Terminology
RATED CURRENT
The input control current (exclusive of null bias current) of either polarity rated flow. Expressed in milliiamperes mA. Rated current is usually specified for individual, differential, or parallel coil connection. For a series coil connection, rated current is half the rated current mentioned above.
QUIESCENT CURRENT
The dc current passing through each valve coil in a differential coil connection with zero differential current.
OVERLOAD CURRENT
The maximum admitted passing through coils.The recommended overload current is twice the maximum current going through the coils.
COIL RESISTANCE
The DC resistance of torque motor expressed in ohms(Ω). The tolerances for coil resistance difference of the coil pairs in the same servo valve should be within 5% of the nominal specified resistance.
COIL INDUCTANC
The apparent induction component comprising the coil impedance, expressed om henrys(H). Coil inductance will vary with supply pressure, signal frequency amplitude, and other operating conditions. It is specified to measure coil inductance at 60 Hz, with a peak-to-peak current amplitude equal to 25% of rated current and under rated supply pressure.
Coil inductance is normally measured at series coil connection.
RATED FLOW
The specified control flow corresponding to rated current and given supply and load pressure conditions. Rated flow is normally specified as the no-load flow corresponding to the specified valve pressure drop equal to rated supply pressure and is expressed in liters/minute(L/min).
FLOW GAIN
The slope of the control flow versus input current curve, expressed in L/min-mA.
INTERNAL LEAKAGE
The total internal valve flow from supply to return with zero control flow, expressed in L/min. Leakage flow will vary with input control current, generally being a maximum at the valve null (called NULL LEAKAGE).
HYSTERESIS
The maximum difference in valve input current required to produce the same valve control flow output as the valve is slowly cycled (usually not exceeding 0.1Hz)between plus and minus rated current, expressed as percent of rated current.
LINEARITY
The degree to which the normal flow curve conforms to the normal flow gain line with the other operational variables held constant. Nonlinearity is measured as the maximum deviation of the normal flow curve from the normal flow gain line, and is expressed as percent of rated current.
SYMMETRY
The degree of equally between the 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 percent of rated current.
LAP
The relative axial position relationship between the fixed and movable flow-metering edges, with the spool at null. For a servo valve, lap is measured as the total separation at zero flow of straight line extension of the nearly straight for each polarity, expressed as percent of rated current.
Zero lap is the lap condition where there is no separation of the straight line extensions of the normal flow curve. Overlap is the lap condition which results in a decreased slope of the normal flow curve in the null region. Underlap is the lap condition which results in an increased slope of the normal flow curve in the null region. The specified tolerance for zero lap is 2.5%overlap to 2.5% underlap.
PRESSURE GAIN
The rated of change of load with input control current at zero control flow, expressed in MPa/mA.Pressure gain is usually specified as the average slope of the curve of load pressure drop versus current between ±40% of maximum load -pressure drop. And also pressure drop expressed as percent of rated supply pressure with input current equal to 1% of rated current at valve null.
NULL BIAS
The condition where the valve supplies zero control flow at zero load pressure drop is called as NULL. Null bias is the input control current required to bring the valve to null, excluding the effects of valve hysteresis, expressed as percent of rated current.
THRESHOLD
The minimum increment of input control current required to produce a change in valve output. Threshold will vary with the amplitude of a given input current and the stayed time of the input current. Valve threshold is normally specified as the maximum value among these minimum increments and is expressed as percent of rated current.
FREQUENCY RESPONSE
The complex ratio of no-load control flow to input control current as the current is varied sinusoidally at constant amplitude over a range of frequencies. Valve frequency response may vary with the input-current amplitude, temperature, supply pressure and other operating conditions. Frequency response is normally measured at rated test conditions and constant in-put-current amplitude whose peak-to-peak value equals to 50% of rated current, and is expressed both as the frequency at which the phase lag is 90°. The maximum amplitude ratio of valve frequency response should not exceed +2dB.
