Electrohydraulic servovalves, capable of controlling pressure, flow, or position in proportion to an electrical input, offer unsurpassed performance in a fluid-power system. At one time, conventional servovalves used on aircraft or machine tools provided operating characteristics exceeding those required for industrial and mobile applications. They were also too expensive and too sensitive to environmental contamination.
However, with the development of custom designs specifically for industrial and mobile equipment, the use of servovalves is moving into the critical circuits of these systems. For example, most robotic applications involve relatively low hydraulic resonance, in the range of 3 to 5 Hz. This low resonance is a result of the high load masses and large fluid volumes under compression in a typical robotic system. Also, the typical flow range for these systems is 10 to 40 gpm. Conventional servovalves supplying this level of flow have frequency response capabilities of 120 to 130 Hz at 90° phase lag, far higher than is required.
The valves can be modified to trade frequency response for increased spool operating force by enlarging the spool area exposed to pilot pressure. Increasing this area by 2 to 2.5 times, for instance, increases spool actuating force by an equivalent amount while lowering frequency response to 20 to 80 Hz at 90° phase lag. Higher spool actuating force enables the valve to overcome spool seizure caused by silting, thus improving system reliability.
Servovalves were adapted to the mobile market primarily by opening critical clearances in the pilot stage, making them more compatible with practical filtration levels. The wider clearances, and the use of hardened materials, allows contaminants to simply wash through the valves without clogging or causing damage.
Today, the use of servohydraulics usually is justified when one or more of the following characteristics is required: high load stiffness (both static and dynamic), good stability, precise positioning, good velocity and acceleration control, good damping characteristics, and predictable dynamic response.
In a servovalve, a given electrical signal produces a definite position of the main-stage spool, but it does not necessarily produce a fixed flow. Flow is a function of the square root of the difference between supply pressure and load pressure. Thus, as load pressure increases, both flow and effective pressure drop across the valve decrease.
Gain-compensated valves incorporate internal feedback to correct for load-fluctuation effects on output and, thus, more nearly approach the ideal steady-state curve. Amount of correction depends on the valve design. Where high accuracy is needed for either velocity or position, experts recommend an external transducer and feedback loop. Steady-state characteristics are used to specify valve operating properties to ensure that the required system operating characteristics are not compromised by a limiting action of the valve due to inadequate flow, pressure, or input current; excessive null shifts; or other foreseeable causes. However, some of the steady-state valve parameters must be based on dynamic analyses to ensure required system response.
Flow: A flow curve is obtained by cycling the valve over the rated input current range and recording a continuous plot of output flow versus input current for one cycle. It is used to measure valve flow gain, hysteresis, linearity, and symmetry.
Generally, the flow curve forms a closed loop. The locus of the midpoints of this loop is the zero-hysteresis flow curve; portions corresponding to the two polarities may differ (symmetry). The flow curve is generally divided into three operating ranges -- null region with low input currents, region of normal flow control at intermediate input currents, and the flow-saturation region near rated input current.
Flow gain is change in output flow per unit change in input current, under zero load unless otherwise specified. Therefore, the slope of the normal flow curve in any specified region is the flow gain. When the term flow gain is used without reference to a particular region, normal flow gain is implied. Ideally, rated flow gain would be equal to normal flow gain, which is the slope of a straight line drawn from the zero flow point to equalize deviations of the flow curve from a straight line.
Pressure characteristics: Pressure gain is related to the rate of pressure increase per unit current increase. Pressure gain characteristics are very important because in a practical system some usually small, but finite, value of output-pressure change is required to overcome load friction before load movement is possible. Thus, the entire system is affected by the pressure-gain characteristics of the valve.
Null characteristics: Ideally, a valve produces zero output at zero current input. In practice, this ideal condition is seldom attained. The null shift may be due to changes in temperature, supply pressure, and return or load pressure. It is expressed in terms of null bias, or the current change required to produce zero flow.
Dynamic characteristics: have an important effect on the dynamics of the system. The transient response of the valve can be so slow that it limits the transient behavior of the entire system. An inherent resonance can cause system instability or oscillation. Limits for the dynamic characteristics of each component must be specified if the required system dynamics are to be maintained. Valve dynamics can be determined either by a transient-response or a frequency-response test, depending on the specific system application.