In my last post, I discussed the planning step of designing an electrical system. Using CAD and other tools, a FRC electrical system can be planned out efficiently. Now that the design is completed, I’ll move on to the actual wiring process. There are many facets of wiring in FRC that must be considered, and all of them have some effect on the quality of the final product.
What are “harnesses?”
A wire “harness” is a method for containing a large group of wires that are going to similar places. These harnesses can be made using any number of materials, including plastic or metal tubing, velcro, and zipties.
An example of a wire harness, detached from its system.
The fundamental goal of wire harnesses is to make the wiring of the robot modular, and therefore easier to build and to work with. As seen in the image above, this is accomplished by assembling groups of wires offboard the robot, then installing these groups, instead of running each wire one-at-a-time. This offers many advantages- by assembling the wiring separate from the robot, cable runs can be made more cleanly. It also means that electrical team members won’t have to compete as much with mechanical students for precious hands-on robot time. However, building off-board harnesses can be very tricky, and small mistakes can cause big headaches. Knowing a few basic tips for harnessing can make these mistakes much less likely, and can help put your electrical team on a path to success.
Reading the Spreadsheets
For my own sanity, I will assume that the reader is using the Autodesk Inventor Harness Environment. This tip, however, applies regardless of what tool you use.
After designing your electrical system in CAD, you will be able to print out spreadsheets that explain the harnesses. These spreadsheets, which typically come in an XML or CSV format, contain everything you could ever want to know about your harness. However, the most important thing these spreadsheets contain are the wire definitions. Wire definitions are everything you need to know about each individual wire, including their lengths, colors, diameters, and routes. Using the spreadsheets properly, you can length out every wire confidently, and route them the right way through the harness. I’d suggest adding about 0.5-1″ to each length, to account for tolerances and crimping, and to reduce the amount of tension in the harness after installing it.
Although the spreadsheets are very comprehensive and can be very powerful once you understand how to read them, they are loaded with difficult-to-read, and occasionally useless information. However, your harness tool has a smart solution to this problem that makes harness building much easier- nailboards.
This image, from Autodesk’s website, shows a harness with its nailboard representation below.
The nailboard is a powerful tool for visualizing how the harnesses will be shaped. Essentially, the software calculates the length of each part of the harness, and what angles they must be at in order to fit properly. If your harnesses are small, it’s worth the extra paper to print off a 1:1 nailboard representation, then use it quite literally as a nailboard. Even if your harnesses are too big for that, using the nailboard tool makes reading the dimensions of the harness much easier.
Picking the Right Materials
There are a variety of ways a harness can be made. Different wire types and harness materials each have their own merits, and choosing the right materials can make your harnesses a lot better.
Types of wire:
Standard: good ol’ wire. Pros: inexpensive, easy to find. Cons: gets tangled easily, many companies sell very cheap standard wire with terrible insulation and poor bend radius.
Zip Cord: a pair of wires, normally red and black, fused together. Pros: relatively inexpensive, cleans up bulky power runs. Cons: only bends one way, don’t use it in IGUS energy chain as it will stiffen the chain significantly.
Double Core: rarely used in FRC, but has some unique traits. Pros: double shield makes it safer to use in areas with many moving parts, while still retaining a fairly tight bend radius. Cons: more expensive and a little harder to work with than other options.
Ribbon: the most popular signal-line cable in FRC. Popular for PWM lines and sensors. Pros: easy to find, relatively inexpensive. Cons: only bends one way, it’s hard to find good quality 3-conductor ribbon cable, as most hobby sites sell ribbon cable that uses low-quality insulation.
Twisted Pair: an alternative to ribbon cable. Less popular, but has huge advantages in some situations. Pros: usually is very high quality, comes in a huge variety of colorways and wire counts. Cons: the huge variety makes finding one specific type more difficult, twists can get loose and messy in high-vibration scenarios.
Types of Harnesses:
Zipties: The quintessential cable management device. The image above is just for fun; I have no idea why you’d ever want to do that. When making a harness with zipties, place them at a 4-6″ spacing, and at each point where some wires go different ways. Pros: extremely cheap and easy to find. Adjustable diameter means one length of ziptie can be used for many harness sizes. Odds are, you’re going to use them anyway when you mount the harnesses to the robot. Cons: very ugly, offers no protection from moving parts or other physical damage, leaves a prickly thing that is annoying when you’re trying to work on the robot.
Velcro: very similar to zipties, but velcro straps can be removed, reused, and adjusted. Pros: very easy to work with, adjustable size, makes adding/removing wires from the harness very easy. Cons: can get costly, depending on the type of velcro, doesn’t completely cover wires, tends to slide out of position.
Split top tube: Ribbed, plastic tubing with a cut in the top to allow wires to fit in. Not as pretty as sleeving, but better than zipties. This stuff is insanely cheap, but still offers many of the benefits of more difficult harnessing materials, so it’s a good way to get started in harnessing. Pros: inexpensive, easy to use. Offers a decent amount of protection from physical damage. It’s very easy to replace a wire, even while the harness is installed in the robot. (not that you should need to) Cons: non-adjustable size, poor bend radius. Bending split top tube too far can open up the gap and allow wires to escape the tube.
Spiral tube: Similar to split top in its functionality, but offers a couple specific benefits. Wires can be pushed straight through the tube, and routed more precisely out the edges of the spiral. Pros: decent bend radius, one size of spiral tube can be used for many sizes of harness. allows for very precise positioning of wires. Cons: can be time-consuming to get wires into the tubing, won’t completely cover the wires on bends.
Sleeving: The fancy option. Sleeving a harness, then heatshrinking the ends makes for a very pretty harness. Comes in a wide variety of colors, and can be made of plastic or metal. Pros: very good looking, excellent bend radius, metal varieties offer a little noise shielding. Protects wires from physical damage very well. Cons: very costly, doesn’t allow for replacing wires, requires experience in order to use it well.
Using these tips, assembling a wire harness is a lot easier. Although the change in workflow makes the transition to harnessing more challenging, it is a method used in industry that can be valuable to an electrical student’s FRC experience. Building harnesses can seem very daunting the first time, but with experience and time, you can find excellent results.