Benjamin P. Dolgin

Novel Horn Designs for Ultrasonic/Sonic Cleaning Welding, Soldering, Cutting and Drilling

A variety of Industrial applications exist where power ultrasonic elements such as the ultrasonic horn are used. These included the Automotive, Instruments, Foods, Medical, Textiles and Material Joining and Fabrication Industries. In many of these devices the ultrasonic horn is the key component. The standard transducer used in these devices consists of three main parts, the backing, the piezoelectric elements and the horn. Standard horn designs have changed very little since their inception. There are four common types of standard horns. They are; constant, linear, exponential and stepped, which refer to the degree to which the area changes from the base to the tip. A magnification in the strain occurs in the 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 in general determined by the mechanical Q (attenuation) of the horn material and radiation damping. 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. Although these standard horns are found in many current industrial designs they suffer from some key limitations. In many applications it would be useful to reduce the resonance frequency however this would require device lengths of the order of fractions of meters which may be impractical. In addition, manufacturing a horn requires the turning down of the stock material (eg. Titanium) from the larger outer diameter to the horn tip diameter, which is both time consuming and wasteful. In this paper we will present a variety of novel horn designs, which overcome some of the limitations discussed above. One particular design that has been found to overcome these limitations is the folded horn. In this design the horn elements are folded which reduce the overall length of the resonator (physical length) but maintain or increase the acoustic length. In addition initial experiments indicate that the tip displacement can be further adjusted by phasing the bending displacements and the extensional displacements. The experimental results for a variety of these and other novel horn designs will be presented and compared to the results predicted by theory.

Virtual reality robotic telesurgery simulations using MEMICA haptic system

There is increasing realization that some tasks can be performed significantly better by humans than robots but, due to associated hazards, distance, etc., only a robot can be employed. Telemedicine is one area where remotely controlled robots can have a major impact by providing urgent care at remote sites. In recent years, remotely controlled robotics has been greatly advanced and the NASA Johnson Space Centers robotic astronaut, Robonaut, is one such example. Unfortunately, due to the unavailability of force and tactile feedback the operator must determine the required action by visually examining the remote site and therefore limiting the tasks that Robonaut can perform. There is a great need for dexterous, fast, accurate teleoperated robots with the operators ability to feel the environment at the robots field. The authors conceived a haptic mechanism called MEMICA (remote MEchanical MIrroring using Controlled stiffness and Actuators) that can enable the design of high dexterity, rapid response, and large workspace haptic system. The development of a novel MEMICA gloves and virtual reality models are being explored to allow simulation of telesurgery and other applications. The MEMICA gloves are being designed to provide intuitive mirroring of the conditions at a virtual site where a robot simulates the presence of a human operator. The key components of MEMICA are miniature electrically controlled stiffness (ECS) elements and Electrically Controlled Force and Stiffness (ECFS) actuators that are based on the use of Electro-Rheological Fluids (ERF). In this paper the design of the MEMICA system and initial experimental results are presented.

Electrorheological fluid based force feedback device

Force feedback from remote or virtual operations is needed for numerous technologies including robotics, teleoperated surgery, games and others. To address this need, the authors are investigating the use of electrorheological fluids (ERF) for their property to change the viscosity under electrical stimulation. This property offers the capability to produce feedback haptic devices that can be controlled in response to remote or virtual stiffness conditions. Forces applied at a robot end-effector due to a compliant environment can be reflected to the user using such an ERF device where a change in the system viscosity in proportion to the force to be transmitted. This paper describes the analytical modeling and experiments that are currently underway to develop an ERF based force feedback element.

Ultrasonic/sonic drilling/coring (USDC) for planetary applications

Future NASA exploration missions are increasingly seeking to conduct sampling, in-situ analysis and possibly return samples to Earth for further tests. Missions to Mars are the more near term projects that are seeking such capabilities. One of the major limitations of sampling on Mars and other low gravity environments is the need for high axial force when using conventional drilling. To address this limitation an ultrasonic/sonic drilling/coring (USDC) mechanism has been developed that employs an ultrasonic horn driven by a piezoelectric stack. The horn drives a free mass that resonates between the horn and drill stem. Tests have shown that the USDC addresses some of the key challenges to the NASA sampling objectives. The USDC is lightweight (450 g), requires low preload (< 5N) and can be driven at lower power (5W). The device has been shown to drill rocks with various levels of hardness including granite, diorite, basalt and limestone. The hammering action involved with the coring process can produce cores of various shapes, which need not necessarily be round. Because it is driven by piezoelectric ceramics, the USDC is highly tolerant to changes in its operating environment. These actuation materials can be designed to operate at a wide range of temperatures including those expected on Mars and Venus. Although the drill is driven electrically at 20 kHz, a substantial sub-harmonic acoustic component is found that is crucial to drilling performance. An analytical model has been developed to explain this low frequency coupling in the horn, free mass, drill stem and rock.

