Conventional planar MOSFET is limited to high power management. For high-powered applications, MOSFET is divided into two-dimensional VMOS or VMOS known as Power MOSFET.

What is Power MOSFET?

Power MOSFET is a three terminal (Gate, Drain and Source), four layers (n + pn – n +), Unipolar (most existing carriers only) semiconductor device.

1. MOSFET is a multi-carry device, and since most carriers do not have the re-integration delays, MOSFET achieves the highest bandwidth and switch times.
2. The gate is electrically separated from the source, and while this provides MOSFET with high input impedance, and creates a good capacitor.
   MOSFETs do not have a second breakage location, their resistance to the source of the resistance has a positive temperature coefficient, so they tend to protect themselves.
3. It has very low ON resistance and no junction in the jungle when leaning forward. These features make MOSFET a very attractive power switch.

MOSFET Symbol:

The symbol for n-channel MOSFET is given below. The direction of the arrow on the lead that goes to the body region indicates the direction of current flow. As this is the symbol for n channel MOSFET, the arrow is inwards. For p-channel MOSFET, the arrow will be towards outside.

MOSFET Structure

1. Power MOSFET has a four-layered vertical layout of P and N (n + pn – n +) alternating layers.
2. The middle layer of the P type is called the MOSFET body. In this region, a channel is formed between the well and the water supply.
3. The n-layer is called the drift region, which determines the voltage separation of the device. This n-region is only found in MOSFET Power that is not at MOSFET signal level.
4. The terminal gate is separated from the body by a silicon dioxide layer.
5. When a positive gate voltage is used relative to a well, a n-type channel is formed in the center of the suction source.
6. As shown in the figure, there is a parasitic npn BJT between the source and the water discharge.
7. To prevent the BJT from opening up, the p body region is shortened to become the source area by skipping the source metallization in the p-type body.
8. The result is a parasitic diode formed between the drain to the source terminals. This critical diode plays an important role in partial and complete bridge conversion circuits.

MOSFET VI Characteristics

What makes a “Base Sensor” different than a “Smart Sensor”? Before we get to that question, let’s review what a base sensor is, what it does, and how it is integrated into process control loops. A “base sensor” is a device that “senses” something. For many years we’ve had sensors that can see, feel, hear, smell, and even taste.

In the world of instrumentation and process control we define a “Sensor” as a device that detects changes in physical properties and produces an electrical output in response to that change. A “Thermocouple” is a temperature sensor that will produce an increasing voltage across it when exposed to an increasing temperature. In industry today, thousands of thermocouples are connected to transmitters in temperature process control loops. In process control, we condition the thermocouple voltage and convert it to an industry standard signal that represents our controlled temperature range.

OK…..so….what if we had a sensor that did more than sense singular basic physical properties? What if we had a sensor that also performs data conversion, digital processing, and can communicate to external devices and “The Cloud”? In very general terms a “Smart Sensor” has a “base sensor”, a “microprocessor”, is “communication-capable”, and has some form of onboard diagnostics. Smart Sensors are capable of a variety of functions and options. Smart Sensors can perform self-assessments and self-calibration. They can detect issues such as “sensor contamination”, “switch failures”, and “open coils”. Some Smart Sensors are capable of multi-sensing and can measure pressure, temperature, humidity, gas flow and more. Smart Sensors play a very important role in the new era of “manufacturing intelligence”. They will become more and more important as industry develops increasingly sophisticated and complex processes.

The Smart Sensor plays a very important role in Industry 4.0 which is considered as the fourth revolution of the manufacturing industry. A Smart Sensor can do more than sense singular basic physical properties. A Smart Sensor can perform data conversion, digital processing, and can communicate to external devices and “The Cloud”. A “Smart Sensor” has a “base sensor”, a microprocessor, is communication-capable, and has some form of onboard diagnostics. Smart Sensors are capable of a variety of functions and options. Some Smart Sensors are capable of multi-sensing and can measure pressure, temperature, humidity, gas flow and more.

