Sojourner F17: MicroMetal DC Motor Section Study

DC Motor Calculations and Comparisons:

The objective of this study is to compare different dual-shaft micro metal gear motors and replace the current dc micro metal gearmotors on the Sojourner Rover [9].  The criteria include motor dimensions like that of the current motors, high stall torque, low stall current, dual-shaft design. Since the rover will need six motors, the current draw must be as low as possible while maintaining a good speed-torque ratio. All the motors selected for the tradeoff study are micro gear dc motors, they were chosen because of their relatively low mass and volume.

Discussion:

The micro gear motors chosen from Spring ’17 were ideal due to their compact size and low power consumption, i.e. current draw [9]. However, the current sojourner rover design requires a slip differential for each motor, color sensors, a proximity sensor, and a phone mount/solar panel mount module which creates additional weight. The reason for replacing current N20 motors is the need of Hall Effect sensors, the sensors require a dual shaft motor design for a magnetic disk to rotate at the same speed as the main shaft.

Another concern when deciding on the gear-motor is the amount of current drawn at the stall torque, also known as stall current. The current draw needs to be as low as possible while maintaining enough torque and enough speed. Due to the low current requirement, a low ratio of the load torque to the stall torque will yield a low ratio of current drawn compared to the stall current. Meaning the motor needs to have a high stall torque compared to the load torque and a relatively low stall current.

An alternative to the Pololu gear-motor is the DF Robotics motor, this motor has the same dimensions as the current N20 gear-motor and Pololu gear-motor. Unlike the Pololu model, the DF Robotics model includes an attached Hall Effect rotary encoder, but the encoder is much larger than the Pololu which would require a redesign of the suspension motor mounts. Another issue with the DF Robotics model is that the stall current is not given but instead a current rating at 170mA [3]. However, a stall current was measured from a forum community member albeit for a model like that of the 210:1 DF Robotics model at 480mA [12].