There are two ways to approach proportional control in a solenoid valve. The first approach is to use a simple on-off valve with a variable PWM input signal to meter fluid flow. The primary advantage of this strategy is a simpler valve design. The emphasis of the valve design is on controlling the fluid flow response time and the solenoid response time to fast and repeatable values. This allows the duty-cycle and frequency of the signal to be prescribed correctly to control fluid metering on the controls side.
The second approach is to use a simple DC or PWM input signal with current control and also design the valve actuator for positional control. This can be achieved with a needle style valve or a spool valve, both of which vary the outlet orifice size as a function of solenoid actuator position. The advantage of the spool valve is the ability to control more than one fluid flow path as long as flow requirements between the paths are directly related. While a needle valve can more easily control bypass leakage, a spool valve’s effectiveness will depend on the fit of the spool and valve body. Getting to a low value for bypass leakage can often require grinding, honing and match fitting components. The emphasis on this type of valve design is matching the dynamic flow geometry to the required fluid control characteristics and gaining excellent positional control from the integrated solenoid actuator.
Challenges and Solutions for Proportional Solenoids and Accurate Positional Control
The precision of positional control is the modern challenge. Accuracy of positional actuators is controlled by traditional means, shaping magnetic components of the actuator to provide a linear and predictable force vs. position characteristic, then balancing that force with an opposing linearly increasing force, in its simplest form, a spring. With better precision the need for error compensation decreases. As a result, valves and actuators are designed with shorter working stroke and lower output force requirements resulting in lower power consumption and better system function and efficiency when integrated. Hysteresis is the most indicative characteristic of precise solenoid function. Measuring and characterizing actuator hysteresis exposes both frictional losses in the system and magnetic materials losses.