Load sensing is a term used to describe a type of variable pump control used in open circuits. It is so called because the load-induced pressure downstream of an orifice is sensed and pump flow is adjusted to maintain a constant pressure drop (and therefore flow) across the orifice. The 'orifice' is usually a directional control valve with proportional flow characteristics, but a needle valve or even a fixed orifice can be employed, depending on the application.
In hydraulic systems that are subject to wide fluctuations in flow and pressure, load-sensing circuits can save substantial amounts of input power. This is illustrated in Exhibit 1. In hydraulic systems where all available flow (Q) is continuously converted to useful work, the amount of input power lost to heat is limited to inherent inefficiencies. In systems fitted with fixed displacement pumps where 100 percent of available flow is only required intermittently, the flow not required passes over the system relief valve and is converted to heat. This situation is compounded if the load-induced pressure (p) is less than the set relief pressure - resulting in additional power loss due to pressure drop across the metering orifice (control valve).
Exhibit 1. Flow-pressure-power diagrams for fixed, variable and
load sensing controlled hydraulic pumps
A similar situation occurs in systems fitted with pressure controlled (pressure compensated) variable pumps, when only a portion of available flow is required at less than maximum system pressure. Because this type of control regulates pump flow at the maximum pressure setting, power is lost to heat due to the potentially large pressure drop across the metering orifice.
A load sensing controlled variable pump largely eliminates these inefficiencies. The power lost to heat is limited to the relatively small pressure drop across the metering orifice, which is held constant across the system's operating pressure range (see bottom of Exhibit 1).
A load sensing circuit typically comprises a variable displacement pump, usually axial-piston design, fitted with a load sensing controller, and a directional control valve with an integral load-signal gallery (Exhibit 2). The load-signal gallery (LS, shown in red) is connected to the load-signal port (X) on the pump controller. The load-signal gallery in the directional control valve connects the A and B ports of each of the control valve sections through a series of shuttle valves. This ensures that the actuator with the highest load pressure is sensed and fed back to the pump control.
Exhibit 2. Typical load sensing circuit. Enlarge
To understand how the load-sensing pump and directional control valve function together in operation, consider a winch being driven through a manually actuated valve. The operator summons the winch by moving the spool in the directional valve 20% of its stroke. The winch drum turns at 5 RPM. For clarity, imagine the spool-adjusted metering notch in the directional valve is now a fixed orifice. Flow across an orifice decreases as the pressure drop across it decreases. As load on the winch increases, the load-induced pressure downstream of the orifice (directional valve) increases. This decreases the pressure drop across the orifice, which means flow across the orifice decreases and the winch slows down.
In a load sensing circuit the load-induced pressure downstream of the orifice (directional valve) is fed back to the pump control via the load-signal gallery in the directional control valve. The load-sensing controller responds to the increase in load pressure by increasing pump displacement (flow) slightly so that pressure upstream of the orifice increases by a corresponding amount. This keeps the pressure drop across the orifice (directional valve) constant, which keeps flow constant and in this case, winch speed constant. The value of the pressure drop or delta p maintained across the orifice (directional valve) is typically 10 to 30 Bar (145 to 435 PSI). When all spools are in the center or neutral position the load-signal port is vented to tank and the pump maintains 'standby' pressure equal to or slightly higher than the load sensing control's delta p setting.
Because the variable pump only produces the flow demanded by the actuators, load-sensing control is energy efficient (fewer losses to heat) and as demonstrated in the above example, improves actuator control. Load-sensing control also provides constant flow independent of pump shaft speed variations. If pump drive speed decreases, the load-sensing controller will increase displacement (flow) to maintain the set delta p across the directional control valve (orifice), until displacement is at maximum. To further your knowledge on load sensing and other variable pump controls, read pages 152 to 170 of Industrial Hydraulic Control.
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