Differences

Performance

Significant improvements in smart transmitter accuracy, static pressure effects, temperature performance, and time stability provide a complete improvement in performance.

Rangeability

The power switch is increased to 400: 1 with a smart transmitter against a standard transmitter from 6: 1

The most noticeable variation helps to

1. Reduce the cost of backup management,
2. Increase the transfer of transmission equipment through crop rotation, too
3. It offers other benefits such as the freedom to define the distances directly when ordering units.
   A clear example of real savings with increasing variability is in applications where two or more standard units are used on a single orifice plate to improve the accuracy of various flows.

The difference in durability of intelligent transmission means that only one unit is required, thus only the other units and the cost of installing and maintaining them can be maintained and also prevent the low reliability that occurs in two or three loops against each other. In the case of a smart transmitter, the range can be changed manually using the speaker for various plant operating conditions.

Remote control adjustment

The range of the smart transmitter, water drain, etc., can now be adjusted from 1500 m away by contact over the 4-20 mA signal line. Adjustments can be made by connecting a smart field communicator (SFC) to all two signal lines at any location in their path.

1. The output can be seen as a straight or square root
2. The reduction time can be changed in several steps
3. LRV / URV can be adjusted to any value in the pressure range.
   Adjustments are made to any engineering units that can be selected in PSI, m bar, bar, k Pa.

Redesigning the smart transmitter without applying pressure is possible. Measurement corrections can be made with minor errors caused by ascending angle effects. Zero testing, and adjustments if necessary, can be done from the control room where the plants are closed or during regular maintenance. Remote diagnoses and remote control can now be performed back in the control room, thus taking a fraction of the previous maintenance period that usually requires two men several hours to do any field testing or switch.

All of the above can be done by one man from easily in the middle.

Reliability

The main purpose of intelligent transfer is built on reliable and stable work as a prerequisite. Few materials are used, and protection is provided against harmful influences, such as reverse polarity, over-pressure, and high voltage.

Many analog sensors, such as pressure and temperature sensors, are inexpensive, and good quality sensors. More complex flow, level, and analytical sensors do cost more, but these still only require a single pair of wires to allow the process variable, or measured variable, to be transmitted to the control system. Another positive feature of analog sensors and transmitters is that the signal can be carried a great distance along a single pair of wires with little or no signal loss.

A current signal can be transmitted up to 1000 meters through 18-gauge wires with no appreciable signal loss. Lastly, 4-20 milliamp current loop signals provide a basic level of diagnostics. Since 0% equals a 4 milliamps signal, a broken wire would break the circuit and 0 milliamps would be sensed. This “live zero” feature, where 0% is equal to a value of greater than 0 milliamps, allows the control system to detect a broken wire at 0 milliamps. But analog sensors can only send one “value” over a single pair of wires to the control system. And the granularity, or precision, of the data is limited by the type of analog to digital converter (or A-to-D converter”) used by the control system electronics.

However, with modern electronics, this is not as much of an issue. An A-to-D converter with 16-bit precision can report the range of values for an analog sensor in 65,535 increments. This means that for a 0 – 1000 psi pressure sensor, the granularity of the signal value is1000 divided by 65,535, or 0.015 psi. This level of precision would be sufficient for most applications.

Before we answer the question of what HART is, let’s look quickly at another analog device; the analog telephone. Analog telephone communication is similar to analog sensor signals used in industrial plants. Analog telephone lines transmit voice as 48 Volt DC electrical signals. When you speak into the handset of your phone, the microphone converts the soundwaves into analog electrical waves. These waves propagate over the telephone line to their destination. The receiving phone then converts the electrical signals back into sound waves through the speaker of the handset. One pair of copper wires for voice transmission, and one conversation (or transmitted value) at a time.

All just like an industrial analog sensor. In the late 1970’s, Bell Labs invented the Bell 202 modem standard. In 1980, the Bell 202 standard was adopted as the communications standard for subsea oil and gas production control systems. Bell 202 specifies a modulation method known as audio frequency-shift keying (AFSK) to encode and transfer digital data at a rate of 1200 bits per second, half-duplex (meaning, transmission only in one direction at a time). Basically, it provides a continuous signal, as an AC sine wave, that shifts its frequency from 1200Hertz, indicating a binary value of 1, to 2200 Hertz to indicate a binary value of 0. Here’s the kicker. If we superimpose a Bell 202 signal on top of a standard analog telephone line signal, we gain the ability to send digital data AND analog data at the same time on the same pair of wires. This was used to transmit the caller’s telephone number along with the voice call. This feature is well-known as Caller ID. So what if we superimposed a Bell 202 signal on top of a standard analog sensor line signal? We gain the ability to send digital data AND analog data at the same time on the same pair of wires. This is HART communication! With  HART, we can send analog data, the measured value of the process variable, along with digitally-transmitted data, such as a tag name, or calibration settings, or sensor diagnostics.

