Vice President Engineering
Bosch Rexroth AG
Lohr am Main
A major challenge facing mobileequipment designers is dealing with a large number of actuators that require hydraulic flow. Machines frequently have up to 10 hydraulically powered functions, with three or more operating simultaneously. And often, due to space, weight, or cost reasons, a common pump supplies all the circuits.
Thus, as an operator actuates various subsystems, control valves must handle the demanding task of compensating for flow changes and disturbances. The goal is to ensure adequate flow to all actuators, even when operating several systems simultaneously, so there is no drop off in machine performance.
To ensure flow matches demand, hydraulics manufacturers fit control valves with pressure compensators that control flow rates or distribution. Two designs predominate. One, called load sensing (LS), uses an upstream pressure compensator. The other, termed flow sharing (LUVD, from the German term Lastdruck Unabhängige Durchfluss Verteilung, has a downstream pressure compensator.
In both, the pump operates as a hydraulic-mechanical (HM) pressureclosed-loop control that ensures supply pressure exceeds the highest load pressure by a fixed excess pressure Δp. Because supply pressure constantly adjusts to the highest load pressure, HM-LS and HM-LUVD controls save energy compared to open-center controls that divert some flow to the reservoir.
In addition, load-sensing technology has advanced to the point where even in demanding applications — such as wheeled excavators with complex, combined movements — machines can generally operate with one pump without compromising performance.
Despite the benefits of HM-LS and HM-LUVD controls, there is room for improvement in terms of system response and energy efficiency. To raise performance, engineers first need to examine how pumps supply control valves, so that the working hydraulics quickly provides highly stable flow, without appreciable interaction between subsystems, and with minimal energy losses.
Here is a closer look at some key parameters.
Energy efficiency. As stated above, in HM-LS and HM-LUVD systems the pump supply pressure exceeds the highest load pressure by a fixed excess pressure Δp. The excess pressure is set so the pump can transport oil to the valve across all flow resistances and under the most unfavorable conditions (such as cold oil or maximum flow rate).
However, under certain conditions, the predetermined excess pressure is too high and unnecessarily wastes energy. A better approach does not preset Δp. Instead, the system compensates for pressure losses between pump and valve independent of the maximum operating pressure.
Dynamic stability. LS pumps operate in a pressure-closed-loop control mode where the highest load pressure can change significantly, depending on operating conditions and the task at hand. And every time load pressure changes, a pressure signal via the LS line instructs the pump to adjust the flow and reestablish Δp.
Many factors affect this pressurecontrol loop, such as different oil temperatures as well as natural frequencies and damping levels of the operating equipment. Therefore, a fixed setting of the pump's control parameters must be a compromise across all operating conditions. Unfortunately, some operating conditions approach or exceed the stability limit of the closed-loop controls.
In HM-LUVD systems, control-valve pressure compensators interact with the pump's pressure controller via the LS line. This can increase the hydraulics' tendency to oscillate under certain operating conditions. A preferred method would supply oil according to flow demand.
Response behavior. Some machine functions require extremely fast response. That is, the working hydraulics must react quickly to operator commands at the joystick. HM-LS and HM-LUVD systems frequently satisfy these demands, but not always. That's because a sequence of operations must take place between command and response. Simplified, the list includes:
- Joystick generates pilot pressure.
- The valve activates and displaces.
- The highest-load-pressure signal travels through the LS line to the pump.
- Pump displaces and generates flow.
- Hydraulic pressure increases between pump and valve.
The time sequences and individual processes that take place after actuating the joystick show the pump can only respond after the valve spool moves and a load signal has been sent to the LS line. Improving response means at least a part of the sequential processes must run in parallel. Therefore, the pump and valve should react simultaneously to operator commands.
Electrohydraulic flow matching
EFM can improve machine hydraulics' efficiency, stability, and dynamic response. EFM systems replace the pressure-controlled pump in HM-LS and HM-LUVD circuits with an electrically controlled, swivel-angle-adjusted pump that supplies required flow at the same time the valve opens. It results in the following improvements:
- The proportional pump modulates flow, letting excess pressure between pump and valve be set independent of the system's maximum load pressure. In certain cases, EFM's excess pressure is lower than the predetermined Δp for LS and LUVD systems, which saves energy.
- The pump in an EFM system does not operate as a pressure controller, but as an electroproportional variable pump in an open control loop. Thus, the pump no longer responds to changes in load pressure and, instead, operates independently without interaction with the pressure compensators.
- The pump and valve are controlled almost synchronously. Therefore, EFM eliminates delays between joystick inputs and the LS signal arriving at the pump. This, in turn, improves system response, and it increases stability with respect to disturbance variables. That is, the working hydraulics is more agile and less susceptible to oscillations.
- Another benefit, particularly appealing for machine manufacturers and hydraulics providers alike, is that EFM can use well-established components (variable pumps with electrohydraulic load-sensing valves). Thus, development work is limited to functional interactions between components in specific applications.
EFM in practice
Here's a look at the behavior of EFM systems in two different applications. We reviewed a municipal shoulder mower that trims weeds and other vegetation along roadsides to evaluate energy consumption. The task is rather unique because the mower requires continuous and high flow rates at medium pressure, but obstacles frequently interrupt mowing. Then, the motorized drive must stop and the entire mower lifted over the obstruction.
Comparing HM-LS and EFM-LS systems shows the latter consumes about 5% less energy. Energy efficiency heavily depends on operating flows and pressures. But, in general, the lower the average hydraulic power (p × Q), the greater the relative energy advantages of EFM.
Another test compared the dynamic behavior of EFM-LS and HM-LS on a front-end loader filling a trailer. The accompanying graphics summarize measurements of the most important hydraulic and mechanical variables.
Clearly, the pump swivel angle, the generated load pressure, and loader hoist speed are significantly smoother without overshoots or vibrations for the EFM-LS compared to significantly rougher behavior for HM-LS.
In addition, comparing excess pressures again shows the energy advantages of EFM-LS, except when the pump completely swivels out or the system is undersupplied.
Current results with EFM solutions are quite encouraging. It simplifies the working hydraulics, improves stability with respect to disturbance variables, speeds response to command variables, and enhances energy efficiency — all while using proven electrical and hydraulic components.
Additional work in this area will focus on refining interactions between valves and pumps in all quasi-static and dynamic operating ranges. And verifying applications in which EFM offers an economically viable alternative to traditional load-sensing systems.