Solenoid Design - Parker
Parker solenoids are good examples of indirect-acting pilot solenoids. The K-series is the smallest "subminiature" solenoid valve available from Parker.
All Parker subminiature solenoids consist of KxHxx. The second number in the model defines the valve as being three-way (3) or four-way (4).
The last two digits are used to describe the method that the valve attaches to the pressurized components as well as the attaching electronics. H01 is a manifold base-mount with wire leads. H03 is a manifold base-mount that attaches to a circuit board PCB.
These solenoids are composed of some variety of the polymer Polyphenylene Sulfide, which is an easily-assembled plastic that penetrates to create a clean, air tight seal throughout the surface. The sections of the coil are assembled, then the rectangular housing is simply poured into position around the functional parts. This is a cheap and easy way to produce them, however the coating is certainly known to flake apart relatively easily. Additionally, if there is an internal problem with the coil, it will be unrepairble since it's integrated to its housing. Other manufacturers such as Humphrey have developed their own methods to keep the coil separate from the housing, and in some cases the two can even be separated and replaced if needed, which isn't possible with Parker solenoids.
Most indirect-acting solenoid valves operate slower than their direct-acting counterparts. This is becuase air pressure is routed in and out of the pilot to push the spool. These characteristics slow the valves down by a considerable amount, but still leaves room for them to cycle into the speeds of 40 cycles per second if properly configured
Parker's four-way solenoids are used for examples on this page. Here are some renderings to show the positions of parts in the valve:
While the solenoid rests idle, the core spring pushes it forward against the pilot base. The purpose of the core is to seal against a very small pressure port facing it, which becomes open when the solenoid is energized.
When energized, the coil around the core pulls it away from the pilot base, whcih allows airflow into the pilot itself. This pushes the pilot against the spool, shifting the valve's airflow as long as the coil is energized. When resetting, the core spring pushes it against the pilot to seal off airflow and also open the pilot's exhaust ports, which let the pilot reset to its idle position.
Some solenoid pilots have a feature known as the manual override as well. This is a small button on the outside of the pilot (between it and the core) which manually pushes the core away from the pilot to fire the marker. The override is fitted with a spring to reset it and prevent it from interfering with the core during normal operation. Here are some renderings of the manual override in the idle and overidden state:
Three-Way Solenoids:
As hinted toward above, the only difference between a four-way and three-way pilot-actuated valve is the porting for the spool. One of the ouptuts is simply removed, and the air travels nowhere. While the spool lies closed (idle), air pressure will enter the solenoid spool but not exit. The air that is trapped in the outlet port is always vented to the outside air through the firing assembly. I would like to make a three-way normally-opne and normally-closed animation, but I'm not sure when that will happen.