This would be a real productivity enhancement for the process plant! And because HART-enabled sensors require only a single pair of wires for communication, to upgrade an existing non-HART sensor loop to a HART-enabled loop, no wiring changes are required! Of course, both the sensor and the analog input card at the controller would need to support HART. The good news is that HART is built-in to most commercially-available analog sensors and HART-enabled analog input cards are available from nearly all DCS and PLC manufacturers. There are even add-on hardware devices to convert your HART sensors into wireless transmitters!

Now that we know what HART is, let’s finish the discussion of how HART works. First, HART is an acronym for “Highway Addressable Remote Transducer”. This simply means that a small network can be formed with up to 63 HART devices, each having its own address, or node number. Because a sensor can be remotely accessed using HART, the name really does say it all: “highway (or network) sensor (also called a transducer) that has an address so that it can be accessed remotely and directly”. The remote capability of HART sensors is very useful and powerful. A HART sensor connected to a PLC analog input card. We can access data in the sensor remotely using the HART communication protocol from the PLC programming software. That means we do not have to be at the location of the sensor to access its data. We can configure, calibrate, and retrieve diagnostic data from a control room or other location where the HART data is accessible. Data from a HART sensor must be requested by the master node, which controls all conversations on the loop. The master node is typically the DCS or PLC analog input card that the sensor is wired to. Each message from the master includes the request type, such as “send measurement value”, the node number of the sensor the message is intended for, and any data that needs to be transmitted to the sensor, like a new value for the upper range limit.

By using a hand-held programming and configuration device, often called a “HART communicator”, the sensor data can be accessed wherever the opportunity exists to connect the hand-held device in parallel to the loop wires. This can be in a junction box, marshalling panel, or at the sensor itself. So if a sensor is in an inaccessible or hazardous area, configuration or maintenance of the sensor can be done from a safe, remote location. Networking HART devices, in most control systems applications, is not practiced. Because of its limited speed and its cumbersome multi-drop network topology, we generally assign only one node, or sensor, to each HART signal loop. Fortunately, HART allows for multiple master nodes, so that the control system AND a hand-held communicator can both be connected to the loop and can communicate with the device at the same time.

With HART, the analog 4-20 milliamp signal AND the digital HART protocol are both available to the control system and instrument technician. If a sensor loop is upgraded from ‘analog only’ to ‘analog plus HART’, the control system programming and configuration for the measurement value can stay the same. You can imagine that superimposing an AC signal on top of a DC signal might interfere with the 4-20 milliamp signal. But this is not the case. The AC HART sine wave oscillates at either 1200 Hertz for a 1 value or at 2200 Hertz for a 0 value. The amplitude of the AC sine wave remains the same, and for every oscillation, the amplitude of the first half of each sine wave above the DC current curve exactly equals the amplitude of the second half of each sine wave below the DC current. The net effect of the sine wave is then zero. So the analog value of the sensor measurement data is not affected by the HART signal, just as a telephone voice conversation is not effected by the caller ID signal using the same Bell 202 protocol.

Every HART device is capable of sending and receiving 35-50 different information items, including the process variable (that is, the same measurement value as provided by the 4-20 milliamp analog signal); device status; diagnostic alerts, like “sensor value under range”; basic configuration parameters, like upper and lower range limits; and the tag name of the device. HART is a perfect choice for multivariable instruments, like mass flow meters, where mass flow, volumetric flow, temperature and density can all be communicated to the control system over a single cable. The HART protocol is governed by a vendor-independent association, The HART Communication Foundation, so HART sensors from any manufacturer can be interchanged with those of other manufactures. This makes implementation, maintenance, and troubleshooting very easy.

Also, HART is used extensively for final control devices, such as control valve positioners, with the same benefits and diagnostic capabilities. Even though the HART standard requires manufactures to provide a minimum number of specific data items with every HART sensor, vendors can also extend the data set to include vendor-specific items, like sensor model numbers or firmware versions or advanced diagnostic counters. In order for the control system to recognize the type and values for these custom data, a special description file, called a Data Description (DD) file is required. This file is loaded on the DCS or PLC configuration station or downloaded to the hand-held communicator and becomes directly associated to the sensor. This file simply allow the data stream from the sensor to be correctly parsed, or interpreted, and allow the technician or engineer to make the correct requests for sensor data.