In this article , we’re going to talk about what a sensor is, what it can do, and how it can
be used in process control. A sensor is a device that “senses” something. Today we have sensors that can see, feel, hear, smell, and even taste. Without sensors, our home and work lives would be quite difficult.

For example, as you drive to work, the traffic lights at an intersection are controlled by sensors embedded in the road. These sensors detect your arrival at the intersection. As you approach the grocery store, the door automatically opens because of a sensor.

In your plant, the batch process temperature and pressure are displayed and controlled as a result of output from Sensors. In the world of instrumentation and process control, we define a Sensor as a device
that detects changes in physical, electrical, or chemical properties and produces an electrical output in response to that change.

What are the typical physical properties that sensors are detecting? Let’s name a few… Level, Temperature, Flow, Pressure, Speed, and Position. From a process control perspective, we can classify sensors as either Passive or Active.

A Passive Sensor requires an external source of power to operate while an Active Sensor does not. A Thermocouple is an Active Sensor as it does not require any external power supply to operate. As a thermocouple is exposed to an increase in temperature, it will develop an increasing voltage across it. Another example of an Active sensor is a piezoelectric sensor. A Resistance Temperature Detector or “RTD” is a Passive Sensor. It is a device that’s resistance will change with a change in temperature. To take advantage of this change in resistance, an external supply, or an excitation circuit is required to produce a change in voltage. Another example of a Passive sensor is a Strain Gauge.

Now that we’ve talked about sensors and the physical properties that they can sense, let’s discuss how they are used in the industry. Almost every sensor used in process control will be connected to a Transmitter because a sensor’s output needs to be conditioned or amplified. Here’s an example… We’ve already talked about a thermocouple and the voltage output created when it is heated. Unfortunately, the voltage output of a thermocouple is minuscule! In our example, the thermocouple will produce a voltage output from 8 mV to 18 mV over a 450 degree Fahrenheit change in temperature! In process control, we condition that 8mV to 18mV thermocouple voltage and convert it to a 4 mA to 20 mA industry standard signal that represents our controlled temperature range.

Let’s review what we’ve covered today… Sensors are a part of everyday life at home and work A sensor is a device that can See, Feel, Hear, Smell, and even Taste. In process control, sensors are classified as Passive, requiring an external excitation to produce an electrical output, or Active, producing a voltage
output without any external excitation In process control, sensors are used to measure physical variables such as Level, Temperature, Flow, Pressure, Speed, and Position. Sensor output voltages are very small and therefore require a Transmitter to amplify or condition the output to make it useable in process control applications.

The first thing we need to know about a load cell is a definition of what we are talking about. A load cell is a force gauge that consists of a transducer that is used to create an electrical signal whose magnitude is directly proportional to the force being measured. There are four common types of load cells. They are pneumatic, hydraulic, strain gauge, and capacitance.

Let’s begin by looking at how a pneumatic load cell works. Since it is pneumatic, we know that it will deal with air pressure. A pneumatic load cell consists of an elastic diaphragm which is attached to a platform surface where the weight will be measured. There will be an air regulator which will limit the flow of air pressure to the system and a pressure gauge. Thus, when an object is placed on a pneumatic load cell it uses pressurized air or gas to balance out the weight of the object. The air required to balance out the weight will determine how heavy the object weights. The pressure gauge can convert the air pressure reading into an electrical signal.

Next, let’s talk about a hydraulic load cell. The word hydraulic should let us know that this load cell will work by using fluid, whether water or oil. These load cells are similar to pneumatic load cells but instead of air, they use the pressurized liquid. A hydraulic load cell is consisting of an elastic diaphragm, a piston with a loading platform on top of the diaphragm, oil or water that will be inside the piston, and a bourdon tube pressure gauge. When a load is placed on the loading platform the piston applies pressure to the liquid contained inside it. The pressure increase of the liquid is proportional to the applied force or weight. After calibrating the pressure, you can accurately measure the force or weight applied to the hydraulic load cell. The pressure reading can be read as an analog gauge or it can be converted into an electric signal from a pressure sensor.

