Sensors, Actuators, and Other Control System Components

5  Sensors, Actuators, and Other Control System Components

CONTENTS

1.·  1.Sensors

·        2.Actuators

·        3.Analog-to-Digital Conversion

·        4.Digital-to-Analog Conversion

·        5.Input/Output Devices for Discrete Data

Contact Input/Output Interfaces

Pulse Counters and Generators

To implement process control. the computer must collect data from and transmit signals to the production process. In Section 4.1.2, process variables and parameters were classified as either continuous or discrete. with several subcategories existing in the discrete class. The digital computer operates on digital (binary) data, whereas at least some of the data from the process are continuous (analog).Accommodations for this difference must be made in the computer process interface. In this chapter, we examine the components required to implement this interface. The components are:

    sensors  for measuring  continuous  and discrete  process  variables

    actuators  that drive  continuous  and discrete  process  parameters

    devices  that convert  continuous  analog  signals to digital  data

    devices  that convert  digital data  into analog  signals

    input/output   devices  for discrete  data.

The types of computer input/output interface for the different categories of process variabies and parameter; are summarized in Table 5.1

     SENSORS

A wide variety of measuring devices is available for collecting data from the rnanufactur ing process lor usc in feedback control. In general.a measuring device is composed of two components: a sensor and a transducer. The sensor detects the physical variable of interest (such a~ temperature. force. or pressure), The transducer converts the physical variable into an alternative form (commonly electrical voltage),quantifying the variable in the con. version.The quantified srgnat can be interpreted as the value of the measured variable. In some cases, the sensor and transducer are the same device; for example, a limit switch that converts the mechanical movement of a lever to close an electrical contact.