The Research On The Effect Of Background Gravity Gradient In The Measurement Of G With Angular Acceleration Method

Not only the gravitational constant G is significant in the theoretical physics, but also plays an important role in aerospace and other engineering fields. The truth-value of G is only measured through experiments. As far, although many experiments have been worked to measure the G on the world, there are only 8 results with uncertainties less than 50 ppm,however, the G values are disagreement in the claimed uncertainties among each experimental group with different methods, that is to say, there are some systematic errors still haven't be properly evaluated. In order to get a more credible result and find the unknown systematic errors in two different methods within the 25 ppm level, our laboratory launch a new experiment with angular acceleration method, as the same time the measurement of G with the time-of-swing method is still carried on and improved.As the measurement of G with the angular acceleration method is a complex scientific experiment with a wide range of contents. The main works of this paper is studying the impact of the effect of background gravity gradient on the measurement of G,the background contains the relatively static background as the Yuga Mountain and the rotating background as the Ultra-Low-Expansion(glass-ceramic) shelves. When the two turntable are moving, the effect of static background gravity gradient is only associated with speed of the air-bearing turntable. Therefore we can separated this part effect by setting the modulation frequency. However, the rotating background move with the balls,and it's gravitational effect of the torsion balance coincide with the modulation signal. We can't distinguish this effect from the modulation signal. So the latter has a greater impact on the G value.According to the measurement results, we find the effect of rotating background contributes more than 900 ppm uncertainty to the G value. It's much larger than the goal we set(uncertainty < 25 ppm). Therefore, this part of the effect need to be measured andcompensated. We design two feedback control loops for two turntables to ensure a constant value of their speed difference, and let the air-bearing turntable track the torsion balance to keep the torsion fibre without twisting. We can obtain the the effect of the rotating background gravity gradient at the signal frequency by measuring the twist angle of the torsion balance. Based on the measurement results, we design different size masses to compensate this part effect. It is decreased to 1.1±1.5ppm after several compensation,and it isn't a major error in this experiment. What's more, we measure and compensate the part of static background gravity gradient, it lays the foundation to obtain a further precision G value.