Page 510 - Mechatronics with Experiments

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directional flow and pressure control valve functions (i.e., proportional flow metering,
pressure relief, check valve, unloading valve).
Cartridge valves for small flow rates can be directly actuated by a solenoid. Cartridge
valves for large flow rates typically use a two-stage actuation mechanism which includes a
first-stage electrical actuator and a second-stage pilot amplifier. For instance, let us consider
a proportional spool type cartridge valve (Figure 7.70). The valve has three ports: inlet port
(main input pressure, i.e., pump supply), tank port, and output port. The output port pressure
is regulated to be somewhere between the inlet and tank port pressures. It is essentially a
pressure reducing valve. The outlet pressure is regulated to a value between the two limit
pressures (inlet and tank pressures) by the pilot stage presssure. The pilot stage supply
pressure is obtained by a pressure reducing valve section from the main inlet pressure. The
actual pilot pressure in the pilot control chamber is regulated by the poppet ball which
is controlled by the solenoid. Therefore, the solenoid current regulates a pilot chamber
pressure proportional to it which in turn positions the main stage spool proportional to the
pilot pressure with the balancing force from the spring. The output pressure is proportional
to the main spool position, hence to the solenoid current.
For very large cartridge valves, the valve actuator may take the form of a double
nozzle flapper or jet pipe valve as the pilot stage. Such a valve can support very high flow
rates (i.e., up to 9600 lpm by the Moog DSHR series). However, the addition of a pilot stage
servo valve along with poppet position feedback makes the cost of the valve significantly
higher than the cost of proportional spool-type valves. Screw-in cartridge valves provide
check, relief, flow, pressure, and direction control functionalities for two, three, four or
more port configurations.
The recent design of the direct drive valve concept uses electrical actuators with
higher power density to directly shift the main spool of a two-stage valve. The valve then
becomes a single stage with the flow capacity of a two-stage valve. The electrical actuator
is either a rotary motor (hybrid permanent magnet stepper motor or bushless DC motor
coupled with a rotary to linear motion conversion mechanism, i.e., helical cam, ball-screw),
or a direct drive linear electric motor (i.e., linear force motor, linear brushless DC, linear
stepper motor). The main benefit of the direct drive valve is that it eliminates the pilot
pressure stage between the electrical actuator (solenoid in current designs) and the main
spool motion. Due to power density and cost, current applications are still limited to valves
with low flow rates. As the power density of electric actuators (permanent magnets and
power amplifiers) gets larger and costs gets lower, the application of direct drive valve
technology is likely to increase in the coming years.

7.5.6 Mounting of Valves in a Hydraulic Circuit

In hydraulic motion systems, a set of valve functions can be machined into a single manifold
block (Figure 7.71). The manifold block, a set of cartridge valves, as well as other non-
cartridge valves provide a very reliable and leakage free design for hydraulic valve groups.
The manifold block provides the hydraulic connections from P and T ports to different
valves internally. The hydraulic plumbing functions between P and T and service ports
(A1, B1, A2, B2, etc.) are machined into the block. For instance, in a multi function circuit,
parallel or series hydraulic connection of valve ports to P and T lines can be machined
into the manifold. For each function, there is a valve (typically a screw-in cartridge valve)
that controls (meters) the hydraulic flow between the ports. Technically, a single P port
and T port is sufficient for the whole manifold block. There are as many A and B ports as
there are the number of valve functions used. Furthermore, the manifold block is machined
to accomodate open-center or closed-center functions by selectively plugging certain flow
orifices on the manifold. Check valves, relief valves, and filter connections can also be built