To review, HART is a digital data communication protocol that is layered on top of a traditional analog 4 – 20 milliamp signal which provides advanced data retrieval and configuration options to be executed remotely from a DCS or PLC system or from a hand-held communicator. HART communicates over a single pair of wires, so adding HART to an existing 4 – 20 milliamp sensor loop requires no wiring modification. Only the hardware at the analog input card and the sensor electronics may need to be upgraded so that HART functionality is provided. A high percentage of sensors already installed in 4 – 20 milliamp loops are already HART-enabled. HART may be the fieldbus you already have in your plant. Through simple configuration, a wealth of new process data and diagnostic capability can be obtained with a minimum of effort and expense.

So let’s take a closer look and try to break the pyramid down. Beginning on the bottom of the pyramid is what we will refer to as the “field” level. These are the devices, actuators, and sensors that you see in the field or on the production floor. If you think of it this way, the field level is the production floor that does the physical work and monitoring.

Electric motors, hydraulic and pneumatic actuators to move machinery, proximity switches used to detect that movement or certain materials, photoelectric switches that detect similar things will all play a part in the field level. The next level is referred to as the control level. This is where the PLC’s and PID’s come in to play. The control level uses these devices to control and “run” the devices in the field level that actually do the physical work. They take in information from all of the sensors, switches, and other input devices to make decisions on what outputs to turn on to complete the programmed task. A PID is usually integrated in to the PLC and stands for proportional–integral–derivative. That is what can keep a variable within a set of parameters.

A common example that you probably use every day is your cruise control. You set your cruise control to whatever speed you want, then a set of sensors and the computer in the car will tell it when to accelerate or decelerate according to the set speed. A common industrial PID controlled item is a heater. Many systems in manufacturing plants have to be heated. We control this with a PID block within the PLC. When a set point is entered, the PID will determine when the PLC needs to turn the heater on and off to maintain a constant temperature. The third level of the automation pyramid is known as the supervisory level. Where the previous level utilizes plcs, this level utilizes SCADA.

SCADA is short for supervisory control and data acquisition. SCADA is essentially the combination of the previous levels used to access data and control systems from a single location. Plus it usually adds a graphical user interface, or an HMI, to control functions remotely. Water plants will often employ this technology to control remote water pumps in their systems. The important thing to remember about SCADA is that it can monitor and control multiple systems from a single location. It isn’t limited to a single machine like HMI’s.

The fourth level of the automation pyramid is called the planning level. This level utilizes a computer management system known as MES or manufacturing execution system. MES monitors the entire manufacturing process in a plant or factory from the raw materials to the finished product. This allows management to see exactly what is happening and allows them to make decisions based on that information. They can adjust raw material orders or shipment plans based on real data received from the systems we talked about earlier.

The top of the pyramid is what is called the management level. This level uses the companies integrated management system which is known as the ERP or enterprise resource planning. This is where a company’s top management can see and control their operations. ERP is usually a suite of different computer applications that can see everything going on inside a company. It utilizes all of the previous levels technology plus some more software to accomplish this level of integration. This allows the business to be able to monitor all levels of the business from manufacturing, to sales, to purchasing, to finance and payroll, plus many others. The integration of the ERP promotes efficiency and transparency within a company by keeping everyone in the same page.

So let’s see what you have learned about the automation pyramid. When you start at the bottom, you are starting on the production floor with all of the sensors, motors, and actuators that make the facility run. As you move up the pyramid, the next level is for controlling the field level. This is where the PLCs and PIDs come in to control the devices at the field level. The next level is the SCADA controls. These are great for controlling and automating large areas or over long distances. The last two levels are mostly for management. One to control a single plant, the last that can monitor an entire company inside and out. I truly hope this gave you a little information on the automation pyramid and how technology is automating industry.

In a common relay system, you would push a button to energize a relay, which in turn will pass current through a set of contacts that close when it is energized. There are many common, everyday items that use a relay to operate. We will dive in to those in a minute, but first I want to talk about why you may need to use a relay system. A very common reason, especially in an industrial setting, is for voltage and current requirements. Many machines and equipment use a higher voltage to run. To make it safer for the operators, we use a low voltage and current for our controls. You wouldn’t want someone pushing a button with high voltage attached to it. I know I wouldn’t want to on a regular basis. It takes a very low amount of current to cause bodily harm. Plus, most push buttons and switches are rated for fairly low current.

When we use a relay, the contacts that close can be rated for much higher current. Earlier it was mentioned that relays are used in many everyday applications. One of those applications are the headlights on your car. When you turn the headlight switch to on, the wires are not connected straight to the light. The switch has a positive wire from the fuse box. It then has a wire from other terminal of the switch to a coil terminal on the relay. Once this coil circuit is closed with 12 volts, the electromagnet inside the relay is energized and a set of contacts will close. The “input” contact will have 12 volts connected to it. The “output” contact will have a wire connected from it to the headlights. Therefore, once the coil is energized on the relay via the light switch, voltage will pass from the input contact to the output contact and turn the lights on.