Characterization of transducers and resonators under high drive levels

In many applications, piezoelectric transducers are driven at AC voltage levels well beyond the level for which the material was nominally characterized. In this paper we describe an experimental setup that allows for the determination of the main transducer or resonator properties under large AC drive. A sinusoidal voltage from a waveform generator is amplified and applied across the transducer/resonator in series with a known high power resistor. The amplitude of applied voltage and the amplitude and the relative phase of the current through the resistor are monitored on a digital scope. The frequency of the applied signal is swept through resonance and the voltage/current signals are recorded. After corrections for the series resistance and parasitic elements the technique allows for the determination of the complex impedance spectra of the sample as a function of frequency. In addition, access to the current signal allows for the direct investigation of non-linear effects through the application of Fourier transform techniques on the current signal. Results determined from resonators of both soft and hard PZT and a ultrasonic horn transducer are presented.

Analysis of the impedance resonance of piezoelectric stacks

Inversion techniques to determine the complex material constants from the impedance data of a zero bond-length stack resonator are studied. The impedance equation examined in this paper is based on the derivation by Martin [G.E. Martin, JASA, 36, pp. 1496-1506, 1964]. The asymptotic solutions for the case where the number of layers n is large (n>8) and n small (n/spl les/2) are presented in terms of the complex material constants of the piezoelectric. When n =1 or 2, it is shown that the wave speed in the stack is determined by the open circuit elastic constant S/sup D//sub 33/. In the limit of large n, the wave speed is determined by the short circuit elastic constant S/sup E//sub 33/. Techniques to invert the impedance data to determine complex material constants are presented for all values of n. The error associated with using the impedance equations derived from fully short and fully open electrical boundary conditions is investigated. Since the model is based on material properties rather than circuit constants, it allows for the direct evaluation of specific aging or degradation mechanisms.

Ultrasonic/sonic drilling/coring (USDC) for in-situ planetary applications

A novel ultrasonic drilling and coring device (USDC) was demonstrated to drill a wide variety of rocks: form ice and chalk to granite and basalt. The USDC addresses the key shortcomings of the conventional drills. The device requires low preload and power. The drill bits are not sharpened and, therefore there is no concern to loss of performance due to warring out. The device is not subject to drill walk during core initiation, and does not apply larger lateral forces on its platform. The USDC has produced round and square cores and 14-cm deep holes and has opened new possibilities to the designers of future NASA planetary exploration missions. USDC can be mounted on a Sojourner class rover, a robotic arm or an Aerobot.

Category V Compliant Container for Mars Sample Return Missions

A novel containerization technique that satisfies Planetary Protection (PP) Category V requirements has been developed and demonstrated on the mock-up of the Mars Sample Return Container. The proposed approach uses explosive welding with a sacrificial layer and cut-through-the-seam techniques. The technology produces a container that is free from Martian contaminants on an atomic level. The containerization technique can be used on any celestial body that may support life. A major advantage of the proposed technology is the possibility of very fast (less than an hour) verification of both containment and cleanliness with typical metallurgical laboratory equipment. No separate biological verification is required. In addition to Category V requirements, the proposed container presents a surface that is clean from any, even nonviable organisms, and any molecular fragments of biological origin that are unique to Mars or any other celestial body other than Earth.

Thermally induced changes in the focal distance of composite mirrors - Composites with a zero coefficient of thermal expansion of the radius of curvature

Calculations are presented of the coefficient of thermal expansion (CTE) of the radius of curvature of the reflector face sheets made of a quasi-isotropic composite. It is shown that, upon cooling, the change of the CTE of the focal distance of the mirror is equal to that of the radius of the curvature of the reflector face sheet. The CTE of the radius of the curvature of a quasi-isotropic composite face sheet depends on both the in-plane and the out-of-plane CTEs. The zero in-plane CTE of a face sheet does not guarantee mirrors with no focal length changes.

Analysis and simulation of the ultrasonic/sonic driller/corer (USDC)

An ultrasonic/sonic driller/corer (USDC) was developed to address the challenges to the NASA objective of planetary in-situ rock sampling and analysis. The USDC uses a novel drive mechanism, transferring ultrasonic vibration into impacts on a drill stem at sonic frequency using a free- flying mass block (free-mass). The main parts of the device and the interactions between them were analyzed and numerically modeled to understand the drive mechanism and allow design of effective drilling mechanism. A computer program was developed to simulate the operation of the USDC and successfully predicted the characteristic behavior of the new device. This paper covers the theory, the analytical models and the algorithms that were developed and the predicted results.