Construction and performance of an inverted pendulum thrust balance

Abstract

The objective of the work in this thesis was to devise a means of profiling the thrust of the MIT Space Propulsion Lab's (SPL) Diverging Cusped Field (DCF) thruster and, more generally, other thrusters of similar size and thrust levels. The former SPL thrust stand, which had been used to characterize the BHT-200 engine, was not suitable for the DCF because of its torsional style design. An entirely new, inverted pendulum-type balance needed to be built. The new design employs a vertical arm with the DCF situated at the top and a counterweight placed at the bottom. The vertical arm rotates at the fulcrum through a flexible pivot attached to a base. A horizontal thrust force from the DCF causes the balance to rotate. This motion is sensed by a linear variable differential transformer (LVDT) and counteracted by a force from a voice coil. The voice coil's neutralizing force nulls the balance back to an equilibrium position and supplies the thrust value produced by the DCF. The inverted pendulum thrust balance was built from an initial design proposed by Professor Manuel Martinez-Sanchez. Many of the electrical components found on the old thrust stand, like the LVDT and the voice coil, were incorporated into the new one. Additionally, the control software and hardware from the old stand required several changes and updates to be compatible with the new design. After the assembly of the new thrust balance, the issues of calibration and thermal drift during use were also addressed. Once a means of correcting the undesired operational forces and thermal effects had been established, the balance displayed thrust measurement within a range of 0mN to 23mN with an uncertainties as low as ±0.5mN.