# Automation of a Hybrid Control for Electrohydraulic Servo-Actuators with Residual Dynamics
###### Abstract  
In this paper, a hybrid position/force controller for electrohydraulic servo-actuators is designed in the presence of residual dynamics. The purpose is to apply a cycle of movement that is mostly used in industrial fatigue test applications, which consists in imposing a specified force on a flexible load after a movement to some position. This cycle can be repeated several times with different magnitudes and frequencies of force and position trajectories. Some damages could occur at switching times, especially in the presence of some residual dynamics. To avoid any damages at switching times, a force trajectory generator is designed. Then, our contribution defines a method to automatically generate the switching signal in order to commute between two controllers with any abrupt change of state. To show the effectiveness of the proposed approach, several simulation tests are carried out on the electrohydraulic system. hybrid control position/force; residual dynamics; trajectory generator; electrohydraulic system; simulation results +
Footnote †: journal: Applied  
gives some degree of decoupling between the position and effort controls. In Ref. [26], using the model-based control, different effort and position control schemes were developed. This suggested control loop was defined by a modified classical P + PID scheme, which has achieved good tracking performance. On the other hand, the effort and position control problem was treated separately, and the switching problem was then not considered. In Ref. [27], the authors presented the control of integrated electrohydraulic servo-drivers via conventional structures of PI and PD in order to separately control effort and position, respectively. The stability proof was not given. The parameters of these structures were calculated via indirect adaptive methods. These methods used identification tools and two control models with requirements for both effort and position tracking. Good performance was recorded for position control, but, for the effort controller, tracking remained efficient only for very low pulsation desired trajectories. Furthermore, the indirect adaptive schemas always have some issues with initial conditions. To overcome such issues and comparing the indirect adaptive method to the multiple PID controller approach, which is sensitive to its parameters, various solutions have been proposed in the literature [28]. In this last one, the authors proposed a multi-input multi-output (MIMO) controller based on fuzzy logic. The effort and position controls were independently tested and provided satisfactory results while consuming less energy. In Ref. [29], a hybrid position/force controller for joint robots based on a decoupling approach was presented. In fact, this approach used a smooth and invertible mapping linking the joint space to the task space in order to design the controllers separately. In Ref. [30], a literature review of the hydraulic control, specifically for manipulator robots, was detailed. Other than the separate hybrid position/effort control approaches, control based on the switching signal approach is considered in this paper. The challenge here consists in controlling the position as well as the second derivative of the position of the same cylinder. Some relevant research works can be cited in the literature. For example, in Ref. [31], the authors synthesized a hybrid position/force controller and applied it to an electro-neumatic system. In this research work, the important purpose was to move a load near a rigid structure and apply a force against it without damages in the process. The control algorithm presented in this paper concerns a linear state feedback strategy for tracking both position and force. In this study, the switching position/effort controllers are carried out only according to the desired position. It should be noted that, despite the satisfaction tracking performance, a large peak of effort and position remains present during the switch occurrences.