However, these models use wearable components such as rolling diaphragms and pulsed solenoid pilot valves to move the piston to regulate pressure. This renders air-piloted valves less suited to applications with constantly changing set points, which produce heavy solenoid cycling rates that can soon wear out these components. Worn components can result in degraded performance and excess air consumption. So these valves may perform well in static control tasks, but may be less appropriate in dynamic applications.
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Finally, air-piloted models typically offer minimal adjustment capabilities to optimize performance.
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Direct-acting proportional coil solenoid valves are less suited than air-piloted models to applications requiring the smallest power consumption or the lowest purchase price.
However, they may actually offer greater advantages than their more popular counterparts. They fit a considerably wider range of applications, and their simpler construction (with fewer mechanical parts) and principles of operation generally result in more dependable performance.
In fact, the more robust construction of direct-acting proportional coil solenoid valves often substantially increases their service life compared to air-piloted proportional valves. This renders the direct-acting types more appropriate in dynamic applications where wearable components prove less durable.
In addition to improved robustness, the direct-acting design provides several other technical advantages. Overshoot is a tendency to accelerate toward the set point and then go past it, having to reverse back toward it &#; perhaps going past it again in the other direction, and so on. Common in many systems that are tuned for short response times, these oscillations typically take less time to settle in direct-acting proportional coil designs. The overshoot settling time is short because the valve directly moves the piston by varying current to the coil.
By contrast, air-piloted solenoid valves must move air into and out of a pilot control chamber. This is a slower process reliant upon pilot valve cycle rates.
Direct-acting proportional coil designs also typically provide a finer resolution of pressure control, which can be critical in some applications. For example, some polishing applications demand small pressure changes to avoid damaging parts. Direct-acting units achieve this by precisely varying coil current to make very controlled pressure adjustments. Solenoid air-piloted units have stepped pressure adjustments based on pilot valve cycle times that can result in pressure swings that are too large in some applications. The direct-acting design possesses greater speed, responsiveness, and resolution that makes it superior to air-piloted designs for a number of applications.
Air-piloted proportional pressure regulators commonly offer a handful of settings to accommodate varying application requirements. However, these canned settings can meet with limited success for a number of applications. And even proportional pressure control valves that can be adjusted for specific applications typically do so only in one of two ways:
Instead, for many applications, users should consider direct-acting solenoid valves featuring newer valve technology that offers digital tuning software. For example, parameter adjustment software incorporating a digital oscilloscope supplies quick, easy adjustments that provide vital benefits for many applications.
Such software places a welcome amount of control in users&#; hands. It gives them the power to change almost any setting &#; including dead band, analog signal type, and shutoff level. In the prototype stage of OEM assembly, design engineers can immediately see the effects of any changes they make; for instance, they can quickly and easily fine-tune the pressure received by a cylinder to obtain a specific force. If necessary, they can make these real-time changes while on the receiving expert advice from the manufacturer&#;s tech support specialists. These features all increase speed to market by allowing an OEM to modify the proportional valve as needed during design revisions.
This type of software enables much faster tuning and resolution of issues or questions. Users can often make and validate a change in seconds, instead of days or weeks. (NOTE: While dynamic applications are often the best candidates for tuning software, applications with ultra-stable pressure control, such as in leak testing situations, may also greatly benefit.)
Contact us to discuss your requirements of hydraulic proportional valves. Our experienced sales team can help you identify the options that best suit your needs.
Proportional solenoid valves offer important advantages in the right applications.
Prospective purchasers should evaluate their choices carefully, considering application demands, type of control required, valve service life, and useful new technologies such as software-based digital tuning. Weighing these factors in making a final proportional valve selection can help save time, effort, and cost for OEMs, integrators, and end-users.
Pneumatic flow control valves circulate air and gases throughout a larger pneumatic system by either allowing or inhibiting the flow of pressurized air. The function of proportional valves falls into a wide spectrum somewhere between on/off solenoid valves and electrohydraulic servo valves. With proportional valves, output flow isn&#;t exactly linear in relation to the control signal. Despite their nonlinear nature, proportional valves offer an inexpensive way to control force, velocity, and position on machinery needing high-speed responses at high flow rates. So, why use proportional control valves?
Many proportional valves are modified versions of four-way solenoid valves in which proportional solenoids replace conventional solenoids. In operation, the solenoid force is balanced by spring force to position the spool in proportion to the input signal. Improving the positioning accuracy is achieved by replacing the centering springs with a positioning sensor at the end of the spool. When the spool reaches the specified position, the sensor signal then cancels the solenoid signal. As a result, the spool in low performing valves have considerable overlap in the null position. This overlap causes flow dead band that can cause errors and instability in positioning systems.
One advantage of the Kelly Pneumatic proportional valve design is the elimination of friction that is common to spool valves. As described above, in a standard spool valve design the mechanical control of the spool position within the valve sleeve creates the resultant flow of the valve. The use of a spool creates friction between the spool and valve seat, and this friction can limit the life of the spool valve. The friction inherent in this design also requires dithering to avoid the spool sticking when in motion. Dithering can cause issues with stable flow control.
In contrast, our proportional valve uses a single armature design, which offers virtually frictionless performance. Precise flow control 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 will result in a variable outlet flow rate, avoiding friction and the need for dithering.
A defined trend in the valve industry is emerging: an increased difficulty in differentiating between proportional valves and servos. Traditionally, proportional valves couldn&#;t match servo valves&#; performance, and they were mostly used in open loop applications. They were mass produced, whereas servos needed meticulous and precise manufacturing, which made them up to 10 times more expensive. Proportional valves also had wider clearance, so they tolerated contamination more than servo valves. All of that will quickly become a thing of the past as proportional valves start to outperform servos.
For example, closed loop proportional valves that function just like servo valves are available in many places. Using high force, continuous action solenoids with minimum friction moving parts and quick response electronics, proportional valves have servo-like performance without the contamination and high pressure drop of conventional valves. It requires precise manufacturing, but it&#;s still cheaper than servo designs.
Proportional valves control flow, actuator position, velocity, toque, and they can synchronize the action of many different cylinders. They work well for applications such as press systems, molding machines, and traditional servo markets like flight simulators and airframe testing. Also, they prove ideal in areas that already use proportional valves but need a performance upgrade with a closed loop system.
The technology advanced, and continues to advance, to the point that some manufacturers can make proportional vales that basically work like mass-produced servo valves with larger tolerance allowances and looser fits than the standard servo line. Adding electronic feedback creates performance that&#;s almost as good as a servo valve at a lower cost as well.
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