Motor arming and kill switch

Mike Shellim 26 April 2016
Updated 30 June 2020

Here's a recipe for a safe arming/kill switch for an electric motor. It's designed on the premise that arming the motor should be difficult, and killing it should be easy - but not so easy that you can do it accidentallly.

Operation

To arm the motor:

1. Throttle to idle
2. Elevator stick back, and hold.
3. Pull and release SH.
4. Release stick

To kill the motor:

• Pull SH for a minimum duration (user settable), and release.

How it works

The system is based on the safety switch recipe with a couple of mods. Key points:

• L1 is the arming gesture: throttle is at idle, elevator is back, and SH is pulled.
• L4 is true while SH is pulled, but not while arming ('!L1') .
• L5 is the disarm gesture. It generates a True pulse after L4 has been true for 2 seconds. In other words, after pulling SH for 2 seconds, not while arming.
• L3 is the arming switch. It's set by the arming gesture (L1), and reset by the disarm gesture (L5). Use this to control the motor's state (idle/running).

Usage

Here's the suggested mixer setup for controlling a motor on CH3:

How it works:

• The first line sets the motor to motor off. This is the default state.
• The second line activates the throttle lever. It over-rides (multiplex=REPLACE) the first line when the motor is armed (L3=True).

[An alternative method of disarming the motor is via the Channel Override special function. However, it's not generally recommended; it's applied at the last stage in the command pipeline, and has no effect on the mixer outputs. This can cause problems for example if you want to query the mixer output to detect if the motor is running. The method shown above is more explicit and easier to debug.]

Safety!

• Remember to set the failsafe, so motor goes to idle on loss of signal
• Use the channel monitor to test operation before connecting up the motor