Failsafes (specifically in Feathers)

Good Point John, We use PCM but have to be very careful upon powering up to make sure the failsafes become active. The default mode for the PCM Rx we have is upon lost of signal to stay in the last good position until the Tx updates it after a few seconds. They are the best type around because theyre in the Rx. We still use other failsafes built into the mixer/interface in case a cable becomes unplugged or the Rx battery becomes flat.
Interesting point though as when we were going into the Arena at Series 7 It failed the quick tech check because the Tx had lost it memory and returned to its default settings. It was quickly fixed and passed the second check. FRA events currently only check this once at each event! If that had not been checked at Series 7 then we would have gone into the arena without working failsafes.

Thats the strange thing with those complicated PCM systems. They have a tendency to forget theyre settings and revert to some default settings. Its not the first time ive heard about this.
The default setting usually is something you dont want for your robot. The PCM receiver we used to have in TAN actually crashed and the robot ran off because it did go on with generating servo signals.

When I started out I used a simple 40MHz set and had the problem John describes. About once per minute on average, the motors would move a fraction as the system found a valid servo signal from random radio noise.

The solution I chose was to alter the failsafe to require four valid pulses within the last second before it would move, still technically possible but extemely unlikely.

Our first Rx was a Dual conversion FM set and I should mention that the failsafe in the mixer were designed for this one.

On our mixer it has two main features for the failsafes. The first is that it effectivly integrates the in incomming signal so that any valid pulses can only change the output by a small amount so a single error would at worst case only change it by a small amount.

The second is that over a long period (100 pulses) it counts the number of in-range & out-of-range pulses and if at any point it is more than 10 pulses in error then it will enter failsafe for a minimum of 2.1 seconds. It worked well until we started using the PCM set when it effectivly overrides this.

Once it has entered failsafe mode it will not come out of it until 100 pulses are received in range. The failsafes work on a per channel basis and also have a 65mS timer so that if that channel does not receive a pulse in that time it will shut that channel down for 2.1 seconds before trying again.

The only problem (from other peoples point of view) is that its my software code running it and so far we have not had it crash but occasonally it still entered failsafe mode. There is a watchdog active that has only one timer reset function in the begining of the main program loop so if it did crash it should correct itself after a very short delay.

I dont know how other failsafes operate but I would guess its not this complex.

All in all I still would not like to approch a robot in failsafe mode as you know it only needs to pickup a valid signal (from any source) and it could activate again.

On the latest version of my radio control software I put in a 28-bit keycode. My robot wont come out of failsafe mode until this is received, so no other radio controller could activate it. Nothings perfect though, as you say, you still have to approach it with caution.

Im continously developing the ultimate rc-relay. If there exists such a thing. The hardware has christalled out nicely, but i still have to do some work on the software side. Right now it only works on one of the two channels. The most important thing to test for is the perriod between pulses. If this is too small its a bad signal.
My relays require 4 pulses to flip over the switch, both for the on-off and off-on transition. If it detects a spike it ignores the previous 3 and next 4 pulses. Failsafe detection is similar to marks system. if 4/60 pulses are bad it failsafes. It only comes out of failsafe after 60 good pulses. If the perriod between pulses becomes too long the system inmediatly goes into failsafe.
This seems to be a relyable system. Yet the most dangerous situation which can occur is when the receiver is switched off and the system is expected to failsafe. Its my experiance that if the failsafe and filtering algorithms arent good enough the relay may trigger in such a situation.
Keep your distance if you turn off your transmitter, you never know what will happen just before your system failsafes :)

yeah and i use one in mine works very well never had a problem unless i damaged it !!!

i have tried 4or 5 switches now and this is the best

I know it sometimes happens with my pcm receiver that it goes back to the default continue doing what yo are doing setting if you turn of the transmitter too quickly the first time you attempt failsafe. The answer to that is to power down the robot and rx, start tx first, start rx is it has a seperate battery pack, or the robot if it has a BEC. and allow the TX to send its settings to the RX for 30 sec to a minute to be sure.

You should always start the receiver first with PCM.

The failsafes are normally transmitted during the first 10 seconds and normally 30 seconds thereafter. If you turn the transmitter on first it will transmit before the receiver is on.

As a helpful hint, the PCM indicator flashes on the futaba screen (Field Force 6) when the parameters are transmitted.