Switching solenoid valves, which are normally closed, form the basis for Bürkert proportional valves. Making constructive modifications to switching solenoid valves can achieve a balance of the spring and magnetic force for any coil current. The level of the coil current and magnetic force determines the stroke of the plunger or the valve opening, whereby the valve opening and the current are ideally linked by a linear dependence.
Servo-assisted proportional valve
The Figure shows the operating principle of such a valve. When it is closed, the medium at the input side has a pressure of p1, the core or plunger (3) has dropped out and therefore presses on the pilot seat (4). As a result of this and the force of the piston spring, which acts on the piston (2), the main seat (5) is closed. A restrictor port (6) allows the medium to enter the control chamber (1) and press on the diaphragm or gasket from above with a pressure px.
If the restrictor port, pilot seat and area ratios on the main stage are rated accordingly, the compression forces on the piston reach equilibrium when the seat is opened by a certain position. With proportional pilot control, ideally, the piston follows the continuous axial movement of the plunger precisely at the distance that creates this equilibrium.
Diagram of a servo-assisted proportional valve
Direct acting proportional valve
1 Valve body, brass or stainless steel
2 Plunger, FKM (standard)
3 O-ring, FKM (standard)
4 Glide ring, PTFE compound
5 Return spring, stainless steel
6 Stopper with integrated adjusting screw, stainless steel
7 Plunger, stainless steel
8 Coil, Epoxy-encapsulated