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Unread 24-12-2013, 19:08
RRLedford RRLedford is offline
FTC 3507 Robo Theosis -- FRC 3135
AKA: Dick Ledford
FRC #3135 (Robotic Colonels)
Team Role: Mentor
 
Join Date: Jan 2010
Rookie Year: 2009
Location: Chicago, IL USA
Posts: 286
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Re: [FTC]: Motor encoders Love'em? or leave'em?

DavisDad, regarding the VEX mechanum wheels, I am not sure if their square axle material is considered legal for FTC. We also used these wheels this year and made 1st seed in our 1st regional, captaining the winning alliance. We also received a Rockwell Design Innovation Award (primarily for the drive system) at our second regional. Our robot was very consistently best positioned for flag winding and still able to get back on the bridge in correct location for executing a potential double hang, when possible.

We used Tetrix motors at 1:1 with external encoders and chain direct driven wheel attached sprockets. We were doing consistent autonomous block drop and getting on bridge from the level of precise driving we could get with encoders on our practice field, but at competition, we kept hitting the wall enroute to block drop.

We opted to drill & ream out the VEX wheel square holes to .188" and then used extra long, hardened 10-32 SHCS with unthreaded portion polished for fit to terix sleeve bearings. Protruding overlong Tetrix hub screws engaged into holes in plastic of the VEX 7-spoke wheel hubs, and Shoe Goo urethane adhesive strengthened the hub-to-wheel attachment. Bolt axles free spin in nested channel bearing blocks, and threaded portion captures wheel to adjacent hub/sprocket. Direct driven chain sprocket that is close mounted to wheel is our preferred FTC drive scheme. Driving wheels through axles has proven too unreliable for us.

Double bar H-frame of 20mm Bosch-Rexroth slotted aluminum, using 80/20 10-32 slot nuts. Sliding motor plate adapter and sliding bearing block on opposite sides of T-slot frame allow variable positioning for setting wheelbase length and chain tensioning. This encoder supplemented drive was very precise & reliable in competition and had excellent traction. The VEX wheels are an excellent value.

PICs below:







-Dick Ledford
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FTC 3507 RoboTheosis
FRC 3135 Robotic Colonels

Last edited by RRLedford : 24-12-2013 at 19:34.
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