Engineering topics

Sherrit, Stewart

Publications and source records attributed to Sherrit, Stewart.

147 records · Page 9

NASA NTRS2001

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. Our results indicate that care is required when interpreting impedance data at high drive level due to the frequency dependence of the dissipated power. Although the transducer/resonator at a single frequency and after many cycles may reach thermal equilibrium, the spectra as a whole cannot be considered an isothermal measurement due to the temperature change with frequency. Methods to correct for this effect will be discussed. Results determined from resonators of both soft and hard PZT and a ultrasonic horn transducer are presented.

transducers↗

NASA NTRS2001

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

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 dnves a free mass that resonates between the horn and drill stein. 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 low 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.

Peterson, Thomas↗

NASA NTRS2000

Modeling of Horns for Sonic/Ultrasonic Applications

JPL has a requirement for telerobotic tools for planetary sample acquisition, which require low power and have the ability to work in harsh environments. We are currently investigating the possibility of using ultrasonic horns to develop a family of ultrasonic tools for these environments. In an effort to determine control parameters a one-dimensional Mason's model for a stepped ultrasonic horn assembly was developed which includes the effects of mechanical and electrical losses in the piezoelectric material and acoustic elements. The model is separated into three regions; the piezoelectric stack including stress bolt the backing layer and the horn. The model is found to predict the impedance data of the horn assembly very accurately up to the first coupled (radial) resonance. The model also allows for the calculation of the velocity and force and power delivered to each acoustic element. FEM modeling and accelerometer data from the horn tip were used to corroborate the model. The difficulties associated with modeling the load impedance of various devices will be discussed and current directions noted.

Sherrit, Stewart↗