## 1 Introduction
[9] studied the effect of the bypass valve in a conventional lifting system with a vertical servohydraulic cylinder and experimentally compared the improvement in energy efficiency. The study found that the energy efficiency is higher at low pressure and gradually decreases when the operating pressure becomes high. The energy efficiency is inversely proportional to the operating pressure for the larger load. When the weight of the cylinder load is doubled, the energy efficiency also increases. Maximum energy efficiency can be achieved by keeping the pressure as low as possible to overcome gravity, and the required speed can be achieved by maintaining the maximum load on the cylinder. Studies have also been conducted in the industrial sector to increase the energy efficiency of hydraulic injection molding machines (HIMMs). Clamping force control (FC) requires a high pressure but a low flow rate, so only a small amount of energy is needed. Widely used hydraulic valve-controlled cylinder systems, in which the maximum supply pressure is set with a relief valve and constant-displacement pumps, offer good servo control performance but poor energy efficiency. To increase energy efficiency, energy-saving control methods based on an electro-hydraulic variable-displacement pump system (EHVDPS), such as load-sensing control (LSC) and constant supply pressure control (CSPC) are used. From the studies of [PERSON] et al. [24], it emerged that the energy consumptions of (FC + LSC) and (FC + CSP) are 17.9 and 67.4% of the input energy of the EHVDPS method, respectively. Table 1 summarizes solutions presented in the literature for energy saving in industrial hydraulic applications. \begin{table}
\begin{tabular}{c c c c c c} \hline
**Ref.** & **Method** & **Field** & **Pump** & **Motor** & **Energy Saving** \\ \hline \multirow{3}{*}{[11]} & Control of an electro-hydraulic servo system, with two proportional directional valves, and four switch valves & Industrial & Fixed displacement & Servo motor & Not declared \\ \cline{2-6}  & Low–high pressure system with accumulator & Industrial (fine blanking press) & Fixed displacement & Fixed speed & 50\% \\ \hline \multirow{3}{*}{[13]} & Multiple motor pumps—system control & Industrial (hydraulic press) & Variable displacement & Fixed speed & 26.97\% \\ \cline{2-6}  & Direct-driven hydraulic system & Industrial (hydraulic press) & Fixed displacement & Servo motor & 53–87\% \\ \hline \multirow{3}{*}{[15]} & Two presses combination & Industrial (hydraulic forming presses) & Variable displacement & Fixed speed & 36\% \\ \cline{2-6}  & & & & & \\ \end{tabular}
\end{table} 
Table 1: Energy savings obtained in the literature with different methods. The high power-to-weight ratio and increasing miniaturization of components make hydraulics increasingly suitable for applications in robotics: in exoskeletons [25; 26], in humanoid, bipedal, or quadrupedal robots [27; 28], and in industrial and collaborative robots [29]. Energy efficiency is fundamental in these systems because their autonomy must be maximized. There are numerous studies in the literature that address the energy efficiency of these systems and propose innovative implementation solutions [30; 31; 32]. System power consumption can also be an indicator of operating conditions. In particular, application-independent variations in power consumption can be associated with hydraulic system malfunctions [33; 34; 35; 36; 37; 38]. Condition monitoring techniques based on artificial intelligence algorithms can be used to prevent system malfunction. The following is an overview of the reviews available in the literature on hydraulic systems and their energy efficiency. Some reviews on the energy efficiency of hydraulic systems energy can be found in the literature. [PERSON] et [PERSON] [6] provide an overview of energy-saving approaches for hydraulic drive systems.