Bosch Rexroth Proportional Solenoid Valves
Pressure Control • Flow
Sizes 1/2˝ thru 2˝
1.3 thru 250 GPM • 5100 PSI
Directional Control Models
Principles of Operation
Proportional solenoid action can be applied to directional
control, pressure control, and flow control valves. The principle
is to control the movement of the valve spool against a return or
centering spring by regulating current flow through a DC solenoid
coil. Current is varied by varying the applied voltage to the coil
through the appropriate amplifier card. Bosch
Rexroth amplifier cards are specifically designed to
operate a particular series of valves, and can, therefore, be set
up with a minimum of field adjustments. This is particularly
important when an amplifier has to be replaced in the field and
down-time must be kept to a minimum.
Two types of solenoids are used with Bosch
Rexroth proportional valves. The first is called a
"stroke-controlled solenoid" because the length of stroke is
controlled by feedback from the actuator to cause the valve spool
to move to a precise position for the desired valving action.
Metering notches on the spool lands are carefully designed to
produce a flow orifice directly proportional to the distance the
spool is shifted. The second type is called a "force-controlled
solenoid" because there is no feedback from the load. The spool
compresses a spring and stops when the applied force is equal to
the spring resistance. This type of solenoid is commonly used in
pressure control applications, and is also used on the
Bosch Rexroth 4WRA series of directional control
A proportional solenoid valve operating in an open loop
configuration receives no feedback to let it know that the actuator
has reached its commanded position or that the commanded pressure
or flow has been reached. It is possible, through the use of
additional control components such as pressure transducers,
encoders, and magneto restrictive feedback devices to use these
valves in closed loop applications. Certain applications which once
required the use of servo valves can now be accomplished with
proportional solenoid valves at greatly reduced cost and increased
Selection Criteria for Proportional Solenoid
Selection of proportional solenoid valves is a more demanding
process than selection of standard directional valves. The goal is
to provide true control of the load rather than simply commanding
it to start or stop. It is necessary to have a complete
understanding of the machine cycle and the characteristics of the
load in order to properly apply a proportional solenoid valve.
Following are several important criteria to be considered when
designing control circuits:
- The natural frequency of the system.
- The range of flows required.
- The influence of changes in mass, velocity, pressure, and
- Any time delays required.
- If pressure compensators are required.
- Pressure intensification when using single rod cylinders or
oversize rod cylinders and meter-out pressure compensators.
- Whether closed loop control must be used or open loop control
- Will valve response time affect system-operation? If so, do
servo valves need to be considered?
Size selection is not made on the lowest possible pressure drop
through a valve as is commonly done when selecting other
directional control valves. The most important function of a
proportional solenoid directional valve is to be able to meter the
flow accurately; and to accomplish this there must be a moderate
pressure drop through the valve. A valve model must be selected and
a spool variation be chosen so there will be at least a 145 PSI
pressure drop through the valve when the spool is fully shifted. An
accurate selection can only be made with the aid of the flow graphs
in the Bosch Rexroth bulletin for that model.
Model numbers listed in this manual are not complete. Additional
numerals and letters must be added to specify spool choice and
other options. Contact your local Womack office
for assistance in selecting the correct valve for your
Rexroth Amplifier Cards
The amplifier cards offered by Bosch Rexroth for
operation of solenoid proportional valves perform many functions in
addition to simply amplifying the input signal to a level required
to drive the proportional solenoid. Some of these functions may
- A voltage stabilization circuit to provide constant voltage for
- Internal potentiometers that can be adjusted to limit the
maximum output command signal. Some amplifier cards contain up to 4
of these potentiometers which allow individual settings of multiple
- Internal relays that facilitate selection of pre-set
velocities. Some cards contain up to 4 of these relays and also
provide LED's to indicate the relay which is activated.
- A ramp generator to control acceleration and deceleration rates
which provide shock-free operation.
- Potentiometers which allow ramp control for each individual
- A function generator which eliminates valve deadband, allowing
instantaneous reversal with smooth crossover characteristics.
- Pulse width modulated (PWM) output which reduces system
hysteresis and allows amplifiers to be used at high ambient
- 10-volt differential amplifier input which enables interface
with programmable controllers, microprocessors, or computers.
The following spool types
Please consult Rexroth Catalog for more
View the Bosch
Rexroth Proportional Solenoid Valves web