Earth rotation sensing has many applications in different disciplines, such as for the monitoring of ground motions, the establishment of UT1 and the test of the relativistic Lense-Thirring effect on the ground. We report the development of a 1 m × 1 m heterolithic passive resonant gyroscope (PRG). By locking a pair of laser beams to adjacent modes of the square ring cavity in the clockwise and counter-clockwise directions, we achieve a rotation resolution of about 2 × 10 −9 rad/s at an integration time of 1000 s. The sensitivity of the PRG for rotations reaches a level of 2 × 10 −9 rad/s/ √ Hz in the 5-100 Hz region, currently limited by the detection noise, residual amplitude modulation and the mechanical instability of the cavity. Our initial results improve the reported rotation sensitivity of the PRGs and indicate that PRGs have a great potential for highresolution Earth rotation sensing.Many large scale ring laser gyroscopes (RLG) have been built over the last three decades with extremely high sensitivity and stability [1]. These large ring lasers along with the developed fiber optic gyros have found applications in different fields like geodesy [1], seismology [1][2][3][4] and fundamental physics tests [5][6][7]. The monolithic G-ring experimentally resolves a rotation rate of 3.5 × 10 −13 rad/s over 1000 s and detects the Chandler and the annual wobble of the Earth [8,9]. The successful operation of two really large heterolithic RLGs, namely UG-1 and UG-2 with enclosed areas of 367 m 2 and 834 m 2 , demonstrate the feasibility of constructing giant rotation sensors [10,11]. Laser gyros are also used to improve the seismic isolation system for ground-based gravitationalwave antennas [12,13]. Moreover, an ambitious plan named GINGER has been proposed, focusing on measurement of the Lense-Thirring effect in a terrestrial laboratory [14]. Highly sensitive interferometric fiber optical gyroscopes (I-FOG) are developed mainly for applications in navigation and platform control, with a sensitivity in the order of 10 −8 ∼ 10 −9 rad/s/ √ Hz. Such rotational sensors with low self-noise are also suitable for applications in geosciences [15][16][17].The measurement principle of RLGs is based on the Sagnac effect: the counter propagating beams in a ring cavity will see a different round trip time if the ring cavity is rotating in the optical plane. This effect was first described by Sagnac in 1913 [18]. In 1963, Macek and Davis utilized a ring cavity that contains He-Ne gas to lase at the 1.153 µm line, which demonstrated rotation sensing [19]. This is considered to be the first active RLG. A passive gyro was successfully realized by Ezekiel and Balsamo in 1977 [20], where an external laser is split into two beams and locked to a passive ring cavity in the clockwise (CW) and counter-clockwise (CCW) direction. The rotation rate is determined by measuring the resonance frequency difference of the ring cavity in the opposite directions. A modern PRG, used as a tilt sensor, recently reported a sensitivity of 10 −8 rad/s/...

Water hydraulic motor (WHM) is an important transmission component in a water hydraulic system. Because of different physicochemical properties of raw water when compared with mineral oil, the design for the WHM would be different from that for a mineral oil one. A static equilibrium-type water hydraulic axial piston motor (SEWHAPM), which is equipped with a global-moment-balanced plate valve and a hydrostatic slipper bearing with a concentric gap damper, is developed in this research. The mechanism of the global-moment-balanced plate valve is analysed to eliminate the irregular wear of the plate valve. Next, the relevant design guideline is constructed. A mathematical model of the hydrostatic slipper bearing is established, and the performance characteristics of the bearing are discussed. The theoretical analyses indicate that the load-carrying capacity of this hydrostatic slipper bearing is not correlated with system pressure, water viscosity, temperature, or rotor speed, and that the lubrication film thickness of the bearing possesses high rigidity. The fundamental principle for designing the hydrostatic slipper bearing is also derived. On the basis of a material-screening experiment, a SEWHAPM is fabricated and tested on a water hydraulic-component-performance test stand. The experimental results show that developed SEWHAPM is feasible, with its volumetric, mechanical, and total efficiencies being 86, 80, and 70 per cent, respectively, under a pressure of 10 MPa. The maximal output torque can reach 19.5 N m; the maximal speed can reach 3000 r/min. As a result of the aforementioned development, the SEWHAPM has been successfully used in an underwater tool system since 2002, and its performance has been tested, demonstrating that the developed equipment can meet the regulated requirements for its practical applications.

In hydraulic power systems with water hydraulic valves, cavitation is more likely to happen than in an oil hydraulic system. A two-step throttle, which consists of two ori ces, is an effective way to avoid or reduce cavitation damage. In this research, experiments are conducted to investigate the ow-pressure and cavitation characteristics of throttles of different shapes. The working medium is tap water. A new indicator is introduced to detect cavitation inception, namely that a particular low-frequency component of the outlet pressure uctuation of valve appears and grows rapidly when cavitation occurs. From the experimental results, it can be concluded that the two-step throttle has different features to a single-step throttle.

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