Ground testing of release impulse for the aluminum cubic test mass with a compound pendulum for the TianQin project.

In the space-borne gravitational wave detection TianQin project, the locking and releasing of test mass is one of the key technologies. The test mass will be locked during the spacecraft launch and then released to free fall for the science phase. The residual release impulse is required to be on the order of magnitude of 10-5 kg m/s, which allows us to capture the test mass by the force authority of the capacity control. In this paper, the release impulse of the aluminum test mass is measured with a compound pendulum for the TianQin project. The test mass is locked by two tips from opposite positions, and the release impulse is obtained from the oscillation of the pendulum. When the aluminum test mass is locked and released by the stainless steel and aluminum tips, the release impulses and their uncertainties are on the order of magnitude of 10-5and 10-7 kg m/s, respectively. This provides a feasible measurement scheme for the impulse testing in the TianQin project.

A newly designed decoupling method for micro-Newton thrust measurement.

A decoupling method is proposed for micro-Newton thrust measurement with a torsion pendulum. The basic approach is to reduce the influences introduced by the propellant tube and wires of the thruster. A hollow aluminum tube is used to hang the torsion pendulum and is also chosen as the transport pipe for the propellant of the thruster. The electric control box of the thruster is mounted on the pendulum body, which is powered by an externally installed power supply through a liquid metal conductive unit. The control of the electric control box is performed through wireless transmission. With this design, the influences of the propellant tube and connection wires between the torsion pendulum and the outside device are reduced and the stability of the torsion spring constant of the system can be improved. The use of the liquid metal conductive unit reduces the coupling between the wires and the measurement system. The feasibility of the wireless transmission is analyzed. The error sources during the thrust measurement are analyzed, and the expected three σ uncertainty of the thrust is 0.032+(0.10%*F)2µN for the measurement of the cold gas thruster. The scheme provides a thrust measurement with higher precision and stability.

Apparatuses for verifying the precision of gravimeters with lifting spherical source masses.

Two apparatuses with lifting spherical source masses are built and used to verify the precision of gravimeters. The 333-kg source mass produces a maximum acceleration of 200 nm/s2 with an uncertainty of 0.31 nm/s2, which corresponds to a relative uncertainty of 0.16%. After evaluating the temperature effect, drift of the gravimeter, the atmospheric effect, and the tidal effect, a combined uncertainty of 1 nm/s2 is obtained. One CG6 gravimeter is tested using two apparatuses, the measured accelerations agree with the theoretical values within the error range. Differential measurement with two CG6 gravimeters on one apparatus is performed, which provides a common-mode rejection of the effects due to ambient noise, such as the gravity tide, atmospheric effect, and drift. The difference in acceleration measured by the two gravimeters is determined to be 199 ± 6 nm/s2, which agrees well with the value 200 ± 1 nm/s2 obtained by using apparatus II. Our apparatuses provide a verification of the precision of gravimeters with an uncertainty of 1 nm/s2, which is one of the lowest uncertainties reached so far. The determination of geometrical metrology and mass distribution and detailed error analysis are presented. The methods on error analysis as well as differential measurement used in our work are helpful for gravity measurement.

A compound pendulum for thrust measurement of micro-Newton thruster.

A thrust stand is developed for testing micro-Newton level thrusters on the ground. The stand is composed of a compound pendulum that is symmetrically suspended by two thin beryllium copper strips, and it is precisely calibrated by gravity. The stiffness of the stand can be adjusted in 3 orders of magnitude by a counterweight. When the stiffness is larger than 1 Nm/rad, the stand demonstrates a fast response to thrust. The measured range of the stand reaches 1000 µN, and the noise is less than 0.1 µN/Hz within 1 mHz-1 Hz. To calibrate the resolution of the stand, an electrostatic force is applied to the stand with an actuator. The equivalent thrust is determined to be 0.09 µN with a standard uncertainty of 0.02 µN. Using the stand, a micro-Newton colloid thruster is tested. The output of the colloid thruster changes with the applied voltage as 0.015(1) µN/V. When changing the voltage by 50 V, the change in thrust is measured to be 0.7 µN with a standard uncertainty of 0.1 µN.