Output Control Device

A variety of output control devices can be operated by the PLC output to control traditional industrial processes. These devices include pilot lights, control relays, motor starters, alarms, heaters, solenoids, solenoid valves, small motors, and horns. Similar electrical symbols are used to represent these devices both on relay schematics and PLC output connection diagrams. Figure shows common electrical symbols used for various output devices. Although these symbols are generally acceptable, some differences among manufacturers do exist.

An actuator, in the electrical sense, is any device that converts an electrical signal into mechanical movement. An electromechanical solenoid is an actuator that uses electrical energy to magnetically cause mechanical control action. A solenoid consists of a coil, frame, and plunger (or armature, as it is sometimes called). Figure shows the basic construction and operation of a solenoid. Its operation can be summarized as follows:

• The coil and frame form the fixed part.
• When the coil is energized, it produces a magnetic field that attracts the plunger, pulling it into the frame and thus creating mechanical motion.
• When the coil is de-energized the plunger returns to its normal position through gravity or assistance from spring assemblies within the solenoid.
• The frame and plunger of an AC-operated solenoid are constructed with laminated pieces instead of a solid piece of iron to limit eddy currents induced by the magnetic field.

Solenoid valves are electromechanical devices that work by passing an electrical current through a solenoid, thereby changing the state of the valve. Normally, there is a mechanical element, which is often a spring, that holds the valve in its default position. A solenoid valve is a combination of a solenoid coil operator and valve, which controls the flow of liquids, gases, steam, and other media.

When electrically energized, they open, shut off, or direct the flow of media.

Figure  illustrates the construction and principle of operation of a typical fluid solenoid valve. Its operation can be summarized as follows:

• The valve body contains an orifice in which a disk or plug is positioned to restrict or allow flow.
• Flow through the orifice is either restricted or allowed depending on whether the solenoid coil is energized or de-energized.
• When the coil is energized, the core is drawn into the solenoid coil to open the valve.
• The spring returns the valve to its original closed position when the current coil is de-energized.
• A valve must be installed with direction of flow in accordance with the arrow cast on the side of the valve body.

Stepper motors operate differently than standard types, which rotate continuously when voltage is applied to their terminals. The shaft of a stepper motor rotates in discrete increments when electrical command pulses are applied to it in the proper sequence. Every revolution is divided into a number of steps, and the motor must be sent a voltage pulse for each step. The amount of rotation is directly proportional to the number of pulses, and the speed of rotation is relative to the frequency of those pulses. A 1-degree-per-step motor will require 360 pulses to move through one revolution; the degrees per step are known as the resolution. When stopped, a stepper motor inherently holds its position. Stepper systems are used most often in “open-loop” control systems, where the controller tells the motor only how many steps to move and how fast to move, but does not have any way of knowing what position the motor is at.

The movement created by each pulse is precise and repeatable, which is why stepper motors are so

effective for load-positioning applications. Conversion of rotary to linear motion inside a linear actuator is accomplished through a threaded nut and lead screw. Generally, stepper motors produce less than 1 hp and are therefore frequently used in low-power position control applications. Figure shows a stepper motor/drive unit along with typical rotary and linear applications.

All servo motors operate in closed-loop mode, whereas most stepper motors operate in open-loop

mode. Closed-loop and open-loop control schemes are illustrated in Figure. Open loop is control without feedback, for example, when the controller tells the stepper motor how many steps to move and how fast to move, but does not verify where the motor is. Closed loop control compares speed or position feedback with the commanded speed or position and generates a modified command to make the error smaller. The error is the difference between the required speed or position and the actual speed or position.

Figure illustrates a closed-loop servo motor system. The motor controller directs operation of the servo motor by sending speed or position command signals to the amplifier, which drives the servo motor. A feedback device such as an encoder for position and a tachometer for speed are either incorporated within the servo motor or are remotely mounted, often on the load itself. These provide the servo motor’s position and speed feedback information that the controller compares to its programmed motion profile and uses to alter its position or speed.

Sensor End

Ultrasonic sensor

An ultrasonic sensor operates by sending high-frequency sound waves toward the target and measuring the time it takes for the pulses to bounce back. The time taken for this echo to return to the sensor is directly proportional to the distance or height of the object because sound has a constant velocity.