The next type of load cell we will discuss is the strain gauge. This is the most popular style of the load cell. A strain gauge load cell is a transducer that changes in electrical resistance when under stress or strain. The electrical resistance is proportional to the stress or strain placed on the cell making it easy to calibrate into an accurate measurement. The electrical resistance from the strain gauge is linear therefore it can be converted into a force and then a weight if needed. A strain gauge load cell is made up of 4 strain gauges in a “Wheatstone” bridge configuration. A Wheatstone bridge is an electrical circuit that measures unknown electrical resistance by balancing two legs of a bridge circuit, one of the legs contains the unknown component. The “Wheatstone bridge” circuit provides incredibly accurate measurements. The strain gauges that are in the Wheatstone bridge are bonded onto a beam which deforms when weight is applied.

The last type of load cell we are going to discuss is a capacitive load cell. Capacitive load cells work on the principle of capacitance, which is the ability of a system to store a charge. The load cell is made up of two flat plates parallel to each other. The plates will have a current applied to them and once the charge is stable it gets stored between the plates. The amount of charge stored, the capacitance, depends on how large of a gap between the plates. When a load is placed on the plate the gap shrinks giving us a change in the capacitance which can be calculated into a weight.

Now that we have discussed the different types of load cells lets discuss some applications. The first application we are going to discuss is a salt bag filling process. In this application, empty bags are loaded into a machine where arms will swing down and pick up an empty bag and place it underneath a funnel. Above the funnel, there is a fill bin that will dispense salt onto a conveyor belt with a built-in load cell in order to dispense the correct amount of salt into the bags. As the fill bin is dispensing salt, the load cell is giving an analog input to a plc which is the current weight on the load cell. Once the load cell is reading a weight close to the full bag weight the fill bin will close to a trickle until the correct weight is determined. Once the load cell has the full bag weight on it, the conveyor will start dropping the salt into the funnel and down to the waiting bag. The bag will be sealed and removed from the machine so another empty bag can be loaded.

Next, let’s discuss how a load cell can be used in a pressing application. In this example, we will be looking at door panel press. Sheets of aluminum will be rolled into a die which will be closed down onto the aluminum creating a pattern on the door panel. As the die closes, a load cell is sensing the amount of force applied on the die and the aluminum. Once the applied force has reached a predetermined limit the die will open and the panel will now be removed. If the applied force is too light or too heavy the panel could be damaged or not pressed to the correct pattern.

Determining which load cell your application requires depends on how sensitive and accurate your application needs to be. The accuracy and sensitivity are very high with capacitive. A strain gauge type of load cell would be the next in line when it comes to accuracy and sensitivity. While still useful in certain applications, pneumatic and hydraulic load cells would be the less sensitive and accurate types.

In closing, we discussed the four different types of load cells. They were pneumatic, hydraulic, strain gauge, and capacitive. Also, we discussed how a load cell can be used in different industrial applications. In closing remember that the determining factor in choosing a load cell comes down to how accurate and sensitive your application requires.

First, we need to have a definition of vibration before we get too far into our discussion. Vibration can be defined as the mechanical oscillation about an equilibrium position of a machine or component. Or simply the back and forth motion of a machine or component. Vibration in industrial equipment is sometimes part of the normal operation but sometimes it can be a sign of trouble.

In machine monitoring we are dealing with two types of vibration; Axial (or Thrust) Vibration and Radial Vibration. “Axial” Vibration is a longitudinal shafting vibration or parallel to the shaft of a motor. For example, a shaft misalignment could cause axial vibration. “Radial” Vibration occurs as a force applied outward from the shaft. Radial vibration would occur if there is a heavy spot in the motor as it rotates. If there is a deformed fan blade, as the fan spins the deformed fan blade would pull outwardly on the shaft of the motor causing radial vibration.

