If you're looking for good solenoid valve, you'd be pressed to find a better brand than Parker valves or its subsidiaries, most of whom sell Parker's products, identical but for repackaging.
Parker valves are used for motion control in a vast number of industries - so vast a number, actually, that entering any large scale factory or production facility is bound to bring you into contact with machines that contain at least a few of the company's products.
solenoid valve and servo motors are, to mince no words, absolutely crucial to the process of intelligent automation. We say 'intelligent' because solenoid valve are designed in such a way that they can respond to stimuli - so that, in some way, the condition of the whole machine is sensed by the servo, a sense to which the device then adjusts its operation accordingly.
An example of this technology of transduction, as it's called, can be found in cars that utilize cruise control. Cruise control systems usually estimate the ground speed of a car by the speed of the rotation of the car's driveshafts. On the basis of that information, cruise control systems change or maintain the position of the throttle by means of a solenoid (the same kind used in solenoid valves). Thus for a device to be a servo, be it a valve or a motor, it needs to perform some kind of error correction in response to outside conditions.
Another example was the earliest incarnation of the world famous thermostat, which was designed and patented by one Albert Butz in 1886. This device was designed in such a way that when a room's temperature fell below a certain level, the device would close a circuit and in so doing energize a solenoid which would suck in its armature, allowing the main motor shaft to turn one half revolution, pulling a chain connected to its crankshaft which in turn would open the furnace grate wider, allowing air to enter, giving the flames more oxygen to feed on. Once the temperature rose above a certain level, the solenoid would energize again, allowing the engine to turn another half revolution, pulling the chain back and closing the grate.
Parker valves are generally very sophisticated systems of this nature, all utilizing in some way the guiding concept of negative feedback - the means by which the desired or 'control' position of the device is contrasted with the actual position, and the 'negative' difference adjusted for.
The majority of electrohydraulic valves (an electromechanically enable form of hydraulic valve) are solenoid valve, as well as being, technically speaking, solenoid valves. Their response is to the pressures at play on the valve. By various means, usually computerized, Parker valves can calculate the pressure bearing down on their intake pipes and thus calculate the adjustment necessary to allow the appropriate amount of fluid through when desired.
Hydraulic pneumatic valves of this nature are capable of actuating a great deal more force than solenoid valves or pneumatic valves. However, because liquids are incompressible relative to gases, hydraulic Parker valves are generally reinforced to a far greater degree than pneumatic mechanisms, to ensure that the machine doesn't break under the onslaught of its own strength.