Figure 1 demonstrates the block diagram of the position controller. It employs a cascade P-PID controller for both longitudinal and lateral control channels, along with a PID controller for height control.

In-depth, in the longitudinal control, the reference x position is determined by user input, and it is subtracted from the quadrotor's measured x position (x _Measured) from the sensor to calculate the x position error (x_e). This error is then passed to a proportional (P) controller. The output of the x position controller is the reference longitudinal velocity, denoted as u_ref. Utilizing the measured longitudinal velocity (u_Measured), the velocity error (v_e) is processed through a LimPID controller, generating the reference pitch angle (θ_ref) for input into the attitude controller. This LimPID controller generates the reference pitch angle, which is then fed into the attitude controller.

The same approach is applied to the lateral channel control for controlling the quadrotor's lateral position.

The quadrotor's vertical position (z) is regulated by a LimPID controller, which produces the throttle increment command for altitude adjustment. The base thrust, which represents the minimum required thrust to overcome the weight of the quadrotor, should be added to the thrust increment.This net command is then sent to Mixer, where the allocation of forces and moments takes place, enabling the generation of the appropriate motion.

When comparing this controller to the PositionAttitude controller, the structure of this controller is more accurate because it possesses greater information regarding the quadrotor, including velocity feedback. However, it is less stable, particularly when subjected to rapid changes in input.

LimPID

The LimPID controller can be finely adjusted by modifying its proportional gain, integrator, derivative time constants and saturation limit. The saturation limit can be configured within the constraints defined by yMax and yMin, taking into account model limitations such as actuator deflection limits or motor RPM. Once the controller type in LimPID is selected, a combination of proportional, derivative and integral controllers can be configured to meet the desired control objectives.

Note: The controller gains should be tuned based on the specific quadrotor and mission requirements.