For TSIS, tuning was performed via direct frequency response measurements that applied sinusoidal  command signals to the hardware and then measured the system response. Performing these  measurements was quite time consuming and also had problems associated with the test hardware that  further increased measurement time. On the other hand, the ability to take direct measurements of the  system dynamics was enormously beneficial in the ability to tune the system, correlate models, and improve  confidence in the system performance.  Tuning was performed quite frequently throughout the integration and test (I&T) program in order to match  controller tuning to the specific hardware configuration (mechanical and electrical) that evolved and  changed numerous times (10+) during I&T. Taken together with the duration of each measurement, tuning  added significant, yet important, time to the program schedule. In the future, a software-based tuning  approach may be developed that could reduce the measurement time for each of these tuning activities.  Center of Gravity Sensitivity  Because the BLDC actuators are directly coupled, they are particularly sensitive to offsets in the center of  gravity (CG) of the instrument payload mass relative to the intersection of the rotational axes in two main  ways. First, on-orbit translation disturbances couple into rotational disturbances via the offset CG. Second,  ground testing may require offload/balancing ground support equipment (GSE) to account for gravityinduced torques. Additionally, the configuration change associated with any balancing GSE may  necessitate separate controller tuning. For these reasons, it is highly desirable to locate the CG of the  moving mass as close to the rotational axes intersection as is reasonably possible.  Bearing Friction Sensitivity  Bearing friction is a dominant dynamic that affects BLDC motor control and the limits of performance. In  general, the friction should be as small as possible and as linear as possible. Of course, this is challenging  to achieve given other constraints on the system. In particular, bearings that carry significant launch loads  need to be appropriately large and with sufficiently high preload, both of which increase bearing friction.   Although suggested methods exist to select proper preload and Hertzian contact stress, a perfect approach  does not exist that is as good as experience with actual hardware. For this reason, early testing and  measurements are particularly valuable to understanding and compensating for bearing friction.  Unpowered Free Motion  An obvious direct consequence of a direct-drive system with low friction is that it will experience free motion  when not powered. This was accounted for by including brakes in the actuator design that operate in the  unpowered state. But as an additional mechanism, the brakes added their own complexities to the system  design and operation that would have been easily accounted for by detent torque in a stepper-based  actuator. Nonetheless, the brakes did function properly and prevent free motion when TSIS is unpowered  on-orbit.   Structural Coupling  Even though the primary structural mode associated with the stepper-based harmonic drive was eliminated  by selecting direct-drive BLDC actuators, structural dynamics still had to be accounted for to ensure  successful actuator operation. In particular, it is important to provide frequency separation between the  controller bandwidth and the structural modes in order to avoid resonant excitation of those modes. This  was accomplished through frequent mechanical design iteration and communication between the  mechanical engineers and the controls engineers. Additionally, to reduce risk, modal testing of the TPS  structure was performed that was able to identify the structural modes and damping. The measured modes  were actually closer to the controller bandwidth than was desired, but frequency response measurements  of actuator control loops showed acceptable bending margins without any excitation of the structural modes.  This was attributed to higher than originally predicted structural damping, which was attributed to the  specific mechanical architecture of the TSIS system. One important lesson learned was that structural  damping can be extremely beneficial to ensuring compatibility between the structural and control design,  NASA/CP—2018-219887 277