A better motor arming system for OpenTX

With the high power available from modern e-flite motors, motor safety is of vital importance. In this post I'll review a couple of so-called safety features, explain why they're actually unsafe, and describe a couple of simple and secure solutions for OpenTx.

Throttle check

On most radios including OpenTx, the first line of defense against accidental spinups is the 'throttle check':

It seems like a good idea... the radio is blocked from transmitting unless the throttle stick is in the idle position. 

There are however a couple of issues:

First, the throttle must be unlocked in order to check the RF link. That means the pilot is left unprotected from the moment he carries out his pre-flight checks, even before carrying the model to the flight line (the most risky phase).

Secondly, it's easy to get in the habit of skipping the check when programming the transmitter, then forgetting that you have a real model with a live motor.

Throttle lock

Some transmitters offer a throttle lock function, whereby the throttle control can be disabled by a switch. It's better than a simple throttle check. However, it's only secure if the pilot keeps the switch in the correct position. 

Is there a better solution?

Virtual switch

The solution I use (a) doesn't rely on physical switches and (b) does not disable the RF link. This means that arming can be carried out at the last moment prior to launch.

To arm the motor the pilot carries out a simple sequence, much like a game controller:

1. Motor to idle
2. Up elevator + right aileron, and hold
3. Pull momentary switch until confirmation (2 seconds)
4. Release

To disarm the motor, it's a quick pull on the momentary switch. (Some folks prefer that the throttle should be at idle before disarming. No problem, it's just another logical switch or two. This is OpenTX, remember!).

One slight drawback of a virtual switch solution like this, is the lack of a tactile cue as to the state of the arming system. To get round this, the system generates a short beep every 12 seconds while the motor is armed. And if you're using a tx with colour LCD, my ShowItAll widget has an option to display the armed state.

Smart switch

Another method is to use a 'smart' switch. It's just a bunch of logical switch sitting behind a real switch. At startup, the LS reports 'off' regardless of the position of the real switch. In order to arm the motor, an explicit transition is required from off to on. 

This method allows quick arming/disarming, and provides a visual cue as to the arming state. However it's not as secure at the first method, and so is better suited to advanced users. 

Goodbye to startup checks

Using either method, the motor is off by default, so no startup switch checks are needed. (I hate startup checks.)

In the field

Both the above arming methods have been tested with my MPX Cularis. To minimise the risk window, I arm the motor immediately before launching, and disarm immediately upon landing. Very reassuring.

Update

Here's a code snippet for safe arming system. All my e-flight templates incorporate a choice of safe arming gestures.

Taranis and Multiplex Cularis testbed for E-soaring setup