Motor arming

Mike Shellim 26 April 2016
Updated 20 April 2023

Motor arming is a common topic of discussion in the forums. In this article I'll demonstrate a simple framework for designing your own custom arming system, using two examples.

Note that I use the term 'system' rather than 'switch', because arming can be much more refined than just simply flipping a switch on and off.

But first a few definitions:

• Arming: an action which allows the motor to run.
• Disarming: an action which prevents the motor from running, and stops it if already running.
• Kill switch: a switch which immediately and unconditionally disarms the motor.
• Gesture: a user action which involves moving switches and/or sticks in a particular sequence.

The Sticky logical switch

Arming systems are typically based on a Sticky switch which is used to activate or deactivate a throttle mix.

A Sticky logical switch is essentially an on/off switch where the 'on' and 'off' actions can be customised:

Switch is a physical or a logical switch.

In the context of an arming system, the 'on' switch represents the arming gesture and the 'off' switch represents the disarming gesture.

This method provides great flexibility since the arming and disarming gestures can be different.

We'll see later how the sticky switch is integrated into the motor mix.

Gesture design

As an example, we'll design an arming system with the emphasis on safety. For this kind of system, (a) arming should be difficult and (b) disarming should be easy (but not so easy that you can do it accidentallly whilst flying the model).

We'll use the following gesture to arm the motor:

1. Throttle to idle
2. Elevator stick back
3. Pull SH

And the gesture to disarm the motor:

• Pull SH for 2 secs

Note that SH disarms the motor unconditionally, so it also functions as a kill switch. A condition could be added that the throttle must be at idle, in which case SH would no longer qualify as a kill switch.

The code

The logical switches are shown in the Companion screenshot below.

• L1 is the arming gesture: throttle to idle + elevator fully back + pull SH.
• L5 is the disarm gesture:
  • L5 goes True briefly, after L4 has been true for 2 seconds.
  • L4 is true while SH is pulled, but not while arming ('!L1').
• L3 is the Sticky arming switch. V1 is the arming gesture (L1), and V2 is the disarm gesture (L5).

Note that 'Thr' and 'Ele' refer to sticks (not inputs) - make sure to make the correct selection (inputs are prefixed with 'I', sticks have no prefix)

Integrating an arming switch into the motor channel

With the arming logic done, we can apply it to the motor channel:

How it works:

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

Smart switch

One of the great benefits of a Sticky based system is that switch checks are not needed at startup. This is because the initial state of a sticky switch is false equating to 'disarmed'.

We can apply this to make a regular 2-position switch 'smart', whereby the default state is Off regardless of the actual position of the switch.

To do this,

1. Select a 2-position switch to use.
2. Create an Edge logical switch to trigger when the 2-pos transitions from Off to On
3. Create a sticky to handle the on and off actions.

Here's an example with SF↑ = Off and SF↓ is On.

Regardless of the position of SF, the sticky switch (L2) will always be false at startup. If SF is in the On position, it will need to be moved Off then On in order to activate it.

Using the Channel Override special function (don't!)

An alternative method of disarming which is often mentioned is the Channel Override special function. I don't recommend it, for the following reasons.

• It can't be integrated into a Sticky-based arming system.
• The override is applied to the final channel value - the mixer value is not impacted. This makes it impossible to determine, via a logical switch, whether the motor is running.
• It's more difficult to debug.

Download demo

Safety first!

• 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