In addition to the control valves described above, we will refer to some other devices that commanded by the controller, eventually serve to regulate a process according to the desired values. Among them briefly mention the solenoid valves and variable speed motors.
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SOLENOID VALVES
Solenoid valves are often used in a variety of on-off or interruption with control valves. In addition to the potential applications of control can be used in security operations or to connect a line of instruments to another. Because we are interested here in automatic control function of fluids, but we will refer to this aspect in particular.
While it is true that the most common application is referred to on-off control is important to note that a regulatory system, these valves can be used in a PID control provided that the controller outputs were discrete (PWM type) and permitting process (characteristics of the fluid flow rate, pipe size, etc.).. Typically these valves are available for it among pipelines / 4 inch and 2 inches in diameter, though there are larger.
These valves are control units that are electrically energized or when de-energized, allow or disallow fluid flow. It depends if they are normally open or normally closed. The actuator takes the form of an electromagnet. When it is energized, a magnetic field appears to attract a truss rod or against the action of a spring. When de-energized, the one element returns to its original or rest position by spring action.
According to the mode of action makes a distinction between directly-operated valves, pilot valves piloted internally or externally. Another distinguishing feature is the number of ports or the number of tracks.
In a solenoid valve is driven directly, the seat seal is attached to the core of the solenoid. In the condition of de-energized, closes a hole in the seat, which opens when the valve is energized. Can be found for these two-way valves and three-way. The former have a port of entry and exit and the latter are at rest one input and two outputs, one open and one closed, a situation that is reversed to be energized.
1. Valve Body, 4. Solenoid head 7. Piston
2. Input, 5. Coil, 8. Spring
3. Output 6. Signal cables, 9. Orifice
Figure 17 View of cut and parts of a solenoid valve
Internally piloted valves are used to handle higher pressures than can drive the valves described above. In this case the valves are fitted internally with a valve 2 way or 3 way. In particular applications such as pneumatic or hydraulic, especially for the handling of double acting cylinders, you can use 4-way valves. In externally piloted valves used an independent means to power them.
Some things to consider when selecting a solenoid valve is the pressure range in PSI with, that can work, the flow will move in them, which depends on the nature of design and type of fluid; coil type to use (can be ac or de) and its size and the response time of the same, which is defined as the time between the application of a control signal and the completion of the operation respective mechanical closing or opening, as the case.
In this latter respect, the small volumes and relatively high magnetic forces involved, allow quick response times. For special applications we can find several valve response times.
SPEED DRIVES
The drive is one of the most important both for the speed control of electric motors to position control (servo drives). The development of power electronics, manufacturing equipment has enabled drives that control virtually all the important parameters of the engine, allowing its use in all applications. From the point of view of technology today we speak of devices based on microcontrollers. If we talk i type that can control motors, drives generally are divided into those who drive DC motors and applied to AC motors.
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