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This article takes an in-depth look at proportional solenoid valves.
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A proportional solenoid valve is a mechanism for controlling fluid flow through the use of a restrictor. In many applications, the flow rate regulation affects process systems regarding level, pressure, temperature, weight, thickness, humidity, density, color, and viscosity. The actuator for a proportional solenoid valve is the solenoid used for variable valve positioning.
A traditional solenoid valve is a two-way valve that can be open or closed. A proportional solenoid valve provides the same functions as a traditional solenoid but with greater control over the positioning of the plunger and spool. They have a wide range of positions to meet the needs of any flow control application.
To regulate the flow rate, proportional solenoid valves vary the position of the valve plunger. The positioning of the plunger is electromagnetically operated and controlled by a magnetic field that is created by power supplied to the solenoid coil using the input it receives from the driver. The positions of a proportional solenoid valve are open and closed, both of which are held in position without a source of power.
In simple terms, a proportional solenoid valve works against a spring to determine the size of the valve opening. When working with fluids, their characteristics are changed by flow, pressure, temperature, and purity, requiring changes to the spool position. Additionally, changes in the pressure and thickness of a fluid affect the speed of an actuator.
Although servo valves are ideal for monitoring the characteristics of fluids, proportional solenoid valves provide the same accuracy as servo valves and are far less expensive. They are used to improve the performance of machines by reducing pressure on output lines.
The unique nature of proportional solenoid valves allows them to be activated remotely to control the flow of air, gas, or fluids. A common use for proportional solenoid valves is to control fluid flow in engines and hydraulically operated machinery. Their more specialized functions include controlling the flow of power and compressed air in marine applications, construction, food processing, woodworking, material handling, and metalworking industries.
The defining factors for proportional solenoid valves are their linearity, frequency response, and hysteresis, how much the valve lags. These three factors determine how accurately the valve will control flow.
The purpose of a proportional solenoid valve in irrigation is to control water flow and automatically operate sprinkling systems. Initially, the valve is adjusted in accordance with how much it will open and close. Once set, proportional solenoid valves operate automatically if the upstream pressure is greater than the downstream pressure.
They can be set to be open or closed before being actuated.
A coolant proportional solenoid valve controls the flow of antifreeze and cleaning solutions in a coolant system. They have an adjustment screw or knob used to set how much the valve will open when actuated. When installed, all coolant proportional solenoid valves remain closed until actuated.
An electronically activated coolant proportional solenoid valve changes the directional flow of coolant, adjusts the flow rate, and proportionally distributes coolant volume.
Brake proportional solenoid valves limit the pressure that reaches the rear brakes to prevent rear wheel lock-up. The control of the valve allows a vehicle to stop safely and smoothly. The rear brakes of a vehicle require less pressure than the front brakes and are the reason for using proportional solenoid valves.
The types of brake proportional solenoid valves vary according to the type of braking system and are normally custom designed to fit the system.
Proportional solenoid valves are widely used in the medical field to control the flow of liquids. Medical applications require precise control and exceptional accuracy. For these reasons, proportional solenoid valves are used since they are known for their excellent control and safety.
The use of proportional solenoid valves in medicine includes providing doses and controlling the release of liquids.
There are several different types of proportional solenoid valves for use with hydraulic equipment, including flow, directional, and pressure relief valves. The reason proportional solenoid valves are used with hydraulics is their many positions. They can hold any position along with the proportional solenoid rated stroke. Proportional solenoid valves have a more sophisticated design than traditional on-and-off solenoids.
In a hydraulic system, proportional solenoid valves are part of a closed-loop system requiring precision and accurate control. Common types of proportional solenoid valves for hydraulics are bolt, screw, and three pin. Regardless of the type, they control the flow, pressure, and direction of the hydraulic media.
The gas supply to a furnace must be precisely controlled to avoid overheating the furnace and maintain a controlled temperature in a residence or business. Proportional solenoid valves regulate the gas supply to ensure proper temperature control. A differential temperature signal gives feedback regarding the gas flow when the desired temperature is reached. The proportional solenoid valve shuts off gas flow at that time.
The purpose of a ventilation machine is to send and receive respiratory air for the lungs. They are used when a patient is having problems breathing during treatment or due to an injury. The air must be transferred to the patient at the correct pressure, flow, and oxygen ratio. Precision flow control is exceptionally important for the life of the patient.
