Spring 2017 Mini Pathfinder Motor Trade Off Study

By: Moses Holley (Electronics and Control)

Introduction

The objective of this trade off study is to find a motor that meets our robots requirements and also satisfies mission verification.

Subsystem Relation

Motor selection ultimately depended on the Mini Pathfinder’s size. Initially, the customer wanted a rover that could fit in one’s pocket and also in the palm of their hand. With this perspective we looked in to micro vibration motors. After a few meetings with the customer, we can to an consensus to have the Mini Pathfinder scaled down to 1:4.57 for length and height of the sojourner rover to minimally encompass the 3DoT Board. That in turn caused the wheel size to be 28 mm high and have the motors encased making the motor not visible.

Micro Vibration Motors

The vibration motors that we were interested in for our initial design can be seen here[1].  These motors require a gearbox to bring down the RPMs. Some of these motor RPMs were as high as 15000 RPM. In creating the gearbox we would have to alter the shaft and place the gearings on top of the shaft.

Figure 1. Micro Vibration Motor

Micro Metal Gearmotor HP 6V

The updated design scale granted to me to to choose a motor that must not exceed 19mm. The 19mm was given by the 2mm clearance of the 3D print of the wheels on top and bottom of the motor. The motors chosen has a 12mm diameter [2]. It also already had its own gearbox that produced 30 RPMs according to the specs. I figured this would be a great motor size just in case we make additional adjustments to the wheels. We later did make adjustments by adding shaft encoders. There was still enough clearance for the wheel and the shaft encoder.

Figure 2. Micro Metal Gear Motor

Conclusion

When choosing a motor for the Mini Pathfinder, we came to find out the size of the robot is a huge factor. Our first idea of the motor caused me to research for micro vibration motors. Those motors required many adjustments to benefit the Mini Pathfinder. Fortunately, we were able to resize our robot which actually granted us a motor that not only met clearances but also ideal performances through the assembled gearbox. After this trade off study, we decided to use the Micro Metal Gearmotor HP 6V motors for our design.

References

[1]https://www.digikey.com/product-detail/en/jinlong-machinery-electronics-inc/Z4TL2B124064X/1670-1016-ND/6009921

[2]http://www.ebay.com/itm/131747528794?_trksid=p2060353.m2749.l2649&ssPageName=STRK%3AMEBIDX%3AIT

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