stress shown in red.

The stress calculations determined that the deformation of the PZT was negligible since the ceramic material has a very high stiffness compared to the flexure. The SolidWorks design was modeled and tested in COSMOS and the results were compared with IDEAS values. After choosing the manufacturing technique and material (Ti-6Al-4V) we were able to determine proper dimensions of the flexure and its gap to allow for a 20 MPa pre-stress when the flexure was pulled to produce a 0.13 mm increase in distance from the nominal separation of the 9.2 mm gap in order to accommodate the 9.33mm PZT stack thickness. The titanium flexure could be opened and the PZT placed in position with a factor of safety of approximately 1.89. The highest stress occurred on the inside of the flexure at the inside edges. This surface was thickened and given a large fillet to reduce the stress concentration. Another consideration was the risk of fatigue failure, since the actuator is operating at 30 kHz enduring thousands of cycles each second. In order to investigate rapid prototyping in the manufacture of horns the monolithic horns were manufactured in Titanium (Ti-6Al-4V) using electron beam melting/manufacturing EBM by CALRAM Inc. A schematic of the EBM manufacturing process is shown in Figure 2. In the EBM process the titanium parts can be made to an accuracy of about 0.4 mm with comparable

Figure 2. Schematic of the EBM manufacturing process.

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Figure 3. Impedance spectra of the bare Piezomechanik Gmbh bipolar stack.

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Piezomechanik Gmbh. The bi-polar stacks were nominally 25.4 mm OD. and 9.33 mm thick. The impedance spectrum of the first length extensional mode for these stacks is shown in Figure 3. An analysis of the small signal resonance data of the bare stack gave an effective piezoelectric constant of 480 pC/N for the material and a capacitance of 261 nF. The coupling was determined to be k33 =0.56 and the elastic compliance at constant field in the 33 direction was 5.4x10-11 m2/N. The mechanical Q was in the 40 - 80 range.

length. Also, the removal of the stress bolt removes a potential discharge point internal to the piezoelectric and removes a free surface for cracks to initiate on.

Figure 5. Photographs of two assemble flexure horns (Ti-6Al4V) manufactured using EBM next to a quarter.

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Figure 4. Part of the support rig that was used to open the flexures. The horn tip is clamped between a vice and the bolt is threaded to pull the backing away from the horn base. The inner set of plates is compressed in a vice and aluminum block is fixed to vice surface.

Admittance (S)

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In order to open the flexure to install the stack a support structure was designed to pull the flexures apart and allow for the piezoelectric stack coated with 3M 2216 epoxy to be inserted. When the flexures were released the voltage on a 10 microfarad capacitor connected in parallel with the piezoelectric stack was monitored. Upon releasing the flexure the charge generated on the piezoelectric indicated a pre-stress greater than 25 MPa. The support structure to open the flexures is shown in Figure 4. III.

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Figure 6. Impedance spectra of the horns shown in Fig. 5. primary extensional resonance is about 29 kHz.

RESULTS

IV.

The impedance spectra of the assembled horns were tested and the resonance frequencies are 28.5-29.3 kHz and the coupling is k=0.20-0.21 which is significantly better than similar horns produced with a stress bolt[6]. The two prototype horns are shown in Figure 5. The impedance spectra were measured on a HP 4294a Impedance analyzer and the admittance spectra is shown in Figure 6. The mechanical Q’s of the horn were measured to be 106 and 115 which is larger than the Q of the bare piezoelectric stacks (Q ≈ 40-80). These results for the flexure stack horns suggest that we can produce high power ultrasonic horns that have coupling coefficients that are equal to or greater than standard horns manufactured with a stress bolt. In addition these horn structures have the advantage that we can increase the piezoelectric volume for a given

The

DISCUSSION

The horns discussed in this paper were generated using the EBM process however if large quantities were required they could also be manufactured using investment casting or other high production run techniques. In addition one could also include sacrificial features like a larger diameter horn tip or a threaded hole in the backing to aid in the assembly. It was noted that by reducing the spring constant and increasing the displacement we require less energy in the spring material at resonance. In a previous study of the stepped horn with a stress bolt[6] we found a coupling k = 0.18. a re-analysis of that data shows that if the stress bolt in the model is removed, which

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corresponds to a very soft spring, the coupling was found to increase to k = 0.34. In order to demonstrate that these horns could be used to do useful work the horns shown in Figure 5 were designed to be implemented in a Barth motor[12] as shown in Figure 7. Initial testing of the motor with one horn demonstrated a rotor speed of 15 RPM with a torque of approximately 0.3 N-m could be produced. The torque was determined by hanging a mass around the shaft, then measuring the constant rate at which the mass was lifted against gravity.

