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Build to the target event’s current rulebook, not to a generic combat-robot recipe. Choose the organizer and weight class first, then design a robot whose power cutoff, radio failsafe, weapon lock, batteries, guards and testing procedure can all be demonstrated to safety inspectors. National Havoc Robot League (NHRL) 2026 rules provide a useful example, but their requirements are not universal.
The safest build is one that can be made harmless quickly, remains harmless after a lost radio signal, is physically locked whenever people handle it, and is simple enough for an inspector to understand. Do not select motors, batteries, weapon dimensions or chassis materials until the class, event, design concept and approval requirements are known.
Start with the competition and weight class
Sanctioned events do not share one safety standard. Organizers set different weight limits, voltage rules, weapon definitions, cutoff designs, testing areas and approval deadlines. The event’s rulebook and safety officials are the authority; general advice cannot override them.
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- Organizer, country and exact event date.
- Weight class and whether the limit includes batteries, armor, weapon locks or other removable parts.
- Robot category, such as remote-controlled, autonomous-assisted or fully autonomous.
- Weapon type and whether it is kinetic, non-kinetic, pneumatic, hydraulic, combustion-based or flame-producing.
- Electrical architecture, battery chemistry, expected voltage and charging method.
- Venue rules for pits, machine shops, charging areas, tools and transport.
- Inspection deadlines and any requirement to submit unusual designs in advance.
Download the current rulebook, safety guidance and judging criteria before buying hardware. Rules, class limits and deadlines can change between seasons. Keep a copy with the robot and ask the organizer’s safety staff about any clause that could affect the design.
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Use a requirements sheet before designing
Write each requirement as a testable statement. For example: “An official can remove all drive and weapon power without disassembling the chassis,” or “After the transmitter is switched off, the drive and weapon stop within the event’s published limits.” This turns safety from an intention into an acceptance test.
| Requirement area | Question to answer before fabrication |
|---|---|
| Power isolation | Where is the cutoff or removable link, and can it be operated in the robot’s required orientation? |
| Radio loss | What exactly must stop after receiver signal loss or transmitter shutdown? |
| Weapon handling | What physical lock prevents actuation while batteries and weapon systems are installed? |
| Testing | Where does the organizer permit powered drive and weapon tests? |
| Restricted systems | Does the design require written approval before the event? |
| Inspection | Which functions must be demonstrated with the robot in its match configuration? |
Design a power system that can be made safe immediately
Main cutoff or removable link
The cutoff must remove power from both drive and weapon systems. It may be a switch or removable link when the event permits it, but it must be reachable without taking the robot apart. Plan its location around the robot’s required fighting orientation, not merely around workshop convenience. Protect wiring from impacts and provide the strain relief specified by the rulebook.
In NHRL’s 2026 example, the cutoff must be deactivated within 15 seconds. For 12 lb and 30 lb robots, it must be accessible in the primary fighting orientation and use a dedicated removable link with adequate strain relief. A 3 lb robot may use battery connectors as removable links under those rules. NHRL also says the cutoff must be operable with one hand, although competitors may use both hands at the event. These details apply to that league and season only.
Separate the cutoff from ordinary software controls
A transmitter command is not a substitute for physical power isolation. A software arm switch can prevent an accidental command, but an official still needs a dependable physical method to remove energy. Label the cutoff clearly, make it resistant to vibration, and ensure an official can identify it without removing armor.
Route and protect high-current wiring
Keep power conductors away from rotating parts, sharp chassis edges and areas likely to be crushed in a collision. Secure connectors so an impact cannot partially disconnect them and create arcing. Provide access for inspection without requiring an unsafe reach near the weapon. The exact conductor size, connector and fuse choice depends on the event, electrical design and current demand; the available information does not establish universal component ratings.
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Make radio-loss behavior predictable
A failsafe is the behavior that occurs when the receiver loses the transmitter signal or the transmitter is powered down. Configure it so drive commands go to neutral or stop and weapon systems go to their safe state. Test the actual receiver, transmitter, speed controllers and weapon controller together; a bench test of one component does not prove the complete system.
NHRL’s documented pass criteria
For NHRL 2026, a safety inspector oversees the test. After signal loss or transmitter shutdown, the drive must stop within 30 seconds and the weapons within 60 seconds. Autonomous robots still require radio control for remote activation and deactivation. Treat these timings as that event’s pass criteria, not as a universal engineering target.