In an industrial setting, relay systems are used regularly. One very common example is when an electric motor needs to be turned on and off. Commonly called contactors and motor starters, they are a prime example of a relay system. We use a low voltage, low current circuit for our motor controls. These can be push buttons or sensors that turn on the motor. In this case we will use a photoelectric sensor to turn on a motor that runs a conveyor belt. Whenever a box is placed on the conveyor belt, the photoelectric sensor is blocked. The sensor acts as a switch that will send the low voltage to the coil of the motor starter. Once the coil is energized, the electromagnetism closes the contacts of the motor starter allowing the high voltage to pass to the motor and run the conveyor belt. Once the box is past the photoelectric sensor, it will turn off the motor starter by removing the low control voltage.

Let’s go back over this one more time. A relay is an electrical device that closes one circuit by being energized by another circuit. These can be used for many reasons. One that we covered today was for safety. When we use a relay to power a high voltage or high current device, while a lower voltage is used to power the controls that energize the relay. Relays are commonly controlled with switches, push buttons, and sensors.

A simple, everyday example of automation would be an ordinary pull in and park automatic car wash. If we break it down step by step, it should help you understand how automation works. Once you select your wash, that input will run a certain program on the controller. Next, you drive forward until a sensor is flagged, starting the wash cycle and another sensor tells the driver when to stop. From there, the wash will continue through the cycle that the controller is running. In a typical car wash, the nozzle spraying the water and chemicals will travel alongside a vehicle and a sensor will detect when it is past the vehicle. This saves time, water, and chemicals by controlling when the nozzles spray, as well as serves as an input to advance to the next step of the wash. Once the car is sprayed, the program tells the wash to spray the next chemical. This is done by the controller turning on and off each pump as it is needed. The controller then runs through the rest of the cycle and wash that was selected in the beginning. Once all steps have been completed, the driver will be signalled to pull out of the wash.

Essentially, the only human intervention in the process is the driver selecting what wash they want. Industrial automation works exactly the same way. Each industrial process needs to be started by some sort of input. That input can be a sensor, pushbutton ,switch, among many other possibilities. Typically a person will start the process with one of these input devices or it could be a sensor that detects an object automatically. Those inputs will go to a PLC to which will then make decisions based on how it was programmed. The PLC will then activate whatever  output the program says to run. An output can be anything that does work such  as a motor, solenoid, heater, or a light. In an automated process, that output will typically act as an input to the PLC and combined with other input devices or programming, to keep the process running. With this programming, it can act like a chain reaction with one output device starting before the next is allowed to run. This is also called sequential starting. Automation has to have many aspects working together in order to function properly. An automated process will continue its cycle until it receives a stop signal. That can come from a physical input such as a stop button or sensor, or something programmed like a timer.

Let’s look back at what we now know. Automation is an automatic process, typically controlled by computers and sensors. There is usually very little human intervention. Input devices, such as sensors and switches, will work with a controller or PLC to activate output devices like motors or other machinery. The PLC will advance as it was programmed. Industrial robots are great examples of process automation as they are commonly being used to replace humans in pallet loading operations.

Take ice cream, for example. The company can mix, heavy cream, half egg yolks, sugar, and salt in a kitchen dish is to make the base for the ice cream. This can then be sent on to Company B, which will be combined with flavors like vanilla, strawberry, and chocolate. While the components of the product have been made to separate, and the product is handled and did not involve himself in the creation of the ice cream. This is just one of the many examples of this.

Take it easy a procedure to show that an analogue of process automation. I will tell you about the water purification process. The first thing you’ll need to have our waste water treatment plant, water. Let’s assume that our facility has a variety of water sources, a reservoir, a lake, and a number of underground wells. And, of course, who doesn’t want a drink of water out of the tank, and the thought of drinking a bit of water, but the most snobbish of fish, and, of course, the majority of the people.

These sources will require cleaning and disinfection, it is not only because we prefer to, but because the act is usually like this. We have been providing water to the plants, and that it is not so desirable, and it’s the first thing we want to do is remove it from the “big stuff”. It can be leaves, sticks, debris and even small fish or other animals into the water, and is the first line of defense for the removal of those things that can have a nervous system.

As soon as the water leaves the filter in the system, you can have a fast mixing process, where it will be used to balance the pH, if necessary, as well as how to add special chemicals to ensure that the molecules of the water of the application, and a dust of larger particles, referred to as petals of a flower. The petals of a flower can be go to the pool, where everything slows down a little bit, and the petals of a flower that can still grow.

