Many control methods have been developed to provide high-performance operation of the machinery, and to prevent the shaft stress and oscillations [
3,
4,
5,
6]. A considerable part of them is based on well-known proportional-integral-derivative (PID) controller due to its simple implementation, intuitive tuning methods and limitation of the physical variables. The complexity of considered plant causes that several modifications of PID control scheme have been proposed. These are based on additional reference signal filtering, introduction of additional feedback or modification of the controller structure [
3,
4,
5,
6]. As it was shown in [
7], the state feedback speed controller provides satisfactory performance of the two-mass system. Recently, the SFC is getting significant attention by the researchers and it is applied in wide-range of applications, e.g., cascade-free permanent magnet synchronous motor (PMSM) speed control [
8], dynamic voltage restorer [
9], grid-connected inverter that operates under distorted grid [
10], synchronization of doubly-fed induction generators [
11]. It is caused by good disturbance compensation and cope with constraints [
8]. On the other hand, the main drawback of the SFC is still related to the tuning process. Due to the lack of cascade control structure all coefficients of SFC have to be selected simultaneously. It should be pointed out that simultaneous selection of coefficients seems to be non-trivial task, especially for complex control system. There are two the most commonly used tuning methods: pole-placement technique and linear-quadratic regulator optimization (LQR) [
12]. It is worth to point out that it is possible to force the system to have closed-loop poles at the desired locations, by choosing an appropriate gain matrix for state feedback controller. Then, the first approach requires to set closed-loop poles at desired location. The proper selection of poles location is mainly based on experience and expert knowledge in a field of control theory. Alternatively, to compute the state feedback control gain matrix in systematic way, the LQR can be used. This approach minimizes the performance index that takes into account the relative importance of respective state variables and the energy amount needed for control process. This importance is defined by user during selection of the weighting matrices
Q and
R. The initial guess of these matrices can be obtained with the help of Bryson’s method [
13]. In this approach, initial values are selected using maximum value of states variables, what in most practical cases can be easily defined. As it was mentioned earlier, the Bryson’s method provides just an initial guess, therefore it requires an additional manual tuning. It can be concluded that both considered methods use the trial-and-error approach. For this reason the commonly used tuning procedures are very time-consuming and/or require an expert knowledge. Due to this researchers have taken into account the automatic tuning process. In [
14], the auto-tuning of SFC for speed control of permanent-magnet synchronous hub motor (PMSHM) was proposed. In order to assure the optimal control, the Gray Wolf Optimization algorithm was employed to acquire the weighting matrices Q and R in linear-quadratic regulator optimization process. The authors proposed the complex fitness function with novel element, which is responsible for minimizing the overshoot of the step response. Comparison with different optimization algorithms, such as Particle Swarm Optimization and Genetic Algorithm, as well as with classical approach to speed control are also included. The obtained experimental results proved proper operation of proposed method. A similar approach to automatic selection of weighting matrices Q and R was proposed in [
15] and [
16]. The first article focuses on comparison of nature-inspired optimization algorithms, and three different fitness functions in speed control of PMSM, while the second one presents an auto-tuning method for state feedback voltage controller for DC-DC power converter. It is worth to point out that, the proposed approaches took into account the constraints of selected state and control variables of PMSM and DC-DC power converter, respectively. Both articles provide satisfactory step response thanks to SFC coefficients obtained from proposed tuning approach. The further research in this field is related to auto-tuning of SFC with constraints [
17]. In this paper, the comparison of two constraint-handling methods applied in automatic tuning of SFC is included. The authors compare simple parameter-less Deb’s rules with augmented Lagrangian method. It is worth to pointing out that parallel implementation of auto-tuning methods is also taken into account in the research [
18]. Due to commonly used in personal computers multi-core processors, parallelisation of time-consuming processes, e.g., constrained auto-tuning of SFC, is justified to reduce computation time. An application of new meta heuristic algorithm called Whale Optimization Algorithm for design of two-degree-of-freedom state feedback controller (2DOFSFC) for automatic generation control problem is presented in [
19]. The Integral of Time multiplied Squared Error (ITSE), Integral of Time multiplied Absolute Error (ITAE), Integral of Squared Error (ISE), and Integral of Absolute Error (IAE) are taken into account during construction of the performance index required for controller design. The proposed automatic tuning method of 2DOFSFC provides stable and satisfactory operation of the interconnected power system, and better dynamic performances in terms of settling time, overshoot, and undershoot values in comparison to PI, PID and 2DOFPID controllers. In [
20], design of state feedback and state feedback plus integral controllers for rotary inverted pendulum system have been presented. The optimal values of the state feedback controller were obtained by Particle Swarm Optimization algorithm. It was shown that optimized coefficients provide a better robustness and time response specifications in comparison to other controllers commonly applied for inverted pendulum system, i.e., fuzzy logic and fuzzy PD.
In this paper automatic tuning of SFC for two-mass system is presented. This paper includes entire process of auto-tuning procedure: (i) assumptions on desired step-response characteristics, (ii) definition of objective function based on the assumptions made and, (iii) application of nature-inspired optimization algorithm to optimize defined objective function. The ABC is applied to select optimal weighting matrices Q and R. The assumptions take into account practical aspects of the machinery operation i.e., the stress-free and chattering-free operation of the two-mass system. To present advantages of the proposed approach, comparison with analytically calculated coefficients is included. Additionally, the stability analysis for the control system, optimized using the ABC algorithm, is presented. Finally, robustness of the proposed control scheme is also investigated on the laboratory stand with elastic joint and variable moment of inertia.