53 New Supplier - Hardware Duplication – Some Pitfalls 

Edwin Joscelyn* 

Abstract    Companies, for many reasons, often need to task a new supplier to duplicate the design and manufacture of a product that has alr eady been qualified and flown on one of their earlier systems. This paper deals  with such a situation. The conflicts that arise when required to include characteristics other than those defining the specific performance and dimensional requirements of the product is explored. While this paper uses the “reverse engineering” of a motor as the means to describe some of these problems, the basic conflicts and conditions can apply to other areas and products.    Introduction  The task was to duplicate the design of 3 different motor configurations. Although the performance specifications were clear and well written, there were no other mechanical design details provided other than the outline dimensions, winding location, and weight. The real challenge came from additional non-performance requirements that were specified. These requirements stemmed from the fact that these motors pre-existed from an earlier build cycle and the program was extremely sensitive to anything which differed from this earlier build in the new design. Winding profile, material, torque constants, resistance, inductance, harmonic distortion, detent torque, and drag torque parameters could not differ from the earlier designs. In addition some of the dimensions had tight tolerances on the OD and ID of the designs (e.g. 13 µm / 0.0005 inch). While these conditions alone might not seem to be in themselves presenting that difficult a design situation certain underlying factors proved otherwise.   In many cases, the task of “reverse engineering” is greatly simplified when there is a unit available which  allows for physical inspection and disassembly of the product and the ability to make performance parameter measurements. This was not the case for this exercise.  Designing a duplicate and identical motor under these conditions is daunting at best. Attempting to get all of the performance parameters within specification is problematical. For example, one generally does not design a motor for a specific inductance. Inductance generally falls out as a by-product of the size of the motor, number of winding turns, laminations, etc. The motor’s torque constant Kt ( N-m/amp) is generally  determined by the motor’s dimensions, materials, air gap and winding  turns. The Kt is directly  proportional to the number of turns while the inductance is determined by the number of turns squared. So, one can see the dilemma of attempting to converge these two parameters. This was only one of the issues…getting a pure sinusoidal BEMF waveform (< 2% harmonic distortion for the 3, 5 th and 7th) was  another of the major difficulties to solve.    We will attempt to walk through the iterative and somewhat frustrating process of arriving at a successful  series of motors. This paper is not intended as a design review but rather as lessons learned when unrelated design restrictions are added to the design process. The work began in May 2008 and was completed in January 2010.                                                                 * Aeroflex, Hauppauge, NY 

Proceedings of the 41st Aerospace Mechanisms Symposium, Jet Propulsion Laboratory, May 16-18, 2012