Electronic Instrumentation and Control Principles

Electronic Transmitters: Balance of Forces

The balance of forces consists of a rigid bar supported at one point, where two forces are in equilibrium:

• Mechanical force: Exerted by the measurement element.
• Electromagnetic force: Generated by a magnetic unit.

An imbalance between these forces leads to a change in the relative position of the bar, driving a displacement transducer such as a differential inductor or transformer. This feeds an associated oscillator circuit, which adjusts the magnetic force to reposition the balance beam. This feedback circuit varies the output current in proportion to the process variable range.

Wheatstone Bridge and Diffused Silicon

Semiconductor sensors utilize the electrical properties of materials subjected to stress. The diffused silicon model is manufactured as a semiconductor film that functions as a dynamic Wheatstone bridge circuit. Applicable to pressure, differential pressure, and level measurement, it consists of a silicon chip containing resistors (Ra, Rb, Rc, and Rd). The bridge imbalance caused by changes in the variable results in a 4-20mA DC output signal.

Electronic Amplifiers

Electronic amplification is based on operational amplifiers. The device features two inputs—inverting (-) and non-inverting (+)—and one output. The output depends on the specific connections made to the inputs:

• Inverting Amplifier: The input is connected to the inverting terminal through resistor R1, while the non-inverting input is connected to ground.
• Feedback: Resistor R2 provides a feedback path from the output terminal to the inverting terminal.

Split Range Control

Split range is a basic control configuration where the controller output is sent to two or more final control elements. Only one element acts to control the process variable at a time, while the other valves remain in a fixed position until the controller output reaches their specific range of action.

Valves in Opposition

In this configuration, valves act simultaneously to achieve the control objective, which differs from split range control where valves never act at the same time.