Using precision calculations, proportional solenoid valves are designed and engineered with the correct voltage to provide the type of control a ventilation machine requires.
Leak detection is necessary in a wide range of applications because both fluid and gas leaks can cause major damage. Proportional solenoid valves are designed to apply increasing pressure at a defined profile to test a system. In some configurations, the proportional solenoid valve is attached directly to the control panel and turns off the supply when a leak is detected.
The position of a proportional solenoid valve is adjusted according to the voltage that crosses the coil. The higher the voltage, the greater will be the magnetic current and the more the valve spindle moves. The difference between a proportional solenoid and a regular solenoid is the ability to control the position of the spool using a regulating magnet.
When a proportional solenoid valve is used in a system, it is referred to as proportional technology. They are designed to regulate load build-up in modern fluid technology systems. Their varying positions soften the change in forces directed against the actuator.
Proportional directional control valves are mainly used in hydraulic systems. They can be direct or pilot operated and can operate in a closed-loop application. Directional proportional solenoid control valves direct the flow of fluids to an available cylinder. They are the solution to the problem of opening multiple valves with multiple solenoids. Since they have an infinite number of positions, they can easily adjust to differences in flow volume.
Proportional solenoid flow control valves control the flow rate by varying the position of the valve plunger. It is a method for controlling pressure, level, and temperature. The plunger on a proportional solenoid flow control valve is changed by the power sent to the solenoid coil, which changes the stroke positions. The power for the valve is controlled by a pulse width modulator (PWM).
Pressure relief proportional solenoid valves are a remote method for increasing or decreasing pressure. They are used to limit the amount of pressure in a system using electronically transmitted signals. Proportional solenoid pressure relief valves are designed to operate at various pressure ranges to increase controllability. They can be used for PTO, transmission, pressure, and hydraulic control.
A proportional solenoid pressure-reducing valve keeps pressure constant and regulated on the secondary side of a system regardless of the pressure fluctuations on the primary side. They eliminate the need for an additional pressure relief valve. They reduce pressure in relation to the solenoid current and work independently of the inlet pressure. When inactive, they leave the connection to the tank fully open with a minimum of pressure.
A proportional solenoid pressure reducing valve keeps pressure constant and regulated on the secondary side of a system regardless of the pressure fluctuations on the primary side. They eliminate the need for an additional pressure relief valve. They reduce pressure in relation to the solenoid current and work independently of the inlet pressure. When inactive, they leave the connection to the tank fully open with a minimum of pressure.
The 3-way proportional pressure-reducing valve is used for pressure control by converting a single input into a proportional pressure signal. It is connected with a subplate and has exceptional precision and sensitivity. As the flow demands vary, the opening of the valve will change to maintain proper circuit pressure. They are designed to vent a load when there is a pressure spike.
Many applications use a 3-way proportional solenoid reducing valve, including hydraulics, turbochargers, transmissions, oil pumps, and brake systems.
A canister proportional solenoid valve is used to control the purge or flow of vapors from the canister to the engine manifold. The proportional solenoid valve provides precision control of fluid flow and simplifies the operation of the system. It can be used for canister diaphragm flow control and vacuum regulation, providing a multifunctional unit for improved performance and cost savings.
Piston proportional solenoid valves open when energized and close when de-energized. When the coil of the valve is energized, its pilot valve opens and relieves the pressure above the piston. The relief of pressure causes the piston to be lifted from its seat by the plunger. When the coil is de-energized, the pilot valve is closed by the spring, and a bleed passageway opens to allow pressure to build above the piston and force it back into its seat.
A piston proportional solenoid valve must be mounted horizontally with the solenoid enclosure in a vertical position and on top. The valve operates from zero to maximum differential pressure.
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Pilot proportional solenoid valves use the differential pressure of the medium at the inlet and outlet, with the pressure at the inlet being higher than the outlet. The valve has a two-way pilot solenoid that uses a piston or diaphragm to seal the main seat. The construction of the diaphragm necessitates a pressure drop across the main valve seat in order to remain in the open position using the solenoid plunger.
When the solenoid is energized, its plunger opens the pilot orifice, releasing the pressure on the top of the diaphragm to the outlet side of the valve through the pilot channel of the main body. The change results in an unbalanced condition that causes the main line pressure to open the diaphragm on the main seat.