Following the herd will take time to develop, you can use the pump to a different pool, where it was allowed to settle. This process is the large flakes sink to the bottom of the pool, and it has to be removed, while the top of the water to flow out of the swimming pool is a kind of filtration system. The filtration system is made mostly from gravel, sand, and a kind of coal called anthracite.

If water is not flowing through these levels, the smaller the molecules, and organisms are removed from the water. Then, the water can enter the disinfection system. It can be as complex as for any other type of filtration system, such as membranes, UV, reverse osmosis, ozone, or of a different type. The disinfection process is largely dependent on the quality of the water, and the ability to remove particles and organisms. If there are, either bacterial or viral problems, and if the water is small, and it doesn’t taste very friendly and helpful, these are the problems, which may require additional treatment. If the problems are reduced to a minimum, disinfection, it can be as simple as adding chlorine to the water to make sure that any other items are to be removed. The government is generally required that a number of the chlorine before the water in the distribution system. As soon as the water is properly disinfected, you can turn it into a container that is used for the storage of the final product, whether it is water that can be pumped up to the junction for the switching devices in the line all the way to the city’s needs.

Other examples of analog, automation, continuous flow of oil and gas, pulp and paper, chemicals, and so on. While the majority of these products are to be measured, to some extent, a gallon of gas in a quarter of the olive oil in a drum of chemicals, but again, this is definitely not treated as a separate automation is a consequence of the way in which the process has been dealt with. How to create a widget, which is derived from a company, it is a lot easier to quantify than that of a water treatment product. The definition of an analogous process automation, it is more related to what is happening in order to get the final result. Analog automation is the process of a mix of things, whilst the discrete automation, it is more about the installation of the various components of the final product. As I have already mentioned in the discussion of the discrete automation and the start and stop of the process of automation can be carried out by using the discrete and analog system. however, with the start and stop of the discrete process is as simple as stopping a car. How to start and stop the analog process automation requires careful planning, but also in the preparation process. For example, in order to stop the water treatment facility, the city must first find out where the water is the place to come. If there is a lot of potential for other urban waste-water treatment plants, the city may wish to consider a number of short and long-term options, such as the filling of the tanks are at full capacity and will to ensure that it’s time to stop, and resume of the process is carried out in a relatively short period of time.
L

Let’s just say that this was the option that was selected earlier. First, you will need to stop it with the power of plants. This may not be an easy one. If it has a motor-driven control valve, then you can use this valve to close and the start of the process in order to stop it. If they are not, the motorized valve, the staff would have to go to the boiler’s location and manually close the door. Although it may seem relatively easy, and some of these shut-off valves in the pipes, with a diameter of 96 cm. It was, indeed, a very big mouth, so you might be able to manually turn on until it is completely closed. As soon as this procedure is complete, the staff will be forced to come to a stop with the addition of any chemical substances, and, where appropriate, the removal of the quick to mix a swimming pool. The reason for the draining of the pool, that is, the flocculant will sink to the bottom of the pool and will cause a lot of problems for the fluid to be pumped to the next station, as well as for the operation of the equipment. Of course, if you have to drain the pool, it is possible they will need to have in order to dry up, and the swimming pools of the same reasons. The filters that are needed in order to dry, as the water in the filter, it would be to create the conditions for the reproduction of the algae, and other unwanted substances. In other words, all of the processes that are required for the wastewater treatment plant can be affected by both short-term and long-term costs. As you can imagine, the consequences, and the exceptions that can be substantially larger in the case of analog, automation, in the case of the discrete automation.

Let’s first discuss an assembly process that would place a cap on top of a bottle. The procedure to attach that cap may be by compression, meaning that the lid is pressed onto the bottle, or the lid may be twisted or turned onto the bottle. This automation process would take that bottle into some sort of piece holder while another piece holder would hold the cap. The automated process would attach that cap to the bottle in a single action, either by pressing the parts together or one of the piece holders may spin in order to twist the cap onto the bottle. This automated process produced a product that can be physically counted. Of course, before the cap was attached, the bottle was probably filled with some sort of liquid and the possible final step of this process may be to attach a label to the front of the bottle. It really doesn’t matter how many steps it takes to complete the product, the end result is a countable item. However, just a small process such as this one isn’t the only type of thing that would be considered discrete automation.