Figure illustrates a practical application in which the returning echo signal is electronically converted to a 4- to 20-mA output, which supplies a monitored flow rate to external control devices. The operation of this process can be summarized as follows:

• The 4-20 mA represents the sensor’s measurement span.
• The 4-mA set point is typically placed near the bottom of the empty tank, or the greatest measurement distance from the sensor.
• The 20-mA set point is typically placed near the top of the full tank, or the shortest measurement distance from the sensor.
• The sensor will proportionately generate a 4-mA signal when the tank is empty and a 20-mA signal when the tank is full.
• Ultrasonic sensors can detect solids, fluids, granular objects, and textiles. In addition, they enable the detection of different objects irrespective of color and transparency and therefore are ideal for monitoring transparent objects.

Strain/Weight Sensors

A strain gauge converts a mechanical strain into an electric signal. Strain gauges are based on the principle that the resistance of a conductor varies with length and cross sectional area. The force applied to the gauge causes the gauge to bend. This bending action also distorts the physical size of the gauge, which in turn changes its resistance. This resistance change is fed to a bridge circuit that detects small changes in the gauge’s resistance. Strain gauge load cells are usually made with steel and sensitive strain gauges. As the load cell is loaded, the metal elongates or compresses
very slightly. The strain gauge detects this movement and translates it to a varying voltage signal. Many sizes and shapes of load cells are available, and they range in sensitivity from grams to millions of pounds. Strain gauge–based load cells are used extensively for industrial weighing applications similar to the one illustrated in Figure.

Temperature sensor


The thermocouple is the most widely used temperature sensor. Thermocouples operate on the principle that when two dissimilar metals are joined, a predictable DC voltage will be generated that relates to the difference in temperature between the hot junction and the cold junction ( Figure ). The hot junction (measuring junction) is the joined end of a thermocouple that is exposed to the process where the temperature measurement is desired. The cold junction (reference junction) is the end of a thermocouple that is kept at a constant temperature to provide a reference point. For example, a K-type thermocouple, when heated to a temperature of 300°C at the hot junction, will produce 12.2 mV at the cold junction. Because of their ruggedness and wide temperature range, thermocouples are used in industry to monitor and control oven and furnace temperatures.

Flow Measurement

Many industrial processes depend on accurate measurement of fluid flow. Although there are several ways to measure fluid flow, the usual approach is to convert the kinetic energy that the fluid has into some other measurable form. Turbine-type flowmeters are a popular means of measurement
and control of liquid products in industrial, chemical, and petroleum operations. Turbine flowmeters,
like windmills, utilize their angular velocity (rotation speed) to indicate the flow velocity. The operation of a turbine flowmeter is illustrated in Figure. Its basic construction consists of a bladed turbine rotor installed in a flow tube. The bladed rotor rotates on its axis in proportion to the rate of the liquid flow through the tube. A magnetic pickup sensor is positioned as close to the rotor as practical. Fluid passing through the flow tube causes the rotor to rotate, which generates pulses in the pickup coil. The frequency of the pulses is then transmitted to readout electronics and displayed as gallons per minute.

Velocity and Position Sensors

Tachometer generators provide a convenient means of converting rotational speed into an analog voltage signal that can be used for motor speed indication and control applications. A tachometer generator is a small AC or DC generator that develops an output voltage (proportional
to its rpm) whose phase or polarity depends on the rotor’s direction of rotation. The DC tachometer
generator usually has permanent magnetic field excitation. The AC tachometer generator field is excited by a constant AC supply. In either case, the rotor of the tachometer is mechanically connected, directly or indirectly, to the load. Figure illustrates motor speed control applications
in which a tachometer generator is used to provide a feedback voltage to the motor controller that is proportional to motor speed. The control motor and tachometer generator may be contained in the same or separate housings.

An encoder is used to convert linear or rotary motion into a binary digital signal. Encoders are used in applications where positions have to be precisely determined. The optical encoder illustrated in Figure uses a light source shining on an optical disk with lines or slots that interrupt the beam of light to an optical sensor. An electronic circuit counts the interruptions of the beam and generates the encoder’s digital output pulses.

