常平架

此内转台可在其自身平面内绕其轴线A转动，并装在一个位于圆环里面的常平架上。

This turnable can be rotated in its own plane about its axis A. and is mounted on gimbals within a ring .

宏控制系统为方位、俯仰两轴常平架结构，实现快速粗跟踪，同时隔离粗平台的剩余摇摆误差，稳定光电系统的视轴。

The macro-control system is used to achieve a fast tracking , while isolating the remaining swing error of primary platform and stabilizing the LOS optical tracker .

由于机械加工精度和常平架的调平精度有限，天顶摄影仪的视轴、垂直旋转轴与铅垂线总是存在不一致，导致测量精度与理论精度存在很大的偏差。

Due to machining accuracy and limited leveling precision of gimbals , there is always an inconsistency between the visual axis and the vertical rotational axis of the digital zenith camera and the plumb line , which leads to a big deviation between measurement precision and theoretical precision .