Spool valves

Figure 8-74b is an example of a pneumatic positioner/actuator. The input signal is a pneumatic pressure that (1) moves the summing beam, w ch (2) operates the spool valve amplifier, which (3) provides flow to and from the piston actuator, which (4) causes the ac tuator to move and continue moving until (5) the feedback force returns the beam to its original position and stops valve travel at a new position. Typical positioner operation is thereby achieved. 

Figure 4.19 shows a spool-valve eontrolled linear aetuator. When the spool-valve is moved to the right, pressurized hydraulie oil flows into ehamber (1) eausing the piston to move to the left, and in so doing forees oil in ehamber (2) to be expelled to the exhaust port. 

The following analysis will be linearized for small perturbations of the spool-valve and aetuator. 

Fig. 4.20 Pressure-Flow-rate characteristics fora spool-valve.

Hectrolysis Cell. The cell (Figure 2) consists of an electrode assembly two rectangular Teflon reservoirs, 1- and 0.5-L, for the sample and the 2 X 10"5 M Hg(N03)2 solution, respectively a cell pump (peristaltic, variable speed. Cole Parmer 7545-15) and three in-house-built, solenoid-operated spool valves that allow for the transfer of solutions via Teflon tubing. The cell consists of essentially two solution loops one recirculates the Hg solution through the electrodes, and the other recirculates the sample through the electrodes. Circulation of the Hg solution and sample through the electrodes is sequential and mutually exclusive. Two more peristaltic pumps are also external to the wet box. They connect the seawater reservoir to input and exhaust lines. 

Fig. 10.2 Two chambers separated by spool valves Mo, Ml, M2. Mo is impermeable. Mi is permeable to gas (1), and M2 is permeable to gas (2). The valves can be switched independently. The chambers are placed in a thermostat...

Suppose the left chamber is filled with a one-component gas of type (1) and the right chamber is empty. The spool valve is closed. Initially the total entropy is 5s = 5 since the right chamber is empty (5" = 0)m... 

If we open the spool valve, the gas is distributed between both chambers. At the end we have an entropy of 5 ... 

After opening the spool valve we have just the situation of a single system with increased volume and mol number in comparison to the two separated systems according to Eq. (10.1) with n n[ + n [ and V V + V". Thus, we get... 

After opening the spool valve the gases will mix and the final entropy is... 

Alternatively, the pneumatic pressure can be held constant and a valve used to control the fluid flow. Needle valves, spool valves, and rotor valves are commonly used in this type of device. In all devices of this type, the fluid flow rate (0 is directly proportional to the material viscosity. Those factors that change the viscosity should be controlled carefully or the equipment must be able to compensate for the changes in order to achieve consistent results. 

A nearly infinite stiffness can be attained using controlled restrictors such as servo valves, diaphragms, or spool valves (Morsi 1969 Rowe 1969). However, these systems suffer from dynamic instability due to the resonance of control valve components. The orifice restriction and the constant flow supply system are also sensitive to lubricant viscosity change. When lubricant... 

Like sources, resistive elements may be modulated. For instance, variable hydraulic orifices in spool valves controlled by the displacement of the spool may be modelled by displacement controlled R elements. 

Let us illustrate the use of these functions and other tools utilizing the CAMPG/SIMULINK system using a nonlinear hydraulic system example. The system under consideration here is a 4-way closed-center spool valve. The valve shown in Fig. 11.47 is connected to a hydraulic ram. 

The system has a hydraulic fluid source that is supplied at a constant pressure to the system. The spool valve can be moved back and forth. When the spool valve is moved in either direction, a port that leads to the hydraulic ram is opened. At the same time on the other side of the ram a port that leads to a hydraulic fluid return tank opens. In actuality, there is really only one port on each side of the hydraulic ram. Each is used as either a supply or a return port depending on the direction the... 

Spool valve is displaced. Figure 11.47 shows how the flow is diverted into the ram or the flow from the low-pressure side of the ram is diverted away from the supply and into the return tank. 

Parameters are entered into this system by double clicking on the 4-way spool valve block in the center of the window. This window has been modified for the actual simulation to run by adding the input and output blocks. All of the initial conditions, constants, and physical system parameters are entered into this system by double clicking on the 4-way spool valve block in the center of the window. 

Pig. 11.51 Spool valve and housing displacement relative to ground... 

A number of features common to most sliding-spool valves are shown in Figure 9.10. The small ports at either end of the valve housing provide a path for fluid that leaks past the spool to flow to the reservoir. This prevents pressure from building up against the ends of the pistons, which would hinder the movement of the spool. When these valves become worn, they may lose balance because of greater leakage on one side of the spool than on... 

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