Electrohydraulic valves are commonly categorized as either servo or proportional, which generally indicates their expected performance. However, this broad classification often oversimplifies and obscures the specific differences among the various electrohydraulic valves available on the market.

To select the appropriate Automotive Lift Repair Tampa Florida valve for a given application, it’s essential to understand the different types of electrohydraulic valves and their roles in controlling pressure or flow.

Traditionally, “servo valve” refers to valves that utilize closed-loop control systems. These valves monitor and feedback the position of the main-stage spool to a pilot stage or driver through either mechanical or electronic means. Conversely, proportional valves adjust the main-stage spool directly in response to a command signal, but typically lack automatic error correction (feedback) within the valve itself.

Confusion can arise when a valve appears to be a proportional valve but includes a spool position feedback sensor (such as an LVDT, or linear variable differential transformer) that enhances its performance to levels comparable to a servo valve. This highlights the importance for designers and suppliers to use clear terminology and focus on the specific performance needs of each application.

Automotive Lift Repair Tampa Florida Proportional valves generally use one or two proportional solenoids to move the spool against a set of balanced springs. The spool’s displacement is proportional to the current driving the solenoids, and the springs help to center the main-stage spool. The accuracy of the spool position relies on the symmetry of the springs and the design’s ability to minimize nonlinear effects such as spring hysteresis, friction, and machining tolerances.

 Automotive Lift Repair Tampa Florida Servo Valve Overview

The term “servo valve” often brings to mind mechanical feedback valves, where a feedback wire connects a torque motor to the main-stage spool. Displacement of the spool causes the wire to apply torque to the pilot-stage motor. The spool remains in position when the torque from the feedback wire’s deflection equals the torque from the electromagnetic field generated by the current through the motor coil. These valves typically feature a pilot stage or torque motor and a main or second stage, sometimes referred to as the power stage. They can be classified mainly into nozzle flapper and jet pipe types.

In nozzle flapper and jet pipe servo valves, the electromagnetic circuit is similar, but the hydraulic bridge design differs. The Automotive Lift Repair Tampa Florida hydraulic bridge regulates the pilot flow, which in turn controls the main-stage spool movement. In a nozzle flapper, the torque from the magnetic field moves the flapper toward one of two nozzles based on the command signal. This movement creates a pressure imbalance that drives the spool. In a jet pipe valve, the armature movement deflects the jet pipe, creating an asymmetrical flow between the spool ends through the jet receiver. This pressure imbalance persists until the feedback wire returns the jet pipe or flapper to a neutral position.

Historically, nozzle flapper and jet pipe servo valves have competed for applications requiring high dynamics. The nozzle flapper typically offers better response due to its first-stage dynamics, while the jet pipe’s design provides higher spool driving forces due to improved pressure recovery.

Both valve types require low command currents, resulting in a significant mechanical advantage. Motor current for these valves is usually less than 50.0 mA. 

Automotive Lift Repair Tampa Florida  Direct-Driven Electrohydraulic Valves

Unlike hydraulically piloted two-stage valves, direct-driven valves physically link the spool to the motor armature. These valves fall into two main categories: those driven by linear force motors (LFM) and those actuated by proportional solenoids. Within these categories, they can be further classified as proportional or servoproportional, depending on whether they include a position transducer for spool position feedback. Servoproportional valves use closed-loop feedback to improve repeatability and accuracy for high-control applications. Generally, servoproportional direct-driven valves exhibit lower dynamic response compared to hydraulically piloted two-stage valves with similar flow characteristics, due to the larger mass of the armature and the time constant associated with the coil’s induction and resistance.

Direct-driven valves maintain consistent performance regardless of supply pressure changes, making them suitable for applications where pilot flow for the first stage is unavailable. They are also less sensitive to oil viscosity compared to nozzle flapper and jet pipe valves, which perform best with oil viscosities below 6,000 SUS. However, direct-driven valves typically cannot generate the high spool driving forces of hydraulically piloted valves.

LFM-based direct-driven valves enable bidirectional movement by incorporating permanent magnets, making the armature sensitive to command polarity. The LFM must overcome spring force and external flow and friction forces during the outstroke. During the backstroke to the center position, the spring provides additional driving force, making the valve less susceptible to contamination. In contrast, proportional solenoids are unidirectional and require continuous current to hold the spool at midstroke, making them less energy-efficient than LFM or dual-solenoid designs. However, single-solenoid designs may cause uncontrolled load movements as the spool moves through an open position.

 Automotive Lift Repair Tampa Florida Multistage Valves

All the aforementioned designs can be used to create multistage hydraulic valves, with each approach tailored to specific application requirements. Most designs do not exceed three stages. A nozzle flapper, jet pipe, or direct-driven valve can be mounted on a larger main stage to meet most dynamics and flow requirements.

In some cases, a jet pipe valve may be configured with electronic feedback, replacing the mechanical feedback of a traditional jet pipe. This servojet configuration has pilot characteristics typical of a jet pipe. Depending on the control requirements, many multistage valves can close a position loop around the main stage using a linear variable differential transducer (LVDT) to monitor spool position. Springs on either side of the main-stage spool return it to a neutral position in the event of hydraulic power loss.

 Detailed Look at Automotive Lift Repair Tampa Florida Servo Valve Designs

Examining the internal components of a flapper-nozzle servo valve, the torque applied from a torque motor to the flapper causes it to move closer to one nozzle, increasing flow through it, while moving away from the other nozzle, decreasing flow. This results in a pressure increase at one end and a decrease at the other, driving the main valve spool in the direction of the higher pressure and facilitating fluid flow between different ports.