Let’s take a look at the process of making a cell phone. There are several parts that are used to create a cell phone. Those parts may include a plastic or metal case, an LCD screen which can be a touch screen or just a display, possibly a keypad if it’s not integrated as a touch screen, main circuit board, battery, etc. Now each of these components may be made on a different production line or in a different facility or even in a different country. The process of creating each of those parts would be considered a discrete automation process. Each process is creating a quantifiable or countable part. When those parts are created in separate processes, they would then be shipped or moved to another production line that would then assemble those products into the whole, complete cell phone. The process to assemble the cell phone may take the cell phone case and first install the screen. After that screen is installed, it may then be followed by the circuit board, the battery, the memory card, etc. When the cell phone is assembled, again, itis assembled using a discrete automation process. There are all of those components that were created by discrete automation processes meaning, they could count a component or part at the end of the process. Then you can take all of those countable part sand assemble them. The final product is a countable product which makes it a discrete automation process.

In discrete automation, the process is not necessarily continuous. The process of creating something via discrete automation could be started and stopped at any time. In other words, let’s say that plant A would create the cell phone case in a discrete automation process. Plant A needs to ful fill an order to produce 50,000 cases in various colors. The order is for 10,000 cases in stainless steel, 10,000 black, 10,000 white, 10,000 gold, and 10,000 rainbow tie-dye. The plant’s process can create 1000 cases an hour of a solid color and 500 an hour of a mixed or multi color. The plant has only 1 shift and that shift works 8 hour days and no weekends. So Monday, plant A starts production on the stainless steel case. The shift was able to make 6,500 cases and plan to resume production the next day. On Tuesday, the plant resumes production of the stainless steel case for the final 1500 count. If it takes an hour to change over the machine to produce a different color, the order for plant A would take at least 8 working days to complete. If however, plant A had a 24-hour shift, this order would be complete in approximately 3 days. Basically, discrete automation production of components may be started and stopped no times or multiple times. The starting and stopping of an automation process can be done in either discrete or analog automation, however, starting and stopping a discrete process is as easy as stopping a machine after the last component is produced and then starting it up again when you want to resume production. This is one the easiest forms of process automation. In a future lesson I will contrast the discrete and analog automation processes and demonstrate that with an analog process, the chore of starting and stopping is a much more difficult task. So to sum it all up, discrete automation is the production of parts that are of a quantifiable nature. That may include cell phones, soda bottles, automobiles, airplanes, toys, etc. As you know, an automobile contains many, many parts. The parts required for an automobile are also quantifiable in nature. A car requires 2 or 4 doors, measurable, 1hood and 1 trunk, measurable, 1 engine and 1 transmission, again measurable. Obviously, each of those required parts is not necessarily a single part in and of itself but an assembly of other quantifiable parts. Once all of those parts are put together, in a discrete automation process, you are left with a countable part, an automobile. This is discrete automation.

Now let’s see how the wiring will change when we utilize a Profibus-DP network. Okay, at the moment our I/O modules are situated right next to the CPU. To use Profibus-DP we can put an enclosure near the sensors in the field area and then move the I/O modules into it; then we can connect the sensors to the I/O modules.

Now, to enable data transfer between the PLC in the control room and the I/O modules in the new enclosure we are going to install an IM or Interface Module here; then we can use Profibus-DP and an RS-485 cable to transfer all data. So, previously the PLC’s I/O modules were arranged centrally but by introducing a network bus between the main controller and its I/O modules we DECENTRALIZED the I/Os, moving them to this enclosure in the field area. That’s why this type of profibus has the added suffix of “DP” or “Decentralized Peripherals”. We usually call these Decentralized Peripherals “Remote I/O.” So using Profibus-DP, instead of wiring each individual sensor, actuator or other facility to the PLC individually, we can install a set of “remote I/Os” next to these facilities in the field area and then transfer the data to the control area using a single RS-485 cable. This can decrease the cost of wiring dramatically and on the top of that, since the data transmission method here is “digital”, industrial environment noise has less of an impact on data, so data communication between the control and field areas will be more robust. But there are a number of disadvantages with Profibus-DP. For instance, because we are transferring data with a single cable, ifit malfunctions in any way we’ll lose all the data from the field facilities.

To prevent this problem, some applications have some of the main signals connected directly to the PLC and transfer the rest of the signals using Profibus-DP. With this wiring configuration, even if the data cable goes down for any reason, the signals from important facilities are not lost. This method is called hybrid. Another solution is to utilize Profibus-DP as a “redundant” network. In this method we use two “RS-485”cables to connect the remote I/Os to the PLC. In this configuration if the main cable goes down, the data transfer can be switched to the reserve cable with no problems. So to recap, “Profibus-DP” is a type of industrial network which decreases the amount of wiring required by “decentralizing” the PLC’s I/O modules.

