## 1 Introduction  
The crewed lunar roving vehicle (CLRV) is essential in Chinese plans to build a lunar base. The CLRV is an essential and indispensable exploration tool that not only transports astronauts to great distances from the lunar module but also ensures their safety as they move on the lunar surface [1]. To mitigate the potential issues of the CLRV, it is imperative to conduct a comprehensive ground test of the full rover before launch. Furthermore, the differences in gravity between the Moon and Earth give rise to distinct steering properties and driving sensations for the astronauts using the CLRV. Therefore, it is essential to construct a ground test facility on Earth that accurately simulates lunar gravity to facilitate astronaut training and evaluate the CLRV's performance. A problem common to the ground test facility is simulating a low-gravity environment similar to the spacecraft's natural working environment [2]. Simulating a low-gravity field is an essential issue in the motion performance experiment of the planetary rover. The simulation method of offsetting part of the lunar rover's gravity by an external force is called gravity compensation. The primary ways to achieve artificial micro-gravity include free-fall testing [3], the air-bearing table [4], neutral buoyancy [5, 6], and the suspension system [7, 8]. Among the available methods, the suspension system [9, 10] is widely adopted because of its relatively simple structure, easy construction, and 3-D simulation with unlimited time. The track-following servo subsystem is a critical component of the suspension gravity compensation system, which is used to precisely track the movement of the test object in the horizontal direction. In the suspension gravity compensation system of the CLRV, due to the large motion velocity, acceleration, and range of the CLRV, the track-following servo subsystem also requires a corresponding motion capability and a large scale. In this context, the crane as the servo motion mechanism exhibits significant inertial characteristics [11]. At the same time, the subsystem is also faced with various constraints such as speed, acceleration, and position. These characteristics often result in significant tracking errors and even control failure. Currently, research on the effectiveness of the track-following servo subsystem in the suspension gravity compensation system of the CLRV remains unexplored. The active response gravity offload system (ARGOS) at NASA's Johnson Space Center is a typical suspension system [12]. However, the track-following servo subsystem of this system is only applicable for walking tests of astronauts with low velocity in a single direction. The Harbin Institute of Technology built a suspension lunar gravity compensation system, which was only suitable for slow-moving unmanned lunar rovers [2]. The max speed of the Yutu lunar rover is about 200 m/h, while the theoretical maximum speed of the CLRV in pre-research is about 4 m/s. The Soviet Union's planetary rover ground test used an active tracking constant tension suspension scheme [13], which generated a constant vertical pulling force using a parallelogram with a spring. However, this solution is only suitable for ground tests on Mars rovers and may not work well for driving a large vehicle on simulated soft lunar soil. NASA proposed a suspension system scheme suitable for CLRV ground tests in the last century, but the track-following servo control in the program lacks simulation validation and practical implementation [14]. The suspended gravity compensation system is more commonly used in slow-motion microgravity experiments, such as deployable antennas [7] and satellites [8], and its tracking servo subsystem is not suitable for the CLRV. To meet the CLRV's test requirements, this paper conducts research on the system design and control scheme of the track-following servo subsystem in an overhead-crane-like Lunar Gravity Compensation facility (LGCF) based on a single-cable gravity compensation system.