Absolute Advantage: Proportional Valves for Pressure ...
Absolute Advantage: Proportional Valves for Pressure ...
Absolute Advantage: Proportional Valves for Pressure Regulation
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FeaturesBy John Kelly, Engineering Manager, Kelly Pneumatics
For decades, electronic pressure regulators (or IP transducers) have been a staple of pneumatic control systems. Some electronic pressure regulators still use solenoid valves, but this can cause various issues in pressure control applications. Many of these issues can be solved by incorporating proportional valve technology as both flow inspiration (fill) and flow expiration (vent) for pressure control.
To better understand the advantages of proportional valve technology for pressure control, we need to consider the general properties of electronic pressure regulators and proportional-valve design.
Electronic pressure regulators have many advantages over mechanical regulators. A mechanical regulator maintains pressure downstream, most effectively with constant flow rates. However, downstream flow and pressure often fluctuate in most applications. Pumps and compressors deliver pulses, and the pressure fluctuates with demand. When a mechanical pressure regulator lacks accuracy or can&#;t respond fast enough, the result is uneven pressure output. This simple device might be good enough when accuracy isn&#;t as crucial. However, an electronic or digital pressure regulator delivers better accuracy and dynamic output pressure regulation by using a control signal and the feedback signal to create closed loop pressure control.
Electronic pressure regulators maintain a constant output pressure within a pressurized system even if there are fluctuations in the incoming pressure to the regulator. Mechanical pressure regulators are typically only adjustable by hand, limiting their usability in high-tech applications. When using an electronic pressure regulator with a digital control system, the precise output pressure is controllable via slight adjustments to the control signal. An internal pressure transducer creates a true closed loop control device and offers a feedback voltage of the current output pressure. By reading this feedback signal from an electronic regulator, control systems can make real-time adjustments, greatly improving consistent pressure regulation in the most demanding applications.
Electronic pressure regulators are a big improvement over mechanical regulators; they are fairly simple and effective at allowing better control. They use a fill valve and a vent valve to maintain the outlet pressure at the desired set point. A small internal pressure sensor monitors the output pressure, and the digital controller adjusts the flow output of the internal valves to retain the required set point (i.e., closed loop control). That means that when you set the desired pressure, the regulator maintains this pressure even with changing flow rates through the system. The internal pressure transducer gives immediate feedback to adjust the flow of either the fill or vent valve so the output pressure remains static.
When the control signal increases, the internal proportional valve activates, increasing pressure into an internal pilot chamber. When this happens, more of the inlet pressure passes through the proportional fill valve and into the pilot chamber. The pressure in the pilot chamber grows and causes the upper surface of the diaphragm to operate. Because of this, the air supply valve linked to the diaphragm opens, and a portion of the supply pressure becomes output pressure. This output pressure goes back to the control circuit using the pressure sensor. The fill valve continues to slightly adjust until the output pressure is equivalent to the desired set point of the control signal.
As we&#;ve shown, in standard electronic regulator design, using very accurate and fast fill and vent valves greatly affects the overall pressure control characteristics. But how do proportional valves achieve this?
Proportional valves
Traditional electronic pressure regulators typically employ a dual on/off valve design, with both an inlet and exhaust valve. Changes in pressure are controlled by cycling the on/off valves to open and close via pulse width modulation. Using pulse width modulation can be problematic for control systems for various reasons. Problems associated with using pulse width modulation of on/off valves and frequent valve cycling include a much shorter product life, product noise, control circuitry deadband, pressure oscillation, and inconsistent performance during dynamic output conditions.
Some pressure regulators incorporate an inlet proportional valve to compensate for these performance issues. However, these units typically use a single on/off solenoid exhaust valve to vent excess pressure and compensate for changing flow rates to the system. Typically, this is achieved with rapid valve cycling, so applications with dynamic output pressure rates are more susceptible to oscillation, as the exhaust valve must continually cycle at a rapid hertz to compensate for the dynamic pressure changes.
The critical component of a Kelly Pneumatics electronic pressure regulator is the dual proportional valve design. One valve controls the inlet and the other the exhaust; each valve proportionally increases or decreases the output pressure, respectively.
The incorporation of two proportional valves avoids many problems associated with traditional on/off valve regulators. This is mostly due to the inherent design of the proportional valves that control pressure regulation. Each proportional valve uses a single armature design, which offers virtually frictionless performance. Outlet pressure/flow is acquired by proportionally moving the single valve armature away from the valve inlet orifice. The total travel distance of the valve armature is only thousands of an inch. A slight increase or decrease in the armature distance from the valve orifice results in variable outlet flow rates and pressures.
Conversely, on/off valves do not typically offer variable flow rates and use an armature with a single calibrated travel distance. The valve armature during the off state is completely closed against the valve seat, while during the on states it is completely opened against the valve housing.
On/off valve cycling can be can be problematic when considering the aforementioned use of pulse width modulation in other pressure control units that must cycle valves at a particular hertz, which can substantially lessen the life of the solenoid valves being cycled and subsequently the product itself. This is especially true of applications with consistent outlet flow or frequently changing output pressures to the system when valve cycling is more consistent. Therefore, in many applications, valve cycling can be constant for these products, severely limiting product life. In the Kelly Pneumatics electronic pressure regulator, however, the internal proportional valves need only change their respective armature distance to change the output pressure and flow of the pressure controller. For example, when an increase of flow occurs downstream, the inlet proportional valve immediately adjusts to compensate by slightly adjusting the internal valve armature distance in a single movement. Additionally, during steady states, the proportional valves maintain the valve armature distance, with no wear to the valves themselves.
Using on/off valves for either fill or vent valves in pressure controllers can lead to problematic deadband in control systems. Due to the pulse width modulation of cycling on/off valves, this can cause rates of deadband in the control range at certain set points because the control algorithm may not calculate a small enough adjustment when cycling the fill and vent valves at particular frequencies. Since the proportional valves are able to finely adjust the internal armature height, fine tuning the output pressure is much more effective, preventing deadband for most set points and fine tune adjustments to the output pressure.
Using proportional valves also avoids the occasional pressure oscillations caused by pulse width modulated on/off valves. In static output conditions, the constant valve cycling of on/off valves can cause sudden missteps in pressure output with varying downstream flow conditions. These oscillations can affect applications that require constant output pressures over long periods between set point adjustment.
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In comparison, any changes in downstream flow conditions are quickly adjusted for by the proportional fill and vent valves, since the internal valve armatures only need to make slight adjustment to the immediate armature position (oftentimes only thousands of an inch).
Another issue with using pulse width modulation is the audible sound generated by rapidly operating on/off valves. Many of these older designs with on/off valves for filling and venting must operate the valves at an oscillation in hundreds of cycles a minute, meaning that the valves have to fully close and open, generating audible clicking.
In contrast, since the proportional valve design does not require oscillation, there is no sound generated by the valves when changing flow outputs for either filling or venting. This is because the proportional armatures are adjusted through a single armature movement, rather than requiring a full on/off cycle.
Proportional valves for both fill and vent valves in electronic regulators offer huge improvements for pressure control. From extended product life, to improved accuracy and quieter operation, proportional valves in electronic regulators make all the difference.
Understanding proportional valve designs and uses
By Josh Cosford, Contributing Editor
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.
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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