By Josh Cosford, Contributing Editor
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The lever-operated directional valves used to control the fore and aft movement of cylinders, or motors&#; rotation, offer a performance advantage well above electric solenoid valves. Although inexpensive options are typical for either option, lever valves tend to flow more than their solenoid counterparts. In addition, because pressure and flow forces against the spool resist shifting, those forces may hang up solenoid valves and prevent them from shifting entirely or at all. As a result, you&#;ll find that the industrial stack valve is limited to size D05 by flow forces and requires pilot operation for anything more than 30 gpm (about
120 lpm) or so.
However, the flow forces that sideline electrically shifted valves are easily overcome with good old-fashioned bicep power. A lever valve large enough to handle 80 gpm (300 lpm) requires a giant handle to shift the spool. However, these giant valves with giant handles come at a giant price comparable to pilot-operated valves with similar flow capacity.
Even large valves such as the D08 pilot-operated solenoid valve may flow upwards of 160 gpm (600 lpm) or more. They still cannot perform a simple task so easily achieved with a lever valve &#; a partial opening. Solenoid valves, by their nature, shift between two or three discrete positions where one or two coils pull the spool from its spring-biased position to its fully open position. Indeed, a lever valve under a steady hand may flow anything from droplets to fully open and everywhere in between. But unless you&#;re prepared to hire many workers to operate many valves, anything more than a single operator working your machine&#;s lever valves may motivate you to consider the electrical alternative &#; proportional valves.
Proportional valves, in many cases, look exactly like the spool valve they&#;re based upon; cartridge or CETOP valves, for example, are hard to tell apart from their &#;bang-bang&#; counterparts. The difference between standard solenoid valves might be the spool and coil in many cases. The spool of a primary solenoid directional valve allows little or no partial flow off-center; once the valve shifts, full flow potential is available. On the other hand, proportional valve spools require metering notches so that even a minute valve shift allows a throttled volume to flow.
Proportional valve coils must translate their incoming power signals into a variable magnetic field that tugs the plunger, which in turn shifts the spool to varying degrees. The Pulse Width Modulated signal produced by the electronic valve controller maintains a constant voltage but varies the length of time the signal is &#;on&#; (see Diagram 1). By varying the pulse width, the valve controller essentially varies the current to the valve to control the strength of the magnetic field, thereby the metered flow output from the valve.
The performance range from the poorest to the best proportional valves spans a gap nearly as wide as hydraulic pumps. Basic &#;dumb&#; prop valves accept only a simple PWM signal from the valve driver with little attention given to accuracy. These basic, direct operated, open circuit valves act much like a machine operator moving the handle of a lever valve without paying attention to how far she&#;s moving the handle or the effect her operation has on the machine.
There is plenty of value in the basic proportional valve because they still offer variability where standard solenoid valves have none. Although the basic valve is not very accurate, you can still call upon it to offer variable flow output anywhere from just past its dead band*1 to open fully. The speed at which the valve opens may also be programmed to prevent the downstream actuator from starting abruptly. The valve driver also controls the ramp rate and may be set to open quickly like a solenoid or even as slowly as five seconds or more. The adjustable ramp rate alone often justifies the cost of entry, especially for applications where a jarring stop or start would be detrimental, such as a bucket boom.
As electronics evolved, becoming smaller and more powerful, engineers developed valves to utilize sophisticated controls. Previous iterations of proportional valves used separate valve driver cards, which were customarily mounted to the rack in electrical cabinets. However, the compact electrical circuits installed into the proportional valve&#;s wiring box gave the valves a compact and economical solution to separately purchased cards.
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The valve driver installed directly to the proportional valve has advantages other than cost and simplicity. The number of control options may be reduced because valve performance parameters are known. The dither frequency*2, dither amplitude, dead band and often the input mode come factory programmed to suit the performance of the particular valve.
If your application requires a series of proportional valves, and especially if those valves each perform differently, a separate valve driver suitable for multiple outputs might make sense for your application. However, dedicated valve drivers must accommodate proportional valves from all manufacturers, so parameters must be adjustable to meet the requirements of each valve.
The universal valve driver should contain adjustable parameters to meet the dither, dead band, ramp time and other features required by the valve(s) it controls. Additionally, the valve driver must recognize the analog control input as created by the PLC or controller, and the best drivers offer universal inputs. The most popular analog inputs are 0-5 VDC, 0-10 VDC and 4-20 mA, which for the most part, depend on designer preference rather than performance (although many designers use 4-20 mA because of its natural resistance to interference over long distance).
This wouldn&#;t be the &#;s if all electronic valve controllers were still using rotary potentiometers to dial in all the adjustable valve parameters. Modern valve controllers use wired CAN inputs or wireless Bluetooth inputs to drive valves. Every performance parameter may now be adjusted using a smartphone or desktop app. Once operating, these new controllers utilize Industry 4.0 concepts such as diagnostics and data logging, which help improve productivity or troubleshooting. Imagine knowing immediately that the analog X-axis input coming from a joystick is mysteriously attenuated compared to the Y-axis input, resulting in erratic operation of your excavator. The previous method of diagnostics might have started with hours of hydraulic troubleshooting before the technician even began to test the electronics. How long before an accurate measurement of each analog input would yield the same result?
Further to high-performance valve controllers are the options within high-performance valves themselves. For example, an intelligent controller operating a dumb prop valve will not offer the accurate flow resolution or response time as higher performance valves. To increase performance, manufacturers added a spool position feedback to the hardware package, which measures the spool position and compares the actuator position to the target position.
The most basic form of spool position feedback uses an inductive sensor that measures the position of a magnetic plunger to relay to the controller the actual spool position. Any flow forces, contamination or inherent imprecision that prevents the valve spool from accurately positioning (and therefore accurately flowing) results in the controller compensating with a modified output signal in either direction until the spool is measured in the correct position, once again.
In actuality, the spool position may be measured and corrected hundreds of times per second, especially when the method of spool position feedback is an LVDT. Standing for Linear Variable Differential Transformer, these devices are the gold standard for position sensing technology. Because of their accuracy, the valve can respond more quickly to and accurately to its target position.
Although larger proportional valves are inherently less accurate because it&#;s more challenging to control the inertia of the larger spool, you&#;d be surprised at just how well large valves can perform. The cream of the crop D08 valves uses a pilot valve with the aforementioned LVDT to accurately control pilot flow while also employing an LVDT on the main-stage spool. A PID control circuit compares the target input value to the actual position of the main-stage spool and then corrects its output as needed.
High precision proportional valves are used on anything from flight simulators (smaller valves) to injection molding (larger valves). Although high-end valves are many times more expensive than even the largest lever valves, in this Industry 4.0 world of automation, flexible manufacturing and big data, proportional valves will continue to cement hydraulics as the primary control method for powerful machinery.
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