Figure 2. This plot shows a typical pressure metering curve for a valve exhibiting significant overlap between the spool and ports. Click on image for larger view.
Figure 2 shows a "typical" pressure metering curve for a proportional valve that exhibits significant overlap. In proportional valves, with their significant overlaps, pressure metering takes place throughout the entire dead zone. The dead zone is not really dead, because the finite spool-to-bore clearance results in internal leakage. Note, also, that such valves do not fully shut off. There is still some miniscule amount of flow metering too, which results in the actuator creeping very slowly through the null zone. So the pressure metering zone extends over the entire overlap region and even extends somewhat into the region where the lands move into the flowing region.
A general rule is to view the pressure metering zone of an over-lapped valve to cover the width of the dead zone, plus about 5% more of spool travel. Thus, it is possible to have pressure metering from about 10% of spool travel up to about 40% of spool travel in a proportional valve. On the other hand, in the nominally zero-lapped servovalve, the pressure metering zone is confined to about the center 5% of spool travel and may extend to about 10% of spool travel in some valves. Not surprisingly, pressure gain is directly proportional to the supply pressure, but inversely proportional to the dead zone width.
Servovalve null characteristics
The major manufacturing effort in servovalves goes into making the null. Null for the valve is defined as that condition where the differential dead head output pressure is zero. Because the valve has no output flow, all leakage is confined to the valve's four lands.
Certain system parameter changes will cause the null to shift. System parameters that affect the null are:
• supply pressure,
• valve temperature, and
• tank port pressure.
Null sensitivity tests — Imagine a situation where we have set up a valve at a fixed supply pressure, a fixed temperature, and a fixed tank port pressure. Set up will consist of adjusting the mechanical null on the valve until both work ports reach the same dead head pressure while the coil current is zero. The valve is now nulled.
Next, we change one of the three parameters, say, supply pressure. If we increase supply pressure, and if the valve was perfect, it would maintain its null at any supply pressures. But it is imperfect, requiring that we adjust the coil current to a new value in order to re-acquire the null. That change in current is a measure of null sensitivity to (in this case) the change in supply pressure. That value is published for servovalves. Similar figures are published for thermal and tank port pressure null sensitivity.
To design a servo system, you must take into account all the sensitivity factors of the valve in the context of the application. This requires calculating all the expected null shifts as amperes of coil current. Start by understanding the following definitions: SPS is the published sensitivity of the valve null to changes in supply pressure.
S is the published sensitivity of the valve null to changes in temperature.
SBP is the published sensitivity of the valve null to changes in tank port back pressure.
If the valve's manufacturer has published a given sensitivity factor, SX, then the change in current caused by a change, (XAPP) of the affecting parameter is given by:
∆IX = SX × IR × ∆XAPP
where ∆IX is the amount the valve current must change, in amperes, to compensate for the null shift,
SXis the specific sensitivity factor taken from catalog data on the valve,
IR is the rated current of the valve in amperes, and
∆XAPP is the change in one of the three parameters that is expected in the application.
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