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Common failsafe mistakes
- A receiver retains the last throttle command instead of returning to neutral.
- The drive stops but a weapon controller continues its previous command.
- An autonomous routine can start without a valid radio activation.
- A reset or power cycle unexpectedly re-arms the weapon.
- Failsafe settings are lost when a different transmitter model or receiver channel map is used.
Document the settings and label the receiver ports. If the event requires a particular channel arrangement or arming sequence, copy it exactly rather than relying on memory at inspection.
Use a physical weapon lock whenever people handle the robot
A weapon lock must prevent actuation, not merely make the weapon difficult to reach. Design it as a robust mechanical part that can survive transport and handling, and make its installed and removed states obvious to an inspector.
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Match the lock to the weapon
NHRL requires locks for weapon and weapon-like systems. Its rules distinguish the permitted movement of rotating kinetic weapons from non-kinetic weapons and prescribe special arrangements for some designs. Because those limits are design-specific, confirm the exact allowed movement in the current rulebook before machining the lock.
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Cover hazards during transport
Cover sharp edges that could cause serious cuts or punctures while the robot is carried through the venue. Covers should stay in place during movement and be removable without placing hands near an energized weapon. Mark the safe lifting points and provide a transport method that does not require gripping armor edges or weapon assemblies.
Test only in the organizer’s controlled area
Powered testing belongs in the event’s designated test box or other area controlled by officials. NHRL requires weapon and drive testing in a test box. Its wheels-up exception applies only when the robot has a second dedicated switch for the weapon system. A bench, driveway or workshop is not an informal substitute for the event’s test area.
Prepare a controlled test sequence
- Confirm the robot is in the test area and that spectators and unnecessary personnel are clear.
- Install all guards, covers and the weapon lock required for the current step.
- Verify the cutoff position and radio control before applying power.
- Remove the lock only when the official permits it and the test requires it.
- Test drive at the minimum movement needed to demonstrate control, then stop and isolate power.
- Test the weapon only under the official’s direction and within the area’s permitted procedure.
- Reinstall the weapon lock before leaving the test area.
Bring a written checklist so a rushed setup does not skip the cutoff, lock or failsafe demonstration.
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Choose a weapon that is both legal and safely demonstrable
Weapon selection is not just a performance decision. The mechanism must fit the class allowance, accept a secure lock, stop under radio-loss conditions and satisfy the organizer’s definition of an active weapon.
NHRL’s 2026 rules require an active weapon designed to visibly and consistently harm an opposing robot. Its official rules Q&A states: “This doesn’t require weapons to be capable of destruction, but it does require intentional, effective use of the active weapon that makes the robot legal at NHRL.” A decorative mechanism or one that cannot be used meaningfully may fail the legality test even if it is mechanically safe.
Compare concepts against the same criteria
| Decision criterion | What to verify |
|---|---|
| Legality | Is the mechanism permitted by the named event’s current weapon and judging rules? |
| Lockability | Can a physical lock prevent actuation while batteries are installed? |
| Failsafe behavior | Does the controller stop the weapon after signal loss and prevent unintended re-arming? |
| Inspectability | Can an inspector see the cutoff, lock, guards and critical connections? |
| Mass and class fit | Does the complete robot remain within the class limit in match configuration? |
| Approval burden | Does the design trigger a special review before the event? |
Get approval before building restricted systems
NHRL identifies pneumatic and hydraulic systems, combustion engines, flame systems and exposed moving components above its stated speed threshold as restricted or approval-sensitive designs. Requirements and lead times depend on the system. Contact safety staff early and provide the information they request.
For a flame design, approval guidance may require details about fuel, ignition, weapon safety and tank protection. Do not complete a restricted robot on the assumption that an inspector will approve it at check-in. If approval is denied or conditions change, redesigning late can make the robot unsafe, overweight or impossible to test properly.
Handle batteries and tools as part of the safety system
Charging and battery storage
NHRL requires charging batteries to be attended and requires balance leads when charging lithium-chemistry packs. “Attended” means a responsible person remains present and able to respond; do not leave a pack charging in an empty pit or vehicle where the venue prohibits it. Follow the venue’s designated charging location, fire-containment rules and storage limits.
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Inspect packs, connectors and balance leads before charging. Do not charge a damaged, swollen, wet or unusually hot battery. Keep the battery electrically isolated from the robot until the event’s approved setup, and protect terminals from accidental short circuits during transport.
