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JWST499-Cetinkunt
JWST499-c07
ELECTROHYDRAULIC MOTION CONTROL SYSTEMS 497
into the manifold design. The main advantages of the manifold block approach to multi
valve hydraulic circuits instead of individual valves are
the manifold block modularizes and simplifies hydraulic plumbing in installation and

maintenance,
it reduces leakage and can support higher pressure circuits,

their compact size.

The manifold block port locations and sizes are standardized by ISO-4401 standards (i.e.,
ISO-4401-03, -05, -06, -07, -08, - 10 specify different number of ports, sizes, and locations
for external connections). These are also refered as CETOP-03, … , CETOP-10 standards.
For pressures up to 3000 psi, aluminum manifolds, and for higher pressures (5000 psi)
cast iron manifolds are recommended. In addition to manifold blocks, valves are also made
with stackable standard mounting plates which makes connecting supply and tank lines
between valves easier.
Other mounting methods for hydraulic plumbing are sub-plates, inline bar mani-
folds, mounting plates, valve adaptors, and sandwich style mounting plates . In particular,
sandwich style mounting is typically used to integrate a directional or proportional valve
and a number of relief, check, and pressure reducing functions in one stack. Sandwich type
mounting plates also have DIN, ISO, and NFPA standard interface dimensions.

7.5.7 Performance Characteristics of Proportional
and Servo Valves

The spool and orifice geometry around the null position is an important factor in servo
valve performance. The spool may be machined so that at null position it overlaps, zero-
laps, or underlaps the flow orifices. The zero-lapped spool is the ideal spool, but difficult
to accomplish due to tight manufacturing tolerances. The overlapped spool results in a
mechanical deadband between the current and flow relationship. The underlapped spool
provides a large gain around the null position (Figure 7.72).
The deadband helps reduce the leakage in a valve, and hence allows the actuator to
hold position better in open loop control in neutral position. In operator controlled open-
loop speed control applications, where the speed of the actuator is desired to be proportional
to the operator command signal which is proportional to the valve spool displacement, the
deadband is quite often a desirable feature of the valve. Because small changes in the
operator command due to human motion resolution or vibrations in the environment (hence
the small changes of the valve spool position around neutral position) are desired to not
create any motion, so that operator hand vibrations do not create unintended motion. On
the other hand, in closed loop position or force or velocity control applications, the valve
deadband acts as a nonlinearity and delay in the system response. This limits the closed loop
control system bandwidth. The gain of the valve (flow rate divided by the spool displacement
under a constant pressure drop condition across the valve) about null-position is zero if
there is a finite deadband, that is 5 or 10% of total valve spool travel. Likewise, the gain
of under-lapped spools is higher than those of zero-lapped or over-lapped spools. Hence,
the closed loop system stability and dynamic performance of an electrohydraulic motion
control system will be different for valves with different null-position characteristics. In a
closed loop control system, having a different null-position gain for a valve is equivalent to
having a different loop gain in the root locus analysis of the closed loop system. Different
gains will result in different stability margins and closed loop pole locations.
Figure 7.73 shows various valve spool position-orifice area characteristics. The spool
position versus the orifice area opening is directly determined by the way the valve is