proportional control valves

III. PROPORTIONAL CONTROL VALVES

PROPORTIONAL VALVES

1. What are Proportional Control Valves

• In modern electro-hydraulic systems, proportional control valves are used as an alternative to servo valves.

• The function of proportional control valve is to provide a smooth and continuous variation in flow or pressure in response to an electrical signal.

• Proportional control valves offer a high degree of flexibility in automatic programmed circuits.

• Proportional valves are available to regulate and control movement and pressure by varying an electrical input signal. These input signals may be provided by a simple potentiometer or by complex electronic hardware.

• Similar to servo valves, proportional valves also use a variable force direct current solenoid to control the output from the main spool. Therefore the output is directly proportional to the input signal.

• The main advantage of proportional valves is their ability to control the pressure and flow rate proportionately and provide remote function.

2. Types of Proportional Control Valves

Three important types of proportional control valves are :

1. Proportional pressure relief valve,

2. Proportional pressure reducing valve, and

3. Proportional direction control valve.

3. Proportional Pressure Relief Valve

1. Construction

The construction and operation of a typical proportional pressure relief valve is illustrated in Fig.6.33(a).

2. Operation

The proportional pressure relief valve' works on exactly the same principle as the conventional pressure relief valve (described in Section 6.13), except that the compression (adjustable) springs are replaced by a proportional solenoid. In this valve, the operating pressure of the valve depends on the current in the solenoid.

The input current to the solenoid energies it coil and hence produces an armature deflection by an amount proportional to the input current. This armature deflection exerts a force on the poppet. The poppet keeps the valve closed until the inlet pressure at port P overcomes the force exerted by armature deflection. When the inlet pressure overcomes this force, then the poppet valve opens. Thus the opening and closing of the valve, and hence the operating pressure of the valve can be varied by proportionately varying the input current to the solenoid.

3. Graphic Symbol

Fig.6.34(b) shows the graphic symbol of the proportional pressure relief valve.

4. Proportional Pressure Reducing Valve

1. Construction

The construction and operation of a typical proportional pressure reducing valve is illustrated in Fig.6.34(a).

2. Operation

The proportional pressure reducing valve works on exactly the same principle as the conventional pressure reducing valve (described in Section 6.17), except that the control spring is replaced by a proportional solenoid. Also unlike the conventional pressure reducing valve, this valve is normally closed when the solenoid is not energized.

When the solenoid is energized, its corresponding armature deflection will move the spool to the right. This opens the control orifice A and allows the fluid to flow to the output port X. The outlet pressure of this valve is dependent upon the openings of control orifices A and B, which in turn depends on the input current supplied to solenoid. Thus by proportionately varying the input current to the solenoid, the operating pressures of the valves can be varied.

3. Graphic Symbol

Fig.6.34(b) shows a graphic symbol of proportional pressure reducing valve.