When the proportional solenoid de-energizes, the pilot orifice closes, and full line pressure is applied to the top of the diaphragm. The bleed orifice in the diaphragm applies pressure to the inlet side of the valve, which supplies sufficient force to close the seat of the valve.
Pilot proportional solenoid valves are continuous-duty valves that can be constantly energized without overheating. The unique nature of pilot solenoid valves allows them to use line pressure as part of their operation. The two proportional solenoids open and close the pilot orifice in order to change the pressure between the inlet and outlet side of the diaphragm and valve seat.
A proportional solenoid valve controls variations in flow and pressure in accordance with electrical input. They are found in places where the pressure and flow continuously change and replace fixed flow or pressure valves. A proportional solenoid valve increases the accuracy and efficiency of a system and reduces the number of components.
A common place where proportional solenoid valves are found is in conditions where precision control is required for acceleration and deceleration. They can reduce flow variations that cause changes in inlet and outlet pressure.
A proportional solenoid valve spool has "V" notches to provide control over fluid flow, the actuator&#;s speed, and the fluid&#;s direction. This allows a single valve to control direction and actuator speed. The fact that the spool does not shift completely at one time is what makes a proportional solenoid valve unique.
A proportional solenoid valve receives its instructions from a programmable logic controller (PLC) or a computer. The driver card is a pulse width modulator (PWM) that can control other devices. The input from the PLC sends current to the valve&#;s solenoid. The spool shifts due to the electromechanical force. The driver uses a comparator with hysteresis to pulse a switch, which results in a constant amplitude PWM current signal.
The solenoid coil is the central part of a proportional solenoid valve. It is made up of a coil, tube, and armature. Copper wire is used to make a solenoid coil and is divided into Classes F, H, and N. Each of the three classes can withstand varying temperatures from 311 °F (155 °C) up to 392 °F (200 °C). When the proportional solenoid valve is electrified, the solenoid coil creates a magnetic field that forces the plunger to move proportionally.
The plunger is the part of the proportional solenoid valve that is connected to the device being controlled and opens or closes a valve. Plungers can take various shapes, including square or rectangular bars and round pistons. When the proportional solenoid valve is electromagnetically activated, the plunger is positioned by the directions from the PLC and moves proportionally to actuate the valve.
When the spool shifts, its motion is measured by a position feedback transducer. The purpose of the feedback transducer is to check the position of the spool. Its data is sent to the driver card. The signal the driver card receives will be compared to the input from the PLC. If the two sets of data do not match, the driver will reset the position of the spool until its signal matches the input data.
The typical solenoid valve has the simple function of opening or closing a valve or activating a switch. They are electromagnetically driven and have little controllability. A proportional solenoid valve has controllability and can regulate and monitor the positioning of a valve. They monitor load build-up to reduce the load on mechanical components.
The operations of a proportional solenoid valve can be performed by a servo motor, which is more expensive than a proportional solenoid valve. Since a PLC or computer can easily control proportional solenoid valves, they are a less expensive method for process control.
The foundational principle upon which a proportional solenoid valve operates is that it is possible to control the positioning of a solenoid plunger by varying the DC current. In order to control the static friction of the plunger, the inlet signal is transformed into a pulse width modulation (PWM) signal using controlling electronics.
A PWM is a method for controlling the power supplied to electrical instruments and devices. It is a type of signal that is produced by a digital integrated circuit (IC), such as a microcontroller that takes the form of pulses. For proportional solenoid valves, the PWM frequency is low in the range of 25 Hz to 200 Hz to allow the valve to respond over the duty cycle of control.
Some factors influence the operation of the PWM, the most important of which is temperature. Heat rise in a PWM&#;s coil can influence its output. Their coils should be able to compensate for the increased resistance caused by a rise in temperature. This aspect of the PWM&#;s operation is determined by the operating conditions of the proportional solenoid coil.
With digital control, the proportional solenoid valve moves fully and is held there. Position control moves from zero to full stroke according to the time average current being applied. The voltage to the plunger is turned on and off, which puts the plunger in a fast, weak amplitude oscillation, a balanced condition to ensure constant sliding friction.
