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A step beyond load sensing

Aug. 13, 2009
Advanced hydraulic controls speed machine response, eliminate oscillations, and save energy.

Designers of mobile equipment must deal with many actuators requiring hydraulic power. Machines may have a dozen 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 uses various circuits, control valves must compensate for flow changes and disturbances. The goal is to ensure adequate flow to all actuators, even when operating several systems simultaneously, to avoid reducing machine performance.

Load-sensing controls
To ensure that flow matches demand, system designers specify control valves with pressure compensators that control flow rates or distribution. Two designs predominate. One, load sensing (LS), uses an upstream pressure compensator. The other, flow sharing (LUDV, from the German term Lastdruck Unabhngige Durchfluss Verteilung), has a downstream pressure compensator.

In both, the pump operates as a hydromechanical (HM) closed-loop pressure control that ensures supply pressure exceeds the highest load pressure by a constant differential pressure (ΔP). Because supply pressure constantly adjusts to the highest load pressure, hydromechanical load sensing (HM-LS) and hydromechanical flow sharing (HM-LUDV) controls save energy compared to open-center controls that divert some flow to the reservoir. Plus, load-sensing technology has advanced to the point where machines can generally operate with one pump without compromising performance.

Flow management
Despite the benefits of hydromechanical load sensing and hydromechanical flow sharing 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.

Electrohydraulic flow matching circuit shows an electrohydraulically controlled proportional pump supplying flow at the same time as the valve opens, improving response over loadsensing and flow sharing circuits.

Energy efficiency — Again, pump supply pressure in HM-LS and HM-LUDV systems exceeds the highest load pressure by a fixed ΔP. The excess pressure is set so the pump can transport oil to the valve across all flow resistances under the worst conditions — such as cold oil or maximum flow rate.

However, the ΔP is too high under certain conditions and unnecessarily wastes energy. A better approach does not use a preset ΔP, but instead compensates for pressure losses between the pump and valve independent of the maximum operating pressure.

Dynamic stability — LS pumps operate in a closed-loop pressure control mode where the highest load pressure can change significantly — depending on operating conditions and the task at hand. Every time load pressure changes, a pressure signal in the LS line instructs the pump to adjust the flow and establish a new ΔP.

Many factors affect this pressure-control loop, such as changing oil temperatures and 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-LUDV systems, controlvalve pressure compensators interact with the pump’s pressure controller via the LS line. This can increase the hydraulic system’s 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:

1. Joystick generates pilot pressure.
2. The valve activates and displaces.
3. The highest load-pressure signal travels through the LS line to the pump.
4. Pump displaces and generates flow.
5. Hydraulic pressure increases between the 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
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-LUDV circuits with an electrically controlled, swivel-angleadjusted pump that supplies required flow at the same time the valve opens.

The following improvements result:

• 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 P is lower than the predetermined P for LS and LUDV 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 interacting 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 are more agile and less susceptible to oscillations.

• Another benefit is that EFM can use well-established components, such as variable pumps with electrohydraulic load-sensing valves. Thus, development work is limited to functional interactions between components in specific applications.

EFM in practice
To evaluate the energy consumption of EFM systems, we reviewed a municipal shoulder mower. The application is unusual because the mower requires continuous and high flow rates at medium pressure, but obstacles frequently interrupt mowing. Then, the entire drive must stop and be 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 energy savings of EFM.

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.

Christoph Latour isVice President of Engineering at Bosch Rexroth AG, Löhr am Main, Germany.

For more information, call Bosch Rexroth in the US at (330) 263-3317 or visit www.boschrexrothus.com.

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