Wiring Best Practices
The article Wiring the FRC Control System walks through the details of what connects where to wire up the FRC Control System, this article provides some additional "Best Practices" that may increase reliability and make maintenance easier.
An FRC Robot is an incredibly rough environment when it comes to vibrations and shock loads. While many of the FRC specific electronics are extensively tested for mechanical robustness in these conditions, a few components, such as the radio, are not specifically designed for use on a mobile platform. Taking steps to reduce the shock and vibration these components are exposed to may help reduce failures. Some suggestions that may reduce mechanical failures:
- Vibration isolation - Make sure to isolate any components which create excessive vibration, such as compressors, using "vibration isolators". This will help reduce vibration on the robot which can loosen fasteners and cause premature fatigue failure on some electronic components.
- Bumpers - Use Bumpers to cover as much of the robot as possible for your design. While the rules require specific bumper coverage around the corners of your robot, maximizing the use of bumpers increases the likelihood that all collisions will be damped by your bumpers. Bumpers significantly reduce the g-forces experienced in a collision compared to hitting directly on a hard robot surface, reducing the shock experienced by the electronics and decreasing the chance of a shock related failure.
- Shock Mounting - You may choose to shock mount some or all of your electronic components to further reduce the forces they see in robot collisions. This is especially helpful for the robot radio and other electronics such as co-processors, which may not be designed for use on mobile platforms. Vibration isolators, springs, foams, or mounting to flexible materials all may reduce the shock forces seen by these components.
Unfortunately there are few places in the FRC Control System where redundancy is feasible. Taking advantage of opportunities for redundancy can increase reliability. The primary example of this is wiring the barrel connector to the radio in addition to the provided PoE connection. This ensures that if one of the cables becomes damaged or dislodged, the other will maintain power to the radio. Keep an eye out for other potential areas to provide redundancy when wiring and programming your robot.
For any connections on the Robot or Driver station that may be frequently plugged and unplugged (such as DS joysticks, DS Ethernet, roboRIO USB tether, and Ethernet tether) using a "Port Saver" or "pigtail" can substantially reduce the potential for damaging the port. This type of device can serve double duty, both reducing the number of cycles that the port on the electronic device sees, as well as relocating the connection to a more convenient location. Make sure to secure the port saver (see the next item) to avoid port damage.
Wire Management and Strain Relief
One of the most critical components to robot reliability and maintenance is good wire management and strain relief. Good wire management is comprised of a few components:
- Make sure cables are the correct length. Any excess wire length is just more to manage. If you must have extra wire due to additional length on COTS cabling, secure the extra into a small bundle using separate cable ties before securing the rest of the wire.
- Ensure that cables are secured close to connection points, with enough slack to avoid putting strain on connectors. This is called strain relief, and is critical to minimizing the likelihood that a cable comes unplugged or a wire breaks off at a connection point (these are generally stress concentrators).
- Secure cables near any moving components. Make sure that all wire runs are secure and protected from moving components, even if the moving components were to bend or over-travel.
- Secure cables at additional points as necessary to keep wiring neat and clean. Take care to not over secure wires; if wires are secured in too many locations, it may actually make troubleshooting and maintenance more difficult.
A great way to make maintenance easier is to create documentation describing what is connected where on the robot. There are a number of ways of creating this type of documentation which range from complete wiring diagrams to excel charts to a quick list of what functions are attached to which channels. Many teams also integrate these lists with labeling (see the next bullet).
When a wire is accidentally cut, or a mechanism is malfunctioning, or a component burns out, it will be much easier to repair if you have some documentation to tell you what is connected where without having to trace the wiring all the way through (even if your wiring is neat!)
Labeling is a great way to supplement the wiring documentation described above. There are many different strategies to labeling wiring and electronics, all with their own pros and cons. Labels for electronics and flags for wires can be made by hand, or using a label maker (some can also do heatshrink labels), or you can use different colors of electrical tape or labeling flags to indicate different things. Whatever system you choose, make sure you understand how it complements your documentation and make sure everyone on your team is familiar with it.
Check all wiring and connections
After all wiring on the robot is complete, make sure to check each connection, pulling on each, to ensure that everything is secure. Additionally, ensure that no stray wire "whiskers" are sticking out of any connection point and that no uninsulated connections are exposed. If any connections come loose while testing, or any "whiskers" are discovered, re-make the connection and make sure to have a second person check it when complete.
A common source of poor connections is screw-type or nut-and-bolt fasteners. For any connections of this type on the robot (e.g. battery connections, main breaker, PDP, roboRIO), make sure the fasteners are tight. For nut-and-bolt style connections, ensure that the wire/terminal cannot be rotate by hand; if you can rotate your battery wire or main breaker connection by grasping the terminal and twisting, the connection is not tight enough.
Another common source of failures is the fuses at the end of the PDP. Ensure these fuses are completely seated; you may need to apply more force than you expect to seat them completely. If the fuses are seated properly they will likely be difficult or impossible to remove by hand.
Snap-in connections such as the SB-50 connector should be secured using clips or cable ties to ensure they do not pop loose during impacts.
Re-Check Early and Often
Re-check the entire electrical system as thoroughly as possible after playing the first match or two (or doing very vigorous testing). The first few impacts the robot sees may loosen fasteners or expose issues.
Create a checklist for re-checking electrical connections on a regular basis. As a very rough starting point, rotational fasteners such as battery and PDP connections should be checked every 1-3 matches. Spring type connections such as the Wago and Weidmuller connectors likely only need to be checked once per event. Ensure that the team knows who is responsible for completing the checklist and how they will document that it has been done.
Take good care of your batteries! A bad battery can easily cause a robot to functional poorly, or not at all, during a match. Label all of your batteries to help keep track of usage during the event. Many teams also include information such as the age of the battery on this label.
- Never lift or carry batteries by the wires! Carrying batteries by the wires has the potential to damage the internal connection between the terminals and the plates, dramatically increasing internal resistance and degrading performance.
- Mark any dropped battery bad until a complete test can be conducted. In addition to the mentioned terminal connections, dropping a battery also has the potential to damage individual cells. This damage may not register on a simple voltage test, instead hiding until the battery is placed under load.
- Rotate batteries evenly. This helps ensure that batteries have the most time to charge and rest and that they wear evenly (equal number of charge/discharge cycles)
- Load test batteries if possible to monitor health. There are a number of commercially available products teams use to load test batteries, including at least one designed specifically for FRC. A load test can provide an indicator of battery health by measuring internal resistance. This measurement is much more meaningful when it comes to match performance than a simple no-load voltage number provided by a multimeter.
Check DS Logs
After each match, review the DS logs to see what the battery voltage and current usage looks like. Once you have established what the normal range of these items is for your robot, you may be able to spot potential issues (bad batteries, failing motors, mechanical binding) before they become critical failures.