Direct acting solenoid valves require a significant amount of electrical power to change from a normally closed state to an open or actuated state. Once the valve is fully actuated, the amount of electrical energy required to maintain this state is significantly lower than the initial energy required to initiate the actuation. Therefore, in direct acting solenoid valve applications where battery life and/or heat generation is a concern, the utilization of a “hit and hold” electrical signal can result in significant power savings and heat reduction while still maintaining the required pneumatic function. This article explains basic direct acting solenoid valve theory, test methods to characterize actuation and drop-out current parameters, PWM hold control methodology, and general recommendations/typical examples to support end-users in reducing power consumption and heat generation in their applications.
This blog is part one of a three-part series that comprises a complete white paper "Efficient and Effective Direct Acting Solenoid Valve Control: The Benefits of Using “Hit and Hold” Electrical Control on Direct Acting Solenoid Valves". Download a copy here.
Basic solenoid valve operation
To understand the theory and benefits of hit and hold electrical control, we must first have a basic understanding of direct acting solenoid valve design and operation. A typical direct acting solenoid valve is comprised of the following sub-components: (Reference Figure-1).
- Insulated copper coil
- Pole, flux bracket, flux bushing, armature
- Plunger seal, stem, o-ring, body
- Return spring
Figure1: 2-way/2-position direct acting solenoid valve.
Note: Iron core/solenoid components are shown in blue. Pneumatic components are in green and the return spring is orange.
When an electrical potential is applied to the coil and current flows through the copper conductor, the coil component transforms into a magnet or solenoid. The magnetic field generated from the solenoid is then captured and directed through the iron core components of the valve as illustrated in figure 2.
Figure-2: Magnetic field captured and directed through iron core components
As the magnetic field is established, opposing magnetic poles are formed between the armature and pole subcomponents as illustrated in figure 3. (Also illustrated are the lines of magnetic force, or flux density, connecting the poles in green and blue).
Figure-3: Magnetic pole locations
The resulting magnetic force generated between these components is inversely proportional to the distance squared between the two components as illustrated in Figure-4. This distance is referred to as the magnetic gap.
Figure-4: Magnetic force on armature
When the magnetic force on the armature exceeds the installed load of the return spring, (and any supplemental pneumatic differential force), the armature will actuate/move to contact the pole as illustrated in Figure-5. Specific to the 2-way/2-position solenoid valve illustrated in Figure-5, once the actuation occurs, the fluidic path is opened between the valve ports.
Figure-5: Actuated valve state (open fluidic path)
To close the fluidic path and return the valve to a normally closed state, the voltage source is removed/disconnected from the coil. Once the power source is removed, the magnetic field collapses, and the spring returns the armature to the normally closed position.
It is important to understand that solenoid valves are current driven devices where the magnetic field strength of the solenoid is proportional to the amount of current traveling through the coil. Therefore, the two valve states described above, (actuation and drop-out), will ultimately be defined by the effective current traveling through the coil.
Now that you have an understanding of the basic solenoid valve operation, read the second part of this three blog series titled: Capitalizing on Valve Mechanics. This second part gives an in-depth understanding of how these principles tie into reducing consumption and heat generation within the solenoid valve.
This blog is part one of a three-part series that comprises a complete white paper "Efficient and Effective Direct Acting Solenoid Valve Control: The Benefits of Using “Hit and Hold” Electrical Control on Direct Acting Solenoid Valves". Download a copy here.
Paker Precision Fluidics has been designing and building solenoid valves for over 25 years. Our engineers specialize in helping OEMs update original valves that are producing low yields. Our applications engineering team is always available to provide recommendations and customize equipment to customer specifications. To learn more about Precision Fluidics' miniature valves, flow controllers, pumps and accessories, please visit our website, or call 603-595-1500 to speak with an engineer.
This article was contributed by Phil Dodge, senior engineer at Parker Precision Fluidics.
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