## 7 Conclusions  
This article brought several new results concerning the design of control structures with reconstruction and compensation of constant input disturbances based on proportional-derivative-accelerative (PDA) controllers. It was shown how it is possible to create a simple series PIDA controller with a disturbance observer (DOB) for reconstruction and compensation of input disturbances consisting of a positive feedback in the form of a low-pass filter from the (limited) output of the PDA controller. Here, the disturbances are reconstructed by evaluating steady states. The importance of this interpretation of DOB was confirmed by comparing the settings of the original PDA controller and its DOB-based modification, both of which satisfy multiple real dominant pole (MRDP) conditions. The DOB functionality, based on the evaluation of steady states, results in a much longer low-pass filter time constant \(T_{i}\) than the closed-loop time constant of the stabilized loop. The article also pointed out the terminological misnomer of the proposed structure as a series PIDA controller, since it does not contain an explicit integral action. It was shown that the MRDP method provides PIDA tuning that results in setpoint and disturbance step responses with nearly ideal input and output shapes. However, the constrained series PIDA controller with the MRDP setting already results in transient responses with some distortions (overshoots) at the process input and output. Experimentally, the given controller proved to be equivalent to the MRDP-based parallel PIDA controller with anti-windup circuit using the conditioning technique. The simplest way to eliminate these swings is to factorize the transfer function of the controller, where we neglect the imaginary parts of the numerator zeros and choose the smallest of their real time constants as the integrating (automatic reset) time constant. A simplified version of this procedure has already been proved for the series PID controller [13]. Even with the optimal setting of the series PIDA controller derived using the performance portrait method, a significant improvement in control performance occurred. Controllers with automatic reset realized by filtered positive feedback from the stabilizing controller output still represent the basis of industrial automation. The equivalence of the transfer functions of the individual solutions in the proportional control band contributed to their subsequent interpretation among the integrating controllers. However, for the disturbance responses in Figure 7, or for the real-time responses in Figure 8, the control signal lies in the linear regions for only a small portion of the transients. Add to this the fact that the PIDA controller tuned by PPM ensures much better dynamics than the parallel PIDA controller modified by the conditioning technique, while its design and structure is also much simpler, and the question arises whether the subject of PID control design is not ripe for a thorough revision. As for the novelties of the present paper, besides the unique simple solution to the saturation control problems, it is worth highlighting the clear solution to the proposal of the implementation filters, which proved to be dominant in the recently published paper in terms of robust stability and the resulting dynamics of the circuit with PIDA controllers. In future research, we will focus on saturation control and robust stability evaluation when higher-order derivative terms are used. Writing--original draft preparation, M.H. and D.V. Simulations, M.H. Editing, D.V., M.H. and P.B. Project administration, M.H. and P.B. All authors have read and agreed to the published version of the manuscript. This research was supported in part by the following grants: research program P2-0001 financed by the Slovenian Research Agency; \"Developing a laboratory of mechatronics, based on smart technologies\" (No.