| | 21 | * Exercise: Check state of charge of a battery |
| | 22 | * Materials required: Battery with cable attached, Battery beak, senior student or mentor |
| | 23 | * Activity: |
| | 24 | * Lift battery (from the body, not the wires) to get a feel for its weight |
| | 25 | * Plug the battery beak into the connector |
| | 26 | * Press the beak button and observe the measurements (ask a senior student or mentor for explanation) |
| | 27 | * Unplug the battery beak, holding the battery connector and the beak (don't pull by the wires) |
| | 28 | * What happened: you learned to handle a battery and measure its readiness for use in competition |
| | 29 | |
| 28 | | [http://www.banebots.com/product/M7-RS775-12.html 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. |
| | 37 | [http://www.banebots.com/product/M7-RS775-12.html 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. [http://www.andymark.com/Motor-p/am-2235.htm snowblower motors] are used in electric snow throwers and can provide up to 20W of force. Some things to note: |
| | 38 | * 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). |
| | 39 | * 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''. |
| | 40 | * 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. |
| | 41 | * 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. |
| | 44 | |
| | 45 | * Exercise: Identify and test various FRC-legal motors |
| | 46 | * Materials required: [http://www.andymark.com/CIM-Motor-p/am-0255.htm CIM motor], [https://www.vexrobotics.com/217-3371.html mini-CIM] or baby-CIM, [http://www.andymark.com/Motor-p/am-2194.htm RS-775], RS-775 with [http://cdn3.volusion.com/vyfsn.knvgw/v/vspfiles/photos/am-pgseries-2.jpg?1496901653 PG-775] gearbox or [http://www.andymark.com/Motor-p/am-2235.htm snowblower motor], motor mounts, PP45 connectors, motor test box (aka "Johnson" or "Saunders" box - named after the mentors who created them) |
| | 47 | * Activity: |
| | 48 | * Examine motors and mounts |
| | 49 | * With power off, plug a motor into the motor test box |
| | 50 | * Turn on the motor test box and gradually increase/decrease the motor speed |
| | 51 | * 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. |
| | 52 | * Try different motors to get a feel for different speeds and torques |
| | 53 | * Try a motor with a gearbox (PG-775 or Snowblower) to get a feel for what gearing does |
| | 54 | * 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. |