Sensor Part 2

Magnetic Reed Switch

A magnetic reed switch is composed of two flat contact tabs that are hermetically sealed (airtight) in a glass tube filled with protective gas, as illustrated in Picture. When a magnetic force is generated parallel to the reed switch, the reeds become flux carriers in the magnetic circuit. The overlapping ends of the reeds become opposite magnetic poles, which attract each other. If the magnetic force between the poles is strong enough to overcome the restoring force of the reeds, the reeds will be drawn together to actuate the switch. Because the contacts are sealed, they are unaffected by dust, humidity, and fumes; thus, their life expectancy is quite high.

Light Sensors


The photo voltaic cell and the photo conductive cell, illustrated in image , are two examples of light sensors. The photo voltaic or solar cell reacts to light by converting the light energy directly into electric energy. The photo conductive cell (also called a photo resistive cell ) reacts to light by change in the resistance of the cell.

A photoelectric sensor is an optical control device that operates by detecting a visible or invisible beam of light and responding to a change in the received light intensity. Photoelectric sensors are composed of two basic components: a transmitter (light source) and a receiver (sensor), as shown in Picture . These two components may or may not be housed in separate units. The basic operation of a photoelectric sensor can be summarized
as follows:

• The transmitter contains a light source, usually an LED along with an oscillator.
• The oscillator modulates or turns the LED on and off at a high rate of speed.
• The transmitter sends this modulated light beam to the receiver.
• The receiver decodes the light beam and switches the output device, which interfaces with the load.
• The receiver is tuned to its emitter’s modulation frequency and will only amplify the light signal that pulses at the specific frequency.
• Most sensors allow adjustment of how much light will cause the output of the sensor to change state.
• Response time is related to the frequency of the light pulses. Response times may become important when an application calls for the detection of very small objects, objects moving at a high rate of speed, or both.

The scan technique refers to the method used by photoelectric sensors to detect an object. The through-beam scan technique (also called direct scan) places the transmitter and receiver in direct line with each other, as illustrated in Picture . Because the light beam travels in only one direction, through-beam scanning provides long-range sensing. Quite often, a garage door opener has a through-beam photoelectric sensor mounted near the floor, across the width of the door. For this application the sensor senses that nothing is in the path of the door when it is closing.

In a retro reflective scan, the transmitter and receiver are housed in the same enclosure. This arrangement requires the use of a separate reflector or reflective tape mounted across from the sensor to return light back to the receiver. The retro reflective scan is designed to respond to objects that interrupt the beam normally maintained between the transmitter and receiver, as illustrated in Figure. In contrast to a through-beam application, retro reflective sensors are used for medium-range applications.

Fiber optics is not a scan technique, but another method for transmitting light. Fiber optic sensors use a flexible cable containing tiny fibers that channel light from emitter to receiver, as illustrated in Figure. Fiber optic sensor systems are completely immune to all forms of electrical interference. The fact that an optical fiber does not contain any moving parts and carries only light means that there is no possibility of a spark. This means that it can be safely used even in the most hazardous sensing environments such as a refinery for producing gases, grain bins, mining, pharmaceutical manufacturing, and chemical processing.

Bar code technology is widely implemented in industry to enter data quickly and accurately. Bar code scanners are the eyes of the data collection system. A light source within the scanner illuminates the bar code symbol; those bars absorb light, and spaces reflect light. A photo detector collects this light in the form of an electronic-signal pattern representing the printed symbol.
The decoder receives the signal from the scanner and converts these data into the character data representation of the symbol’s code. Figure 6-31 illustrates a typical PLC application which involves a bar code module reading the bar code on boxes as they move along a conveyor line. The PLC is then used to divert the boxes to the appropriate product lines according to the data read from the bar code.

To Be Continue.......

Sensor Part 1

Sensors are used for detecting, and often measuring, the magnitude of something. They convert mechanical, magnetic, thermal, optical, and chemical variations into electric voltages and currents. Sensors are usually categorized by what they measure, and they play an important role in modern manufacturing process control.

