Design Discussion  Driving Requirements Prior to the ISAD, the planned method of acquiring ice-bearing soil samples used a set of passive blades  mounted to the exterior of the Robotic Arm digging scoop. Once a layer of potentially icy soil was  encountered, these blades would be dragged across the surface of the hard icy soil by the Robotic Arm to  produce piles of loose material, which would then be acquired by the scoop and deposited at the  instrument inlet ports. Several blade geometries were tested in dry analog material to determine the  feasibility of the sample acquisition method. It was found that the method of scraping material and using  the scoop to acquire it produced an insufficient amount of sample for the TEGA instrument within a  reasonable amount of time. Due to potential sublimation loss of water ice, a faster, more productive  method of sample acquisition was required. Honeybee Robotics Spacecraft Mechanisms Corporation was contracted by the Jet Propulsion Laboratory to provide an end-of-arm sample acquisition device that  would produce at least an order of magnitude improvement on the passive scraping method, which had  been generating on the order of tenths of cubic centimeters.  The ISAD was a late addition to the payload of the Phoenix Mars Lander. Most lander subsystems had  already undergone their critical design reviews and were well into the fabrication, assembly and test  phases by the time the ISAD was conceived. As a result, the ISAD had to comply with existing  subsystems that had not been designed with an active end-of-arm sampling device in mind. This led to a  number of requirements that tightly constrained the design space of the ISAD. These requirements, as well as a limited budget and rapid development schedule led to innovative design solutions and  compromises.    One of the main drivers throughout the design process was the overall ISAD mass. The Robotic Arm was  designed and analyzed to support a hollow scoop, with relatively light weight digging and scraping blades.  The Robotic Arm’s severe launch load environment required that the ISAD mass be limited to 0.75 kg in  order to maintain sufficient strength margin.   The design of the ISAD was also limited by available resources of the lander electrical subsystem. There  was one switched circuit available for an active device, limiting the device to one motor. A preset drive  voltage of 24.6 to 29.3 VDC limited the speed of the motor to a narrow range of velocities. This voltage input was also unipolar, meaning that the ISAD would need to function unidirectionally. A restriction in the  overall power available to the device resulted in a current limit of 1.0 Amp, causing a constraint on the  available motor torque. Further constraining the amount of torque available for sampling was a  requirement to start motion of the mechanism when preloaded against the ground with a torque margin of  1.0. Although an encoder was integral to the motor selected, there was no means of reading the encoder  signal, resulting in an open loop device architecture, with no direct data feedback to tell if the device  functioned properly. By reading the overall Robotic Arm subsystem power data, it could be determined  that the device ran, however there would not be enough resolution in the data to calculate motor torque or  speed. Due to limited budget and schedule resources, motor selection was limited to existing Jet  Propulsion Laboratory flight spare stock from the Mars Exploration Rover project.  Productivity of the sampling device was also a key driver in the design process. The TEGA instrument  requires at least 1 cm 3 of icy soil material at its inlet ports to capture enough sample to perform an  analysis. To fulfill this requirement, multiple sampling placements of the ISAD can be performed, however  sublimation loss of water ice places a constraint on the total duration of these groups of sampling  occurrences.  During the conceptual stage, it was determined that vibration greatly improved the movement of icy soil  over metallic surfaces at Mars ambient temperature and pressure. This resulted in a self-imposed  requirement for the device to produce its own vibration to help fluidize particles and move them over internal transfer surfaces.  Several other requirements placed constraints on the geometry, function and material choices of the  device, including operational and survival temperatures (-80 °C to +45 °C and -108 °C to +110 °C  respectively), required interior volume for digging operations (300 cm3), structural survival of digging loads  during soil overburden removal, and surface coatings for thermal balance, durability and imaging.  290