Sojourner F17: Power Budget
By Piyush Jain ( Missions, Systems, Testing) | Fall 2017 | Sojourner Team
Introduction
For Sojourner to have a successful mission, the battery must be able to supply enough current to the entire system. The power budget outlines the amount of current Sojourners systems and peripherals consume. A generic power budget was supplied by Professor Hill and modified by each project
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Project Sojourner’s current budget is seen above. The important asepcts to notice are the motor current draws and shaft encoder draws. During the preliminary design phase, each of our motors would draw 120mA which caused Sojourner’s struggle to meet the required current requirements. With the new MicroMetal Gear Motors, we reduced the current draw from the motors to a fourth. A current study was conducted [1], and the maximum current draw value of 208.6 mA was chosen under a load of 450 grams because we wanted to leave a current draw margin of several mAs because the load did not take into consideration the mass of an iPhone.
A current study of the shaft encoders [2] was also conducted, which measured the current draw of each encoder from a 3.3v power source. The study did not consider the current draw with a load, so the shaft encoder current draw has a margin of 10%-20%.
The total current draw for Sojourner is roughly 623.04mA. This leaves Sojourner with enough of a margin to allow excessive load to be placed on it without having an excessive effect on the mission.
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The current budget gives Project Sojourner information regarding the mission duration. The maximum mission duration was found by:
(60min/h)*(RCR123A Nominal Capacity – Depth of Discharge / Sojourner Total Current Draw)
= 60*(523mAh/625.08mA)
= 50.20 minutes
From [3] , an engineer calculated the estimated average mission duration for Sojourner at one-third speed. Since the projects will not know the maze path until the finals, the estimated 13.31 minutes and margin of 73.49% will suffice.
Conclusion
Project Sojourner uses much less current than initially estimated. This is due to several changes made throughout the iteration process including better motor selections, removal of sensors, and accurate current studies. The margin of roughly 50% allows Sojourner to include peripherals and additional mass without worrying too much about the current.