Now that we know what vibration is let’s look at the different types of sensors to monitor vibration. First, we will talk about an accelerometer. Accelerometers are devices that measure the vibration, or acceleration of motion of a structure. They have a transducer that converts mechanical force caused by vibration or a change in motion into an electrical current using the piezoelectric effect. There are two types of piezoelectric accelerometers, high impedance, and low impedance. High impedance accelerometers produce an electrical charge which is connected directly to the measurement instruments. They require special accommodations and instrumentation so they are found in research facilities or high-temperature applications. Low impedance accelerometers have a charge accelerometer as its front end as well as a built-in micro-circuit and transistor that converts that charge into a low impedance voltage. This type of accelerometer easily interfaces with standard instrumentation which makes it commonly used in the industry.

Now, let’s talk about a strain gauge type of vibration sensor. Just like it sounds a strain gauge measures the strain on a machine component. A strain gauge is a sensor whose resistance varies with applied force; It converts force, pressure, tension, weight, etc, into a change in electrical resistance which can then be measured. When external forces are applied to a stationary object, stress and strain are the results. When there is a strain applied to any metallic wire the length of that wire increases and the diameter decrease. This increase in length and decrease in diameter will change the resistance of the wire which will give us our measurement of strain on our machine component.

The last type of vibration sensor we will discuss is an Eddy Current or Capacitive Displacement sensor. Eddy-Current sensors are non-contact devices that measure the position and/or change of position of a conductive component. These sensors operate with magnetic fields. The sensor has a probe which creates an alternating current at the tip of the probe. The alternating current creates small currents in the component we are monitoring called eddy currents. The sensor monitors the interaction of these two magnetic fields. As the field interaction changes the sensor will produce a voltage proportional to the change in the interaction of the two fields. When using Eddy-Current sensors it is important for the component to be at least three times larger than the sensor diameter for normal operation; otherwise, advanced calibration would be required.

When choosing a vibration sensor for your application it is important to look at factors such as; range and accuracy, environment conditions, and the shape of the measuring surface. Out of the three sensors that we have discussed the accelerometer is the most common because it has a good range of frequency, meaning it can sense slow and fast applications. Along with the frequency, accelerometers are priced affordably and are durable. They do have to be mounted directly to the machine which is common for vibration sensors. Eddy current or capacitive sensors have medium accuracy and are not optimal for high-resolution applications. They are very durable making them a good option for dirty environments. Just like the accelerometers they have to be directly mounted to the machine being monitored. Lastly, strain gauges are both versatile and accurate while still suitable for hazardous environments. Unfortunately, they can be hard to install correctly and to get proper data your application will need amplifiers which can drive up the price.

In closing, we discussed the differences in Axial and Radial vibration and their effect on machinery. We also identified three different types of vibration sensors, the accelerometer, Eddy Current or Capacitive, and strain gauge. These sensors are the most common vibration sensors but they are not the only option for your application. When determining the correct vibration sensor for your application it is important to consider the range and accuracy, environment conditions, and the shape of the measuring surface so that the sensor will perform the best in your application.

A pressure sensor simply monitors this pressure and can display it in one of the several units known around the world. This is commonly the “Pascal”, “Bar”, and “PSI” or pounds per square inch in the United States. The pressure of the air in your tire is a great example of pressure and how it is measured. As we air the tire up, the force it exerts on the tire increases, causing the tire to inflate. This is monitored with a pressure sensor inside the tire on newer vehicles.

So how does a pressure sensor work? In a nutshell, it converts the pressure to a small electrical signal that is transmitted and displayed. These are also commonly called pressure transmitters because of this. Two common signals that are used is a 4 to 20 milliamps signal and a 0 to 5 Volts signal.

