## 2 The Principle Analysis and Modeling for ALS-EHA
### The Load Sensing Principle Description  
Load-sensitive EHA includes two types: DLS-EHA and ALS-EHA.  
The DLS-EHA schematic is displayed in Figure 3. the load-sensitive servo mechanism uses a shuttle valve to guide the pressure of the high-pressure chamber of the EHA to the swash plate mechanism of the variable pump. Thus, the function of passively regulating the displacement with the load is realized. Adding a damping hole between the valve port and the variable mechanism can effectively alleviate the fluctuation of system performance caused by load shock.  
According to the principle of direct load sensitivity, it can be known that, when the external load is large, the pressure introduced into the pump variable mechanism increases, so the displacement of the pump decreases.  
According to the analysis in Section 2.2, it can be known that the motor current can be effectually alleviated in the stalled condition; in the non-stalled condition, the motor current can be validly subsided within a certain adjustment range. Therefore, the DLS-EHA can effectively reduce the copper loss of the motor within a certain operating range. However, because the pump displacement is passively adjusted with the load, the system stiffness is reduced, resulting in a significant reduction in system dynamic performance under heavy load conditions. the existing improvement method is to appropriately increase the spring stiffness of the variable displacement mechanism of the plunger pump, thereby which is in favor of increasing the system stiffness and reducing the gradient of displacement with load pressure, but will increase the loss of the motor at the same time. In addition, the view that the smaller the pump displacement is regulated, the less the heat is generated by the motor is still to be further verified: According to the analysis in Section 2.2, it can be known that the conclusion that the smaller the displacement is, the less heat the motor generates is a one-sided and inaccurate conclusion. Under medium/high speed and heavy load conditions, it may be happened that within the displacement adjustment range, the motor heat generation will first fall and then rise as the pump displacement drops. Under these conditions, the use of DLS-EHA may even outweigh the benefits.  
So as to deal with the direct discrepancy between dynamic performance and efficiency in DLS-EHA, a novel active load sensitive technology is proposed. the ALS-EHA is based on the DLS-EHA, and a pressure follow valve (PFV) that can actively adjust the pressure enforcing the variable pump swash plate is added. the schematic of the ALS-EHA is shown in Figure 4.  
The principle of the ALS-EHA is that the load pressure is imported to the PFV through the shuttle valve and then the valve outlet pressure can be controlled at range of 0 to the maximum load pressure by the external control signals, which is connected to the swashplate variable mechanism of the pump. It can be known that by controlling the PFV input current, the pump swashplate angle can be adjusted between the maximum and the value corresponding to the load pressure.  
Since the pump displacement can be actively controlled in the ALS-EHA, by designing a desirable control law, it can effectively avoid the problem that the decrement in displacement leads to the increment in motor heating and the decrease in dynamic. Meanwhile, the system needs better tracking performance and the external load is relatively large, the motor efficiency can be sacrificed to ensure the dynamic performance. Therefore, under the trade-off, the ALS-EHA is relatively superior in performance.  
Figure 3: DLS-EHA schematic.  
Compared with the conventional EHA-VPVM, the ALS-EHA need not have a separate power source, and can realize the active adaptive control on displacement through a PFV, which has the advantages of small size, light weight, and simple structure.