Proportional Valve

Overview

A proportional valve is an electro-hydraulic directional control valve that modulates flow and pressure in proportion to an electrical input signal (0–10 V analog or CAN message). Unlike on-off solenoid directional valves (step response), proportional valves enable smooth, infinitely variable speed control of cylinders and motors. The valve combines a precision Spool Assembly, dual Proportional Solenoid actuators, and closed-loop Proportional Valve LVDT Feedback position feedback with an onboard Proportional Valve Servo Amplifier servo amplifier. Application examples: smooth acceleration of cranes, variable-speed excavator arms, load-adaptive pump flow control.

How it works

An operator command (analog voltage or CAN) is sent to the servo amplifier. The microcontroller drives the proportional solenoid current proportionally to command, creating solenoid force that balances against a spring. Spool displacement varies continuously (0–20 mm full stroke). As the spool slides, its lands open and close the work ports (A and B), modulating flow area to the load. The LVDT core, mechanically coupled to the spool, continuously senses position; feedback voltage is compared to command, and the amplifier adjusts solenoid current via a closed-loop control loop (P, PI, or PID tuning). This feedback ensures precise spool position even under varying loads.

Spool design and land overlap

The Spool Assembly is a hardened steel cylinder with precisely ground lands (shoulders) that overlap or expose port openings. Center-open spools (lands at null expose tank return) allow pilot-operated cartridge loads; center-closed (lands block all ports) require pilot pressure to open. Spool stroke is typically 5–20 mm. Land surfaces are hard-chromed and honed to resist wear; surface finish < 0.2 µm Ra. Leakage past spools is 0.5–2 cc/min at rated pressure; external drain line routes this back to tank.

Solenoid and feedback architecture

Each Proportional Solenoid pair (one for each spool direction) pulls proportionally to applied current (0–2 A typical). A spring centered between them returns the spool to null when current is zero. The Proportional Valve LVDT Feedback (linear variable differential transformer) is a purely inductive sensor; its ferrite core is attached to the spool, and primary/secondary coils are fixed to the body. As core moves, the differential output voltage varies linearly from −10 V (spool full left) through 0 V (null) to +10 V (spool full right). This signal is demodulated and compared to the input command inside the servo amplifier's microcontroller.

Onboard servo amplifier

The Proportional Valve Servo Amplifier card contains a 24 VDC-powered microcontroller (Cortex-M4), LVDT signal conditioning, and a MOSFET gate driver pushing proportional current to the solenoids. Closed-loop control frequency is 10–50 Hz; tuning (proportional gain, integral time, derivative time) is done via configuration file or DIP switches. CAN interface allows networked control (ISO 11898 500 kbaud); analog input (4–20 mA or 0–10 V) is standard. Integrated dither (100–500 Hz 0.1 V AC superimposed on command) reduces static friction and stick-slip in the spool.

Pressure and load compensation

Proportional valves are often used with load-sensing pump controllers; a pressure compensator maintains pump outlet at a fixed margin (≈ 20 bar) above the highest work pressure. The Load-Check Cartridge (integrated cartridge) prevents runaway pressure if the load jams. Dual proportional valves in series enable independent metering (proportional control of inlet and outlet pressure independently), maximizing energy efficiency in load-lowering circuits.

Maintenance and diagnostics

Annual inspection includes electrical continuity check of solenoid coils, LVDT output verification (sweep command 0–100%, verify output 0–10 V), and flow/pressure mapping under a dummy load to confirm proportionality curve. Fluid cleanliness to ISO 16/14/11 is mandatory; contamination causes spool stiction (sluggish response) and accelerates seal wear. Seal replacement interval is 1500–3000 operating hours. Built-in CAN diagnostics report solenoid current draw, LVDT feedback value, and amplifier temperature for predictive maintenance.