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Open and Closed Loops in Aircraft Management

December 25, 2011  by: Madan G Singh  Points: 15   Category: Gadgets    Views: 792

Open and closed loops are servo mechanism to evaluate working parameters on board an aircraft

         

One of the systems of monitoring the performance of operating parameters of an large aircraft is the 'open' and 'closed' systems. These represent a control process that amplifies the parameters. There are two types of CNC ( Computer Numerik Control) systems. As already mentioned they are the 'open' and 'closed' systems. Most aircraft engines operate at a frequency of 400 hertz. To monitor and give a feedback we have these CNC systems.
'Open loop' signifies a line where the controller system and the motor have a one way interaction. The output of the motors is monitored through a feedback system through the closed loop. Errors in position, velocity, and acceleration can also be corrected through the closed loop and in case the error is large then the fault system will be operative.
Diagram of an Open Loop system



Firing the engine from cold and the achievement of an rpm of 400 is the time when the system goes into 'open loop'. The signal from the Oxygen (O2) sensor will be ignored by the ECM in an 'open loop'. The air/fuel ratio will be calculated based on inputs from the coolant and MAF sensors, including a pre-programmed table in the memcal.
The system will stay in 'Open Loop' until the following conditions are met:
1. The voltage output of the O2 sensor is varied and it is tepid enough to operate properly.
2. The specified temperature of the coolant sensor has touched 40oC/104oF.
3. After starting the engine a specified amount of time has elapsed.
Temperature in relation to the aircraft must be understood in relation to the servo mechanism involved for air-conditioning in large aircraft and the power electronics on board. These require thermal regulation and directly affect the open loop. Thermal management in an aircraft is through a two way hot loop.
In an aircraft the open and closed loop help in steering the aircraft accurately. In the open loop system a controller is used to define the control inputs in order that the aircraft flies on a particular path. The controller is designed by past experience. However no allowance is made for external factors like wind speed and direction.
In order to steer the aircraft accurately an updated comparison between the actual and required direction is made. This comparison can be narrowed and reduced to zero till the actual direction is achieved. This is closed loop system. Such a closed loop system is the basis of all flight systems.
Different engines will have different specific values which are stored in the mem-cal. The system goes into 'Closed Loop' operation, once these conditions are satisfied. The ECM will calculate the air/fuel ratio (injector on-time) based on the various sensors but mainly the O2 sensor in a 'closed loop'. However closed loop systems are not cost effective and are also more complex. Closed loop controllers are harder to tune and the failure rate of their parts is higher.




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