• Rated Flow – Servovalves are rated at 1,000 psi drop, while Proportional Valves are rated at 150 psi valve drop.The flow under a no-load condition, QNL will vary with supply pressure.
• Frequency Response - Servo or Proportional Valve frequency response will vary with signal amplitude, supply pressure, and internal valve design parameters.The typical response varies with supply pressure as expressed by the change in frequency of the 90° phase point, as shown in figure 3. Note that Direct Drive Valve response is independent of system pressure.
• Step Response – Servo or Proportional Valve step response will vary with amplitude, supply pressure and internal valve design parameters. See individual series catalogs for specifications. Full amplitude step responses will normally exhibit a straight line portion which represents flow saturation of the pilot stage.The slope of this straight line portion will vary with the square root of the change in supply pressure.
• Flow–Load Characteristics – Control flow to the load will change with various combinations of load pressure drop and electrical input.
• Internal Leakage – There are two sources of internal leakage; first, flow through the hydraulic amplifier (known as “tare flow”) which is relatively constant, and second, flow around the spool which varies with its position. Maximum internal leakage occurs at null. See individual Servo and Servo-Proportional Valve catalogs for specifications.
• Spool Driving Forces – The maximum hydraulic force available to drive the second-stage spool will depend upon the supply pressure,multiplied by the end of the spool. In the case of Direct Drive Valves, spool driving force is created by the linear force motor and does not change with supply pressure.
• Pressure Gain – A measure of the change in control port pressures as the input current is varied about the zero flow point. Pressure gain is measured against a blocked load under no flow conditions. Normally the pressure gain exceeds 30% of the supply pressure for 1% change in rated current and can be as high as 100%.
• Null Bias – Input current to the valve required to adjust the output to zero flow. Most valves have mechanical adjustments which allow the null bias to be externally adjusted.