Proximity Sensor

Proximity sensors or switches, such as that shown in image , are pilot devices that detect the presence of an object (usually called the target) without physical contact. These solid-state electronic devices are completely encapsulated to protect against excessive vibration, liquids, chemicals, and corrosive agents found in the industrial environment. Proximity sensors are used when:

• The object being detected is too small, lightweight, or soft to operate a mechanical switch.
• Rapid response and high switching rates are required, as in counting or ejection control applications.
• An object has to be sensed through nonmetallic barriers such as glass, plastic, and paper cartons.
• Hostile environments demand improved sealing properties, preventing proper operation of mechanical switches.
• Long life and reliable service are required.
• A fast electronic control system requires a bounce free input signal.

Proximity sensors operate on different principles, depending on the type of matter being detected. When an application calls for non contact metallic target sensing, an inductive-type proximity sensor is used. Inductive proximity sensors are used to detect both ferrous metals (containing iron) and nonferrous metals (such as copper, aluminum, and brass).

Inductive Proximity Sensor

Inductive proximity sensors operate under the electrical principle of inductance, where a fluctuating current induces an electromotive force (emf) in a target object. The block diagram for an inductive proximity sensor is shown in image and its operation can be summarized as follows:

• The oscillator circuit generates a high-frequency electromagnetic field that radiates from the end of the sensor.
• When a metal object enters the field, eddy currents are induced in the surface of the object.
• The eddy currents on the object absorb some of the radiated energy from the sensor, resulting in a loss of energy and change of strength of the oscillator.
• The sensor’s detection circuit monitors the oscillator’s strength and triggers a solid-state output at a specific level.
• Once the metal object leaves the sensing area, the oscillator returns to its initial value.

Most sensor applications operate either at 24V DC or at 120V AC. The method of connecting a proximity sensor varies with the type of sensor and its application. Image Below shows a typical three-wire DC sensor connection. The three-wire DC proximity sensor has the positive and negative line leads connected directly to it. When the sensor is actuated, the circuit will connect the signal wire to the positive side of the line if operating normally open. If operating normally closed, the circuit will disconnect the signal wire from the positive side of the line.

Image shows a typical two-wire proximity sensor connection intended to be connected in series with the load. They are manufactured for either AC or DC supply voltages. In the off state, enough current must flow through the circuit to keep the sensor active. This off state current is called leakage current and typically may range from 1 to 2 mA. When the switch is actuated, it will conduct the normal load circuit current.

Image below shows the proximity sensor sensing range. Hysteresis is the distance between the operating point when the target approaches the proximity sensor face and the release point when the target is moving away from the sensor face. The object must be closer to turn the sensor on rather than to turn it off. If the target is moving toward the sensor, it will have to move to a closer point. Once the sensor turns on, it will remain on until the target moves to the release point. Hysteresis is needed to keep proximity sensors from chattering when subjected to shock and vibration,

slow-moving targets, or minor disturbances such as electrical noise and temperature drift. Most proximity sensors come equipped with an LED status indicator to verify the output switching action.

As a result of solid-state switching of the output, a small leakage current flows through the sensor even when the output is turned off. Similarly, when the sensor is on, a small voltage drop is lost across its output terminals. To operate properly, a proximity sensor should be powered continuously. Image illustrates the use of a bleeder resistor connected to allow enough current for the sensor to operate but not enough to turn on the input of the PLC.

Capacitive proximity sensors

Capacitive proximity sensors are similar to inductive proximity sensors. The main differences between the two types are that capacitive proximity sensors produce an electrostatic field instead of an electromagnetic field and are actuated by both conductive and non conductive materials.

Image illustrates the operation of a capacitive sensor. A capacitive sensor contains a high-frequency

oscillator along with a sensing surface formed by two metal electrodes. When the target nears the sensing surface, it enters the electrostatic field of the electrodes and changes the capacitance of the oscillator. As a result, the oscillator circuit begins oscillating and changes the output

state of the sensor when it reaches certain amplitude. As the target moves away from the sensor, the oscillator’s amplitude decreases, switching the sensor back to its original state.