In this article, we are going to familiarize you with the sorts of sensors that we frequently use in industrial equipment. We intend this to be an introductory article for a new engineer just getting started, or for anyone looking for a general understanding of some general controls concepts.

As a controls engineer, we often tend to see things from the electrical end of things, so in this exploration, we will look at two broad categories of sensors: Digital sensors that return on/off signals, and Analog sensors that return a range of values.

Digital sensors

Our first stop is Digital sensors. These are by far the most commonly used sensors in the industrial world. So, what is a “digital” or “binary” sensor? In theoretical terms, we are referring to something that returns one or more bits of information per sensor.

Initially, they were simple: A contact that touched another contact when something got where it was supposed to stop. In those old machines, this often meant 110 Volts exposed for the operator to touch or passing through the machine frame – Unsafe under any condition, and probably illegal in today’s safety conscious world.

Later these became a switch that flipped when something got to a position. These are referred to as “limit switches,” and are still in use. We use these sorts of sensors for anything that we divide into two states – On and Off, True and False, Is and Isn’t.

For example: In Position, Full, Empty, Power On and Running. Let’s look at a few examples of these kinds of sensors. First, as mentioned before, mechanical switches of various kinds are still around. Limit switches are still used in ugly, dirty environments thanks to their “armor-plated” construction. One big reason they have become less popular over the years is that they are huge compared to many of the other sensors available.

Proximity sensors

Proximity sensors, very often called “proxes,” are used for detecting close metal objects using magnetic fields. In many environments, these have replaced limit switches in position sensing applications. Optical sensors have a much longer range than proximity sensors, but they are susceptible to dirt and other environmental and mechanical issues because they use light for sensing.

We often use them where we are not picky about exactly where the target is, but we need to know it is “there,” like boxes on a conveyor where we don’t care whereon the conveyor it is, just that it is passing by. Capacitive proximity sensors are like a proximity sensor, but for detecting non-conductive materials. They are very sensitive to contamination and historically have not been very dependable. Ultrasonic proximity detectors detect solid objects using high-frequency sound but are very susceptible to environmental conditions and dirt. We don’t use them often, but they can solve sensing problems nothing else can.

An auxiliary contact is a part of a relay. These tell us when whatever is controlling the relay has turned it on or off. A pushbutton senses the operator’s action. For now, that is all we will say about digital sensors. Most controls are still designed around on/off signals, so these are the “bread and butter” of a controls engineer’s life. At one time, cars had no fuel gauge, and you had to have a reserve tank – a gas can – so that when you ran out, you could get to a fuel station. Now cars all have fuel gauges, and we will next look at the sensors that make that possible along with many other measurements that automation requires.

Analog sensor

An analog sensor is one that converts a variable physical quantity into a signal that the control system can understand – a voltage or current. By physical quantity, we mean Temperature, Pressure, Humidity, Distance and Speed among others. There is a general category of sensor for each of these. Some sensors combine quantities, like temperature and humidity or distance and speed into a single instrument generating two signals. There are a few general categories of signals generated by these devices.

For temperature sensing, the devices themselves produce either millivolt-range signals in the case of thermocouples, or variable resistances in the case of Resistance Temperature Detectors (RTDs.) Because of their higher accuracy and repeatability, RTDs are generally a better sensing element when we can use them. PLCs have cards that are specifically designed to handle both of these kinds of devices.

The rest of the signal categories are converted locally into a more generally understood form of signal, either voltage or current before being connected to the control system. If the temperature signals have to travel very far to the control system, we usually convert them like this also.

The most commonly used standard today is the 4-20mA signal because of noise immunity and other characteristics, and every type of analog sensor I have mentioned can generally be purchased in that type of output. We have just taken a whirlwind tour of the primary sensors used in almost every industrial control system. With these sensors, we sense everything from which buttons the operator pushes to the height of liquid in a tank to the pressure and temperature of steam in a boiler. These sensors and a few others, with their signals processed by hardware and software, control the industrial processes of the world.

Most likely you’ve heard about “RTDs”, “Thermocouples”, “Thermistors”, “Semiconductor” type elements and so on, which will be addressed here. Before I go into details of this subject, let’s see what a “Temperature Sensor” (Temperature Transducer) is and what does a “Temperature Transmitter” mean.

Generally, a sensor or transducer is a physical device which is capable of transforming one type of process variable to my favorite signal type. To elaborate on this generalized sentence, let me give you an example. Temperature, pressure, flow, etc, are some process variables and actually, they are physical characteristics of our real world. With modern technology and because of tremendous advances in Electrical Engineering in the past century, we like to transform every measurable process value into an electrical signal and a temperature sensor is a device which will transform the temperature into an electrical signal, no matter how tiny the amount of this signal might be!

