284 Many parameters were discovered to have an effect on the performance of this dual pivoting brush  design. These parameters and their relationship to design/performance requirements are illustrated in Figure 6. Following is a summary of findings associated with key brush parameters:    Brush offset distance from center  – A large offset distance made it difficult or impossible to  clear the center of the 45-mm diameter area; a small offset increased the chances of bristle entanglement around the center post or with bristles from the other brush.   Brush separation distance – Naturally if the brushes were too close they became entangled; a  minimum separation distance (~8 mm) was established which yielded best performance (minimal  entanglement).   Brush symmetry  – It was found that with a symmetric brush design, it was impossible to clear  the center of the 45-mm diameter area without severe bristle entanglement; an asymmetric brush design was adopted where one brush (Inner Brush) overlaps and clears the center and most of the area while the other (Outer Brush) assists with clearing the outer part of the area.   Engagement distance (brush angle) – The engagement distance is the distance between the  DRT and the rock surface as measured from the tip of the center post to the rock surface; being too close (<10 mm, shallow brush angle) resulted in a poorly cleaned surface while being further away (>10 mm, steep brush angle) resulted in better clearing and better ability of the bristles to pluck dust out of vesicular pits and crevices.   Brush width – There seemed to be a critical brush width (~15 mm or  1/3 the required cleared  area diameter) where thinner brushes produced very clear surfaces of smaller diameter and wider brushes produced less clear surface of larger diameter.   Wire bend – Straight bristles had a very difficult time removing fine particles from voids and  crevices unless the engagement distance was large enough; forming a bend in the bristles enhanced their ability to “scrub” the surface even at shallow brush angles (close engagement).   Brush speed – Speeds less than 300 rpm did not impart enough energy to effectively remove  particles, instead the brushes just pushed the particles around in a circle; at speeds greater than 300 rpm, the brushes are much more effective at clearing the particles – a big difference between 300 rpm and 500 rpm was observed while a negligible difference between 500 rpm and 1000 rpm was observed.   Approach algorithm – Not spinning the brush block while approaching the rock occasionally  produced large reaction forces and at times no “center clearing”; spinning while approaching rock reduced axial force on DRT, resulted in symmetric pivoting of brushes and helped to clear the center of the 45-mm diameter area.   Retract algorithm  – Spinning at high speeds (or not spinning at all) while retracting tends to pull  dust and debris back into cleared area; spinning at lower speeds (100 rpm or less) tends not to drag or eject material into clean areas. 

Ultimately, all of the prototype testing resulted in the final flight brush design shown in the far right-hand image in Figure 5. The DRT flight brush bristles were made of a material similar to RAT brush bristles [4]. A brush manufacturer delivered straight brushes of a specified width and Honeybee formed the final brush geometry by bending the bristles around mandrels per a template and potting with a suitable flight-grade adhesive. The bristles were then trimmed to length. At two points in the process the bristle wire and brush assemblies underwent ultrasonic cleaning for contamination control (CC) and planetary protection (PP) reasons. Samples were sent to JPL for CC/PP analysis and approved for flight.  The spring-loaded brush pivots (hinges), Figure 7, were designed to keep the brushes lightly loaded against the rock surface across large variations in surface height relative to the DRT. The pivots allow the brushes to rotate 30° from hard-stop to hard-stop. The whole brush block assembly is designed to withstand inadvertent loading by the RA up to 200 N. This is one reason the pivot shaft is supported by bushings as opposed to small ball bearings. Spring-energized Bal Seals protect the bushings against dust ingress. The Bal Seals were match fit with the shaft to reduce the parasitic drag to near zero at standard temperature and pressure – a similar procedure was used with success for the RAT grinding wheel shaft