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Actuator-Locator Design Description

The design of the actuator and positioner must be considered together. The combination of these two devices will greatly affect the static performance (dead zone) and dynamic response of the regulator (control valve) assembly, as well as the total air consumption of the valve. Today, positioners are used in most specified regulator (control valve) applications. When used with conventional digital control systems, the positioner provides high positioning accuracy and a faster response to process disturbances. As people increasingly focus on the economics of process control, positioners should be considered for every valve application where process optimization is important.
The most important feature of a good positioner in reducing process variation is that it is a high gain device. The gain of the positioner consists of two parts: static gain and dynamic gain. Static gain is the sensitivity of a device to sense small changes in the input signal (0.125% or less). Unless the device is sensitive to these small signal changes, it is unlikely to respond to disturbances in the process variable. The high static gain of the positioner is achieved by a preamplifier. This amplifier is functionally similar to the preamplifier included in the hi-fi system. In many pneumatic positioners, a nozzle-plate or similar device has the same function as this high static gain preamplifier.
Once the change in process variable is sensed by the high static gain preamplifier, the positioner must be able to cause the intercepting element to move quickly to provide a timely corrective action on the process variable. This requires a lot of power to move the actuator and valve assembly to a new position quickly. In other words, the positioner must quickly provide a large amount of air to the actuator to make it respond quickly. This ability comes from the high dynamic gain of the positioner. Although the preamplifier is capable of providing high static gain, it typically does not have sufficient capability to provide the required power. Thus, the function of the preamplifier must be compensated by a high dynamic gain power amplifier. This high dynamic gain power amplifier can quickly deliver the required air flow as needed. The function of this power amplifier is generally provided by an amplifier or spool valve.
Spool valve positioners are relatively common due to their simplicity. Unfortunately, many spool positioners achieve this simplicity by eliminating high gain preamplifiers at design time. The input stage of these locators is typically a low static gain converter module. This converter module converts the input signal (electrical or pneumatic) into the movement of the spool, but this device is generally less sensitive to small signal changes. The result is a large time lag and a very slow overall response time of the regulating valve (control valve) assembly. Some manufacturers attempt to compensate for the poor performance of these devices by using spools with larger diameters and less channel overlap. This does increase the dynamic power gain of the device. This method will improve performance to some extent if it matches the actuator, but it greatly increases the air consumption of these high gain spools. Many high gain spool positioners have more than 5 times the static meter air consumption of a typical high performance secondary positioner. A typical secondary positioner uses a pneumatic amplifier during the power amplification phase. Amplifiers are preferred because they provide high power gain. This high power gain produces the best dynamic performance with minimal steady state air consumption. In addition, they are less susceptible to fluid contamination.
As microprocessor devices become more popular (see Chapter 4), the design of the locator is undergoing dramatic changes. These microprocessor-based locators provide the same dynamic performance as the best common secondary locators. They also provide valve monitoring and diagnostic capabilities to help ensure that the initial good performance does not decrease with use. In summary, a high performance positioner with both high static and high dynamic gain provides the best overall performance for any given valve assembly with reduced process variation.

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