Comparison of electrostatic fins with piezoelectric impact hammer techniques to extend impulse calibration range of a torsional thrust stand

Abstract

With the progression of high-power electric propulsion and high thrust-to-power propulsions system, thrust stand diagnostics require high-fidelity calibration systems that are accurate over a large-range of thrust levels. Multi-mode and variable I(sp) propulsion devices also require that a single stand be capable of measuring thrust from 10's of uNs to 100's of mNs. While the torsional thrust stand mechanic and diagnostics are capable of operating over such a large range, current pulsed calibration schemes are typically limited to a few orders of magnitude of dynamic range. In order to develop a stand with enough dynamic range, two separate calibration methods have been examined and compared to create a combined system. Electrostatic fin (ESF) and piezoelectric impact hammer (PIH) calibration systems were simultaneously tested on a large scale torsional thrust stand system. The use of the these two methods allowed the stand to be calibrated over four orders of magnitude, from 0.01 mNs to 750 mNs. The ESF system produced linear results within 0.52% from 0.01 mNs to 20 mNs, while the PIH system extended this calibration range from 10 mNs to 750 mNs with an error of 0.99%. The two calibration methods agreed within 4.51% over their overlapping range of 10-20 mNs.

Recommended Practice for Thrust Measurement in Electric Propulsion Testing

“…To damp thrust stand oscillations, passive eddy current dampers [17,[19][20][21][22][23][24] and active electromagnetic dampers [7,25,26] have primarily been employed, although oil dampers have also been used [9,10]. Eddy current dampers are simple, noncontact mechanisms consisting of two stationary magnets between which a conducting plate (typically copper) moves.…”

“…It is generally easiest to change the distance of the center of mass from the rotation axis. Counterweights are often used on the side of the beam opposite the thruster to place the center of mass of the beam system very near the axis of rotation [8,9,[14][15][16]24,28].…”

“…Several contact calibration methods involve the swinging of known masses [4,41] or the use of impact pendulums [2] or impact hammers [8]. Noncontact methods include calibration using electrostatic forces between planar electrodes [14], free molecular gas flow from underexpanded orifices [9], electrostatic combs [24], and electromagnetic coils [17]. Of these methods, we recommend piezoelectric impact hammers and electrostatic combs, which are common practice and provide precise, controllable impulses over a broad range.…”

“…By accurately controlling the amount of charge, or voltage, on the combs as well as the time that the charge is applied, a well-known impulse can be created. ESCs can accurately produce forces from tens of nanonewtons [42] to tens of millinewtons [24] with errors typically less than 1%. By varying the pulse width and amplitude of the applied voltage, Pancotti et al demonstrated impulses ranging from 0.01 to 20 mN ⋅ s, which produced responses of a torsional thrust balance that were linear to within 0.52% [24].…”

“…ESCs can accurately produce forces from tens of nanonewtons [42] to tens of millinewtons [24] with errors typically less than 1%. By varying the pulse width and amplitude of the applied voltage, Pancotti et al demonstrated impulses ranging from 0.01 to 20 mN ⋅ s, which produced responses of a torsional thrust balance that were linear to within 0.52% [24]. The capacitance of the combs may lead to voltage overshoot at the beginning of the pulse.…”