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There are three basic types of control valves that are available as proportional control valves. The first type is called the directional control valve. This type of valve has travel functions and flow functions available for control. The travel function includes directional control for a valve. When a valve is connected to a cylinder, this type of control allows the cylinder to extend or retract. The flow function controls the amount of hydraulic fluid flow through the valve. In most applications, fluid flow will be similar to current flow in electricity. This means the greater the fluid flow, the more power the valve will control. When fluid flow is used to control a hydraulic motor, the greater the flow, the faster the motor can turn and the larger the load it can move.
Ill. 1 shows an example of a simple proportional valve. From this diagram notice that the valve is controlled by a control amplifier (op amp) and the op amp is controlled by a potentiometer. Note that the output of the op amp will be 0-100% depending on the input signal.
The amount of voltage that the op amp sends to the valve will determine the percentage opening for the valve. If the 100% voltage signal is sent to the valve, the valve will be open 100%. If the voltage is 50%, the valve will be open 50%.
The second type of valve, called a flow control value, uses internal positional feedback to indicate the position of the valve spool when a signal is provided. The feedback helps the amplifier determine the position of the spool and allows the amplifier to provide more or less voltage if the spool has not moved to the proper position inside the valve. The position of the valve spool is very important to controlling fluid flow. Fluid flow is similar to velocity control and these two terms may be used interchangeably.
The third type of valve is called a pressure control valve. In a hydraulic system, the pump control can be designed to allow the pump to run 100% of the time at 100% rpm, or the system can be designed so a variable-type pump is used and its pumping capacity is adjusted 0-100%. If the pump is running at 100% rpm it will produce maximum pressure anytime the valves in the system are closed. At first this may sound like an unwanted condition, but it can be used as a precise method of control.
The valves in this type of system are connected between the main flow lines and the reservoir tank. When the valve is open, the flow is allowed to return directly back to the reservoir tank rather than be used to move cylinders or hydraulic motors. This type of valve is called a pressure relief valve. The valve controls the amount of fluid that's sent to a cylinder or hydraulic motor by the amount of fluid that's not bypassed. That is, if the pressure relief valve is set at 100% open, the cylinder or hydraulic motor will not move because all of the flow from the pump is being sent directly back to the reservoir. If the amplifier sets the valve at 0%, the total flow from the pump is sent to the hydraulic motor or cylinder and this will make them move at maximum speed with maximum power. Ill. 2 shows examples of the directional control valve, the flow control valve, and the pressure relief valve.