Understanding Smart Sensors - Nomads.usp

150 Understanding Smart Sensors7.1.1 Programmable Logic ControllersPLCs are used in industrial applications to control a variety of logic andsequencing processes. A typical PLC consists of a CPU, a programmable memory,I/O interfaces, and a power supply [1]. Smaller, less expensive PLCs can beused and the response time and interference from noise reduced by “combininga sensor and microprocessor into one unit, commonly called a smart sensor”[1]. Functions performed by smart sensors in that role include correctingfor environmental conditions, performing diagnostic functions, and makingdecisions.7.1.2 Open- Versus Closed-Loop SystemsThe simplest control system is an open-loop system. The sensor input goes to aprocessing unit, which produces an output. As shown in Table 7.1, thatrequires basic mechanics and physics knowledge, as well as mechanical-toelectricalinterface knowledge for a simple system such as a fuel indicator [2].The table shows the increasing knowledge level required to implement increasinglymore complex systems with automotive applications as examples.A closed-loop system can use the sensor to modify the control strategyand use a control strategy to improve the performance of the sensor [3].Figure 7.1 is a comparison of a capacitive surface-micromachined accelerometerusing an open-loop signal conditioning circuit and a closed-loop signal conditioningcircuit. The closed-loop system has advantages over the open-loopsystem, including improved linearity and wider dynamic range. Table 7.2 is acomparison of several parameters [3].In general, a single-loop closed-loop system has a comparison node for aset point versus an input (the feedback signal) from a sensor regarding thestatus of the process that is being controlled. On the basis of the comparison,control action is taken to increase, reduce, or maintain speed, flow, orsome other variable controlled by a mechanical actuator such as a valve ormotor. The PID control algorithm is a common technique in a closed-loopsystem [4].7.1.3 PID ControlMost single-loop controllers include at least three principal control actions:proportional, integral (or reset), and derivative (or rate). Each action correspondsto a specific technique for controlling the process set point. Proportionaladjustments amplify the error by a constant amount relative to the sizeand sign of the deviation. The proportional band has a range of deviations in