Implications of Underdamped Stepper Mechanism Performance and Damping  Solution Methodology  Shane Brown*and Scott Starin* Abstract  When driving a stepper motor the control method of stepping the motor has a significant impact on the  performance of the mechanism. When driving a stepper motor system with unipolar or wave method of  control, the back-emf generated in the open, unregulated coil creates a stepper motor system that  provides significantly less inherent damping compared to full bipolar control. Less damping results in  higher overshoot, ringing, and potentially exciting mechanical resonances resulting in fatigue. In addition,  a significantly underdamped system, particularly with significant load inertia, raises many concerns with  mechanism life and performance. The mechanism described herein utilizes a Rotary Accelerometer (RA)  for step counting as well as state-of-health monitoring [1]. Another consequence of an underdamped  mechanism using accelerometers is that the RA data is of limited or no value, due to high overshoot  acceleration. This paper presents a proven design approach to damp a stepper motor driven in a  unipolar, wave fashion, or a system with significant inertia mismatch between the motor and the reflected  load.  Introduction  Ball Aerospace & Technologies Corp. (BATC) is involved in a proprietary space vehicle application that  has several mechanisms. One of the mission critical applications is a single degree of freedom  mechanism consisting of a geared stepper motor actuator coupled to a balanced load inertia, to be  referred to here after as the mechanism. The load of the mechanism must be controlled to precision  interval locations and have full 360° of operation (no hard stops). A geared stepper motor was selected  because of the precision response, simple controls, and proven heritage and reliability.  For various reasons, a three-phase stepper motor with an integral Rotary Accelerometer (RA) was  selected for the actuator. We believed the mechanism was simple enough that an Engineering  Development Unit (EDU) was not required. However, for calibration of the control electronics, a  representative EDU mechanism was produced simultaneously with the flight hardware. During the  electronics calibration procedure anomalous performance of the mechanism was observed, resulting in  the inability of the control electronics to process the RA feedback signal as originally intended. We  believed the erratic performance of the mechanism was a mechanical reliability concern, due to  significant overshoot, operational resonance and mechanical ringing.  This paper details the troubleshooting, analysis and test simulation process, as well as the ultimate  course of action taken to resolve the performance issue. We also detail the implications of the stepper  motor drive control methods and the effects they have on mechanism performance. In addition, we detail the requirements for interfacing with, and signal processing of, the RA signal for step verification. Time  was critical in the decision making and rework process because flight hardware was received, and all  viable solution options required an in-process modification to the flight hardware. Because of these  schedule needs, several solutions that would have been plausible for the initial design phase, were no  longer viable, and are not fully discussed in this paper.  The Mechanism  Mechanical Description                                                  * Ball Aerospace & Technologies Corp., Boulder CO  Proceedings of the 39th Aerospace Mechanisms Symposium, NASA Marshall Space Flight Center, May 7-9, 2008  29