Pressure Control • Flow Control
Sizes 1/2˝ thru 2˝
1.3 thru 250 GPM • 5100 PSI
|Typical 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 valves.
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 reliability.
Selection Criteria for Proportional Solenoid Valves
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 viscosity.
- 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 is adequate.
- 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 application.
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 include:
- A voltage stabilization circuit to provide constant voltage for consistent performance.
- 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 velocities.
- 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 input command.
- 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 temperatures.
- 10-volt differential amplifier input which enables interface with programmable controllers, microprocessors, or computers.
The following spool types are available.
Please consult Rexroth Catalog for more details
Available in: Alabama, Arizona, Arkansas, N.W. Florida, S. Idaho, Kansas, Louisiana, Mississippi, W. Missouri, Montana, New Mexico, W. North Dakota, Oklahoma, W. South Dakota, Texas, Utah and Wyoming