Proportional action amplifier

The use of the flapper-nozzle amplifier increased the sensitivity of the controller but at the expense of its proportional action. The flapper-nozzle was so sensitive that the controller had a proportional range of about 1 per cent of full-scale movement and hence became the equivalent of an on-off controller. Developments during the 1920s concentrated on trying to modify the flapper-nozzle valve action so as to increase its proportional range, and by 1930 several instrument companies offered controllers with a proportional band of 5 to 7 per cent of fiill range. 

The amplifier network provides signal conversion and suitable static and dynamic compensation for good positioner performance. Control from this block usually reduces down to a form of proportional or proportional plus derivative control. The output from this block in the case of a pneumatic positioner is a single connection to the spring and diaphragm actuator or two connections for push-pull operation of a springless piston actuator. The action of the amplifier network and the action of the stem-position feedback can be reversed together to provide for reversed positioner action. 

Figure 24.7. The proportional-plus-derivative controller. Derivative action is accomplished by a shunt capacitor C across Rf. When deviation from the setpoint is rapid, the low reactance of the capacitor causes less negative feedback—hence, greater amplifier gain. The derivative time resistor Ra allows adjustment of the magnitude of derivative control action to a given rate of change of the error signal. Courtesy of the Foxboro Company.

In most controllers, derivative acts on the output. Physically, it is a lag introduced in the feedback path around the controller amplifier. Therefore, if he output of the controller is constant, no derivative action will take place no matter what the controlled variable may be doing. This situation occurs whenever the controller s output has reached one of Its limits. The output of a pneumatic controller, for example, can go as low as zero or as high as 20 psi (the supply pressure), although the range of the control valve-hence the proportional band-is 3 to 15 psi. 

Other aspect to which the majority of researchers do not pay attention, especially in the studies on chemical weathering of insulating materials destined to electrical cables and wires is the evaluation of altered functional characteristics by electrical field. This aspect is quite important for the security of aerial and buried electric transport networks. The channels appeared as the result of the orientation of dipoles in oxidizing polymers die to electrical field allow the further penetration of water which forms electrical trees [39,40]. The length, the density and the size extension of these defects influence deeply the durability of electrical insulator. Moreover, the continuous action of electrical field amplifies the degradation, which increases proportionally the failure of outdoor operating cabled. 

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