针对柔性航天器动力学模型的降阶问题，综述了动力学建模中几种常用的模态集，介绍了用于选择系统保留模态的有关准则。通常的模态价值分析和内平衡理论是用于航天器系统模态的选取，而不是直接用于动力学建模时的部件模态或系统结构弹性变形模态的选取，因而存在如何应用上述准则于动力学建模的问题。为此，提出了一个动力学建模时的降阶策略；还提供了一个数值例子，以对采用不同的模态集建模时的结果进行比较，并表明如何采用上述准则于航天器的动力学建模。

With secondary regulation, a novel hydraulic transmission technology, the secondary regulated loading system described here is oriented to rotational speed experiment of reduction gear or material. In this paper, the hardware structure of the system is introduced with specialties of the transducers, electro-hydraulic servo valves and the computer data collector used. Then, signal-adjusting circuit between computer control system and transducers or servo valves is designed. Next, a VC+ + control program under Windows operating system is worked out. Experiments on Step, Rectangle, Ramp and Sine Responses of the speed sub-system are done under alternative oil pressure and load. Finally, some meaningful results are given by contrasting the experimental curves.