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Powers of attraction

Electromagnetic force is responsible for practically all phenomena encountered in daily life they maintain the relationship between atoms so every action whether push or pull relies on these invisible forces to create movement.

 

 

The use of these forces both magnetic and electrical has a huge impact on our lives. Man has learned to harness them in many ways - generating electricity by movement of magnets or creating magnetism by using electrical current. It is the latter that we rely on the operation of numerous hydraulic valves. We call these solenoid valves where by an electromagnetic actuator provides a force or movement to a hydraulic control element which in turn controls the fluid in a system. Typically a solenoid valve uses an electromagnetic actuator (see Figure 1), consisting of coils of copper wire wound around a bobbin enclosed in an iron yolk, which is encapsulated in a heat resistant thermosetting plastic.

  Figure 1. Component parts of a typical solenoid coil

 

Various types of connector can be molded into the assembly to give varying degrees of water and dust resistance commonly known as the "IP" rating. This coil assembly fits over a non-magnetic tube which contains fixed and moving ferrous armatures. 

When a current is applied to the coil the flux magnetizes the armatures which are attracted together. The level of attraction is dependent on the design and the level of the current. There are two main types of electromagnetic actuator (Figure 2), the pull version and the push version.
Both rely on attraction to work, the only difference being the layout of the fixed and moving armatures and the way they are connected to the part being actuated. (Reversing the polarity of the voltage in a simple coil will not change function of the actuator.) With clever design of the tube and armature the force exerted by the coil can be made to be roportional to the current applied. This allows us to produce proportional directional, pressure and flow control valves.
Figure 2. Pull and push type actuators
Figure 2. Pull and push type actuators
The shape of the pole ends between the fixed and moving armatures along with the design of the non-magnetic infill allows the tube designer to change the force displacement characteristic produced by the combination of the coil and the tube. You can therefore have tubes designed to give proportional movement and those to give proportional force over a small movement. The latter of these designs is typically used for pressure control and the former for flow regulation or directional control. The design of the hydraulic section of the valve enables the oil to pass from port to port due to the movement or force created by the effect of the electromagnetic flux on the two armatures and subsequently on the poppet or spool.
Figure 3. S510A Simple two position, two port directional  control valve (using pull type solenoid)

 

 

 

The simplest device is a two positioned, two ported valve made up of a spool and sleeve connected to the armature (Figure 3). The armature pulls or pushes the spool within the sleeve either closing or opening a ring of holes in the sleeve that connects one port to the other

More ports and a more complicated sleeve/spool assembly can create two position three or four ported valves (Figure 4)

Figure 4. S525 & S542 still two position, but now three and four port directional control valves (again using pull type solenoid)

 

 

By using two coils on a common tube a three positioned valve with four ports can be achieved. There are various designs but common layouts use either one fixed and two moving armatures or two fixed and one moving armature, the example below uses the former. (Figure 5)

Figure 5. S570 Two solenoids on a common tube to give a three position four port directional control valves

 

The poppet valve (Figure 6) consists of a poppet that is forced onto a seat against a spring in the case of a normally open valve and pulled away from a seat against a spring for a normally closed valve. (Poppet valves give minimal internal leakage compared to spool valves.) The balance between the working pressure, the spring force and the magnetic force is very important and in order for the poppet valve to pass higher flows it is necessary to operate the valve in two stages. The armature will force a pilot poppet onto a seat contained within a larger poppet so that when the pilot poppet is opened a flow is created across an orifice allowing the pressure difference to act to open the bigger poppet. By doing this you can control very large flows using a small pilot solenoid valve. 

Figure 6. S501 A typical pilot operated poppet valve - good flow characteristics and minimal internal leakage (using push type solenoid)

When specifying a solenoid valve there are several important characteristics of the assembly the machine designer must take into account.

All coils will come with a rating that indicates the voltageß and the power consumption in Watts. Establishing the requirement for a machine seems straightforward but there are pitfalls that must be avoided.

Most coils are advertised as continuously rated provided they are working within set parameters. When a solenoid is energized it will generate heat, the amount of heat experienced by the coil will depend on the power being applied, the design of the coil and any cooling effects. The temperature of the valve is important because it affects the performance of the encapsulating material. The materials used will have a class given to them.

For example Class 'H' guarantees the material up to 180º C while class 'F' up to 155º C. If the ambient temperature plus this increase in coil temperature rises above the rating of the insulation and encapsulation material then failure will occur. Most catalogs will indicate an ambient temperature of between -20º C and +40º C.

Change in temperature also affects the resistance of the coil and hence the force it will exert, but more of that later.

