Tech Briefs Magazine - February 2022 - MD-6

Discrete Proportional v. Standard Proportional Behavior
100.0%
80.0%
Standard
Discrete
60.0%
40.0%
20.0%
0.0%
0.0%
20.0%
40.0%
Command, %
Figure 2: Discrete proportional valve section.
Issues with Extant Solutions
The combination of above attributes
is not shared by any extant control
valve. Specifically, valves that incorporate
near-zero hysteresis, zero steadystate
power, and a near-zero flow state
are available, e.g.
tuated by
stepper motors.
rotary valves acIn
stepper-driven
rotary valves, hysteresis is
not truly zero but is determined by the
repeatability of the step position and
by the backlash between the motor
shaft and the valve element (backlash
may be zero if the valve element is an
integral part of the shaft). Stepperdriven
rotary valves have some undesirable
attributes, however:
1. A rotating seal is required on the stepper
shaft. Rotating seals are prone to
leakage.
2. Lack of a fail-safe condition; if power
is lost to the stepper, the valve will remain
in the last-commanded position.
3. Stepper motors are relatively complicated
devices, so applications demanding
their excellent levels of control
of angular motion pay a penalty
in cost and potentially in weight.
4. A stepper motor controller is required
as well as (potentially) a position sensing/feedback
system.
DPV Concept as an Alternative
An alternative solution for enabling
all the desired attributes, while relying
on simpler forms of valve actuation and
control, is a discrete proportional valve
(DPV). The DPV concept relies on intelligent
combination of simple binary
(on-off) solenoid valves (Figure 2). Two
60.0%
80.0%
100.0%
or more on-off valves with differing flow
coefficients are combined in a single
manifold to achieve a stepped approximation
of a linear flow response using
selective actuation of the valves. For
example, a system of three valves gives
23 or eight possible flow states. The
flow states can include a zero-flow state
or by design may include a designed-in
minimum flow state. Table 1 shows the
possible states for a three-valve system
where the individual valves are sized
for flows of 1.0, 2.0, and 4.0 volumes
per unit time at a given pressure differential.
Figure 3 shows the resulting
relationship between flow and valve
command for such a system, compared
to the typical response of a continuous
proportional valve.
1
3
2
4
Figure 3: DPV and continuous proportional valve performance curves.
6
Referring to Figure 3, the blue curve
reflects the typical performance of a
continuous proportional valve. At 0 percent
command, the valve has some
minimum flow due to bypass leakage. As
the command is increased, there must
be some built-in deadband to accommodate
part-to-part variation in response.
This is shown by the flat portion of the
curve between 0 percent and 15 percent
command. As command is further
increased, the valve begins to open,
and the flow response follows the lower
blue curve. An upper deadband at 100
percent flow occurs, typically between
85 percent and 90 percent command.
As the valve is commanded to reduce
flow again, the response follows the
upper blue curve. The difference between
the upper and lower blue curves
is the hysteresis of the system and is due
to mechanical friction and magnetics.
Hysteresis also increases the effective
deadband at full flow.
The orange curve in Figure 3 shows
the response of a three-valve DPV system.
The response is not smooth but
stepped. There is no hysteresis by definition;
a given command will always result
in the same valve members opening,
and thus the same flow coefficient. In
addition, there is no requirement for
either lower or upper deadbands. What
appears to be a lower deadband is actually
the controlled true off state. It can
be designed as such or designed to be a
minimum flow value such that there is
always some flow through the system to
avoid pump damage, for instance.
For any given combination of valves
opened, the system will always have the
Motion Design, February 2022
MD Fluid Power Feature 0222_1.indd 6
Cov
ToC
1/18/22 9:11 AM
Flow, % Maximum
http://info.hotims.com/82318-800
Tech Briefs Magazine - February 2022

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