Design of the Novel Double-ring Dynamical Gravimeter

Journal of Automation, Mobile Robotics & Intelligent Systems VOLUME 9, N° 3 2015 Design of the Novel Double-ring Dynamical Gravimeter Submitted: 13th February; accepted 5th May 2015 Igor Korobiichuk, Michal Nowicki, Roman Szewczyk DOI: 10.14313/JAMRIS_3-2015/23 Abstract: In this paper, a description of the design and operation principle of the double-ring dynamically tuned gravimeter is given. This design allows for the gravimeter signal separation from the noise, which makes it possible to increase the measurement accuracy of Earth’s gravitational field anomalies using gravimetric aviation system to 1 mGal. Keywords: gravimeter, gyroscope, aircraft gravimetric system 1. Introduction The gravimeter is an instrument used for measurements of local variations in Earth’s gravitational field. The constant downward gravitational acceleration varies by about 0.5% on the surface of the Earth, due to the planet internal structure, land masses, metallic ores deposits, surface shape etc. The gravimeters operation principle is in essence the same as for other accelerometers, but there is need for exceptional measurement accuracy, due to the extremely small relative changes of the measured value [1]. Gravimeters are used in mineral prospecting, seismology, geodesy, geophysical surveys and research. They display the measured gravitational acceleration value in units of gals (cm2/s), named after the famous Galileo Galilei [2]. Table. 1. Acceleration units conversion Base unit Gal (cm2/s) m2/s m2/s 100 1 Gal (cm2/s) Standard gravity g0 1 980.665 Standard gravity g0 0.01 0.00101972 9.80665 1 0.101972 Existing gravimeter systems are either relative or absolute. Absolute gravimeter system measures the absolute value of the gravity acceleration, ex. by directly measuring the acceleration of a mass during free fall in a vacuum, when the accelerometer is rigidly attached to the ground, and the falling mass forms one of the arms of the Michelson interferometer. Relative gravimeters measure the ratio of the gravity between different measurement points. They can be constructed for example as a mass on a spring. Transportable systems would use extremely stable inertial platform to overcome distortions caused by the device’s movement and vibrations [3, 4]. Most of the precise gravimeter systems are heavy, complicated and rigidly attached to the ground. To study the characteristics of the Earth’s gravitational field in inaccessible parts of the globe, the aircraft gravimetric system (AGS) is necessary [5, 6]. In the available literature there are descriptions of known gravimeters working principles, and some accelerometers characteristics that are used as gravimeters in AGS’s. However, there are no descriptions of the design, functional diagram and the working principle of the double-ring dynamically tuned gravimeter [7–10]. Accuracy of the Earth gravity anomalies measurement using gravimetric aviation system is largely dependent on the choice of system’s sensing element. Today, the gyroscopic gravimeters are considered to be one of the most effective among the available solutions. Additionally, in [7, 11] it was proved that the double-ring dynamically tuned gravimeter (DG) has much greater accuracy and performance than the standard one-ring one. Therefore, further research and development of the double-ring dynamically tuned gravimeter design is advisable, as the most promising of the known aviation gravimeters. The aim of this paper is to provide a description of the design and lay out the working principle of the double-ring dynamically tuned gravimeter. 2. Design Double-ring dynamically tuned gravimeter development belongs to measurement techniques science, and can be used for moving platform gravimetric measurements in geodesy, geology, and in inertial navigation systems. A dynamically tuned gyroscope (DTG) is a rotor suspended by a universal joint with flexure pivots. The flexure spring stiffness is independent of the rotor spin rate. However, the dynamic inertia, from the gyroscopic reaction effect, from the gimbal suspension provides negative spring stiffness proportional to the square of the spin speed. Therefore, at a particular tuning speed, the two moments cancel each other, freeing the rotor from torque, which is a necessary condition for an ideal gyroscope. The design of the double-ring DTG [12], is shown in Fig. 1. The rotor 1, drive shaft 2 and internal suspension elements are driven by engine 3, which provides a constant rotational speed. Internal Kardanov suspension contains gimbal ring 4, two internal flexure pivots 5, 47 Journal of Automation, Mobile Robotics & Intelligent Systems linking gimbal ring to the rotor and two external flexure pivots 6 connecting gimbal ring with a drive shaft. Axes of the internal and external flexure pivots are perpendicular to each other and perpendicular to the axis of the drive shaft. Fig. 1. Schematic diagram of the gravimeter construction 48 The reference voltage generator 7 is coupled with the drive shaft. The generator is used to generate control signals that drive the Msξ and Msη moment sensors, affecting the rotor. The two electromagnetic torque sensors are attached to the body of the device. When supplying power to the sensors they create a torque around η or ξ axis, which does not rotate with the rotor Voltage, is applied between the middle and two extreme points of the windings. The angle sensors Asξ and Asη are also attached to the body of the device. They convert the rotor plane deviation from the neutral position into electrical signals. In particular, the shown angle sensors respond to the change of the air gap between the windings center and the rotor surface, in device body points that do not revolve around the x and h axis. The basis of the design of double-ring dynamically tuned gravimeter was the known two-stage gyroscope based gravimeter [11, 12]. The common essential feature of the double-ring dynamically tuned gravimeter and the known one is that they contain two-stage gyroscope and the torque sensors. However, unlike the double-ring dynamically tuned gravimeter, in the known gravimeter construction the center of mass of the two-stage gyroscope rotor is located on the axis of rotation, and it contains the sensor of the angular velocity of the gyroscope rotor rotation relative to the base. As a result, the output value of the gravimeter is the angular velocity of rotation of the gyroscope rotor relative to the device’s base. Thus, the known gyroscopic gravimeter require additional signal conversion to determine the gravity acceleration value, which contributes additional errors in measurement result. Articles VOLUME 9, N° 3 2015 Furthermore, there is no measurement errors calculation and compensation in the standard device. Therefore, measurement results obtained using this gravimeter contains significant measurement errors. These errors are caused by non-linear distortions of the trajecto (...truncated)