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.

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 2020’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.

2 Static friction, or “stiction,” is a tendency for two moving surfaces to become stuck against one another despite a force’s effort to move them. With proportional valves, that initial stiction may result in the valve over-shooting its desired position when the shifting force finally overcomes that friction. A valve may not even move at all if the command signal is small enough. Dither is a programming feature that essentially vibrates the spool to prevent any static position inside the valve body. The distance moved under vibration is minute and imperceptible to the output flow, but enough to prevent the valve from hanging up.

How Proportional Solenoid Valves Are Used in Manufacturing

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Proportional Solenoid Valves are often leveraged in manufacturing processes. They are utilized for applications where flow or output pressure requires variations, remote control and precision. Proportional Solenoid Valves are widely used in medical device manufacturing and semiconductor production. They are also utilized in life sciences product manufacturing, among other industries.

Proportional Solenoid Valves are ideal for use in the production process because of their:

• Dependability
• Cost-effectiveness
• Fast response times
• Accuracy in control of flow or pressure
• Flexibility

How Does a Proportional Solenoid Valve Work?

A Proportional Solenoid Valve controls fluid flow rate or pressure. It does this by adjusting the opening size within the valve. Controlled remotely, the plunger/spool opens or closes a valve position. It can be partially open – not just open or closed, hence the term “proportional.”

The regulated flow rate adjusts parameters affecting a process in a system. These parameters include pressure or flow. In an automatic control valve, the restrictor (in this valve the restrictor is a plunger/spool) is controlled by a signal from an actuator.

A solenoid is an electric coil with a moveable plunger/spool at the center. A Proportional Control Valve uses a direct solenoid or solenoid pilot as an actuator/controller for variable valve positioning. Direct solenoid control is recommended for dynamic applications.

Solenoid pilot control is a lower cost alternative for static applications. Closed loop control with feedback is also available if precise output is required in dynamic applications.

Are Proportional Solenoid Valves Adaptive?

Proportional valves are flexible. Engineers will have the capability to choose the flow rates that they need. They just need to make sure the rates are within the valve’s range. Manufacturers can provide custom variations on valve design to suit the requirements of a particular system.

What Does the Inside of a Proportional Solenoid Valve Look Like?

Proportional Solenoid Valves are components in manufacturing processes. They offer precision control of fluid/ flow rates or pressure.

This type of valve is known for their:

• Dependability
• Cost-effectiveness
• Response times
• Accuracy in control of flow or pressure
• Flexibility

How Are Proportional Solenoid Valves Used in the Production Process?

The precision of Proportional Solenoid Valves and their variable output capacity makes them indispensable for production applications. They are an integral part of manufacturing processes. These valves are utilized for applications where flow or output pressure requires variations, remote control and precision.

The capabilities of this type of valve make them a suitable option for medical device applications that require precise fluid/pneumatic flow or pressure control.

The medical device manufacturing industry uses Proportional Solenoid Valves in anesthesia and ventilator gas mixing. These valves are in ventilator patient delivery for controlled breathing. For medical components and packaging, they are ideal for leak detection.

Semiconductor manufacturers utilize Proportional Solenoid Valves for precise gas control. The life sciences field uses them in the production of pharmaceuticals. They are used in the manufacturing of biotechnology-based foods and medicines. In the consumables area this type of valve is utilized in food processing and the production of nutraceuticals.

Other uses for Proportional Solenoid Valves include:

• Leak detection
• Valves and coolant
• Braking systems
• Irrigation
• Web tensioning
• Counterbalancing
• Spraying

What Are Safety Features of Proportional Solenoid Valves?

This valve type is known for its safety features. This valve features parameter control as it can be set up on a computer. It has signal control, meaning that the valve will close if it receives an input signal less than the predetermined value. The flow of gas, fluids and air features remote control, which is another plus for safe use.

How Long Do Proportional Valves Last?

The life of a valve is dependent upon its design and level of maintenance. Typically, they do not show wear because of inherent low friction. This type of valve has low mass armatures which have no contact points.

What Are Primary Benefits of Proportional Solenoid Valves?

This type of valve can be advantageous where variable flow rates are needed. They give the user exceptional control of fluid/pneumatic flow rates or pressure. Proportional Solenoid Valves provide an elevated level of precision control for manufacturing applications.

Who can I contact if I have questions?

JHFOSTER is committed to providing solutions that increase efficiencies and exceed production demands. We reduce manufacturing costs while raising your firm’s level of success.

Contact an expert at JHFOSTER today. One of our team members would be happy to assist you in choosing the right valve for your application.

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