RESEARCH ON SPACE DOCKING HIL SIMULATION SYSTEM BASED

ON STEWART 6-DOF MOTION SYSTEM

Junwei HanHarbin Institute of Technology

Harbin CHN

Introduction

Spacecraft docking technique play very important role in human space program. For the docking mechanism is very complex and the on-orbit docking is a complicated dynamics process, it is necessary to research spacecraft on-orbit docking process dependent on HIL (Hardware-In-the loop) simulation.

Before 1975, the docking simulation is called physics simulation, for example• In 1964, Langley Research Center of American firstly established a docking simulator.• In 1969, Langley Research Center established another docking simulator.• In 1971, former USSR designed a docking simulator that had been employed to the test of APAS-89 docking mechanism.

• After the Apollo-13 Disaster in 1970, a new docking mechanism called APAS-75 was developed. For the composition and dynamics of APAS-75 were much more complex than that of the docking mechanism developed before. Physics simulation cannot satisfied to verify its dynamics. Then the research on spacecraft docking simulation has come into the time of HIL simulation.

The famous two HIL docking simulator were made by former USSR and USA.

•In 1980s, Europe and Japan began to research on the unmanned spacecraft docking technology, and the spacecraft docking simulator was also developed respectively.

European proximity operation simulator Rendezvous and docking operation test system

•China began manned space program in 1992.• In 2008, an integrated testing system for docking mechanism is developed.

The research contents of the report include three parts:

1. The compositions, and the model of simulation system included dynamic model of the spacecraft are given, 2. The characteristics of the system is analyzed,3. A simply verifying model is employed to research on the effect of 6-DOF Stewart platform frequency characteristics on the system stability and accuracy of the docking dynamics.

System DescriptionDocking mechanisms are employed for docking a spacecraft with another spacecraft. •The on-orbit spacecraft is called passive spacecraft, and its docking mechanism named passive docking mechanism. •The launched spacecraft is called active spacecraft, and its docking mechanism called active mechanism. •To research on the docking dynamics, five coordinate frames are defined

According to Newton-Euler formula, the dynamic of active spacecraft can be written as eq. (1) and eq. (2)

(1)

(2)

1

3112

2

dd

mt

r FFr

331311111

r

1 dd FrMMωJωωJ t

and the passive spacecraft dynamic formula is given by eq. (3) and eq.(4)

(3)

(4)

the relative movement between the two spacecrafts is gained by eq.(5)

(5)

2

222

2

dd

mt

r4FF

r

442422222

r

2 dd FrMMωJωωJ t

21121

b

21

r

dd

dd rωrr

tt

Then on-orbit docking dynamics is obtained

The diagram of docking dynamics

To research on the docking dynamics, a docking simulator is built, the system is consisted of three parts,1 the dynamic simulation software, calculating the relative motion between the two spacecrafts2 a 6-DOF Stewart platform, which is employed to replicate the relative motion between the two spacecrafts3 a 6-DOF force and torque sensor, is used to measure the impacting forces and torques

There are two problems have effect on the verification for the docking simulation system

⑴ Stability. For Stewart platform has phase lag, which may result in the unstable docking dynamics.

⑵ Docking dynamics validation. The strengths of the dynamics should be proven by experimental validation.

System AnalysisThe transfer function of the overall system is given by eq.(6)

(6) If then the docking simulator may replicate the on-orbit docking process with no error. But Is not equal to 1, and so to ensure the validation of the docking dynamics replication on the simulator, the attention must be paid to the design of system.

)()()()()( sGsGsGsGsG TSDMI

1)()()( sGsGsG TSO)(sGO

VERIFICATION To simplify the research, a simple non-damping collision-rebound device is employed to research on the parameters determination of the system controller. the nature frequency of the system is given by eq.(7)

(7)

the rebound coefficient is defined as eq.(8).

(8)

21

21 )(mmmmK

n

o

iv v

vR

The simulator is employed to simulate the non-damping collision-rebound oscillation device. If the frequency error

and rebound coefficient error is

the validation of docking simulator can be verified.

05.0

n

ncnfe

05.01 vcr Re

Analysis and Experiment It is seen that frequency characteristics of the Steward platform has phase lag, the non-damping collision-rebound dynamics simulation system is unstable

A compensation controller of Stewart platform is designed

With the controller, the simulation system is stable. This is the experiment result when kp =1.

The rebound coefficient is about 0.97, but the frequency of the output is greater than that given by eq.(7).

What does result in the frequency changed?

)()()(sSsTksD p

This is the experiment result of the simulation system when kp <1, the frequency becomes small.

This is the experiment result of the simulation system when kp >1, the frequency becomes large.

It is seen that the gain of the controller can result in the frequency changed.

CONCLUSIONS• Experiment research on docking dynamics is an effective method.• Apart from stability, frequency error and rebound coefficient are defined to verify the validation of docking dynamics simulation.• The frequent characteristic of the Stewart platform has great effect on performances of the docking simulator, its phase lag has influenced on the stability of the simulator, and its gain may effect on the nature frequency of the docking dynamics.

Thank you !