electrical.html - frc971/website GitHub Wiki

We plan on filling out this page with details on how 971 wires robots.  Our goal is to specify parts and such that we use so that we and others can use this page as a reference for designing and building robots.

We have used many different cables through the years.  As the team started building more complicated robots, it needed better and better sensors.  We were slow to learn that we needed to pay attention to our wiring as much as all of the other aspects of the electromecanical and software systems.  We initially used a small gauge unshealded ribon cable for our 2014 arm encoders.  These were light weight and easy to route through the robot.  Unfortunatly, they were unusable.  The encoder ribon cable ran along side electrially noisy motor wires.  The signal at the robot controller was so bad that it was unusable.  We found this out about 2 hours before we had to bag the robot.  This was very stressful and discouraging.  I suspected that the cable was the problem and cut up an Eithernet wire and found that it worked a lot better.  After a while, we figured out that the Eithernent wire was not good enough either.  While it had larger stranded wires in it, there was not any shielding.  Excessive noise would occasionaly get into the cable and corrupt the signal and mess up the control loops for a bit.  This was an intermittent and difficult to diagnose problem.  Again suspecting the wiring, we tried again and found the Belden 8723 cable at Halted, a surplus electrical supplier in the heart of the Silicon Valley, a few minutes before they closed the weekend before we left for the Championship Event in St. Louis.  It is a very nice cable with two pairs of 22 AWG stranded wire with 7 each 30 AWG wires i each conductor.  Each pair is shielded.  Cables made with these worked flawlessly at the Championship event and at 4 post season events.  After researching cables, I figured out that we had found what appears to be the perfect cable for our robots.  

In 2015, we decided to use encoders with an index pulse and a potentiometer for the arms and elevator.  I could not find a nice eight conductor wire and really did not want to use two four conductor wires so we tried using a 6 conductor Belden 5542FE cable and shareing the power and ground wires with both the encoder and potentiometer.  This worked amazingly well.  The Belden 5542FE cable only has a shield aournd the three twisted pairs rather than having each pair individually shielded.  This worked fine though.

After a lot of trouble bringing up the CNC Router the the team made in 2015, we started grounding the shielding right near the RoboRIO controller. We do not ground it at the sensor end.  This helps a little bit more with signal integrity.

• For encoders without index pulses and potentiometers, we use a very high quality Belden 8723 cable that has two pairs of wires where each is individually shielded.  The conductors are 22 AWG made up of 7 strans of 30 AWG wires.  Here is the datasheet.  It can be purchased in 100' long and more lenghts from Mouser ($64/100ft on 2/25/17) and economically in 500' spools from Markertek. ($237/500ft on 2/25/17).
• For encoders with an index pulse, we need a five conductor wire so we use Belden 5542FE three pair sheilded wire.  Here is the datasheet.  The shielding goes around all three pairs rather than around each individual pair like the Belden 8723 cable.  The conductors are 22 AWG made up of 7 strans of 30 AWG wire.  I have not found short lengths of it listed on vender websites.  I have been lucky and found a few odd lengths (100+ ft long) at Anxiter but they don't regularly sell it in shorter lengths than the 1000' spools.  Fortunately it is less expensive than the Belden 8723 cable.  The 5423FE costs $252.90/1000' on 2/25/17 from Anixter and $263.44/1000' on 2/25/17 from Mouser.
• The diagram in the image below shows how we wire an encoder and potentiometer using a single run of three pair Belden 5542FE cable.  The ground and power lines are shared by both sensors.  The encoder index pulse and the potentiometer signal line share the green and black pair.  We used this in both the 2015 and 2016 robots.