A Recommended New Approach on Mo torization Rati o Calculations of Stepper Motors Ruben Nalbandian*, Thierry Blais**and Richard Horth+    Abstract Stepper motors are widely used on most spacecraft mechanisms requiring repeatable and reliable  performance. The unique detent torque characteristics of these type of motors makes them behave  differently when subjected to low duty cycle excitations where the applied driving pulses are only  energized for a fraction of the pulse duration. This phenomenon is even more pronounced in discrete permanent magnet stepper motors used in the space industry. While the inherent high detent properties  of discrete permanent magnets provide desirable unpowered holding performance characteristics, it results in unique behavior especially in low duty cycles. Notably, the running torque reduces quickly to the  unpowered holding torque when the duty cycle is reduced. The space industry’s accepted methodology of  calculating the Motorization Ratio (or Torque Margin) is more applicable to systems where the power is continuously applied to the motor coils like brushless DC motors where the cogging torques are low enough not to affect the linear performance of the motors as a function of applied current.  This paper summarizes the theoretical and experimental studies performed on a number of spacequalified motors under different pulse rates and duty cycles. It is the intention of this paper to introduce a new approach to calculate the Motorization Ratios for discrete permanent magnet steppers under all full and partial duty cycle regimes. The recommended approach defines two distinct relationships to calculate the Motorization Ratio for 100 percent duty cycle and partial duty cycle, when the motor detent  (unpowered holding torque) is the main contributor to holding position. These two computations reflect  accurately the stepper motor physical behavior as a function of the command phase (ON versus OFF times of the pulses), pointing out how the torque contributors combine. Important points highlighted under this study are the torque margin computations, in particular for well  characterized mechanisms. The rationale at CDR level versus TRR/TRB level will be discussed, aiming at avoiding too much conservatism for units that have extensive test and in flight heritage. A critical topic is related to the magnetic losses and how to sort out such phenomena as a function of the motor type being used. For instance, detent torque is a major contributor that has no reason to evolve during life and is not  an uncontrolled torque loss. Introduction A large number of space based mechanisms use stepper motors. They are simple, low cost and reliable. There are several different types of stepper motors, each with different characteristics. The type of  stepper motor discussed in this paper is the discrete permanent magnet stepper motor. Its rotor has a discrete magnet for each pole as opposed to a hybrid stepper motor that has only one annulus magnet in its rotor. The specificity of this type of stepper motor is that it exhibits large detent torque, as implemented in the well-known MOOG Type 3 and Type 5 Rotary Actuators and also by MDA for robotics applications. This detent torque is very useful to hold payloads in position without power when not moving. The effects  on the torque produced and the computed Motorization Ratio of different power on duty cycles is *Moog, Inc., Chatsworth, CA **Astrium SAS, Toulouse, France +MDA Corporation, Montreal, Canada /g51/g85/g82/g70/g72/g72/g71/g76/g81/g74/g86/g3/g82/g73/g3/g87/g75/g72/g3/g23/g21/g81/g71/g3/g36/g72/g85/g82/g86/g83/g68/g70/g72/g3/g48/g72/g70/g75/g68/g81/g76/g86/g80/g86/g3/g54/g92/g80/g83/g82/g86/g76/g88/g80/g15/g3/g49/g36/g54/g36/g3/g42/g82/g71/g71/g68/g85 /g71/g3/g54/g83/g68/g70/g72/g3/g41/g79/g76/g74/g75/g87/g3/g38/g72/g81/g87/g72/g85/g15/g3/g48/g68/g92/g3/g20/g23/g16/g20/g25/g15/g3/g21/g19/g20/g23/g3 349