FRC Robot Electrical Overview

• Prerequisite: Electricity Overview
• Summary: Overview of the major electrical components that are used in FRC robots

Battery

FRC robots use a ​SLA1116 12 volt 18Ah sealed lead acid rechargeable battery. These are large, powerful batteries capable of supplying up to 250A in bursts and 80A continuous power. That's a lot of power. 12v * 250A = 3000W. This is enough power to launch a 120lb robot into the air or to melt steel; these batteries must be treated with respect. Car batteries are a little larger, but watch how a battery can easily ​vaporize a steel nail to get an idea of how much power we're talking about. These batteries are full of lead and are very heavy, so be careful not to drop them on anything either.

To carry all of that power to the robot, FRC batteries have heavy metal terminals that connect to 1-foot long, very thick (6-Gauge) wires using a nut and bolt (preferably a split washer and lock nut). The wires end in a heavy-duty ​SB-50A connector capable of carrying 50 Amps of continuous current. The connectors allow the battery to be plugged into the battery or charger easily.

DO NOT CARRY THE BATTERY BY THE WIRES OR CONNECTOR

It's important that all of the connections to battery remain tight (this is true for your car battery too for the same reasons): a loose connection provides resistance to the flow of electricity and as you know, resistors convert electrical current into heat energy. A loose connection to your battery can result in enough heat to start a fire. The reason we don't carry the battery by its wires or connector is that the weight of the battery will loosen those connections.

The FRC batteries are rechargeable. A lead-acid battery charger must be used; this pushes electrical current back through the battery, causing the spent chemicals to reverse their reactions, restoring the battery to its original state (almost). Reversing the chemical reactions must be done slowly and so charge current is limited to 5A.

The state of charge of a battery can be checked with a special tester. Team 2537 uses a tester called a ​battery beak that plugs onto the battery connector and has a digital readout that describes the battery's condition including an overall assessment of how fully charged it is.

• Exercise: Check state of charge of a battery
  • Materials required: Battery with cable attached, Battery beak, senior student or mentor
  • Activity:
    • Lift battery (from the body, not the wires) to get a feel for its weight
    • Plug the battery beak into the connector
    • Press the beak button and observe the measurements (ask a senior student or mentor for explanation)
    • Unplug the battery beak, holding the battery connector and the beak (don't pull by the wires)
  • What happened: you learned to handle a battery and measure its readiness for use in competition

Motors

FIRST has a long ​list of motors that are legal to use in FRC Robots. The list includes loads of detail and many different kinds of motors. Some of the most commonly used motors are the RS775, CIM, and mini-CIM motors. All of the FRC-legal motors operate on 12VDC. Different motors offer different sizes, weights, speeds, and torque (turning strength). The more force a motor generates, the more electrical current it will consume. When designing a robot it's important to choose the right motor for the task and it's important to make sure all of the motors that may be operating at the same time won't overload the battery or electrical wiring.

Most FRC motors were designed for other commercial purposes (wheelchairs, snowblowers, power tools, car windows or seats, etc.) so they are widely available and relatively inexpensive. The most powerful legal motor is the CIM motor (CIM is an acronym for CCL Industrial Motors - the manufacturer of the motor). CIM motors are capable of ​generating 300W of force and were designed for motorized wheelchairs. FRC rules usually allow a maximum of 6 CIM motors per robot and 4 usually power the drive system (wheels/treads) where the robot needs both speed and power. FRC rules usually do not limit the number of smaller motors (mini-CIM, RS775, etc.) that can be used.

​RS775 motors are commonly used in power tools (e.g. cordless drills) and radio-control vehicles; they can provide up to 150W of force and very high speeds. ​snowblower motors are used in electric snow throwers and can provide up to 20W of force. Some things to note:

• Gear boxes trade speed for torque (strength). For example, the snow blower motor is fairly small but has a built-in gear-box that reduces its speed and increases its torque. The maximum geared-down speed of the motor is 100 rotations-per-minute (RPM), but the maximum torque is 70-ft-lbs (1-ft-lb means the force applied by a 1-lb weight at the end of a 1-foot rod mounted perpendicular to the motor shaft).
• The speed of a motor usually varies with the amount of force it must apply, so a motor with nothing attached to it may spin very fast, but if the motor encounters resistance (e.g. if it is trying to turn the wheels carrying a heavy robot), the motor will slow down. When so much resistance is encountered that the motor can no longer turn even when applying its maximum force (e.g. if that robot runs into a wall), the motor stops turning and is stalled.
• The electrical current drawn by a motor varies with the amount of force it is applying. When there is no load on the motor, its idle current is much lower than when it is applying full force. For example, a snow-blower motor draws 5A when idle but up to 24A when stalled.
• It is important not to apply power to a stalled motor for very long or the motor will burn up and may even catch fire. Consider that when a small motor like a snow-blower motor is stalled, it is drawing 24A at 12V (nearly 300W) and since it can't convert that electrical energy to motion, the energy is converted to heat.

Motors are usually mounted to the robot with special ​motor mounts and their shafts are connected to wheels and axles using ​gears which trade reduced speed for increased torque.

• Exercise: Identify and test various FRC-legal motors
  • Materials required: ​CIM motor, ​mini-CIM or baby-CIM, ​RS-775, RS-775 with ​PG-775 gearbox or ​snowblower motor, motor mounts, PP45 connectors, motor test box (aka "Johnson" or "Saunders" box - named after the mentors who created them)
  • Activity:
    • Examine motors and mounts
    • With power off, plug a motor into the motor test box
    • Turn on the motor test box and gradually increase/decrease the motor speed
    • Try (carefully) to stall (stop) the motor with your hand - if you succeed, don't keep it stopped for long or the motor will burn up.
    • Try different motors to get a feel for different speeds and torques
    • Try a motor with a gearbox (PG-775 or Snowblower) to get a feel for what gearing does
  • What happened: you learned how to identify several FRC-legal motors, to connect/disconnect and run them using a test box, and developed a feeling for the speed/torque trade-offs of different motors and the impact of gearing.

Safety

FRC robots use powerful batteries and motors, powerful enough to injure or kill someone. Safety is a crucial aspect of FRC robotics and for your own safety and the safety of your team-mates, learning to handle the robot and its components properly is essential. Repeated safety violations are among the few things that will cause you to be cut from the team. FIRST provides a detailed ​safety manual and every member of the team is responsible for building and encouraging a culture of safety, from small things like always wearing safety glasses when in the shop or working with robots and tools to critical electrical safety such as:

• FRC robots must have a visible ​Robot Status Light (aka RSL) that is either on solid or blinking when the robot is powered up.
• Long hair must be tied back and no loose clothing when working with motors
• Wear sturdy shoes that cover your feet entirely - if you or someone else drops a battery on it, your foot will break (not the battery)
• Always wear safety glasses when working with tools or the robot. Power tools often throw off sharp pieces of metal and a short circuit will cause flying pieces of molten metal; on your arm, those will cause a minor injury, but in your eye, you may gain a permanent disability.
• Wires and connections must be insulated (covered with non-conducting materials) to ensure no accidental electrical contact. Exposed wires create both a shock hazard and a burn/short circuit hazard. Electrical tape, heat-shrink tubing insulation, and non-conductive barriers should ensure no people and no metal can accidentally contact a live electrical wire or terminal. If you see exposed wires, turn off power and add insulation.
• Treat batteries, wires, and motors with respect. A lot of power is involved in an FRC robot and things get hot.
• Lead-acid batteries contain acid (who would have guessed). This is not citric acid like a lemon, it's sulphuric acid, like burn through your clothes and skin. If a battery breaks or explodes (it happens):
  • Don't panic, it ​takes some time for the acid to do damage to skin, but it is particularly dangerous for your eyes. Either way, wash it off your skin and clothes immediately
  • Do not touch the electrolyte
  • Have several boxes of baking soda on hand to neutralize the battery acid (see [​https://www.youtube.com/watch?v=sEY5bSM3OyQ this video) made by another FRC team for how to clean up a battery acid spill.
• Use appropriate thickness wire (more on this below)
• All circuits must include a fuse or circuit breaker to open the circuit (disconnect power) if it is drawing too much current
• Before turning the robot on, shout (yes shout) "Robot Active"
• Read the FIRST FRC safety manual: ​here

Wiring and Insulation

Fuses and Breakers

Power Distribution

Voltage Regulation

Motor Controllers