Capacitive proximity sensors will sense metal objects as well as nonmetallic materials such as paper, glass, liquids, and cloth. They typically have a short sensing range of about 1 inch, regardless of the type of material being sensed. The larger the dielectric constant of a target, the easier it is for the capacitive sensor to detect. This makes possible the detection of materials inside nonmetallic containers as illustrated in image . In this example, the liquid has a much higher dielectric constant than the cardboard container, which gives the sensor the ability to see through the container and detect the liquid. In the process shown, detected empty containers are automatically diverted via the push rod.

Inductive proximity switches may be actuated only by a metal and are insensitive to humidity, dust, dirt, and the like. Capacitive proximity switches, however, can be actuated by any dirt in their environment. For general applications, the capacitive proximity switches are not really

an alternative but a supplement to the inductive proximity switches. They are a supplement when there is no metal available for the actuation (e.g., for woodworking machines and for determining the exact level of liquids or powders).

Mechanically Operated switch

A mechanically operated switch is controlled automatically by factors such as pressure, position, or temperature. The limit switch, shown in image , is a very common industrial control device. Limit switches are designed to operate only when a predetermined limit is reached, and they are usually actuated by contact with an object such as a cam. These devices take the place of a human operator. They are often used in the control circuits of machine processes to govern the starting, stopping, or reversal of motors.

The temperature switch, or thermostat, shown in image is used to sense temperature changes. Although there are many types available, they are all actuated by some specific environmental temperature change.Temperature switches open or close when a designated temperature is reached. Industrial applications for these devices include maintaining the desired temperature range of air, gases, liquids, or solids.

Pressure switches, such as that shown in image , are used to control the pressure of liquids and gases. Although many different types are available, they are all basically designed to actuate (open or close) their contacts when a specified pressure is reached. Pressure switches can be pneumatically (air) or hydraulically (liquid) operated switches. Generally, bellows or a diaphragm presses up against a small micro switch and causes it to open or close.

Level switches are used to sense liquid levels in vessels and provide automatic control for motors that transfer liquids from sumps or into tanks. They are also used to open or close piping solenoid valves to control fluids. The float switch shown in image is a type of level switch. This
switch is weighted so that as the liquid rises the switch float and turns upside down, actuating its internal contacts.

Manually Operated Switched

Manually operated switches are controlled by hand. These include toggle switches, push button switches, knife switches, and selector switches. Push button switches are the most common form of
manual control. A push button operates by opening or closing contacts when pressed. Image shows commonly used types of push button switches which include:

• Normally open (NO) push button , which makes a circuit when it is pressed and returns to its open position when the button is released.
• Normally closed (NC) push button, which opens the circuit when it is pressed and returns to the closed position when the button is released.
• Break-before-make push button in which the top section contacts are NC and the bottom section contacts are NO. When the button is pressed, the top contacts open before the bottom contacts are closed.

The selector switch is another common manually operated switch. The main difference between a push button and selector switch is the operator mechanism. A selector switch operator is rotated (instead of pushed) to open and close contacts of the attached contact block.
Image shows a three-position selector switch. Switch positions are established by turning the operator knob right or left. Selector switches may have two or more selector positions, with either maintained contact position or spring return to give momentary contact operation.

Dual in-line package (DIP) switches are small switch assemblies designed for mounting on printed circuit board modules ( Figure 6-12 ). The pins or terminals on the bottom of the DIP switch are the same size and spacing as an integrated circuit (IC) chip. The individual switches may be of the toggle, rocker, or slide kind. DIP switches use binary (on/off) settings to set the parameters for a particular module. For example, the input voltage range on a particular input module may be selected by means of DIP switches located on the back of the module.

Contactors

A contactor is a special type of relay designed to handle heavy power loads that are beyond the capability of control relays. Figure 6-5 shows a three-pole magnetic contactor.Unlike relays, contactors are designed to make and break higher powered circuits without being damaged.Such loads include lights, heaters, transformers, capacitors, and electric motors for which overload protection is provided separately or not required. Programmable controllers normally have an output capacity capable of operating a contactor coil, but not that needed to operate heavy power loads directly. Figure 6-6 illustrates the application of a PLC used in conjunction with a contactor to switch power on and off to a pump. The output module is connected in series with the coil to form a low-current switching circuit. The contacts of the contactor are connected in series with the pump motor to form a high-current switching circuit.