This is very interesting and will be useful for studies in fault-tolerance onboard spacecraft.

Mechanical degradation includes increase in friction and change in stiction. The build-in mechanical non-ideal mechanisms include rotor un-balance and associated induction of spacecraft vibrations. These could be a bit nasty in presence of high-precision optical sensors, since sampling would take place at lower frequencies that those induced by the wheel and aliasing could be an issue.

If you have access to the inner torque control loop in your hardware, you might superimpose the change in friction torque as a torque command add-on signal (delta Q), which is “unknown” to your control part. If you have only access at the speed reference level (omega), you might use the output of a simulation: delta omega = I^-1 delta Q, where I is the moment of inertia.

The basic idea is illustrated in a lecture note (a very basic model used for teaching a first course in spacecraft dynamics and control – no imperfections added) [1]

Simulating defects in the drive electronics is more difficult. In a project dealing with AC motors (not connected with space), we made a breadboard to be able to emulate faults including: IGBT bridge defects (short or open transistors) and winding short circuits. We made fault detection and fault-handling as described in:

However, the breadboard was only documented in an internal report.

Kind regards

Mogens Blanke

Technical Editor for Fault-tolerant Systems, IEEE Transactions on Aerospace and Electronic Engineering

References:

[1]  Blanke, M.: Satellite Dynamics in a quaternion formulation. Lecture note on satellite attitude dynamics and control. Automation and Control, Department of Electrical Engineering, Technical University of Denmark, Build. 326, DK 2800 Kgs. Lyngby, Denmark, latest version 2010, 22p

[2]  Blanke, M. and J. S. Thomsen: Electrical steering of vehicles - fault-tolerant analysis and design. Microelectronics Reliability Vol. 46 (2006) 1415-1420