Improvement of ONERA Electrostatically suspended torsion pendulum

In order to meet the objectives of the Next Generation Gravity Mission (NGGM), the French Aerospace Lab ONERA is developing a new accelerometer concept MicroSTAR with three sensitive linear and three true angular acceleration measurements (Dalin et al., session G4). This instrument benefits from previous developed accelerometers which flew in the well-known gravity space missions: CHAMP, GRACE, GOCE, GRACE-FO.

The principle of operation of these accelerometers is to maintain motionless a proof-mass with respect to the surrounding electrodes using a control loop. The applied electrostatic forces needed for this control, are proportional to the accelerations suffered by the proof-mass. The proof-mass is polarized with a very thin (few micrometer) wire in order to measure precisely its position by capacitive detection with the electrodes and to avoid charging in orbit. The stiffness and damping induced by the polarization wire impact the performance of the accelerometer at low frequencies. To quantify on-ground the performance limitation due to the wire, an electrostatically suspended torsion pendulum (PTSE) is used (Willemenot 1997, Willemenot and Touboul 2000).

The PTSE is a six-axes servo-controlled accelerometer, optimized for the measurement of angular accelerations about the vertical axis. The torque noise spectral density is 1.3 10^-14 Nm/√Hz  around 0.05 Hz with a 1/√f increase at lower frequency, corresponding to 10^-8 rad/s²/√Hz , and 2 10^-10 ms^-2/√Hz with a lever arm of 2cm. For instance, regarding a gold wire of 7.5µm diameter and 1.7cm length, we use it to measure theoretical stiffness of 2.5 10^-5 N/m i.e. a torsion of 10^-8 Nm/rad. In our presentation, this instrument will be described before sharing ideas for its improvement.

 

Willemenot, P. Touboul; Electrostatically suspended torsion pendulum. Rev Sci Instrum 1 January 2000a; 71 (1): 310–314. https://doi.org/10.1063/1.1150198

Willemenot, P. Touboul; On-ground investigation of space accelerometers noise with an electrostatic torsion pendulum. Rev Sci Instrum 1 January 2000b ; 71 (1): 302–309. https://doi.org/10.1063/1.1150197

Willemenot, Pendule de torsion à suspension électrostatique, très hautes résolutions des accéléromètres spatiaux pour la physique fondamentale. Ph.D. thesis, Université Paris 11, France, 1997