Failure detection and correction system for redundant control elements

For use in a vehicle attitude control system having a plurality of control elements of the type that may contribute control forces even after failure, at least one of the control elements being redundant, control element failure detection and compensation apparatus, comprising:

• distribution matrix means for distributing to the plurality of control elements a like plurality of control signals derived from a set of attitude commands relating to a corresponding set of axes of the vehicle;
• control element modeling means coupled to said distribution matrix means, for simulating operation of the control elements and generating an estimated value of a performance parameter for each control element;
• measuring means for obtaining an observed value of the performance parameter for each control element;
• failure determination means for comparing said estimated performance parameter with said observed performance parameter and determining whether a control element failure has occurred;
• means responsive to detection of a failure by said failure determination means, for deactivating the failed control element and for modifying said control element modeling means to take the failure into account;
• and
• means for automatically compensating for a failure to ensure continuing performance of the apparatus after deactivation of a failed control element.

For use in a satellite attitude control system having a plurality of speed-controlled, skewed-axis reaction wheels, means for sensing attitude, means for deriving angular momentum commands for three vehicle axes from the sensed attitude and from attitude commands, and distribution matrix means for distributing the angular momentum commands among the reaction wheels, failure detection and compensation apparatus comprising:

• wheel loop modeling means for each speed-controlled reaction wheel, for receiving speed command signals from the distribution matrix means, and for generating therefrom an estimated speed signal for each wheel;
• means for obtaining a speed error signal from the difference between said estimated speed signal and a signal representative of the observed speed of the corresponding reaction wheel;
• failure determination means, for determining whether a failure has occurred by monitoring said error signal;
• means responsive to the detection of a reaction wheel failure, for deactivating the failed wheel and for modifying said wheel loop modeling means to simulate deactivation of the failed wheel;
• and
• means also responsive to the detection of a reaction wheel failure, for reconfiguring the distribution matrix means to exclude the failed wheel.

A skewed-axis reaction wheel satellite attitude control system, comprising:

• means for sensing attitude with respect to three orthogonal vehicle axes;
• at least four speed-controlled reaction wheels having axes skewed to the vehicle axes;
• means for generating a set of three vehicle-axis angular momentum commands from the sensed attitude and attitude commands;
• a wheel-speed distribution matrix for transforming said set of three momentum commands into a set of at least four wheel speed commands to effect the three momentum commands;
• at least four wheel loop models for simulating operation of said speed-controlled reaction wheels, and generating estimated wheel speeds from said wheel speed commands;
• speed measuring means for obtaining the actual speeds of said reaction wheels;
• subtraction means for determining the speed differences between corresponding actual and estimated wheel peeds;
• failure criterion means for generating a failed-wheel condition if any of said speed differences has exceeded a preselected threshold for a preselected time;
• first means responsive to a failed-wheel condition, for deactivating the appropriate one of said reaction wheels, modifying the appropriate one of said wheel loop models to reflect such deactivation, and reconfiguring said distribution matrix to exclude the failed wheel and thereby distribute wheel speed commands to the remaining wheels;
• and
• second means responsive to a failed-wheel condition, for resolving the momentum contributions of all failed wheels in the three vehicle axes, and for subtracting the resolved momentum contributions from corresponding vehicle-axis momentum commands, before distribution by said distribution matrix.

For use in a control system having a plurality of control elements of the type that may contribute control forces even after failure, at least one of the control elements being redundant, a method of detecting control element failure, said method comprising the steps of:

• simulating operation of the control elements and thereby generating an estimated value of a performance parameter for each control element;
• measuring the performance parameter for each control element to obtain an observed value thereof;
• comparing the estimated performance parameter with the observed performance parameter;
• determining whether a control element failure has occurred;
• deactivating the failed control element upon detection of a failure;
• and
• modifying performance of said simulating step to take the failure into account.

For use in a vehicle attitude control system having a plurality of control elements of the type that may contribute control forces even after failure, at least one of the control elements being redundant, a method of control element failure detection and compensation, said method comprising the steps of:

• distributing to the plurality of control elements a like plurality of control signals derived from a set of attitude commands relating to a corresponding set of axes of the vehicle;
• simulating operation of the control elements and thereby generating an estimated value of a performance parameter for each control element;
• measuring the performance parameter for each control element to obtain an observed value thereof;
• comparing the estimated performance parameter with the observed performance parameter;
• determining whether a control element failure has occurred;
• deactivating the failed control element upon detection of a failure;
• modifying performance of said simulating step upon detection of a failure, to take the failure into account;
• and
• compensating for a failure to ensure continuing performance of the control system after deactivation of a failed control element.

For use in a satellite attitude control system having a plurality of speed-controlled, skewed-axis reaction wheels, means for sensing attitude, means for deriving angular momentum commands for three vehicle axes from the sensed attitude and from attitude commands, and distribution matrix means for distributing the angular momentum commands among the reaction wheels, a method of wheel failure detection and compensation, comprising the steps of:

• receiving speed command signals from the distribution matrix means;
• simulating operation of each speed-controlled reaction wheel, and generating therefrom an estimated speed signal for each wheel;
• measuring the actual speed of each reaction wheel;
• obtaining a speed error signal from the difference between the estimated speed signal and a signal representative of the actual speed of the corresponding reaction wheel;
• determining whether a failure has occurred by monitoring the speed error signal;
• deactivating the failed wheel upon detection of a failure;
• modifying said simulating step to simulate deactivation of the failed wheel;
• and
• reconfiguring the distribution matrix means to exclude the failed wheel upon detection of a failure.