So far I took a big “First Step” which was the transformation of “Temperature” into “Electrical Signal”. Based on different sensor technologies, this signal may have different ranges and for industrial applications, I need to have my signals limited to some universally accepted electrical “signal-ranges”. Today some of these globally accepted electrical signal-ranges are 4-20 mA , 1-5 V , 0-10 V , etc.

A “Temperature Transmitter” is a device which transforms the tiny output of a “Temperature Transducer” to one of these standard signal ranges. Now let’s get back to different “Temperature Transducer” technologies. RTD or “Resistance Temperature Detector” is a device the resistance of which varies with the temperature. Since it is a passive device, an external electrical current should be applied to it and then the voltage drop across it can be measured. This voltage is a good indication of the temperature. When referring to such a device as “passive”, it means that the device needs external current (or voltage) source. To state the obvious, a big amount of external current can cause power dissipation in the resistor of RTD and lead to excess heat, so to avoid this type of error, the current should be kept at a minimum level.

There is 2 wire, 3 wire and 4 wire wiring configuration for RTD. More accurate reading calls for 3-wire or 4-wire configurations. In reality, the distance between the temperature sensing point and measuring system calls for wiring and since the real wiring has its own resistance, some measurement error sneaks in hereby! 3-wire and 4-wire solutions are developed to remove this error. One of the most common RTDs is “PT100” which consists of a thin film of Platinum on a plastic film and shows a resistance of 100Ω at 32°F. Its resistance varies with temperature and it can typically measure temperatures from -330 to 1560°F. The relationship between resistance and temperature of PT100 is relatively linear. PT100 is just an example of platinum RTDs and in the industry you may find different RTD types suitable for various applications, e.g.: Copper, Nickel, Nickel-Iron, etc.

Thermistors are temperature-dependent resistors and are widely used in industrial purposes, such as over-current protection, self-regulating heating elements, inrush current limiters and so on. Thermistors can be NTC or PTC. In NTC (Negative Temperature Coefficient) thermistors, resistance decreases as temperature rises. NTC’s are commonly used as “inrush” current limiters. And with PTC (Positive Temperature Coefficient) thermistors, resistance increases as temperature increases. PTC thermistors are commonly used as “overcurrent protection” and in resettable fuses.

A thermocouple or simply “TC” is comprised of a couple of specific dissimilar wires joined together, forming the “sensing point” or “junction”. Based on physical characteristics called “Thermoelectric Effect”, when this junction is placed at different temperatures, different millivolt signals are generated which can be interpreted as an indication of the temperature. In comparison with RTDs, Thermocouples are self-powered and require no external excitation current source. Thermocouples are commonly used for furnaces, Gas Turbine combustion chamber, high-temperature exhaust ducts, etc. The main restriction of Thermocouples is the “accuracy” which doesn’t make it the best solution for precise applications. Also, Thermocouples need a reference measurement point called “Cold Junction”. The thermocouple junction is often exposed to extreme environments, while the cold junction is often mounted near the instrument location. Based on “range” of temperature measurement, “sensitivity” and some other factors in each application, different types of Thermocouples are available, for example E, J, K, M, N, T and so on. For instance, Type “J” is made up  of “Iron-Constantan” combination with a range of −40°F to +1380°F and sensitivity of about 27.8 µV/°F while Type “K” (Chromel-Alumel) is one of the most common general-purpose thermocouples with a sensitivity of approximately 22.8 µV/°F. Type K is inexpensive and a wide variety of probes are available in its −330°F to +2460°F operating range. Since the functionality of thermocouple sis based on Thermoelectric Effect in different types of conductors, when the location of a thermocouple is far from the “measuring instrument” (e.g. electronic transmitter), the proper type of conductors should be used for extension purpose. Otherwise, the tiny signal generated by thermocouple will be added with some error at the point where thermocouple wires are connected to the extension wire! “Semiconductor Temperature Sensor” is based on the fact that the junction voltage across ap-n combination of semiconductors, like a diode junction or “base-emitter” junction of regular transistors, is a function of temperature. This technology is vastly used in electronic devices and IC technologies. Linear characteristic, small size, and low cost are advantages of this technology, but it should be noted that the limited range of around -40°F to 248°F makes it suitable for specific applications. To wrap up this video, the comparison between different types of temperature sensor technologies is a multi-facet task. For example, if “accuracy” is considered as the key performance indicator, usually RTD’s are better than Thermocouples; approximately 10 times more accurate. From the “sensitivity” point of view, while both RTDs and Thermocouples respond quickly to temperature changes, at similar costs, thermocouples are often faster. If I have to measure electronic PCB and/or IC temperature, silicon-based types are the best choices.