The use of a second servo was initially a mechanical circuit whereby the second servo would shadow the main servos movement, connected by linkage and a microswitch. Any difference in position would trip the microswitch which in turn would be wired through a latching relay to break the safety circuit.
My misguided thought following this was that the pot in the main servo would only have an output when out of sync with it's signalled position. I had thought that this would provide an error voltage which could be 'Vin' (in the diagram), and then adjust the resistor 'Rb' to fine tune the switching point. --- Sadly none of this will work as I have since realised that the pots output is purely related to the output shaft position, unaffected by whether the position is correct or not.
The PLC option is far from my knowledge but thanks for the suggestion!
I'm now thinking that two external pots may be used, driven by the main servo and a second slave servo. Perhaps they could be wired such that out in an out of sync situation a forward voltage could then trip the safety circuit?
It's impossible to apply a complete failsafe system. What if the radio is failing? Like a potentiometer or something. Then the failsafe on the servo side would be useless.
In my opinion, the best failsafe is the one that relies mainly on the operator, and also on a simple mechanical system that you can be very sure it doesn't fail.
Here's my idea:
Firstly you need to have a loaded spring that is ready to push the valve closed. It should be actively maintained by a simple electrical system. Should ANYTHING fail (even electrical power shutdown), the spring does the job.
Now the failsafe system would deliberately let the spring shut off the valve based on operator's input.
This is the key. The logic system should know whether or not you have your hands on the sticks (metal sticks +, metal case -, your hand closes the circuit). When you (the operator) feel that something is not allright, you just get your hands off the sticks.
When you are actually done (wanna take a break), you should press a button while taking the hands off. This way the failsafe would not trigger. I believe it would be extremely safe. It's a rough idea though, it's your job to improve and apply it.
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