I. INTRODUCTION The Automaton Rover for Extreme Environments (AREE) is a NASA Innovative Advanced Concepts project to design a rover that can operate for six-months on the surface of Venus. To enable terrain traversal and navigation, AREE must be equipped with a robust obstacle avoidance sensor (OAS), however modern electronics cannot operate in the extreme surface temperature and pressure. Therefore, as part of the NASA “Exploring Hell: Avoiding Obstacles on a Clockwork Rover” challenge, an OAS was developed with an array of mechanical sensors akin to mammalian vibrissae and associated electromagnetic actuators shown in Figure 1. The obstacle detection method of the OAS can be described as a mechanical and electrical relay system.
II. VIBRISSAE MECHANISM The first component of this relay is the vibrissae mechanism, an assembly of three mechanical vibrissa that extend from the front of the rover to the Venusian surface to detect obstacles. The outer two vibrissa are directly in front of the rover wheels, while the third is positioned in between the two. The ends of these vibrissae, which make direct contact with the Venusian surface, are characterized as vibrissa heads. The vibrissae are connected by rigid arches that allow motion hindering obstacles between the vibrissae heads to be detected and negligible obstacles to pass under them. Rotary motion of these arches is possible because universal joints are utilized to connect the arches to the vibrissa heads. The impact of an obstacle on an arch or vibrissa head causes the entire vibrissa to translate backwards. Each individual vibrissa head is also capable of translating vertically by allowing the entire vibrissa to pitch upwards or downwards. To enable this, during the vertical translation of the vibrissa head, the vibrissa head also simultaneously moves backward or forward by allowing the arch to yaw via an arch to bearing connection on the middle vibrissa. This is a result of the rigid nature of the vibrissa arm which necessitates that the distance between the ends of the vibrissa to remain constant; in this case, however, the entire vibrissa does not translate backwards as in the case of an obstacle impact event. To enable the outer vibrissa heads to be stationed directly in front of the rover wheels at all times, the arches are fixed to slider assemblies on the vibrissa heads, and non-extendible ceramic-fiber loop bands connect the outer vibrissa heads to eyebolts that are located on the underside of a rectangular plate atop the top-section of the OAS hull. The slider assembly for the outer vibrissae, shown in Figure 2A, consists of a longitudinal slider, lateral slider, and a self-retracting ceramic-fiber cord reel on the vibrissa heads. The lateral sliders for the outer vibrissae are initially located at the furthest outward position within the vibrissa head. The longitudinal sliders for the outer vibrissae are initially located at the furthest backward position within the vibrissa heads. Both sliders are connected to their individual self-retracting cord reels which exert zero spring force in the initial position. The inner vibrissa slider assembly, shown in Figure 2B, does not include a longitudinal slider, but instead has two lateral sliders, which are initially located at the midpoint of the middle vibrissa head, so that both arches can be attached. These sliders are also attached to self-retracting cord reels. During a vertical extension of the vibrissa head during a pitching maneuver, all sliders translate towards the opposing ends of the vibrissa head as a result of centripetal force that acts towards the middle vibrissa and is induced by the rigid arch. However, any inward motion of the vibrissa head is prevented, because the aforementioned centripetal force is negated by the reaction force between the non-extendible ceramic-fiber loop band and the vibrissa head. The slider assembly allows the vibrissa heads to extend vertically while remaining directly in front of the rover wheels in order to meet the obstacle detection criterion but prevents any further extension. The spring force from the self-retracting cord reel mechanisms then allows all sliders return to the initial position when backing away from an obstacle.
III. TRIGEMINAL MECHANISM The second set of components are characterized as trigeminal mechanisms, which function via flexural-based mechanics to convert vibrissa displacement into an electrical signal. The trigeminal mechanism, of which there are three included within the OAS, translates the two-dimensional movement of each vibrissae, which act as type one levers, into compressions or extensions of three sets of linkages and accompanying spring shafts. It is shown in Figure 3. Much of the trigeminal mechanism is composed of Ti-6Al-4V, apart from the springs, foil, and screws. The linkages are constantly held in a resting state that resists displacement of the vibrissa head from level surface, where rover inclination acts as the reference, but are displaced by the slightest movements of the vibrissa. The restoring force is provided by nine Ti-6Al-4V double-ended pivot bearings, three per linkage, and the spring shafts that hold four springs that function to provide either a tension or compression force given the circumstance. The pivot bearings are a flexural-based device that employs three crossed internal flexure beam springs enclosed in a three-part cylindrical housing sleeve. The system of nine differentiated double-ended pivot bearings provides precise rotation with low hysteresis that allow the trigeminal mechanism to resist inappreciable vibrissae movement, sustain a resting state, and allow angular displacement to specified degrees that define the pin actuating configurations. In addition, there is a shaft adjustment system that holds the circular shaft acting as the fulcrum for the vibrissae in place with two springs, which compress when against an obstacle of significant inclination, allowing for the aft linkage and spring shaft to fully compress.
IV. INCLINATION SENSOR There are many obstacles that can be detected by the vibrissae and trigeminal mechanism. However, when inclination increases or decreases gradually, such as a slope on a hill or mountain, these systems will not detect this change since they are limited to sensing obstacles in reference to the plane tangent to the bottom of the rover wheels (i.e. rover inclination). Therefore, rover inclination in reference to the gravity of Venus must be monitored by the OAS and be able to detect when inclination maximums are exceeded to prevent the rover from losing surface traction or becoming overturned on a steep slope. To mitigate the risk that gradual inclines pose, a mechanical-based inclination sensor was designed for use in the OAS to detect gradual declines and inclines in any directions by referencing the orientation of gravity.
V. ELECTROMAGNETIC ACTUATION SYSTEM The electromagnetic actuation system is the final component, containing four highly-compact solenoids. Using two wires for power and ground connections, an electric circuit which becomes complete with sufficient compression of a trigeminal mechanism linkage was designed to flow current through these various solenoids. Using a cylindrical ferritic slug of metal placed inside the solenoid, the magnetic field generated by the energized coils exerts an axial force on the metal slug. This axial force is used to actuate the pins to relay the detection of an obstacle to AREE. This system allows the detection of a multitude of obstacles but can differentiate them into four distinct signals, which include holes and negative 30-degree inclines, positive 30-degree inclines, 90-degree or near 90-degree inclines, and gradual inclination that accumulate to 30-degree in any direction. These signals trigger the rover to reverse away from the obstacle and seek a different path forward.
VI. CONCLUSION The function of the various mechanisms, along with an extensive material trade study to determine the appropriate composition of OAS components, and failure modes with mitigation strategies, ensure that all problematical obstacles outlined by NASA are detected. This OAS ensures that the AREE is capable of operating in the extreme surface conditions of Venus for an extended period of time, and was officially recognized by NASA as one of the top design solutions.