Z B Zhou, Y Z Bai, L Liu, D Y Tan, H Yin Center for Gravitational Experiments, School of Physics, Huazhong University of Science and Technology, Wuhan , China A two-stage electrostatic-controlled torsion pendulum has been developed to investigate the performances of electrostatic inertial sensors on ground. The twist and translation motions of the proof mass relative to the electrode frame were monitored by high-precision capacitance transducers, and were synchronously controlled by electrostatic actuators. A new two stage pendulum using silica fiber suspension is construing in our laboratory in order to improve the sensitivity, and then to investigate the disturbances of the proof mass due to the environment variance such as temperature, electromagnetic field and so on, and also to investigate the cross coupling between the differential degree of freedom of the inertial sensor. I. Introduction High-precision inertial sensors play a paramount role in space-borne gravitational experiments, such as in the CHAMP, GRACE, GOCE, MicroSCOPE and LISA missions. This type of inertial sensors have more than one DOF, and the DOFs could interfere with each other. For the simultaneous simulation of the in- flight operation of the PM with more than one DOF on ground. We developed a two-stage electrostatically controlled torsion pendulum. The operations of both a translational and a rotational DOF of the inertial sensor are simultaneously measured. A two-stage electrostatic-controlled torsion pendulum facility to investigate performances of an inertial sensor A new two-stage pendulum with silica fiber suspension is also construing in our laboratory. The high quality factor of the silica fiber makes the pendulum has low thermal noise so that the sensitivity of it could be improved a lot. Noise power spectrum density. Left: Twist; Right: translation PM: 4 cm×4 cm×1 cm titanium cubic, 72 g Total mass: 162 g Nominal capacitance gap: 167  m III. Seismic noise limit These ground-based fibre suspension experiments are inevitably disturbed continuously by the seismic noise, which will limit the experimental resolution to a certain level. the experimental limit for the horizontal seismic noise based on the two-stage torsion pendulum scheme is theoretically analysed, and the corresponding experimental result is presented, which is consistent with the theoretical estimation. The transfer function from the seismic noise to the equivalent disturbance a in of the PM, which is used to estimate the seismic noise limit, can be given by For our two-stage controlled torsion pendulum experiment, the CMRR is estimated at about 86 dB at frequencies below f L ~ 2 mHz. For the two-stage torsion pendulum of Stanga et al, the limit by the NLNM, when the experiment is carried out at the quietest site on Earth, is shown as the star line in figure. (CQG 2010) II. Experimental setup Magnetic coils and electrical heaters are mounted near the electrodes inside the chamber along the Z axis to investigate the coupling to the magnetic field and the effects of the temperature and the temperature gradient along the Z axis. (CQG 2010) Torque noise around the X axis and the force noise along the Z axis, as shown in figures, come to 1.4×10 −13 Nm Hz −1/2 and 1.1×10 −11 NHz −1/2 near 30 mHz, respectively, which are mainly limited by the seismic noise at frequencies above 0.1 Hz and the capacitance transducer noise at frequencies below 0.1 Hz. The quality factor of our silica fiber has been measured by a single torsion pendulum with a disk-shape proof mass. The quality factor is measured higher than The pendulum suspended with this silica fiber has been used to measure the stiffness and the loss angle of a thin discharging wire which is applied in some type of inertial sensor. (CQG 2012) To investigate the cross-coupling between the differential degree of freedom of the inertial sensor, a bar-shape mass is used to act as a second-stage torsion balance instead of the original single proof mass, as shown in right figure. The lower bar is perpendicular to the upper one. In this case two translation motions of the proof mass could be simultaneously simulated in flight under this case, and then we can, therefore, investigate their cross-coupling, which is important for LISA and similar projects. H B Tu et al, Class. Quantum Grav. 27 (2010) Z B Zhou et al, Class. Quantum Grav. 27 (2010) L Liu et al, Class. Quantum Grav. 27 (2010) Stanga R et al 2006 J. Phys.: Conf. Ser –5 Stanga R et al 2009 J. Phys.: Conf. Ser References © ESA IV. Experimental results and discussion