# Control Methods Comparison for the Real Quadrotor on an Innovative Test Stand
## 1 Introduction
This approach was used to stabilize the quadrotor in [3; 5; 12; 18; 19; 20; 21]. A backstepping-based inverse optimal attitude controller (BIOAC) was derived in [3] taking into account the input saturation. In [12], command filters are used to avoid a difficult analytic computation of required command derivatives in each step. The double-integral observer was developed in [18] to design a control law based on the Lyapunov function to track a reference trajectory. A decoupling attitude parameterization was presented in [19]. It allows the design of an independent and straightforward position heading tracking control using the backstepping control technique. Some works try to overcome uncertainties (e.g., sensor noise, parametric uncertainties and external disturbance) by designing adaptive controllers [22; 23]. In another recent work [24] a flight controller based on a Neural Network model has been presented for stabilization and trajectory control. Using an AI-based controller seems promising. Several works using these control techniques can be found in [25; 26; 27]. Quaternion describing dynamics of a quadrotor instead of Euler angles is becoming very popular nowadays. A feedback signal in the form of a quaternion can be used to design linear and also non-linear attitude and position controllers [28; 29]. In [13], a cascade attitude controller was proposed. Both an inner and an outer control loop were formed by proportional controllers. The angular velocity and the quaternion were used as feedback signals. From this combination an attitude P2-controller arose. However, the final control law of the P2-controller corresponds to the standard PD controller designed in [2; 30]. The problem of disturbance rejection of the attitude subsystem of a quadrotor was addressed in [1]. An acceleration-based disturbance observer was applied to a quaternion-based integral sliding mode attitude controller. This combination shows a significant improvement of the performance in position control as well as the compensation of large external forces. Generally, many studies where a review of control techniques can be found. Such studies are addressed in [22; 23; 31]. Most of them are pointing out their advantages and disadvantages, but the experiments with the same UAV model and various control techniques are missing in the literature. Authors and scientists usually improve one technique and compare it with the technique from which the proposal originated. For example, authors in [32] improved backsteppingcontroller by own proposed method called robust and saturated backstepping controller (RAS-BSC). Consequently, objective information expressed in the exact control quality indicators is missing and various control techniques cannot be compared in the more objective manner. From this reason, our previous work [9] compared various quaternion-based control methods applied to quadrotor with disturbance observer and position estimator. The comparison was based on simulation results and the results brought qualitative new knowledge, because the objective quality indicators were chosen in order to compare the performance of all controllers. In total, nine control techniques (controllers) were compared. In this article we present a logical outcome of our work. Based on the results in [9], three of the most promising control techniques were chosen (PD, LQR and backstepping) and they were implemented and evaluated on the real quadrotor. However, from the reasons specified in the Section 2.2 a test stand suitable for the complex evaluation of these controllers was not available. We designed an innovative test stand, which is registered as a utility model no. 7766 in Slovak Republic [33]. The control techniques were evaluated by 15 quality indicators and as far as we know, such complex comparison of PD, LQR and backstepping control techniques were not published yet. Moreover, all these techniques are quaternion-based, which is also significant contribution of this paper. The article is structured as follows: Two test stands are presented in Section 2.