Slung Load Controller

Slung Load Controller

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Multirotor Unmanned Aerial Vehicles (MRUAV) have become an increasingly interesting area of study in the past decade, becoming tools that allow for positive changes in today’s world. Not having an on-board pilot means that the MRUAV must contain advanced on- board autonomous capabilities and operate with varying degrees of autonomy. One of the most common applications for this type of aircraft is the transport of goods. Such applications require low-altitude flights with hovering and vertical take-off and landing (VTOL) capabilities.

Similar as before in this project we use the AltaX Flight Stack which is compromised by a Raspberry Pi 3 as companion computer and a naze32 as flight controller.

The slung load controller and the machine learning estimator is running on the RPI3, although of course the training of the recurrent neural network was done offline in a big desktop computer. The RPI calculates the next vehicle position based on the estimation of the position of the slung load, everything is running using our framework DronePilot and guess what? its open source ;). Keep reading for more details.

If the transported load is outside the MRUAV fuselage, it is usually carried beneath the vehicle attached with cables or ropes, this is commonly referred to as an under-slung load. Flying with a suspended load can be a very challenging and sometimes hazardous task because the suspended load significantly alters the flight characteristics of the MRUAV. This prominent pendulous oscillatory movement affects the response in the frequency range of the attitude control of the vehicle. Therefore, a fundamental understanding of the dynamics of slung loads as they relate to the vehicle handling is essential to develop safer automatic pilots to ensure the use of MRUAV in transporting load is feasible. The dynamics of the slung load coupled to a MRUAV are investigated applying Machine Learning techniques.

The learning algorithm selected in this work is the Artificial Neural Network (ANN), a ML algorithm that is inspired by the structure and functional aspects of biological neural networks. Recurrent Neural Network (RNN) is a class of ANN that represents a very powerful system identification generic tool, integrating both large dynamic memory and highly adaptable computational capabilities.

Recurrent neural network diagram

In this post the problem of a MRUAV flying with a slung load (SL) is addressed. Real flight data from the MRUAV/SL system is used as the experience that will allow a computer software to understand the dynamics of the slung in order to propose a swing-free controller that will dampen the oscillations of the slung load when the MRUAV is following a desired flight trajectory.

This is achieved through a two-step approach: First a slung load estimator capable of estimating the relative position of the suspension system. This system was designed using a machine learning recurrent neural network approach. The final step is the development of a feedback cascade control system that can be put on an existing unmanned autonomous multirotor and makes it capable of performing manoeuvres with a slung load without inducing residual oscillations.

Proposed control strategy

The machine learning estimator was designed using a recurrent neural network structure which was then trained in a supervised learning approach using real flight data of the MRUAV/SL system. This data was gathered using a motion capture facility and a software framework (DronePilot) which was created during the development of this work.

Estimator inputs-outputs

After the slung load estimator was trained, it was verified in subsequent flights to ensure its adequate performance. The machine learning slung load position estimator shows good performance and robustness when non-linearity is significant and varying tasks are given in the flight regime.

Estimator verification

Consequently, a control system was created and tested with the objective to remove the oscillations (swing-free) generated by the slung load during or at the end of transport. The control technique was verified and tested experimentally.

The overall control concept is a classical tri-cascaded scheme where the slung load controller generates a position reference based on the current vehicle position and the estimated slung load position. The outer loop controller generates references (attitude pseudo- commands) to the inner loop controller (the flight controller).

Control scheme

The performance of the control scheme was evaluated through flight testing and it was found that the control scheme is capable of yielding a significant reduction in slung load swing over the equivalent flight without the controller scheme.

The next figures show the performance when the vehicle is tracking a figure-of-eight trajectory without control and with control.

The control scheme is able to reduce the control effort of the position control due to efficient damping of the slung load. Hence, less energy is consumed and the available flight time increases.

Regarding power management, flying a MRUAV with a load will reduce the flight times because of two main factors. The first one relates to adding extra weight to the vehicle, consequently the rotors must generate more thrust to keep the desired height of the trajectory controller, hence reducing the flight time. The second factor relates to aggressive oscillations of the load for this reason. The position controller demands faster adjustment to the attitude controller which increases accordingly the trust generated by the rotors. The proposed swing-free controller increases the time of flight of the MRUAV when carrying a load by 38% in comparison with the same flight without swing-free control. This is done by reducing the aggressive oscillations created by the load.

The proposed approach is an important step towards developing the next generation of unmanned autonomous multirotor vehicles. The methods presented in this post enables a quadrotor to perform flight manoeuvres while performing swing-free trajectory tracking.

Don’t forget to watch the video, it is super fun:

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