Autómatas plc's

LCdiscrete sensors - 4.1

4. LOGICAL SENSORS

Topics: • Sensor wiring; switches, TTL, sourcing, sinking • Proximity detection; contact switches, photo-optics, capacitive, inductive and ultrasonic Objectives: • Understand the different types of sensor outputs. • Know the basic sensor types and understand application issues.

4.1 INTRODUCTION
Sensors allow a PLC to detect the state of aprocess. Logical sensors can only detect a state that is either true or false. Examples of physical phenomena that are typically detected are listed below. • inductive proximity - is a metal object nearby? • capacitive proximity - is a dielectric object nearby? • optical presence - is an object breaking a light beam or reflecting light? • mechanical contact - is an object touching a switch? Recently,the cost of sensors has dropped and they have become commodity items, typically between $50 and $100. They are available in many forms from multiple vendors such as Allen Bradley, Omron, Hyde Park and Turck. In applications sensors are interchangeable between PLC vendors, but each sensor will have specific interface requirements. This chapter will begin by examining the various electrical wiringtechniques for sensors, and conclude with an examination of many popular sensor types.

4.2 SENSOR WIRING
When a sensor detects a logical change it must signal that change to the PLC. This is typically done by switching a voltage or current on or off. In some cases the output of the sensor is used to switch a load directly, completely eliminating the PLC. Typical out-

discrete sensors - 4.2puts from sensors (and inputs to PLCs) are listed below in relative popularity. Sinking/Sourcing - Switches current on or off. Plain Switches - Switches voltage on or off. Solid State Relays - These switch AC outputs. TTL (Transistor Transistor Logic) - Uses 0V and 5V to indicate logic levels.

4.2.1 Switches
The simplest example of sensor outputs are switches and relays. A simple example isshown in Figure 4.1.

normally open push-button 24 Vdc Power Supply + V+ sensor relay output V-

PLC Input Card 24V DC 00 01 02 03 04 05 06 07 COM

Figure 4.1

An Example of Switched Sensors

In the figure a NO contact switch is connected to input 01. A sensor with a relay output is also shown. The sensor must be powered separately, therefore the V+ and V- terminals are connected to thepower supply. The output of the sensor will become active when a phenomenon has been detected. This means the internal switch (probably a relay) will be closed allowing current to flow and the positive voltage will be applied to input 06.

discrete sensors - 4.3

4.2.2 Transistor Transistor Logic (TTL)
Transistor-Transistor Logic (TTL) is based on two voltage levels, 0V for false and 5V fortrue. The voltages can actually be slightly larger than 0V, or lower than 5V and still be detected correctly. This method is very susceptible to electrical noise on the factory floor, and should only be used when necessary. TTL outputs are common on electronic devices and computers, and will be necessary sometimes. When connecting to other devices simple circuits can be used to improve thesignal, such as the Schmitt trigger in Figure 4.2.

Vi

Vo

Vi

Vo

Figure 4.2

A Schmitt Trigger

A Schmitt trigger will receive an input voltage between 0-5V and convert it to 0V or 5V. If the voltage is in an ambiguous range, about 1.5-3.5V it will be ignored. If a sensor has a TTL output the PLC must use a TTL input card to read the values. If the TTL sensor is being used for otherapplications it should be noted that the maximum current output is normally about 20mA.

4.2.3 Sinking/Sourcing
Sinking sensors allow current to flow into the sensor to the voltage common, while sourcing sensors allow current to flow out of the sensor from a positive source. For both of these methods the emphasis is on current flow, not voltage. By using current flow, instead of voltage, many of...