FUNDAMENTALS
Josh Cosford • Contributing Editor
Hydraulic symbology 301: electrical and electronic symbols
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Any book or lesson in fluid power worth its weight in gold will discuss the importance of electrical and electronic control of hydraulics. In fact, over the past few decades, most of the advancements in hydraulics has been that of how it’s controlled rather than improvements in the foundational components such as valves, pumps and actuators. Understanding electrical symbology becomes more important as machinery becomes more sophisticated, and you may come across schematics that are hybridized with electronics or even electric actuators like linear or servomotors. This review of hydraulic symbology covers most of what needs to be known to read and create the average hydraulic schematic, since actual electrical symbols are somewhat different. Figure 1
Push Coil
Pull Coil
Dual Opposed Coils
Starting with Figure 1, there are three ways to draw an electric operator for solenoid valves, which most people recognize. The first operator is the symbol for a solenoid coil that magnetically pushes on the armature pin, which makes sense as the diagonal line leans towards the valve body. When you flip the diagonal line to lean outward from the valve body, the coil now shifts the valve by pulling on the armature pin, although it is rare to see the symbol drawn this way. Most often a valve will be drawn as though the inward-facing coil symbols are the standard, and it’s even my personal preference. When the valve is manufactured with dual, opposed coils, such as with a 3-position cartridge valve, you draw the symbol with both of the diagonal directions as shown. Because the end of the valve opposite the coils is buried within a manifold or ported body, the only place to mount two coils is on the top. Inside the core tube, the armature pin is attached to the spool and can be pushed or pulled off-center by either coil.
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Figure 2 G
Proportional Coil
Old Servo Valve
High Response Valve
Figure 2 shows three options for drawing proportional valve symbols. A generic proportional valve symbol is shown on the left and is quite simply a diagonal variable arrow slashed across the coil symbol. This denotes that the current applied to the coil can be varied (typically via pulse width modulation), and the two parallel lines above and below the valve symbol tells us the valve spool is constructed in such a way as to have infinitely variable positions between fully closed and fully flowing. However, like many symbols, it tells us nothing of the construction technique or method of manufacture. The symbol previously used to represent servovalves is still commonly found in old schematics, literature and documentation. It’s an odd symbol, unique to servovalves, but makes perfect sense when broken down. It varies from the newer proportional valve symbol only by the operator (the spring and parallel lines showing infinite positionability remain). The blank circle is surrounded by three triangles, each of which points into or away from the circle. The left side triangle represents the input to the valve, which is the desired signal sent from the controller. The right side line and arrow depict the actual output being applied to the valve. The bottom triangle facing up into the circle is the closed-loop feedback being added to the control signal, which is the amount of correction equal to the difference between the input and output. An actual servovalve has an inherent feedback loop using nozzles and a spool position-sensing flapper that creates backpressures. It’s a complex task, but it’s only important you know what the symbol means at this point. The high response valve on the right in Figure 2 shows another method of closed-loop valve control. This symbol is a mix of the two earlier examples, but instead of inherent feedback of the servo valve, it uses on-board electronics. The big dashed-line triangle is the electronic symbol for an amplifier, although, in the electrical symbology realm, the triangle is an unbroken, solid line. When the symbol is borrowed for fluid power use, the line is
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8/15/19 9:58 AM
