In Section 2.1 it is stated that most of the power supplied to the hydraulic system is lost on the relief valve in order to maintain a constant pressure at the valve intake. It is also discussed that this lost should be minimized if the excess flow passing through the relief valve is reduced by means of regulating the flow rate delivered by the pump.
In order to decrease the power losses on the relief valve, pressure compensated variable displacement pumps are used. This system is also referred as the "demand flow system" because the pump supplies only the required flow rate to minimize the excess flow passing through the relief valve. The schematic diagram of this type of pump is shown in Figure 2-5.
Figure 2-5 Pressure Compensated Pump [23]
In this system, the pump is running at a constant speed; however, the flow rate is adjusted by adjusting the pump displacement. When the pump output pressure comes to its regulated pressure, the pump decreases its pump displacement and supplies right amount of flow only to maintain the pump output pressure.
When a flow is demanded by the load, it increases its displacement and supplies only the required rate of flow, without changing the pump output pressure. By this way, theoretically, the relief valve losses represented by the area 2 of the Figure 2-4 is eliminated totally, thus the new power losses of the system is only on the flow control valve and represented by the dashed area shown in Figure 2-5.
Another technique to increase the energy efficiency is to use load sensing pumps. Like the pressure compensated pump, the load sensing pump delivers only the required flow rate by the load but differently the pump output pressure changes according to the load pressure. In this system, not the valve supply pressure but the differential pressure across the valve is constant. The schematic diagram of load sensing pump is shown in Figure 2-6.
In this system, the load pressure is fedback to the pump compensator. The compensator control valve inside the pump adjusts the pump displacement to maintain a constant pressure drop across the flow control valve and in the mean time delivering the required flow rate. Because the valve supply pressure is not constant, but changes to maintain a constant pressure drop over the flow control
QL Ps
Var. Disp.
Pump Pressure
Compensator Control Valve
PL
PL Ps
QL
Useful Power
valve, the power loss on the flow control valve, which was represented by the area 3 in Figure 2-4, is reduced and represented by the dashed area in Figure 2-6.
Figure 2-6 Load Sensing Pump Schematic [23]
There are also electro-hydraulic load sensing systems where the pump output pressure and the flow rate delivered to the system are adjusted by changing the drive speed of a constant displacement pump. Figure 2-7 shows the circuit diagram of an electro-hydraulic load sensing system circuit diagram.
Figure 2-7 Electro-Hydraulic Load Sensing System with Constant Displacement Pump [8]
QL Ps
Var. Disp.
Pump Pressure
Compensator Control Valve
PL
PL Ps
QL
Useful Power
In Figure 2-7, the pump is driven by an AC asynchronous motor. The drive speed of the motor is controlled by a frequency converter according to the feedback pressure signals of the load pressure, pump output pressure, and the pump angular velocity [8,9].
Except for the relief valve, there occurs a considerable amount of power loss on the flow control valve itself. In recent years, a new valve technology is developed to reduce the power loss on the flow control valve, by mechanically decoupling the meter in meter out ports. The schematic diagram of the new valve control concept utilizing individual metering is shown in Figure 2-8. In the first circuit two 3/3 valves are used and in the second circuit four 2/2 valves are used.
Figure 2-8 Individual Meter In Meter Out Valve Control System [24]
In a 4-way valve, the meter-in port and the meter-out port are mechanically linked together, so that their resistances to flow are also dependent. But in an individual meter-in meter-out valve, all ports are independent giving a control flexibility to improve system efficiency by adjusting the port resistances independently. For example, while extending the hydraulic cylinder with an opposing resistive load, the valve resistance of the meter-in port is adjusted to satisfy the velocity and force requirements. However, the resistance of the meter-out port is adjusted only to deliver the flow back to the oil reservoir. This provides
a considerably energy saving as the power loss on the meter-out port will not be the same as the meter-in port but lesser.
The individual meter-in meter-out valve control concept is a developing research area; despite its complex control strategy it also allows energy regeneration and energy recuperation [24].
Note that in all three techniques discussed above, the final control element is the valve. Therefore, there is always a throttling loss to regulate the flow rate through the actuator. Of course, the most obvious way to get rid of throttling losses is not to use valves. In the next sections valveless hydraulic control systems are discussed.