Motor arming and kill switch

Mike Shellim 26 April 2016
Updated 24 July 2017

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.

It's based on the safety switch recipe with a couple of mods.

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 implemented as a bunch of logical switches, see screenshot below. Key points:

• L3 is the arming status, use this to block or allow motor operation.
• L1 is true when stick is in the corner, and SH pulled.
• L4 is true when SH pulled to disarm
• L5 adds a delay to L4, to prevent accidental disarm
• L5.V2 is the minimum duration to pull SH, for killing the motor (default = 2 secs).

Usage

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

How it works:

• The first line outputs a fixed -100 which represents motor off. MAX is a 'virtual' source which provides a fixed value of 100, negated by setting weight to -100%.
• The second line becomes active if the arming switch is enabled (L3). The source is the throttle stick. The multiplex field is set to REPLACE, so when active it over-rides the first line.

[Aside: an alternative method is to have the motor active by default, and use the Override special function to disable it. However, I don't recommended this. The Override function is applied at the last stage in the command pipeline, and has no effect on the raw channel outputs. This can cause problems if you reference the motor channel e.g. 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