# Novel Pointing and Stabilizing Manipulator for Optical Space Payloads
## 1 Introduction  
Independent pointing adjustment is necessary for the small optical payloads on a large spacecraft to keep LOS aligned with the pointing target without changing the orientation of the whole orbiter. In addition, optical payloads should also keep pointing stable enough in the presence of spacecraft vibration to guarantee the observation or telecommunication quality.  
The gimbal-type photoelectric platform is one of the most classic Line-Of-Sight (LOS) motion control schemes, which is widely used in high-gain antennas and aviation reconnaissance platforms [1; 2; 3]. It can pan and tilt (or roll, pitch, and yaw) in quite a large range and meanwhile compensate for errors induced by low-frequency disturbance. Parallel mechanisms are also popular for payload pointing because of the high precision, stiffness, and load capacity [4; 5; 6]. Some novel pointing mechanisms for specific applications might also be a potential solution to this problem [7; 8; 9; 10]. The schemes use typical actuators that possess large strokes, such as servo motors and linear modules. However, the motion resolutions of typical actuators are usually inadequate for micro-vibration rejection due to the torque ripple and friction. As for micro-vibration control, the multi-dimensional isolator is a commonly used scheme. It usually adopts voice coil motors (VCM) [11; 12; 13; 14], inertial actuators [15; 16; 17], or piezoelectric devices [18; 19; 20; 21; 22; 23] as actuators and transferred motion with flexible structure, and good motion resolution was then achieved. However, the strokes of these fine actuators are usually too tiny to get enough angular workspace, and independent pointing adjustment is, thus, unavailable.  
It is natural to combine several devices to possess motion of large-scale and high-resolution simultaneously [24; 25; 26; 27]. Stratospheric Observatory for Infrared Astronomy (SOFIA) utilized coarse drive, fine drive, and active secondary mirror to provide the telescope independent LOS control. Meanwhile, it also adopted multiple piezoelectric active mass dampers besides the vibration isolation system to reject the disturbance [28; 29; 30].  
However, the scheme is too complex for a small optical payload and would cause high costs or excess performance. A Large Ultraviolet/Optical/Infrared (LUVOIR) Surveyor was planned to adopt a serial integration scheme [31; 32]. It would reject the disturbance with a parallel isolator that adopts the disturbance-free platform concept. A two axes gimbal that connects the isolator and the support module was planned to provide the telescope with independent angular motion. The serial scheme would increase the electrical interfaces and could not reuse space and structure. In addition, the serial scheme might have stiffness problems, and the additional reinforcing structure may be necessary. In a word, a serial integration scheme would have difficulties in miniaturization.  
Aiming at small optical space payloads, this paper introduces a novel pointing and stabilizing manipulator that integrates both coarse and fine actuators in parallel. On this basis, comprehensive research on the motion characteristics was conducted, and a specific operation scheme was proposed so that large-scale pointing adjustment and disturbance isolation could be accomplished by a single manipulator. The parallel scheme could get rid of additional assistant devices in an integrated and compact way and thus would be a potential solution for the miniaturization of optical payload manipulators.  
The remainder of the paper is organized as follows. Section 2 presents the kinematic and dynamic characteristics of the manipulator. Section 3 proposes a specific operation scheme, including adjustment and isolation algorithms. Section 4 introduces the validation of the proposed scheme. The last section summarizes the contents and contributions of the paper.