REFERENCES [1] A. Shoh, “Industrial Applications of Ultrasound- A review 1. High Power Ultrasound”, IEEE Trans on Sonics and Ultrasonics, SU-22, 2, pp. 60-71, 1975 [2] L. Parrini,, “New Methodology For The Design Of Advanced Ultrasonic Transducers For Welding Devices”, Proceedings of the IEEE International Ultrasonics Symposium, pp. 699-714, 2000 [3] W.W. Cimino, L.J. Bond, “Physics of Ultrasonic Surgery using Tissue Fragmentation, Proceedings of the IEEE International Ultrasonics Symposium, pp. 15971600, 1995 [4] K.F. Graf, Process Applications of Power Ultrasonics – A Review”, Proceedings of the IEEE International Ultrasonics Symposium, pp. 628-641, 1974 [5] S. Sherrit, M. Badescu, X. Bao, Y. Bar-Cohen, and Z. Chang, "Novel Horns for Power Ultrasonics," Proceedings of the IEEE International Ultrasonics Symposium, UFFC, Montreal, Canada, August 24-27, 2004 [6] S. Sherrit, B.P. Dolgin, Y. Bar-Cohen, D. Pal, J. Kroh, T. Peterson "Modeling of Horns for Sonic/Ultrasonic Applications", Proceedings of the IEEE Ultrasonics Symposium, pp. 647- 651, Lake Tahoe, Oct 1999 [7] S. Sherrit, S. A. Askins, M. Gradziol, B. P. Dolgin, X. Bao, Z. Chang, and Y. Bar-Cohen, “Novel Horn Designs for Ultrasonic/Sonic Cleaning Welding, Soldering, Cutting and Drilling," Paper 4701-34, Proceedings of the SPIE Smart Structures and Materials Symposium, San Diego, CA, March 17-19, 2002 [8] X. Bao, Y. Bar-Cohen, Z. Chang, B. P. Dolgin, S. Sherrit, D. S. Pal, S. Du, and T. Peterson, “Modeling and Computer Simulation of Ultrasonic/Sonic Driller/Corer (USDC)”, IEEE Transactions on Ultrasonics, Ferroelectrics, and Frequency Control, vol. 50, no. 9, pp. 1147-1160,September 2003 [9] T. Li, Y. Chen, and J. Ma, “Development of a Miniaturized Piezoelectric Ultrasonic Transducer”, IEEE Transactions on Ultrasonics, Ferroelectrics, and Frequency Control, vol. 56, no. 3, pp. 649-659, March 2009 [10] CALRAM - http://www.calraminc.com/projects.htm Downloaded Sept 1st, 2010 [11] RAMCAST- http://www.ramcast.com/ see subdirectory pdf/Ti2%20Conference%20paper.pdf downloaded Sept 1st 2010 [12] Barth H.V., Ultrasonic Driven Motor, IBM Technical Disclosure Bulletin, 16, pp. 2263, 1973

Figure 7. A CAD rendering and a photograph of an example of a Barth motor produced by mounting a flexure ultrasonic horn against a rotor. The high frequency horn impacts the rotor and produces a rotary motion.

V.

CONCLUSIONS

We have presented a novel horn design where the horn, backing and stress flexure are combined in one monolithic piece. The pre-stress is generated by a flexure that is designed to produce the appropriate stress when assembled with the piezoelectric stack. Prototype horns were manufactured using Electron Beam Melting (EBM). The prototype horns were found to have ultrasonic properties that matched or were better than standard horns with pre-stress bolts having coupling constants measured at k > 0.20. In order to demonstrate the utility of these horn designs to do useful work a Barth motor was designed to incorporate the prototype horns. The produced un-optimized motor was found to rotate at 15 RPM and to have a torque of 0.3 N-m.

ACKNOWLEDGEMENTS Research reported in this manuscript was conducted at the Jet Propulsion Laboratory (JPL), California Institute of Technology, under a contract with National Aeronautics Space Administration (NASA). Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise, does not constitute or imply its endorsement by the United States Government or the Jet Propulsion Laboratory, California Institute of Technology.

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