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Programmable Logic Controller Transducer A transducer is any device that converts energy from one form to another. Amplifier Input transducer (microphone) converts sound energy to electric energy Output transducer (speaker) converts electric energy to sound energy Sensors Sensors are input transducers used for detecting and often measuring the magnitude of something. They convert mechanical, magnetic, thermal, optical, and chemical variations into electric voltages and currents. Photoelectric sensor Sensors Sensors provide the equivalent of eyes, ears, nose, and tongue to the microprocessor brain. Microprocessor Optical sensor Gas sensor Microphone Probe Proximity Sensor Proximity sensors or switches detect the presence of an object without making physical contact with it. Proximity Sensor Applications The object being detected is too small, lightweight, or soft to operate a mechanical switch. Rapid response and high switching rates are required. An object has to be sensed through nonmetallic barriers such as glass, plastic, and paper cartons. Hostile environments conditions exist. Long life and reliable service are required. A fast electronic control system requires a bounce-free input signal. Inductive Proximity Sensor Operation Barrel type Block diagram As the target moves into the sensing area, the sensor switches the output ON Capacitive Proximity Sensor A capacitive proximity sensor can be actuated by both conductive and nonconductive material such as wood, plastics, liquids, sugar flour and wheat. Operation is similar to that of inductive proximity sensor. Instead of a coil, the active face of the sensor is formed by two metallic electrodes – rather like an "opened capacitor". Magnetic Switch (Reed Switch) A magnetic switch (also called a reed switch) is composed of flat contact tabs that are hermetically sealed (air-tight). Magnet N S NO Common NC The switch is actuated by a magnet. Reed Switch Activation Reed switch Magnet Proximity motion – movement of the switch or magnet will activate the switch Rotary motion – switch is actuated twice for every complete revolution Shielding – the shield short circuits the magnetic field; switch is activated by removal of the shield Photovoltaic Or Solar Cell The photovoltaic cell, or solar cell, is a common light-sensor device that converts light energy directly into electric energy. Solar cell The solar cell converts light impulses directly into electrical charges which can easily be amplified to provide an input signal to a PLC. Photoconductive Or Photoresistive Cell The photoconductive cell, or photoresistive cell, is is another popular type of light transducer. Light energy falling on this device will cause a change in the resistance of the cell. Ohms 20 Ohms Light resistance 5,000 Ohms Dark resistance Photoelectric Sensor Operation Most industrial photoelectric sensors use a lightemitting diode (LED) for the light source and a phototransistor to sense the presence or absence of light. Light detector Object to be sensed Light source Light from the LED falls on the input of the phototransistor and the amount of conduction through the transistor changes. Analog outputs provide an output proportional to the quantity of light seen by the photodetector. Reflective Photoelectric Sensor Emits a light beam (visible, infrared, or laser) from its light emitting element and detects the light being reflected. Diffused-reflective Retro-reflective typetype Emitter/receiver Operating Operating range range Target Reflector Through-Beam Type Photoelectric Sensor A through-beam photoelectric sensor is used to measure the change in light quantity caused by the target's crossing the optical axis. Emitter Operating range Target Receiver Bar Code Systems Bar code systems can be used to enter data much more quickly than manual methods, and are highly accurate. Diverter Decoder Scanner PLC The decoder receives the signal from the scanner and converts these data into the character data representation of the symbol's code. Ultrasonic Sensor An ultrasonic sensor operates by sending sound waves towards the target and measuring the time it takes for the pulses to bounce back. The returning echo signal is electronically converted to a 4 mA to 20 mA output, which supplies flow rate to external control devices. Strain /Weight Sensors A strain gauge transducer converts a mechanical strain into an electric signal. Hopper Wire type Force The load cell provides sensor input to the controller, which displays the weight and controls the hopper chute. Controller The force applied to the gauge causes the ON/OFF gauge to bend. This bending action also Chute distorts the physical size of the gauge, Control which in turn changes its resistance. Load cell Temperature Sensors Temperature sensors convert heat into an electric signal. There are four basic types used: thermocouple, resistance temperature detector (RTD), thermistor, and IC sensor. The thermocouple consists of a pair The resistance temperature detector of dissimilar conductors fused (RTD) varies resistance value together at in one end to form the TheCircuit thermistor varies in The Integrated (IC) temperature with changes in temperature. "hot" or measuring junction, with resistance value with sensor produces in voltage the free changes ends available for or changes inintemperature current with changesto temperature. connection the "cold" reference junction. A temperature difference between RTD the measuring and reference junction generates a small DC signal voltage. Flow Measurement The usual approach used in measuring fluid flow is to convert the kinetic energy that the fluid has into some other measurable form. Flow Electronic Magnetic Turbine Can beFlow usedMeter with electrically Flow conducting fluids and offers no Coil Meter restriction to flow. A coil in the The turbine blades turn at unita sets a magnetic to field. rateup proportional the If Magneta conductive liquid fluid velocity andflows are this magnetic field, a Turbinethrough magnetized to induce voltage is induced and sensed voltage pulses coil. by two electrodes. Velocity/RPM Sensors A tachometer is a small permanent magnet DC generator which when rotated produces a voltage that is directly proportional to the speed at which it is driven. Controller M Motor Tach Tachometers coupled to motors are commonly used in motor speed control applications to provide a feedback voltage to the controller that is proportional to motor speed. Velocity/RPM Sensors The rotating speed of a shaft is often measured using a magnetic (inductive) pickup sensor. Pickup coil Sensor output 0V Pole piece N S Magnet A magnet is attached to the shaft. A small coil of wire held near the magnet receives a pulse each time the magnet passes. By measuring the frequency of the pulses, the shaft speed can be determined. Output Control Devices A variety of output control devices can be operated by the controller output module to control traditional processes. These include: Pilot light Heater Control relay Solenoid Motor starter Alarm Solenoid valve Small motor Actuators An actuator is any device that converts an electrical signal into mechanical movement. The principle types of actuators are relays, solenoids, and motors. Solenoid The solenoid converts electric current into linear motion. Symbol Plunger AIR Coil Solenoid Valve A solenoid valve is a combination of: a solenoid with its core or plunger a valve body containing an orifice in which a disc or plug is positioned to restrict or allow flow Forward motion of piston Directional solenoid valve When SOL A is energized, the valve spool is shifted to redirect the fluid and move the cylinder forward CR SOL A FWD SOL A CR Stepper Motor A stepper motor converts electrical pulses applied to it into discrete rotor movements called steps. They are used to provide precise position control of movement. Stepper motor control system Module Communicates with the PLC and responds with pulse trains Stepper-motor translator Enables control of the stepper motor Step motor The motor will move one step for each pulse received