By Darren Magner • Director of Marketing & Product Development • Comatrol
Effective use of logic elements is key to designing cost-effective circuits, and is limited only by the imagination of the designer. Logic elements, often called differential sensing valves, are pressure control devices. Like directional control valves, a spring bias holds the spool in one position (open or closed), and it is shifted by hydraulic pressure. Unlike directional control valves, logic elements are modulating devices (not on/off) that maintain a pressure differential. By themselves, logic elements perform no function, but are building blocks for many circuits.
Compensation and unloading
Adding a logic element upstream to a proportional flow control or proportional directional valve, and a shuttle valve downstream, can provide a simple load-independent, compensated flow control to a cylinder or motor. The red line (with compensation) on Figure 1 shows how the flow does not change with the increase in pressure or load. Without compensation, the flow would decrease as the load on the actuator increased, thus slowing down the cylinder or motor and reducing vehicle performance. The benefit is repeatable, precise proportional performance—regardless of the load on the actuator.
Unloading is another key feature of these logic elements. When using a fixed gear pump, it is always a good practice to unload the flow from the circuit when there is no actuation, thus minimizing power loss through the HIC.
“A logic element will not replace a load sense (LS) pump, but a fixed displacement pump in combination with a bypass logic element can cover most of the benefits of an LS pump, with a lower cost,” said Mark Mahony, product application engineer for Comatrol. “As fuel prices go up, customers cannot afford to have un-optimized systems. Using logic element technology allows our customers to optimize efficiency and precision on the high running functions, and optimize cost on functions used less frequently.”
Creating a Big Relief
Proportional pressure relief valves (PRVs) are 2-way valves that provide a relief pressure as a function of electric current. Both normally-open (increasing pressure with increasing current) and normally-closed (decreasing pressure with increasing current) are available. Normally-closed designs are more common for the growing use of proportional fan speed control.
Normally-closed proportional relief valves are available in direct-acting and pilot operated designs. A direct-acting, normally-closed proportional relief valve can be used for low flow applications, such as when piloting a logic element. For high flow applications up to 180 lpm, internally pilot-operated cartridges are more suitable. If the flow requirements (or even pressure requirements) exceed that of the internally pilot-operated valves, then combining the direct acting PRV with the proper logic element is the next sensible step.
Common applications for normally-closed proportional relief valves are electro-proportional control of system relief pressure or a remote pressure compensator control for open circuit piston pumps, but where system requirements dictate full pressure with no electrical signal. Most fan drive systems require the fail-safe mode (no electrical current) to provide full fan speed, which makes normally closed PRVs ideal.
The two schematics shown in Figure 3 illustrate how these devices work in a system—they are proportional directional control circuits, both with compensation and unloading functionality, and a relief valve for over-pressure protection. The left circuit uses a HLE10-CPC as an unloading compensator, managing both functions in one valve—a simple, cost-effective solution. This circuit works well when operating one actuator at a time.
The right schematic uses Comatrol’s HLE10-OPO as a pre-compensator for each of the actuators, and the HLE10-CPC as the unloading valve for the complete circuit. This is a common solution that provides load-independent, proportional directional control of multiple motors and/or cylinders on a vehicle. The example shown has two actuators, but more can be added as long as there is enough flow provided by the gear pump for all the functions.
The HLE10-CPC is a high-pressure logic element (350 bar / 5075 psi), normally-closed, pilot-to-close, and features a rated flow of 80 lpm (21.1 gpm). The HLE10-OPO is a high-pressure logic element (350 bar / 5075 psi), normally-open, pilot-to-open, and offers a flow rating of 60 lpm (15.8 gpm). Comatrol’s PSV12-34 is a proportional, non-compensated, 3-position, 4-way, directional flow control solenoid valve, with closed- or float-center spools. This valve is designed for directional control of hydraulic cylinders and motors, with a flow rating up to 60 lpm (15.8 gpm) and pressures to 260 bar (3770 psi).
The circuit in Figure 4 shows three different solutions to provide proportional fan speed control to a fan motor. The first two combine a spool type, normally closed, vent-to-open logic element with a direct acting electro-proportional relief valve to create a proportional relief function—selecting the logic element size and pressure to match the system requirements. The third schematic shows the single valve advantage of the pilot-operated (smaller size and less space needed), but has flow and pressure (180 lpm or 48 gpm and 210 bar or 3045 psi) limitations.
The first schematic is the high flow example. If the system requires up to 380 lpm (100 lpm) and 210 bar (3045 psi) maximum pressure, then select the proper logic element to match the capacity, like CP703-2 in combination with a PRV08-DAC. For higher pressure requirements (middle schematic), select the HLE10-CVO with the HPRV08-DAC—both are capable of 350 bar and will provide flow up to 100 lpm.
The third schematic is for flows up to 180 lpm (48 gpm). In this situation, using a single cartridge valve—such as pilot operated proportional relief valves PRV10-POC or PRV12-POC—is best when pressures of 210 bar or less are needed.
By using logic element technology, hydraulic system designers can boost the flow and pressure capabilities of proportional relief valve circuits.