SEN 3261
- Open or close your Loop
Open or close your Loop
From Peter Martin
Hello SEN,
Just a comment on the matter of closed-loop control (following my comment in SEN 2682), speaking as a control systems engineer of 20+ years.
The 2024 CIAM General Rules state “Closed loop control systems with active sensors and operating aerodynamic flight controls or moving mass are not allowed, unless allowed in the class rules”.
To have a clear general understanding on what is allowed and what isn’t, first we need to be clear on the difference between feedforward or open-loop control (which is within the rules) and feedback or closed-loop control (which is banned). Both types of control can use sensors (or some mechanical device or property), where a sensed value is used to calculate a control signal that affects an output. The key difference is that closed-loop control monitors the output (the feedback signal) and adjusts its behaviour to reduce the control system error.
As we don’t really have good examples of electronic control in Free Flight, I will propose an idea. An F1B can have a torque-operated variable pitch (VP) propeller – The rubber motor torque is mechanically sensed and a fixed cam used to set (feed forward) the pitch angle. There is no feedback of the controlled pitch angle here, so it’s open-loop. In principle, a flyer could sense torque electronically and use a servo motor to achieve the same outcome, it would still be open-loop. Note that even though rubber energy storage and torque curve can vary from one motor to the next, the pitch angle still follows a fixed relationship with torque.
Now if some ingenious flyer was to successfully use electronic sensors to somehow measure blade angle of attack (AoA) and a servo motor to control pitch angle, an onboard controller could request a tuneable “cam” profile. With feedback of measured AoA, the controller could follow any profile desired – THIS is closed-loop control and could have advantages for faster adjustment and flight optimisation. Further, if the blade was stalled or operating inefficiently with different model trim or air conditions, the controller could move it back to the best position automatically. We banned such ideas due to potential performance gains leading to ever more complex aeroplane systems.
In summary, on Aram’s thoughts of using calculated F1Q energy consumption to trigger a rudder or end the motor run, this does fall into the open-loop camp. Current and voltage sensors are used to find instantaneous power and then cumulative energy use can be found. The proposal doesn’t involve feeding back model behaviour based on the rudder or motor output changes. It’s just an energy value to set (feed forward) the trigger. As with F1B VP and rubber quality variations, even though battery/motor characteristics can vary, the triggered function follows a fixed behaviour when an energy threshold is passed.
Best Regards,
Peter
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