Electro-Proportional Valves are similar in appearance to a regular On/ Off Solenoid valve but offer variable outputs be it flow or pressure control.
Proportional Directional valves allow the user to vary the degree to which a spool is shifted thereby regulating the flow through the valve and selection of either of 2 coils will change the direction of movement. This makes it possible to move an hydraulic actuator backwards or forward eg at a rate controlled by a “joystick” or other input device as desired.
Electro-Proportional Valves can be either simple Flow Control or Pressure Compensated Flow Controls. The difference is “Simple” flow control uses an adjustable orifice to vary the flow rate- the orifice is “fixed” at a given proportional setting- any variation in the pressure drop over the flow control due to changes in the load being controlled or changes in oil viscosity will change the amount of oil passing through the orifice: This will cause a change of speed of the actuator. Pressure Compensated Flow Control is more sophisticated and incorporates designed elements which maintains a fixed pressure drop over the adjustable orifice which regulates the flow rate at the desired level regardless of load or viscosity within tolerances stated in the Datasheet for any particular Hydraforce valve. Flow controls can be single direction or bi-directional valves so a single valve can meter in and out of a cylinder eg.
Electro-Proportional Valves as pressure controls are available as Pressure Relief valve or Pressure Reducing valves. The controlled pressure can be configured as Normally High or Normally Low- this is to say that the pressure can be zero when the coil is Off, or Maximum when the coil is Off. In a Load Sense system eg. the incorporation of a Proportional Relief valve can be used to remotely limit the compensator pressure on the controlled pump- the main relief can be set to the Design Limit while the thrust, torque of force is varied as per operator input.
Electro-Proportional Valves require an amplifier/controller device of some sort to properly control the output. The voltage measured across a proportional solenoid coil will be equal to the nominal system voltage, eg 12 of 24 V DC. This is a false indicator as proportional coils are current controlled at the voltage of the system, the operator input to the controller is by potentiometer, DC voltage (0-5 or 0-10), 4-20mA, or PWM (Pulse Width Modulation). The controller translates this input to a controlled current output to the coil of the Electro-Proportional Valves- the output current is usually up to 2000 milliamps. In order to minimise hysteresis the controllers also apply a dither wave over the top of the output current/voltage and vary the frequency of the PWM to suit particular Hydraforce valves- different valves work best with frequencies and amplitudes as per the Hydraforce Datasheets. Ref. (http://hydraforce.com/Electro/Electcon.htm ). Corridor Connections can assist with the understanding of which controllers suit your application best. We carry stock in Melbourne of a range of controllers and ECUs.