In order to simulate an open loop and a closed loop control system and to analyze the behavior of the vehicle for a given input and disturbance, we’ll need to model it mathematically. This is usually done after some test or measurements have been performed with the system subject to disturbances. This way the vehicle will keep constant speed throughout the hill.įeedforward control system rely on the calibration of the feedforward part. To prepare (compensate) for the additional opposing force, at the beginning of the hill, the driver will press more the accelerator pedal which will increase the torque of the engine (input). Based on experience the driver knows that the vehicle will decelerate. Imagine that, while driving, the driver observes a hill coming up in front. Image: Open loop (with feedforward) control systemįor our vehicle example, the feedforward can be performed by the driver. Knowing or measuring the amplitude and type of disturbance, the input signal can be modified in such a way that it compensate for the changing output. This control systems are called feedforward control systems. We can have an open loop control system which can adjust it’s input based on known disturbances. This way the output vehicle speed will be kept a the same constant value. In order to prevent the vehicle to accelerate to a higher speed than the set one, the engine torque will be decreased. Therefore the opposing forces will decrease. This way the vehicle speed is kept constant regardless of the disturbance. The cruise control function reads the current vehicle speed and increases the engine torque in order to compensate for the road gradient. The positive road gradient causes an additional opposing force on the vehicle which should cause vehicle slowdown. The engine torque is set to a constant value which will maintain the desired set speed. The vehicle speed is kept constant by the cruise control function. Image: Example of closed loop control system (vehicle cruise control) It’s called a feedback control system because the output is fed back to the input, which uses it in such a way that the desired system’s behavior is maintained. This means that any disturbances affecting the system will be compensated by the input. In a closed loop control system, the input is adjusted function of the output of the system. The control loop is open because there is no dependency between the input and the output. This can be seen as an open loop, where the driver didn’t adjusted the accelerator position in order to keep a constant vehicle speed. This happens because the additional opposing force, due to road gradient, becomes null and the engine torque is enough to accelerate back the vehicle.ĭue to the fact that the input (engine toque) wasn’t adjusted function of the vehicle speed or road gradient, the system’s response was affected. The engine torque being the same, the vehicle will accelerate to the same vehicle speed as in zone A. To road gradient decreases at zero (flat road). In this case, the driver didn’t adjust the engine torque (input) function of vehicle speed drop (output), or road gradient (disturbance). Due to the road gradient, an additional opposing force will act on the vehicle, which will cause a slowdown. The driver doesn’t change the position of the accelerator pedal so the input torque will be the same. The road has a positive gradient which acts as a disturbance on the system. If the road is straight (no gradient) the vehicle speed will stabilize at a constant value. Let’s analyze what is happening with our system for different conditions of input and disturbance.įor a given position of the accelerator pedal, the engine will generate a certain amount of torque which will be applied to the wheels. Image: Example of open loop control system (vehicle driving) The road gradient (inclination) is considered to be the disturbance. The driver is adjusting the engine torque (input) by pressing the accelerator pedal. In this case we’ll consider that the input is the engine torque, the system is the vehicle itself and the output is the vehicle speed. The system is open loop because the input is not adjusted function of the output.Īnother example of open loop control systems is a vehicle driving on a road. For the same input (time/level) the output can be more or less “toasted” depending on several disturbances: consistency of the bread, geometry of the slice, outside temperature, etc. In this case, the system contains the electrical resistances of the toaster, the input is the level (time) of toasting and the output is the toasted bread. The output can be affected also by some disturbances which interact with the system.Īn example of an open loop control system is a toaster. The output is the system’s reaction for a given input. The input is the control action applied to the system.
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