As a continuation of the previous research in [
1], this paper addresses the utilization of alternative rotational power sources in Non-road Mobile Machinery (NRMM), such as an Electric Motor (EM) for the powering of the working hydraulics instead of an Internal Combustion Engine (ICE). Electrification and hybridization of the working hydraulics is a means to support the decarbonization of NRMM. In terms of the latest trends, working hydraulics can be divided into two categories: valve- and pump-controlled systems [
2]. Valve-controlled systems have a large variety of structures such as conventional load-sensing, digital hydraulics, independent metering, and common pressure rail. Load-sensing systems are characterized by quite hard falls, such as pump instability and pump-load instability; however, there are several solutions, such as electronic controllers, that can offer improvements [
3] over the conventional low efficiency of 21% [
4]. Digital hydraulic systems require a high number of valves, which allows a binary representation to adjust the flow according to the needs and to significantly reduce throttling losses [
5]. A common pressure rail allows for an increase in efficiency in comparison with load-sensing systems; however, huge metering losses and an inability to recover energy are regarded as a disadvantage from a long-run perspective [
6]. Independent metering also demonstrates less energy consumption than load sensing systems but requires constant system status monitoring [
7]. As an alternative to valve-controlled systems, pump-controlled actuators can be utilized to improve system efficiency. Pump-controlled systems can be categorized as variable speed, variable displacement, or a combination of both. A system that combines speed and displacement controls are highly challenging, due to the complex control structure of such a system with predictive observers [
8]. Examples of single pump and double pump solutions for variable-displacement and speed pump-controlled systems can be found in [
2,
9], respectively. Both of these pump solutions benefit an improvement in efficiency compared to conventional valve-controlled solutions. Most pump-controlled systems require the flow compensation approaches to balance the flow in a single-rod cylinder, which is the most common actuator in NRMM due to limited space requirements. An overview of the flow compensation valve-based solutions is detailed in [
10]. Alternative solutions for flow compensation can be a double pump [
11], an additional charging pump [
12], or the utilization of the hydraulic transformer [
13,
14]. Commonly, these pump-controlled systems utilize EM as a prime mover. While the concept of combining hydraulics with an electric motor is not new, the functioning of electro-hydraulics is often underestimated and can be improved through multidisciplinary domains, as the impact of the electric drive and its control is usually neglected. As an example of a pump-controlled actuator, the authors evaluated the behavior of an electro-hydraulic system only in hydraulic terms [
15]. Utilization of the speed signal instead of a complete mathematical model of EM is common and presented in [
16]. However, the influence of the electric drive on the hydraulic side of the system may be significant. Control of the electric drive (e.g., speed or torque) impacts the flow of the pump in speed-variable pump-controlled systems [
17,
18]. The main parts of the electric drive, which have an influence on operation characteristics, are the EM and Frequency Converter (FC). Consequently, the parameters of the electric drive, such as the quality of feeding voltages and currents, have a direct impact on the behavior of the shaft of the motor (e.g., torque ripples, vibrations, and speed of reaction) and the system as a result [
19]. Moreover, even voltages and currents have a direct relation with FC factors, such as the type of control system, number of FC levels, strategy of FC control, and frequency of Pulse Width Modulation (PWM). Thus, this study aims to evaluate the influence of hydraulics on electric drive operational characteristics in pump-controlled actuators on an example of a speed-variable system. As a part of this study, a stability analysis is performed to investigate the behavior of the system and the factors that affect it. In addition, a step response analysis demonstrated the main system stability characteristics, such as rise time, overshoot, vibrancy, transient process time, and steady-state values. The set of PWM frequencies was compared and the behavior of the system under the different FC frequencies was demonstrated to evaluate the influence of hydraulics on an electric drive. The accuracy of the simulated model was proven with validation in a laboratory test rig. The analysis demonstrated that the system has a stable behavior, and all transient processes are within the required limitations. Furthermore, a PWM analysis under hydraulic load demonstrated that the dependency of the behavior of the system on the PWM frequency is different compared to other types of load. Increasing the PWM frequency improves the accuracy of characteristics and the stability of the system due to a decrease in the noise produced by the frequency converter. This study demonstrated that increasing the PWM frequency by some value decreases the stability. This new phenomenon needs to be observed and analyzed further. The remainder of the paper is divided into five sections.
Section 2 presents the system description with the principles of operation of the selected variable-speed pump-controlled actuator. This section is divided into two subsections: the selected test rig description and modeling.
Section 3 contains the validation of the model.
Section 4 describes an analysis of the simulation results.
Section 5 and
Section 6 discuss the implications of these results and outline areas for future research.