Fall Biped 2016- Motor and Battery
By: Alan Valles (Electronics and Control)
Approved by: Ijya Karki (Project Manager)
Introduction
The purpose of this document is to show all the updated schematic and hardware changes since CDR.
Hardware Design
The Final Hardware design of our robot was slightly different than what we had spec’d for CDR. The Motor stayed the same, it was a pololu 1117 size 130 motor. However, the gearbox that it went into was different. The final motor selected was the Tamiya 907710.[1] This motor was selected based on its gear ratio which was able to provide the Biped with even more torque than the previous model. Our Motor had an output RPM of m 11500 RPM at its shaft. We needed more torque due to the fact that our latest linkage design was larger/ longer than the previous design. The Tamiya 70110 4-speed Crank Axle Gearbox Kit had 4 ratio option, 5402:1, 126:1,441:1 and 1543:1.
Figure 1. Speed Crank Axle.
These ratios dropped the output RPM to 2.2, 7.45, 27, and 16RPM respectively.
The only observed issue with this gearbox was that the gears internally seemed to be rubbing together when under large amounts of mechanical load. I noticed that there was an unsettling amount of play in the gears when it was put together.
Figure 2. Gearbox.
I suspect that once the load past a certain limit, the gears shifted to the left touched the high RPM green gear. This caused a clicking sound during random intervals of the walking motion.
Battery
Figure 3. Battery.
The battery was changed to a hobby people 7.4v 800mA Battery.[2] This battery was chosen over our previous battery because, we did not have to wait for shipping. Lithium batteries have a risk of catching fire which means that they are not shipped by air, only on trucks. Thus, it was much easier to go to the physical location of hobby people and purchase the needed battery and connector right on the spot. The form factor of the battery was absolutely perfect for our design as well. It had the dimensions of 12x35x68 mmm and weighted around 50g. Thus it was the perfect size for a RC Biped. Also, the pololu 117 motor had around 550mA operational current when attached to the 70110 gearbox. This was done by measuring it with a Digital Multimeter. Therefore, a rough calculation of power would be 650mA*5V = 3.25 W and the battery has a capacity of 7.4* 800mAH = 5.92 Wh. Thus, the battery still appears to meet our requirements that state we should play the game for an hour.
Conclusion
In Conclusion, The Battery and motor were changed to the hobby people 2S Lip and the Tamiya 70110 gearbox, respectively. This was done to increase the realized torque demand of the linkage assembly which was updated in order to meet customer request.
References
[1] https://www.pololu.com/product/68 [2] http://www.hobbypeople.net/index.php/hobby-people-7-4v-800mah-2s-30c-lipo-battery-bec-plug.html [3] http://arxterra.com/fall-2016-biped-updated-motor-study/