Reaction wheel fricton compensation using dither

Claim 1

A reaction-wheel stabilized spacecraft, comprising: a spacecraft body; at least one reaction wheel mounted for rotation about an axis on said spacecraft body; controllable torquing means coupled to said spacecraft body and to said wheel for applying torque to said wheel in response to a torque command signal for acceleration or deceleration thereof for transferring momentum to said spacecraft body; sensing means mounted on said spacecraft body for sensing the attitude of said spacecraft body, and for generating a signal in response to deviation of said spacecraft body from a desired rotational position about said axis; control means including integrating means coupled to said sensing means for generating an attitude-sensitive component of said torque command signal; summing means including a first input port coupled to said control means and an output port coupled to said torquing means, for receiving said attitude-sensitive component of said torque command signal from said control means, and further including a second input port, for combining said attitude-sensitive component of said torque command signal with a signal applied to said second input port of said summing means to generate said torque command signal; and alternating signal generating means coupled to said second input port of said summing means for generating a signal of alternating polarity and for applying said signal of alternating polarity to said second input port of said summing means, whereby said torque command signal may include an alternating component.

Claim 6

A method for controlling the attitude of a spacecraft about the axis of a bearing-mounted wheel, comprising the steps of: sensing the attitude of said spacecraft about the axis of a reaction wheel to generate an error signal; integrating said error signal to produce a torque command signal; torquing said reaction wheel in response to said torque command signal in a manner tending to stabilize said attitude of said spacecraft, whereupon the direction of rotation of said reaction wheel may from time to time reverse; sensing the rotational speed of said reaction wheel; and at rotational speeds near zero rotational speed, adding an alternating-polarity signal to said torque command signal.

Claim 7

A method for controlling the attitude of a spacecraft about the axis of a bearing-mounted wheel, comprising the steps of: sensing the attitude of said spacecraft about the axis of a reaction wheel to generate an error signal; integrating said error signal to produce a torque command signal; torquing said reaction wheel in response to said torque command signal in a manner tending to stabilize said attitude of said spacecraft, whereupon the direction of rotation of said reaction wheel may from time to time reverse; sensing the rotational speed of said reaction wheel; and at rotational speeds near zero rotational speed, adding an alternating-polarity signal to said torque command signal.