Workshop and pit PPE
NHRL requires safety glasses and other appropriate personal protective equipment when using power tools in the pits or machine shop. Impact-rated safety glasses are a sensible baseline, but no particular brand or model is approved by the league. Add hearing, hand, respiratory or face protection when the tool, material and venue rules require it.
Secure the workpiece, remove loose clothing and disconnect batteries before drilling, cutting or servicing near a weapon. Keep a clear separation between machining and an energized robot.
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Build in an inspection-friendly order
- Freeze the rule set. Save the current event documents, class limits, weapon definitions, battery rules, testing procedure and approval deadlines.
- Draw the safety architecture. Show the cutoff, removable link, battery, drive power, weapon power, receiver, controllers, lock and guards on one diagram.
- Design the chassis around access. Reserve a visible, reachable path to the cutoff and inspection points. Do not bury the link behind armor that requires tools.
- Prototype the lock and cutoff first. Confirm that an official can install, remove and identify them with the robot in its required orientation.
- Install and label wiring. Secure high-current leads, protect connectors and mark the receiver, cutoff and weapon isolation points.
- Configure failsafe before high-power testing. Test receiver loss with the weapon mechanically restrained and verify that re-arming requires the intended sequence.
- Check mass and clearances. Weigh the complete match configuration, including batteries, locks and covers, and verify that nothing interferes with the required safety controls.
- Request special approval. Submit restricted-system information before committing to final fabrication.
- Run low-energy checks. With the weapon secured, verify switch operation, channel direction, neutral positions, cutoff access and controller status.
- Perform official-style demonstrations. Rehearse the cutoff, failsafe, lock removal, controlled drive test and weapon test in the same configuration you will present at inspection.
Prepare for inspection and event-day handling
NHRL safety inspection includes weapon locks or covers, sharp-edge covers, radio failsafe behavior, controlled movement, power-cutoff operation, active-weapon functionality and other design-specific requirements. Plan to demonstrate each function with the actual batteries, armor, weapon and removable links used in competition.
Inspection checklist
- Rulebook and approval correspondence are available.
- Robot is within the class limit in its match configuration.
- Cutoff or removable link is clearly identified, reachable and functional.
- Weapon lock is installed before handling and transport.
- Sharp edges and puncture hazards have approved covers.
- Drive and weapon failsafes have been tested on the complete radio system.
- Weapon can be shown to function intentionally without uncontrolled motion.
- Battery charging equipment, balance leads and PPE comply with venue rules.
- Restricted systems have written approval, if applicable.
At the venue, obey officials’ instructions even if your workshop procedure differs. Reinstall locks whenever the robot leaves the test area, and never power a weapon in a pit or staging area unless the organizer explicitly permits it.
Troubleshoot safety failures before arrival
| Observed problem | Likely cause | Corrective action |
|---|---|---|
| Cutoff cannot be reached in the required orientation | Link location was chosen for workshop access rather than match access | Relocate the cutoff or redesign the removable-link path; obtain event approval if the rule permits an exception. |
| Drive stops but weapon continues | Failsafe was configured only on drive channels | Configure and test the weapon controller separately, then repeat the complete signal-loss test. |
| Weapon restarts after power cycling | Controller powers up armed or retains a previous command | Require an explicit arming action and verify the event’s re-arm procedure with an inspector. |
| Lock is difficult to install | Lock was added after the weapon and armor were finalized | Redesign the lock interface; do not rely on a loose pin, zip tie or hand placement near the weapon. |
| Battery becomes hot during charging | Damaged pack, incorrect charger setting or poor connection | Stop charging, isolate the pack and follow the venue’s battery-disposal or safety procedure. |
| Inspector cannot verify a safety function | Component is hidden, unlabeled or accessible only with tools | Add labels, access panels or a demonstration procedure that meets the rulebook. |
| Design approval is delayed or denied | Restricted system was submitted late or lacked fuel, ignition, tank or safety details | Contact safety staff early and provide the complete requested design information. |
Know what cannot be chosen from a generic guide
Without a named event, class, budget, design concept and builder capability, there is no responsible universal recommendation for chassis material, motors, battery capacity, weapon dimensions, connector ratings or armor thickness. Those choices depend on mass allowance, electrical demand, fabrication method, target opponent, transport and the organizer’s technical limits.
The correct next step is to supply those design inputs, then review the current rulebook with the event’s safety officials before ordering parts. A robot that is powerful but cannot be isolated, locked, tested and approved is not ready for sanctioned competition.
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