Proportional valves respond to variations in current applied to the solenoid by delivering a corresponding variable flow output. They are becoming a popular choice for machine and device builders desiring more flexibility in flow or pressure output from a PLC based on the conditions of the machine or making the machine "more flexible for the customers automation needs.”
The most popular and cost effective proportional valves have an armature dynamically responding to current or voltage force changes to the coil, against the force of a specially designed return spring. Of course, if coil and spring force were the only forces to be considered within the valve, precise output flow control can be achieved relatively easy. Other forces, typically referred to as friction, are present and more difficult to control, particularly as the valve functions over time. Friction is present whenever metal moves or is guided within a sleeve or within elastomers. Tolerances can create friction variability from valve to valve and as valves wear over time. As friction changes, the valve output will change correspondingly.
The friction exerted by just micrograms will affect the response time along with the amount the armature moves inside the system limiting the preciseness of a flow control and or pressure control. Along with friction, air flow, air pressure and contaminates can affect the response and movement of the armature. Air flow also creates friction on the system whether it is pushing force trying to open or hold close the orifice. Then once the orifice opens, it creates a pressure drop on the opposite side giving the system more to compensate for. These types of friction correlate to the effective response time and preciseness of the control that your proportional valve performance.
Manufacturers attempt to minimize or eliminate as much friction within proportional valves as possible to achieve consistent and reliable performance. When specifying proportional valves, users should consider the amount of friction potential for best results.