Failsafes

A failsafe is what your robot does when it loses contact with your transmitter – because the transmitter was switched off, its battery died, the robot's receiver was damaged, or the signal was blocked. A correctly set failsafe makes everything stop. A wrong one can leave a robot driving or a weapon spinning with nobody in control, which is one of the most dangerous things that can happen at an event. Failsafe checks are part of nearly every safety inspection.

What the rules say

• SPARC: the SPARC Robot Construction Specifications (v1.7) require that all robots stop all motion, drive and weapons, when the transmitter loses power or signal. This can be built into the robot's electronics or set up in the radio. (Earlier versions only required it for robots with active weapons or over 12 lb, so older guides may say otherwise.) Springs, flywheels and autonomous functions must also fail safe.[1]
• NHRL requires a failsafe test overseen by an inspector: when the signal is lost or the transmitter is switched off, the drive must stop within 30 seconds and weapons within 60 seconds. Each part of a multibot needs its own receiver.[2]
• The UK FRA rules also require failsafes to trigger on signal loss, interference, or low receiver power.[3]

How failsafes work

When a receiver loses signal, it does one of three things on each channel. Radio menus use slightly different names:[4]

Setting	What happens	For combat robots
Hold (last position)	The receiver keeps sending the last command it received	Never acceptable for drive or spinning weapons – the robot carries on doing whatever it was doing.
No pulses	The receiver stops sending any signal	Acceptable only if your ESCs stop the motors when the signal disappears. Most modern ESCs do, but you must test it.
Set position (custom / preset)	Each channel goes to a value you choose	Acceptable if the stored value really is "stopped" for each ESC – for example, center for a bidirectional drive ESC and zero throttle for a weapon ESC.

Team Run Amok's radio functions guide explains the options in more detail. Whichever you choose, test it – never assume.

Common radio systems

Menus and defaults vary between radios, receivers and firmware versions. Check your manual, then test.

• ExpressLRS (ELRS): ELRS receivers enter failsafe after about a second without a valid signal. On PWM receivers, the default failsafe is a set position of 1500µs (center) on every channel except output 3, which defaults to low throttle.[5] That's fine for bidirectional drive ESCs, but a forward-only weapon ESC on any channel other than 3 would get 1500µs – about half throttle. Set each channel's failsafe in the receiver's web interface (Set Position, No Pulses or Last Position), or use the "Set Failsafe Pos" option in the ELRS Lua script.[6] Step-by-step instructions are on ExpressLRS and EdgeTX Setup.
• FlySky (AFHDS 2A, e.g. FS-i6): set failsafe in the transmitter's system menu (RX Setup → Failsafe), with each stick at its "stopped" position, and turn it on for every channel you use. Team Run Amok's FS-i6 setup guide walks through it.
• Spektrum: the failsafe positions are stored when you bind, so hold the sticks at their stopped positions while binding.[7] Some Spektrum receivers default to a mode that only drives the throttle channel low and holds the others, which is unsafe if your weapon isn't on the throttle channel – check your receiver's manual.
• All-in-one boards (such as the Malenki): follow the board's instructions, and turn the transmitter on before powering the robot, since some boards behave differently depending on power-up order.[8]

How to test your failsafe

1. Put the robot in a test box or arena, or block it up so the wheels can't touch anything. For the first test, lock the weapon or remove its belt.
2. Power on as normal: transmitter first, then the robot.
3. Run the drive and weapon at low throttle, then switch the transmitter off.
4. Check that everything stops, and that it stays stopped. Rules usually allow a little time for weapons to spin down (NHRL: drive within 30 seconds, weapons within 60), but the motors must not be powered.
5. Repeat with the weapon at higher speed, in a test box.
6. Test again after you rebind the receiver, change trims, mixing or model memory, swap receivers or ESCs, or update firmware. Any of these can change the failsafe.

Common mistakes

• Leaving channels on "hold" (the default on some receivers).
• ELRS's default center position on a forward-only weapon ESC.
• Storing the failsafe with the sticks or trims away from their stopped positions, or before finishing your mixing setup. The failsafe stores what the receiver outputs, after mixing.
• ESCs that weren't calibrated to the transmitter's range, so "stopped" isn't really stopped.
• Not re-testing after changing anything.

See also

• Radios and Radios/Mixing
• ESCs and ESC Firmware – throttle calibration and neutral settings
• Troubleshooting
• Safety – the safety hub
• SPARC Robot Construction Specifications