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Unformatted text preview: Ae104a Homework #4 Development of an indirect counterbalanced pendulum optical lever thrust balance for micro to millinewton thrust measurement. By A.N. Grubii and S.B. Gabriel : Meas. Sci. Technol. 21 105101 (2010). A summary by Garrett Lewis and Dustin Summy. A device is developed to indirectly measure thrust produced by microthrusters similar to those used in spacecraft. The thrust balance is designed to provide a low cost and reliable thrust measurement for microthrusters with a thrust range of 10 3 to 10 5 Newtons. These measurements can then be compared with calibrating measurements performed with a more expensive and accurate system to estimate the absolute thrust. By using the simple system and correcting with comparisons to the more complex system, measurements can be made much more quickly and cheaply than by relying on the complex system alone. The most accurate measurements using this kind of technique come from devices in which the thruster is mounted directly to the pendulum itself. This avoids the uncertainties in the ion target interaction, but requires careful and expensive measures to mount the thruster to the pendulum without causing undesired torques due to the wiring and fuel lines that must run to the thruster. To avoid these expenses, the authors chose an indirect thrust balance setup as shown in Figure 1. The advantages in simplicity are obvious the thruster remains fixed during the test negating the concern over counter torques developed in connections, the hanging pendulum is in contact only at the pivot point, and the counterweight allows for greater sensitivity without extending the pendulum arm (limited by space in a vacuum chamber). The pendulum reflects a laser beam, and the deflection is measured by an optical sensor which reports the position of the beam to the data collection suite. The response of the resulting system is that of an underdamped harmonic oscillator to good precision. An acceptable block diagram for the system is illustrated in Figure 2. The naturally occurring value of is measured by the logarithmic decrement as shown in Figure 3a. In order to shorten the settling time and decrease noise sensitivity, the authors added a ring magnet of 0.1T mounted a distance behind the pendulum to induce magnetic damping. By varying the distance between the magnet and the target, the authors were able to increase by up to 3 orders of magnitude, shortening the settling time (authors definition) from hours to about 12 seconds. An example of the damped response is shown in Figure 3b. Finally, the authors characterized certain sources of noise inherent to the design. A significant zero drift due to thermal heating was found immediately after thruster shutoff, which the authors simply subtract from the steady state thrust measurement. Additionally, the authors measured the power spectrum of noise in the system and attempted to relate resonances to environmental causes.power spectrum of noise in the system and attempted to relate resonances to environmental causes....
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This note was uploaded on 01/05/2012 for the course AE 104a taught by Professor List during the Fall '09 term at Caltech.
- Fall '09