Understanding proportional valve designs and uses

Understanding proportional valve designs and uses

By Josh Cosford, Contributing Editor

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Lever-operated directional valves excel at controlling the movement of cylinders or motors, often outperforming electric solenoid valves. While both types can be cost-effective, lever valves typically offer higher flow rates. This is because pressure and flow forces against the spool can prevent solenoid valves from shifting effectively. As a result, industrial stack valves are limited to size D05 by flow forces and require pilot operation for flows exceeding 30 gpm (about 120 lpm).

However, even the flow forces that hinder electrically shifted valves are easily managed with manual effort. A lever valve designed for 80 gpm (300 lpm) requires a large handle but comes at a high cost, comparable to pilot-operated valves with similar capabilities.

Even larger valves, such as the D08 pilot-operated solenoid valve, can handle up to 160 gpm (600 lpm) or more. Still, they can't match the flexibility of a lever valve, which can achieve partial openings. Solenoid valves shift between discrete positions, while lever valves provide continuous control from minimal flow to full flow. If managing multiple lever valves with operators is impractical, you might consider an electrical alternative: proportional valves.

The Benefits of Proportional Valves

Proportional valves often resemble their traditional spool valve counterparts, such as cartridge or CETOP valves. The key difference lies in the spool and coil. In standard solenoid valves, the spool allows little or no partial flow; once shifted, full flow is available. In contrast, proportional valve spools feature metering notches, enabling throttled flow even with minimal valve shifts.

Proportional valve coils convert incoming power signals into variable magnetic fields to adjust the spool's position and flow rate. Pulse Width Modulation (PWM) signals created by the electronic valve controller maintain constant voltage but vary signal duration. By adjusting the pulse width, the valve controller regulates current to the valve, controlling magnetic field strength and metered flow.

Types of Proportional Valves

Proportional valves vary widely in performance, similar to hydraulic pumps. Basic "dumb" proportional valves accept simple PWM signals with limited accuracy, acting much like a machine operator moving a lever without attention to precise positioning. However, these valves still offer variable flow where standard solenoid valves cannot. Basic valves can provide variable flow from just past their dead band* to fully open, with programmable opening speeds to avoid abrupt starts or stops. Adjustable ramp rates often justify the cost, especially for applications requiring smooth operations, such as boom lifts.

As electronics advanced, valves evolved to utilize sophisticated controls. Early proportional valves used separate driver cards mounted in electrical cabinets. Compact electrical circuits installed in the valve's wiring box now offer economical solutions, eliminating the need for separate cards.

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Modern Valve Control

Directly installing valve drivers on proportional valves offers advantages beyond cost and simplicity. Known performance parameters reduce the number of control options needed. Dither frequency, dither amplitude, dead band, and input mode are factory-programmed for optimal valve performance.

For applications requiring multiple proportional valves, a separate driver suitable for multiple outputs might be beneficial. Dedicated drivers must accommodate proportional valves from various manufacturers, with adjustable parameters for dither, dead band, ramp time, and more. Universal drivers should also support common analog inputs, such as 0-5 VDC, 0-10 VDC, and 4-20 mA, with 4-20 mA preferred for its natural resistance to interference over long distances.

Modern valve controllers no longer rely on rotary potentiometers for adjustments. Instead, wired CAN or wireless Bluetooth inputs drive valves, allowing performance parameter adjustments via smartphone or desktop apps. Industry 4.0 concepts like diagnostics and data logging enhance productivity and troubleshooting. For instance, immediate identification of a degraded analog input signal can quickly pinpoint operational issues, saving hours of traditional diagnostics.

Enhancing Valve Performance

High-performance valves also feature advanced options. An intelligent controller with a basic valve won't match the accuracy or response time of higher-end valves. To improve performance, manufacturers have added spool position feedback, measuring and correcting the spool’s position to match the target.

The simplest spool position feedback method uses an inductive sensor to measure the spool’s position, compensating for any inaccuracies due to flow forces, contamination, or imprecision. More advanced methods use Linear Variable Differential Transformers (LVDTs) for highly accurate position sensing. LVDTs enable proportional valves to respond quickly and accurately to target positions.

Larger proportional valves are inherently less accurate due to the challenge of controlling larger spools' inertia. However, top-tier D08 valves use a pilot valve with LVDT to control pilot flow accurately, along with an LVDT on the main-stage spool for fine control. A PID control circuit adjusts the main-stage spool position as needed to match input values.

High-precision proportional valves find applications in areas ranging from flight simulators (small valves) to injection molding (large valves). Despite their higher cost, these valves will play a critical role in the Industry 4.0 era of automation, flexible manufacturing, and big data, solidifying hydraulics as a primary control method for powerful machinery.

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* Static friction or "stiction" refers to the tendency of two surfaces to stick despite applied force. In proportional valves, stiction can cause the valve to overshoot or not move if the command signal is too small. Dither is a feature that vibrates the spool to prevent static positions. The movement is imperceptible to flow but prevents the valve from getting stuck.