86 K. J. Pederson et al. Fig. 9.6 NEXT-C load cell schematic In other words, the total force in the direction of testing may be accurate; however, the force contribution from each individual load cell likely is not. If Fx or Fy must be used for summing force in another axis, their contribution is likely inaccurate and will make limiting on this other axis problematic. Also, the individual forces cannot be compared to analytical model data accurately. This issue occurred during the NEXT-C testing in the lateral axes where the load cells used for limiting were not aligned with the axis of excitation. For example, the Z axis setup schematic is shown below. As you can see in Fig. 9.6, load cells A and B are not aligned with the axis of test, so each component of the load cells are used along with some trigonometry to calculate their force in the axis of excitation. The C load cell is aligned with the axis of test and no trigonometry is required. In this scenario, the total force in the Z axis can be calculated using the following equation, which assumes that the load cell labels from Fig. 9.6 are equal to the manufacturer-supplied sensitivity multiplied by the voltage coming from the load cell’s charge amplifier (i.e., Ay =<manufacturer sensitivity>*volts). Ftot,Z =Ay cos(ϕ) −Az sin(ϕ) −By cos(ϕ) −Bz sin(ϕ) +Cz (9.7) After performing in situ calibration and applying the sensitivity adjustment factor, this equation can be written as follows: Ftot,Z =γ %Ay cos(ϕ) −Az sin(ϕ) −By cos(ϕ) −Bz sin(ϕ) +Cz& (9.8) Now the total force in the Z axis can be limited to this summed force level. A challenge arose because the NEXT-C test plan also wished to force limit in the Y and X axes. The force sum equation in the Y axis is shown below. Ftot,Y =−Ay sin(ϕ) −Az cos(ϕ) −By sin(ϕ) +Bz cos(ϕ) +Cy (9.9) The force terms in the Y-axis force sum that appear in the Z axis force sum will be inaccurate. For example, the force measured from the Ay load cell will be equal to γAy. This γ adjustment factor was found during in situ calibration in the Zaxis and is likely not accurate for the individual Ay load cell. Therefore, the force sum in theY-axis will carry this inaccuracy. When the ion thruster is configured for Z-axis testing, there is no way to excite it in the Y axis at low frequencies to perform in situ calibration and check the accuracy.
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