• Input Current (input signal). The effective current to the servovalve which produces a given output.
  • Rated Current (rated signal). The specified input current of either polarity to produce rated flow. Rated current must be specified for a particular coil connection (differential, series or parallel)and does not include null bias current.
  • Quiescent Current. A direct current of opposite polarity in each valve coil when using a differential coil connection.
  • Coil Impedance. The complex ratio of coil voltage to coil current at specified conditions which may vary with signal frequency, amplitude and other operating conditions.
  • Dither. A low amplitude, relatively high frequency periodic electrical signal, sometimes superimpose on the servovalve input to improve system resolution.
  • Coil Resistance. The d.c. coil resistance at a specified temperature.
  • Torque Motor. A type of electromechanical transducer, having rotary motion, which is used in the input stages of a servovalve.
  • Valve Polarity. The relationship between the direction of control flow and the direction of input current.


  • Supply Pressure. The pressure (frequently at nominally constant value) at the supply port of the servovalve.
  • Return Pressure (back pressure). The pressure at the return port of the servovalve.
  • Control Flow. The flow through the servovalve control ports to the load.
  • Rated Flow. The specified control flow corresponding to rated current at a specified servovalve pressure drop. Rated flow is normally specified as no-load flow.
  • Flow Gain. The mean slope of the control flow versus input signal curve in any specific operating region.
  • No-load Flow. The control flow with zero load pressure drop.
  • Internal Leakage. The total internal servovalve flow From pressure to return with zero control flow (usually measured with control ports blocked). Leakage flow will vary with input pressure and input signal.
  • Valve Pressure Drop. The sum of the differential pressures across the control orifices of the output stage. The valve pressure drop will equal the supply pressure minus the return pressure minus the load pressure drop.
  • Load Pressure Drop. The differential pressure between the control ports.


  • Flow Linearity. The deviation which exists between the normal flow curve and an idealized flow cure of slope equal to normal flow gain. Linearity is defined as the maximum deviation and expressed as a percentage of rated signal.
  • Symmetry. The degree of equality between the servovalve normal flow gain of one polarity and that of the reversed polarity. Symmetry is measured as the difference in normal flow gain of each polarity, expressed as a percentage of the greater.
  • Hysteresis. The difference in the valve input signals required to produce the same valve output during a single cycle of servovalve input signal when cycled at a rate below that at which dynamic effects are important; it is expressed as a percentage of the rated signal. Maximum hysteresis is normally indicated and is the maximum difference in input signal occurring in a cycle shown as a percentage of rated signal.
  • Resolution. The change of input signal required to Produce a change in valve output at a specified signal level, expressed as a percentage of rated signal.
  • Lap. In a sliding spool valve, the relative axial position relationship between the fixed and moveable flow metering edges with the spool at null. Lap is measured as the total separation at zero flow of straight line extensions of the nearly straight portions of the flow curve, drawn separately for each polarity. Expressed as percentage of rated current.
  • Overlap. A condition where the fixed and moveable flow metering edges do not coincide with the spool at null in such a way that a relative displacement between the metering edges has to occur before a flow path is created.
  • Underlap. A condition where the fixed and moveable flow metering edges do not coincide with the spool at null in such a way that a flow path exists across two or more metering edges with the spool at null.
  • Zero Lap. A condition where the fixed and moveable flow metering edges coincide with the spool at null, giving rise to constant flow gain over the null and operational regions.
  • Pressure Gain. The gain in load pressure drop per input signal with zero control flow (control ports blocked). Pressure gain is specified as the average slope of the curve of load pressure drop versus input signal in the defined region.
  • Null. This relates to geometric null which is the zero control flow at zero load pressure drop.
  • Null Bias. The input signal required to bring the servo valve to null, excluding the effects of valve hysteresis expressed as a percentage of the rated signal.
  • Null Leakage. Total internal leakage from the servo valve in the null (or quiescent) position.
  • Null Shift. A change in null bias expressed as a percentage of the rated signal. Null shift may occur with changes in supply pressure, temperature and other operating conditions.
  • Frequency Response. The complex ratio of control flow to signal as the current is varied sinusoidally over a range of frequencies. Frequency response is normally measured with constant input signal amplitude and zero load pressure drop, expressed as Amplitude ratio and phase lag.
  • Amplitude Ratio. The ratio of the control flow amplitude to sinusoidal input signal amplitude at a particular frequency, normalized by using a specified low frequency as datum at the same input signal.
  • Phase Lag. A measure of the instantaneous time by which the sinusoidal output follows the sinusoidal input signal at a specified frequency. This is usually expressed in degrees as the vector angle between input and output.
  • Transfer Function. A differential equation in Carson notation or using the Laplace operation, which uniquely describes the relationship of the control flow to the input signal at zero load.
  • Response Time. The time lapse between the initiation of an operation and its point of completion; the initiation point being a given value of the control (increasing or decreasing) and the completion point being a given value of the servovalve output.
  • Step Response. Typical transient response curves for standard valves. The straight line portion of the response represents saturation flow from the pilot stage which will increase with higher supply pressures.