Schematic diagram of the thrust stand. The pendulum with a pulsed plasma thruster (PPT) and a counterweight is suspended by a tungsten fiber, whose motion is monitored by an electronic autocollimator. Two steel spheres, placed on the positions A-A′ and B-B′ successively, are used to change the periods of the pendulum for precisely measuring the moment of inertia of the pendulum. Two electrostatic actuators, paralleled to the pendulum body, are used to avoid the abrupt motion of the pendulum in the open loop mode and to provide an electrostatic force to balance the thrust in the close loop mode.
Enlarged schematic diagram of the suspending design. The fiber is embedded in the hole of the split clamp by two screws. The distances from the fiber axis to the thrust vector R and the center of the positioning rings and (similar to d AO and d BO ) could be measured accurately. The mirror M is used to monitor the attitude of the pendulum by using an additional autocollimator.
Block diagram of the thrust measurement in closed-loop mode. The PID compensator, which is commanded by the angle signal of the pendulum obtained by the autocollimator, controls the electrostatic actuators to produce a force to balance the thrust. K P , K I , and K D are the proportional, integral, and differential gains, respectively.
The pendulum twist data from typical cuts in three configurations. Curve A: without the spheres; Curve B: the spheres placed on the rings A and A′ (close to the fiber); Curve C: the spheres placed on the rings B and B′ (far from the fiber).
The equilibrium position of the pendulum varied with time. The square and circle represent the equilibrium position with and without the lead blocks, respectively. The dashed lines are the fitting results, which show that the shift of the equilibrium position is about (0.66 ± 0.30)μrad.
Power spectrum density of 14-h data of the torsion balance. The dashed line represents the TEPO noise requirement for the thruster of 0.1 μN/Hz1/2 from 10−4 to 1 Hz.
Typical data and fit for the torsion balance's response to a single shot pulse. The torsion balance oscillated with tiny amplitude originally. Once the thruster produced a pulse thrust, the pendulum was excited and kept oscillating with a large amplitude. The data has been fitted with Eq. (3) in the first oscillation period and the fitted curve passes so close to the data points that it cannot be distinguished from them in this plot.
Measured parameters of the thrust stand.
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