The second most important consideration is the application and the exposure of the coil to the elements. Coils and tube assemblies are designed to withstand different levels of water ingress. This is called the 'IP' rating. The connectors used will carry their own IP rating such as DIN 43650 at IP65, most Deutsch connectors are IP67. The level of water ingress protection increases with the higher number. Details can be found in BS EN 60529. Some manufacturers also seal the coil/tube joint with an "O" ring to prevent corrosion and possible weakening of the tube.

On machinery where the valve is mounted a long way from the power source voltage loss along the wires has to be accounted for. The coil is rated to give a performance to a valve specified with a tolerance on the voltage. Typically +/- 10% of the nominal voltage. If the voltage at the coil drops then full performance from the valve will not be achieved. Proportional valves in particular require the availability of a constant current to operate effectively and predictably. And it should be remembered that as the coil warms up the resistance will change affecting the voltage and so the reaction of the valve to change in input signal. Utilizing Pulse Width Modulation (PWM) will minimize these adverse effects. Hysterisis within the valve can also be reduced by applying PWM to the signal. Feedback loops within the electronic control can also make the valve more accurate.

Most hydraulic companies will specify a performance figure for their valves that is available at a percentage of the maximum power usage. This is to allow good operation when the coil heats up and there is a resultant loss of magnetic force, therefore when testing a machine it is important to verify the valve function at stabilized temperature/most extreme service conditions.

A coil is an inductor – it stores energy and resists change, so when switched off it will generate a brief high voltage signal which potentially can damage other electronics devices on the vehicle or system.

To protect against these, coils or connectors can be fitted with diodes to permit the energy to dissipate safely.

As yet there are not generally recognized standards as to how response times should be measured, some manufacturers will quote figures from the time when the power is switched to the point where the armature reaches the end of its stroke, others from the time the power is switched to the point where the hydraulic fluid reacts. When looking at different manufacturer's products it is important to compare like for like.

Note: Response times are usually different for switching the power on or off.

With mechanical valves the performance is often based on the pressure drop across the valve at a set flow. With electrically operated valves it is more likely that the balance between the solenoid force, the internal spring force and the flow forces will dictate the working envelope of the valve.

Force on valve components due to pressure is usually straightforward but forces are also generated by the fluid passing over the surface of the valves components, these flow forces can either act with the coil force to help the valve to stay in its operated condition or act against the coil force. There have been occasions where they have caused a valve to switch back to its original position as soon as the flow force overcomes the magnetic force causing an actuator to change direction in the middle of its stroke. Performance data often indicates that a valve will allow more flow in one direction than the other. This can be directly attributed to the flow forces within the valve.

In the case of most solenoid valves the pressure limitation will be determined by the tube design and the factor of safety employed by the manufacturer. But in some cases over pressurization of the valve may cause the valve to open, as the force created overcomes the magnetic force exerted by the coil. A typical example of good hydraulic design using electrical operation is shown in Figure 7


Figure 7. A pressure compensated flow control with pressure switching 
This valve is used on a pilot line to provide a pressure compensated flow at all times with the ability to switch from one pressure to another by energizing the solenoid. The armature compresses a spring within the valve that increases the force on the poppet increasing the setting of the valve.

 

Figure 8 shows a bi-directional poppet valve for flows up to 90 liters/min To give bi-direction to the poppet and so the flow two very small shuttle valves are situated in the poppet. These direct the flow through the opening orifice from the high pressure side of the valve. When the valve is de-energized the poppet is balanced and offset closed by light spring.

Figure 8. S717 High flow bi-directional poppet valve
Figure 9 shows a proportional pressure reducing valve, where with no current applied, the regulated port is connected to tank. As the current increases the pressure in the regulated port will rise to balance the increasing solenoid force.
Figure 9. PPD22A Proportional pressure reducing vavle

Pressure in the regulated port acts on the spool tending to close it against the force of the solenoid. As pressure in the regulated port increases (with a constant current applied to the solenoid) the spool will shift, restricting the inlet, equilibrium is then achieved and a reduced constant outlet pressure held. Varying the current will alter the force applied to the spool and hence the pressure in the regulated line.

Solenoid operated valves are used on most machines and effectively control the required functions. Proportional controls are becoming more common as the cost of the technology is reducing and the advantages of digital control are realized. The variety of solenoid operated valves is wide because of the ingenuity of the design engineers to solve complex and often conflicting demands.

A solenoid valve can give electrical control to almost any hydraulic function. This reduces pipe work, removes most hydraulic lines from the vicinity of the operator, adds flexibility and functionality and ultimately adds to the competitiveness of the machine in which they are used.