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).
The differentiating characteristic of the EFCV when compared to servo and proportional valves is the pressure feedback from the internal sensors. The pre ssure feedback, combined with the positional feedback from the main stage spool, allows the EFCV to control the flow rate based on the command from the supervisory controller. On the contrary, traditional servo and proportional valves do not have internal pressure feedback inside the valves themselves, and therefore cannot accurately control the flow rate across a large pressure drop spectrum.
Internal Model of the EFCV
An illustration of the pilot and mainstage spools in the EFCV is shown in Figure 3. It is important to note that for complete control of a typical power element, an additional pilot and mainstage spool is required. The embedded electronics perform the internal control, the communication between individual valves, and the communication to the supervisory controller. The EFCV has a two-stage configuration. The pilot stage, stroked by a linear force motor (voice coil), can control the pressures on the end chambers of the main stage spool while the main stage spool can then control the flow rate to the power element. In addition, the EFCV can be designed to have independent up - stream and downstream orifice control for the sake of energy savings and application flexibility.
The inverse flow mapping is used to convert the desired flow rate Q1,d to the desired spool displacement xv1,d. The error between the desired and the actual displacement is imposed on the PI controller, whose output will drive the current into the pilot valve voice coil. The controller drives the displacement error to zero. If the actual flow follows the equation above perfectly, then the flow error goes to zero as well, or Q1,c = Q1,d.
In the flow controller design, the above equation was used to approximate the actual flow rate. The equation is a widely acceptable quasi-steady-state model for the flow rate across an orifice. In the following section, an experimental study will be discussed that investigated the accuracy of the calculation.
A prototype of the EFCV was built and tested. For the hydraulic test setup, the valve was set on a fixed displacement test stand. An adjustable relief valve simulates a load on the valve. A flow meter in series with the “load” relief valve is used to measure the actual flow from that service. The type of flow meter used depends on the demanded flow in order to improve the measurement accuracy. For flows below 7 GPM, a 0.95 in3/rev meter motor was used; for flows above 7 GPM, a flow turbine was used.
Given a flow command from the supervisory controller, the “load” relief valve is adjusted so that the pressure drop between P1 and Ps is equal to some predefined value. Then the actual flow rate was measured.
Application of the EFCV: Self-Sensing Cylinder
An application for the EFCV is a selfsensing cylinder, which will estimate the piston displacement of a regular hydraulic cylinder by taking advantage of integrated sensors and flow rate calculation of the EFCV.
In the cylinder, two EFCVs connect each chamber, respectively. The sensors in the EFCVs measure the pressures, P1 and P2. The flow rates, Q1 and Q2, are calculated by using the analytical model with the experimental calibration. In addition, in order to eliminate the integration error, some deterministic displacement information is required (an absolute start position). For instance, a latch sensor could be installed so that the output voltage is high when the piston displacement xp = 0 and is low as xp ≠ 0.
Due to the combined effect of the flow rate and the load applied to the cylinder, the actual displacement is the sinusoidal signal with the higher frequency oscillation. At t=0.1 [sec], the latch sensor is enabled and then enforcing the error to be zero. Without the latch sensor, the observer will still give the similar displacement profile but with an offset error. Integrated pressure sensors and the experimentally calibrated flow rates in the EFCV, together with the proposed observer, make it possible to implement the self-sensing cylinder.
Due to the embedded sensors and microcontroller inside the valve, the flow rate can be controlled through the power elements without the need to know the load or the displacement condition from the power elements. The flow controller utilizes the well-known quasi-steady flow rate equation to approximate the actual flow rate in the internal closed-loop system.
Experimental studies show that the one equation model with constant parameters is not accurate enough to cover all conditions. In particular, for the low-flow-rate, high-pressure-drop case, the flow error calculation is significant. An experiment-based calibration method is then presented.
This article was written by QingHui Yuan, Chris Schottler, and Jae Y. Lew of Eaton Corporation, Eden Prairie, MN. For more information, visit http://info.hotims.com/ 34459-320.