Gyro-Control of a Solar Sailing Satellite

Transcription

1 of a Solar Sailing Satellite Background by Hendrik W. Jordaan Willem H. Steyn Electronic Systems Laboratory Department of Electrical & Electronic Engineering Stellenbosch University January 20, 2017

2 Stellenbosch Situated about 30 minutes away from Cape-Town, South-Africa

3 Stellenbosch University History with microsatellites, SunSat, SunSpace, SumbandilaSat Current main focus is on ADCS research Develop ADCS CubeSat components which is sold under CubeSpace brand Also involved in a number of interesting international projects

4 Attitude Requirements Current solar sailing missions main payload is the solar sail Background Future missions will have other science payloads e.g. image payloads Main specification driver for attitude control is mission payload Attitude control requirements for solar sailing is low, only slow manoeuvres and rough attitude stability relative to a sun angle. High sampling and long exposure payloads very stringent attitude requirements

5 Standard Satellite vs Solar Sail Satellite Largest difference between a solar sail and standard spacecraft is the large MoI, which is obtained when deploying a large space structure. When comparing the MoI of a 80m2 and 100m2 square sail there is a 43.9% increase in the MoI with less than a meter increase in boom length. The attitude control actuator specifications should increase This is dependent on the sail/spacecraft MoI ratio Ratio of the sail MoI relative to the entire spacecraft MoI Parameter 80m2 Sail 100m2 Sail Boom length 6.325m 7.071m MoI Ixx=Izz kg.m kg.m2 MoI Iyy kg.m kg.m2

6 Standard Satellite vs Solar Sail Satellite The sail/spacecraft MoI ratio is determined simply by λ = max(λ) with Λ = I S /I where I S the MoI of the sail and I is MoI of the entire spacecraft. Small λ indicates rotational dynamics of spacecraft is dominant Larger λ indicates that the dynamics are greatly influenced by the sail MoI will greatly influence the attitude performance either increase in sail size or vibration of non-rigid elements Some control attitude control methods will greatly influence this ratio and thus less suitable to scale to larger solar sails. Manoeuvres are limited by actuator specifications and the non-rigid dynamics of the sail

7 Current attitude control methods Current attitude control methods can be separated into two categories: Active methods Thrusters (gas and electric) Standard magnetorquer rods Reaction and momentum wheels Solar thrust methods Changing CoM Translation stage Control boom Mass-balasts Changing CoP Reflective changes Control vanes Sail shape changes Review by Fu et al. 2016

8 Spinning Solar Sail Spinning sail has a number of advantages above stabilised sail Unsymmetrical solar thrust averaged to spin vector Centrifugal force produce internal force Major drawbacks are Satellite bus is rotating, limits mission payload Angular momentum bias resists angular manoeuvres Standard Spinning Solar Sail Slow Spinning Solar Sail

9 Tri-Spin Solar Sail The tri-spin solar sail consists of three sections rotating relative to each other. The nett angular momentum of the spacecraft is zero Similar to connecting two dual-spin satellite to each other The gyro tri-spin solar sail is created by placing these rotating structures on two-axes gimbals. Tri-Spin Solar Sail Gyro Tri-Spin Solar Sail

10 Gyro Control This configuration creates a dual CMG configuration Background Torque multiplication achieved with small changes in gimbal angles Steering laws of gimbal angles, assuming scissoring, can be used to determine required gimbals angels and control inputs for certain torque requirements

11 Gyro Control (cont) Creates a scalable attitude actuator angular momentum of the sail is used to determine the actuator performance Larger angular momentum requires smaller gimbal angles to obtain reference torque

12 Gyro Control (cont) Investigated the effects of some control inconsistencies Background Angular momentum bias, gimbal angle errors and MoI uncertainties are investigated

13 High performance attitude control must be achieved on solar sail satellites to make it appropriate for science missions. Some control methods scale much better to larger sails than others. Gyro tri-spin solar sail produce an attitude control actuator that scales with the MoI of the sail Comes at a cost e.g. large mechanical complexities Starting to exist the realm of ignoring sail vibrations

14 Final Remarks The current trend is for more deployables on smaller spacecrafts, not only true for solar sails. Smaller spacecrafts are more susceptible to non-rigid influence. Future ADCS needs to be able to handle active damping of vibrations to be able to operate mission payloads.

15 Thank you Questions? Willem Jordaan