The operation of a proportional solenoid valve is similar to that of a normal on-and-off solenoid. When the solenoid coil is electrified, it produces an electromagnetic field that moves the armature of the coil. In the case of a proportional solenoid valve, voltage enters at both ends of the solenoid coil, which drives it to open. Eventually, the electromagnetic field and the spring force balance on the spool, causing the position of the valve to remain unchanged.
When the input from the control source changes, a change occurs in the valve opening to meet the demands of the required parameters.
The system of a proportional solenoid valve has these functions.
A process control system is expensive and has a high failure rate. A proportional solenoid control valve, combined with an intelligent controller and sensors, provides precision control that costs far less than traditional methods.
The main benefit of proportional solenoid valves is providing users with exceptional control over flow rates with heightened precision and accuracy for a wide variety of machine cycles at variable speeds with greater safety. The use of a proportional solenoid valve is dependent on pressure variations, back pressure range, the type of fluid to be controlled, the temperature range of the process, and the ambient temperature.
A key aspect of any piece of equipment is its duty cycle, or the amount of time it is turned on or in operation. Duty cycles, also known as duty factors, are expressed in percentages or ratios. In the case of proportional solenoid valves, their duty cycles are expressed in percentages. If a proportional solenoid valve has a high percentage for its duty cycle, it has a higher amount of power being supplied to it.
The PWM provides a measurement of the actual on time and is measured in milliseconds. The only value being recorded is the length of time that the signal is on. Thus, the PWM is used as part of the calculation of the duty cycle of the valve.
Duty Cycle equals time off divided by time on plus time off multiplied by 100
A proportional solenoid valve controls the flow from zero to maximum as a function of its duty cycle, which can run from zero to 100. They can hold 4% to 5% to 100% open value using different inlet flow rates and electric input rates.
Proportional solenoid valves have become a necessity for fluid control in modern technological fluid systems. They have several advantages over traditional methods that have made them a staple in fluid movement and production operations.
Their advantages include:
The key benefit of proportional solenoid valves is their ability to control pressure. This aspect of their properties is the reason that they are so widely used and why they are the first choice for process regulation.
Directional control valves perform only three functions:
These three functions usually operate in combination.
The simplest directional control valve is the 2-way valve. A 2-way valve stops flow or allows flow. A water faucet is a good example of a 2-way valve. A water faucet allows flow or stops flow by manual control.
A single-acting cylinder needs supply to and exhaust from its port to operate. This requires a 3-way valve. A 3-way valve allows fluid flow to an actuator in one position and exhausts the fluid from it in the other position. Some 3-way valves have a third position that blocks flow at all ports.
A double-acting actuator requires a 4-way valve. A 4-way valve pressurizes and exhausts two ports interdependently. A 3-position, 4-way valve stops an actuator or allows it to float. The 4-way function is a common type of directional control valve for both air and hydraulic circuits. A 3-position, 4-way valve is more common in hydraulic circuits.
The 5-way valve is found most frequently in air circuits. A 5-way valve performs the same function as a 4-way valve. The only difference is an extra tank or exhaust port. (Some suppliers call their 5-way valves, &#;5-ported 4-ways.") All spool valves are five ported, but hydraulic valves have internally connected exhaust ports going to a common outlet. Because oil must return to tank, it is convenient to connect the dual tank ports to a single return port. For air valves, atmosphere is the tank, so exhaust piping is usually unimportant. Using two exhaust ports makes the valve smaller and less expensive. As will be explained later, dual exhausts used for speed-control mufflers or as dual-pressure inlets make this configuration versatile.
Following are schematic symbols for commonly used directional control valves.
2-way directional control valves
A 2-way directional valve has two ports normally called inlet and outlet. When the inlet is blocked in the at-rest condition, as shown in Figure 8-1, it is referred to as "normally closed" (NC). The at-rest box or the normal condition is the one with the flow lines going to and from it.
The boxes or enclosures represent the valve&#;s positions. In Figure 8-1, the active box shows blocked ports, or a closed condition, while the upper box shows a flow path. When an operator shifts the valve, it is the same as sliding the upper box down to take the place of the lower box. In the shifted condition there is flow from inlet to outlet. Releasing the palm button in Figure 8-1 allows the valve spring to return to the normal stop flow condition. A 2-way valve makes a blow-off device or runs a fluid motor in one direction. By itself, a 2-way valve cannot cycle even a single acting cylinder.
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