Electronic Flow Control Valve (EFCV) with Pressure Compensation Capability
- Saturday, 01 October 2011
Flow control is one of the most critical functions in the hydraulic industry. Traditionally, flow control is implemented via a proportional or servo valve. When current is applied into the coil of a solenoid (proportional valve) or a torque motor (servo valve), a corresponding electromagnetic force is generated. These forces could either directly stroke the spool (single-stage configuration) or indirectly move the main stage spool via regulating the hydraulic pressures on each end of the main stage spool (multiple-stage configuration).
The motion of the main stage spool leads to the variation of the orifice area. With a given pressure drop, the orifice area is directly associated with the flow rate. Most proportional and servo valves on the market are incapable of providing accurate flow rate control without feedback from the power elements, or without the addition of mechanical pressure compensators. For example, consider a double-ended hydraulic cylinder with the piston area equal to 1 [unit]. If the required speed is 1 [unit], then the required flow rate is actually 1 [unit]. Without knowing the displacement/speed information from the hydraulic cylinder, neither the servo valve nor the proportional valve can correctly provide the desired flow. The reason for this is because the flow rate is related not only to the spool displacement (orifice area) but also to the pressure drop across the orifice. Therefore, feedback from the power elements is often required to achieve accurate flow control.
In real-world applications, the sensors in the power elements are not often available or are too costly to implement. However, accurate flow control is still required for several applications. For example, in a mobile excavator application, the operators are in the loop controlling the motion of machine. The operators use a human-machine interface device (like a joystick) to send the flow command to each individual cylinder. By controlling the angle of the joystick on each axis, one may also control the speed of multiple cylinders. Despite the variance in supply pressure of the system and the changing loads on the power elements, it is preferred that a joystick angle provides a corresponding velocity of the cylinder/motor consistently.
The traditional solution for this problem is to regulate the pressure drop across the metering orifice to be constant, so that the flow rate is essentially only dependent on the orifice area. This is the principle of a pressure-compensated flow control valve. Figure 1 illustrates a typical restrictor-type pressure-compensated flow control valve. The compensator spool has to be added to implement pressure regulation functionality. This methodology adds additional cost and complexity to the system.
The Electronic Flow Control Valve (EFCV) is an innovative flow control valve with pressure-compensation capability. The EFCV distinguishes itself from other traditional flow control valves because of its embedded sensors and microcontroller that have been integrated into the valve. These integrated components make the EFCV “smart” so it can achieve flow control without the need for feedback from the power elements, or the addition of a complicated mechanical system to regulate the pressure drop across the metering orifice. The Electronic Pressure Compensated Flow Control Valve is more cost-effective and scalable compared to its mechanical counterparts.
Figure 2 illustrates the system design of the EFCV. A supervisory controller, which is implemented by an ECU, processes the input from the human as the set point for the flow rate. An EFCV, as well as a Pressure Regulation Valve (PRV), are connected to the supervisory controller via CAN communication. The embedded sensors in the PRV can provide the value of the supply pressure (Ps), and the tank pressure (Pt). The PRV can also control the supply pressure according to the load requirement. The embedded sensors in the EFCV include the LVDTs, which measure the main stage spool displacement, and the pressure sensors, which measure the port pressures P1 and P2. The ports P1 and P2 are connected with hydraulic power elements. It is worth mentioning that the system is designed so that multiple EFCVs may be connected with multiple power elements. Finally, the entire valve stack, including PRV and EFCVs, is connected to the hydraulic source (pump and a relief valve).