Electronics and Control System
All of our decisions with regard to wiring follow the same basic set of rules to maximize reliability. These rules are summarized below.
- Connections are the weakest part of a wire run, use as few connections as reasonable.
- Nothing will be perfect the first time, leave slack and run wires in places which makes them relatively easy to remove or change.
- Avoid Soldering for bigger wires - as wires smaller than 22 AWG, soldering might be needed, but for 12 AWG power wires and signals/CAN, connectors should be used.
- Make things easy to inspect. The more people look at things, the less likely an error is to slip by.
When building our 2022 robot, Brioche, we decided to use insulation displacement connectors (IDC’s) as a replacement for soldering of 22 AWG wires including low power from the VRM and CAN bus. Shown below are the connectors in question.
There are small blades which cut into each wire when the transparent plastic piece is pressed down. This means that the wires do not need to be stripped, which makes installing these connectors very convenient. The connectors also come in a ‘T’ style, which can be used for a CAN or power branch. When viewing the connector with the clear side facing up and when in the orientation which looks like the letter ‘T’, the left side of the branch side connector connects with the top passthrough wire. The product photo below is actually incorrect in this way. 5940 standardizes CANH or V+ on the left side and on top.
This ‘T’ style of connector does not require interrupting the wires which are being branched, and so has advantages in reliability. The conductor of the wire on top does not to be cut for the connector to be installed. Using these connectors, you could wire your robot CAN bus with a single unbroken loop and a short branch for each connector. Since connections are generally the weakest part of an electrical system, this would greatly reduce the number of connections which can “take down” the entire bus.
Start with the connectors open - you might need to use your fingernail or a small tool to open the latches.
In the picture below, notice the loops which contain the wires on the inline and branch side of the tee connector, the part of the upper transparent plastic which forces the wire into the blades, and the blades which cut into the wire.
For the passthrough side of a tee connector, simply untwist or unzip the segment of wire you wish to tap and lay it over the blades. Take care to lay the wires in straight and aligned with the connector itself - notice the blades in the connector and the absence of loops to contain the wire.
Finally, simply squish the connector closed with pliers from the top, like in the picture below.
It should look like this when closed properly. Notice the latch is hooked around the black lower housing and there is no gap at the joint. Also make sure to verify the wires are still straight and aligned inside the connector.
The process is very similar for the inline connectors and branch side of the tee connectors. Start by untwisting/unzipping about one inch of wire. Make sure it’s straight (you can use the edge of a table or similar to straighten it. Stiffer wire will be easier to use with these connectors, but we always recommend stranded wire.
Insert the wires into the connector. Make sure the wires are straight, parallel, and aligned with the connectors. Below is an example of what the connector should look like with wires inserted, before it is closed.
Once you have the wires properly inserted, simply squish the connector closed. It should look like this when closed.
You should be able to see the wire insulation from the top of the connector when closed. You should be able to see the wires going all the way back into the connector and fairly straight.
We decided to switch to using WAGO 221 connectors for motor power for our 2022 onseason robot, Brioche. Like our decision to use IDC’s for low power, the decision to use 221’s was driven by the need for faster assembly, easier inspection, and ease of modification. Below is a picture of a 221 series connector.
These connectors handle relatively low strand count 12 AWG without issue, but are sometimes tedious to install on the much higher strand count wires like the ones found pre soldered to the falcon 500 and rev spark max. Using high strand count wire with 221’s is still a lot better than soldering, but it takes extra care to twist cleanly to prevent stray strands being excluded from the plastic housing of the connector and potentially shorting out.
In order to install a WAGO 221 series connector, the wire must first be striped. The 221 datasheet specifies 11mm of insulation be stripped from the conductor. I personally strip slightly less (around 9mm) to have more margin of insulation inside the plastic connector housing. I’ve found that stripping 11mm leads to the insulation sliding down the conductor and out of the plastic housing. Always ensure the wire is pressed all the way into the back of the connector and the metal teeth of the connector are gripping the conductor only and not the insulation. Wago publishes the instructions below for installation.
This year, we are experimenting with ring terminals to connect power and CAN to kraken motors. We purchased a genuine molex crimper. Using ring terminals with krakens will allow us to pre-fabricate our drivebase CAN loop, and extra-long power pigtails which have zero additional connections. We hope this will offer further improvements in reliability.
Along with the crimper, we purchased 10-12 AWG ring terminals. These are designed to function with the crimper we purchased. If you choose to use a different crimper system, ensure your crimper matches the terminations you use explicitly.
Begin by stripping the end of your wire. We experimented with the 10AWG wire we had on hand and found that 16mm is an appropriate length to strip.
To determine the appropriate length of insulation to remove for your wire-termination combination, experiment until you have 1-2mm of wire extending out the tip of your termination when the wire is pushed fully in.
Once this is accomplished, orient the termination in the direction you want it and seat the assembly into the appropriate spot in your crimper. For the molex crimper, wires are inserted from the direction with colored dots, in the slot which is color coded to match the termination.
It should look like this from the other side.
The molex crimper was preset so the ratchet release point for all three sizes produced quality terminations. Just squeeze until the crimper "pops" back out and you should get a nice crimp. It's important that you test your crimper and terminations a few times with these instructions to ensure it's set properly. Just put the ring terminal in a bench vice and give it some tugs - you should not be physically able to pull 10 or 12 AWG wire out of the termination. It should look like this when complete.
For kraken CAN wires, we follow the same process of
- Determining the appropriate amount of wire to strip
- Stripping the wire
- Inserting the wire into the termination
- Crimping the termination following the mfr instructions
The only thing we do differently for CAN wires, is folding them over and inserting the folded wire into the ring terminal. We do this to ensure there is a continuous can wire all around the drive-train without any interruptions. We used these ring terminals, and folded the wire like this.
And then put them into the termination like this.
Choosing wire is… complicated. Some big considerations include insulation color, strand count, wire gauge, insulation material, availability as zip cord, if the strands are tinned, and price. Zip cord is the name for wires which are sold as a pair who’s insulation is slightly melted together, this is convenient because the wires remain attached to one another but can easily be separated at the ends or even in the middle with fingernails or snips.
Here’s what 5940 tries to use
AWG Insulation Tinned Strand Ct. Zip Cord
Swerve Drive Motor Power 10 Silicone Either >100 Ideally Yes
Regular Motor Power: 12 Silicone Either >100 Yes
Low Power/Signal: 22 PVC Either >20 Yes (Can Twisted)
Mid Power (20A): 18 PVC Either >30 Yes
Basically, we use the wire gauge that’s legal for the things we are connecting. We use silicone insulation for big wires, because the insulation is a big factor in flexibility and bend radius (which we want) but the copper itself is strong enough by itself to not be damaged by normal use. Use PVC insulation on smaller wires because it will limit the bending and stretching of the wire (because PVC is stiffer than silicone). Since we don’t solder very much, we don’t really care if the wire is tinned. Finally, for red and black power wire, we use zip cord, CAN needs to be twisted, and other signals can be individual spools. There are generally very few mid power wires on the robot, but when they exist it’s basically the same philosophy as small wires.