345 Electrical Noise Performance of Gold -on-Gold Slip Rings  

Ron Hayes*, Erik Mumm*  and Kyle Gotthelf* 

Abstract  

During development of a new slip ring design , our panel of subject matter experts was unable to find  examples of how slip rings perform at very low current levels. This paper discusses the performance of  Gold -on-Gold Slip Rings for micro -amp scale signals. We offer a concise overview of slip ring tec hnologies  and common applications and discuss the challenges associated with low current transfers in slip rings. We discuss the testing that Honeybee Robotics has completed, including a description of the test hardware, the test conditions (electrical signals, rotation rate, and lubrication scheme) and the resulting performance under flight -like conditions.  

Introduction  

Slip rings are devices that make use of sliding electrical contacts to transmit power and signals across a rotating interface. In space craft mechanism applications these are commonly used in Sol ar Array Drive  Assemblies , Bearing and P ower Transfer Assemblies , and other applications requiring continuous rotation.  These devices consist of a rotor (or slip rings) and brushes. Common material  combinations used for slip  ring assemblies (SRAs) are silver -alloy rings with self -lubricating sintered silver brushes, gold alloy plated  rings with monofilament gold alloy brushes, and gold alloy plated rings with gold alloy fiber brushes. Of these basic  types, perhaps the most common employed in flight applications over the last few decades has  been the gold alloy rings with monofilament gold brushes, commonly referred to simply as “gold- on-gold”.    These gold- on-gold systems are most typically wet lubri cated with either Brayco or Pennzane®- based oils  depending on the temperature requirements. Of these lubricant types Pennzane® is the more common choice. Several formulations of Pennzane® (formulated and sold by Wm. F. Nye of New Bedford, MA) have been use d successfully in flight SRA applications. These formulations include NYE 2001A (“neat  Pennzane®”, i.e., without additives), NYE 2001 (with anti -wear and anti -oxidant additives), and NYE 2001XPbNp (with anti -wear, antioxidant, and lead naphthenate additiv es, where the X is the mass percent of  the lat ter, typically around 3% which is used in this testing).  

Noise, or variations in the contact resistance between the rotating ring and the stationary brush as a function  of rotation is an unavoidable characteri stic of all slip ring designs. The number of brushes is usually chosen  for current -carrying ability. Typically low -current circuits are designed to have only a few brushes in order  to minimize the overall size, mass, and parasitic torque of a given SRA. This approach is contrary to reducing noise in those circuits because the more contacts a given circuit possesses, the lower the effective noise becomes as there are more parallel current paths involved. This limited contact scheme is by far the most common approach for signal -level circuits. While the parallel contact approach can reduce noise on  a typical circuit; for circuits carrying very low currents this may be counter -productive. Since a substantial  contribution to the electrical contact at the sliding interface is due to quantum mechanical tunneling (with  the primary path being metal to metal contact) the voltage drop at the contact interface plays a role in contact quality and noise [6] (Holm). With high enough voltage, insulating films at the interface can be broken down or tunneled through. For very low current signals, such as those investigated as part of this study, the question arises:  does sufficient voltage drop exist at the contact for these mechanisms to  contribute to conduction? This was addressed in this study.                                                     * Honeybee Robotics Spacecraft Mechanisms Corporation, Longmont, CO  

Proceedings of the 43rd Aerospace Mechanisms Symposium, NASA Ames Research Center, May 4-6, 2016