Alternatively, BLDC actuators can achieve much finer angular resolution due to the way they operate.  Instead of commanding a number of steps (discrete torque step commands), the controller asks the motor  to apply a continuously-variable current that translates directly to torque that results in corresponding  continuous angular motion. Closed-loop control is then applied to achieve the desired angle with much finer  resolution than could be achieved via the stepping approach.  Minimal Self-induced Jitter   Because of the step changes in torque application that are described above, stepper motors generate selfinduced torque disturbances that result in oscillatory pointing jitter and associated torque disturbances  reflected back to the spacecraft. This self-induced jitter is not directly controllable due to the fact that it is a  byproduct of the system mechanics, but it can potentially be reduced by several means during the design  phase (e.g., microstepping). Because these effects are fixed thereafter, they must be accounted for through  appropriate, detailed analysis that demonstrates compatibility with the overall system design requirements.  Doing so requires accurate analytic models of the stepper motor dynamics and the control excitation  approach. However, these models include inherent nonlinearities (step commands) that complicate  standard linear controls analysis. In past LASP programs, significant effort and time was spent modeling,  predicting, and communicating the stepper motor disturbances produced by the solar array drives and the  instrument pointing system to ensure mutual compatibility with each other and the spacecraft’s attitude  control system.  By contrast, because BLDC motors apply torque in a continuous rather than discrete manner, they do not  produce oscillatory self-induced jitter like steppers do. This results in greatly reduced angular jitter and  torque disturbances reflected back to the spacecraft, both of which are highly desirable goals. And because  the torque application is applied continuously, the actuator dynamics of the BLDC motors can be accurately  and simply modeled as linear systems that support standard linear controls analysis.  Increased Active Control Bandwidth  Stepper motor control circuits typically limit the maximum commanded step rate in order to avoid loss of  motor synchronization, and this rate limiting in combination with the discrete step size has the associated  effect of limiting the control bandwidth. While this limitation may be acceptable for coarse pointing  applications, it can be a hindrance for more precise pointing applications where improved tracking and  disturbance attenuation capabilities are needed.   In contrast, BLDC actuators behave linearly and are not rate limited in the same manner, so increased  active control bandwidth can be achieved that allows for improved tracking of dynamic inputs and for  increased active disturbance attenuation.  Structural Separation  A direct consequence of the harmonic drive-based gear train arrangement is that the actuator input and  output are structurally coupled about the rotational axis with the coupling primarily determined by the  stiffness and damping of the harmonic drive.  By comparison, direct-drive BLDC actuators have very little structural coupling between the input and output  about the rotational axes, and the small couplings that do exist are primarily due to bearing friction, back  EMF, and harness routing (e.g., twist capsules). Because the pointing goal of TSIS is to point at the sun,  which is approximately fixed in inertial space compared to the rotational motion of the underlying spacecraft  (ISS), structural separation is an important advantage that will be further expanded upon in the following  paragraph.  Significant Increase in Base Disturbance Attenuation  When TSIS was comparing direct-drive BLDC to stepper-based actuators for the TPS on ISS, simulations  were performed using representative actuator dynamics and load inertias to compare the output pointing  responses of both actuator types to base motion disturbances. For an inertially pointed instrument such as  TSIS, the objective is for the output motion (i.e., SIM and TIM sensors) to be as insensitive to input base  NASA/CP—2018-219887 273