Most pressure sensors work off of the piezoelectric effect. This is when a material creates an electric charge in response to stress. This stress is usually pressure but can be twisting, bending, or vibrations. The pressure sensor detects the pressure and can determine the amount of pressure by measuring the electric charge. Pressure sensors need to be calibrated so it knows what voltage or milliamp signal corresponds to what pressure. This is a basic “Zero” and “Span” calibration or minimum and maximum which is a common job for maintenance personnel.

In the article “What is Sensor Calibration and Why is it Important?” we described the sensor calibration in detail. What are some of the common types of pressure that you can measure with a pressure sensor? There are three common types that we use in the industry. First being “Gauge Pressure”. This is measured in reference to atmospheric pressure which is typically 14.7 PSI. You will show a “positive” pressure when it is above atmospheric pressure and a “negative” when it is below atmospheric pressure. The next type is “Absolute Pressure”. Simply put, this is the pressure as measured against absolute vacuum. A full vacuum will have an absolute pressure of 0 PSIa and increase from there. If you need to read a pressure that is lower than atmospheric pressure, this is the type of sensor you would use.

The last type that is commonly monitored in the industry is “Differential pressure”. This is exactly what it sounds like, the difference between two pressures, a pressure being measured and a reference pressure. In industry, pressure sensors are used for a wide variety of processes. Some common uses are to measure the pressure of steam. Steam is commonly used to heat many processes in manufacturing facilities. This pressure sensor on the steam system can serve multiple purposes though. First and most obvious is to observe and monitor the pressure. Another purpose is to control when and where steam can flow and regulate its pressure. Steam can build up a pressure in a vessel and become dangerous. We can use the pressure sensor as an input device to open and close a control valve to keep the pressure and steam flow regulated. This only requires simple programming in the PLC to achieve this.

Pressure sensors are also installed next to filters in many industrial processes. If the filter begins to clog, the flow will decrease. As the flow of the liquid decreases, pressure can increase or decrease depending on which side of the filter is monitored. If you monitor the pressure, it will give you a simple indication that the filter is clogged and needs to be cleaned or replaced.

A common use that isn’t as obvious is the use of a pressure sensor as a level sensor. In an open tank, you can use the hydrostatic pressure that is measured at the sensor. With a little math, using the size of the tank and specific gravity of the liquid, we can determine how much of that liquid is in the tank. If the tank is closed, it isn’t as simple of an installation. It is still a viable option though. This will require at least two sensors to measure differential pressure. The high-pressure sensor would be located at the bottom of the tank measuring the liquid pressure and the low-pressure sensor near the top measuring the air pressure inside. A calculation can then be performed to figure out how much liquid is in the tank.

Let’s take a look back at what we have learned. Pressure is an expression of force exerted on a surface per unit area. The standard units are the Pascal, Bar, and PSI or pounds per square inch. Pressure sensors convert the pressure into an electrical signal that can be transmitted and displayed. This is why many sensors are referred to as transmitters. These sensors commonly measure Gauge Pressure, Absolute Pressure, and Differential Pressure. Gauge pressure is measured against the atmospheric pressure, absolute is measured against a vacuum, and the differential pressure is the difference between two pressures. Pressure sensors are commonly used to monitor pressures in different processes. A common thing to monitor is steam pressure. That pressure sensor can be used to control a valve to keep steam pressure at a constant level. Another common but lesser known use is to monitor the level of a liquid in a tank. Filter clogs are a common use of differential pressure monitoring. By knowing the pressure before and after the filter, you can determine if it is clogged.

1. Point level measurement and
2. Continuous level measuring.

Point level measurement indicates when a product is present at a certain point and continuous level measuring indicates the continuous level of a product as it rises and falls. The sensors for point level indication are “Capacitance”, “Optical”, “Conductivity”,“ Vibrating” (Tuning Fork) and “Float Switch”.

The sensors for continuous level measuring are “Ultrasonic” and “Radar” ( Microwave). We will talk about how they work and which applications are best suited for their technology, as well as their limitations in certain applications. Let’s talk about point level indication sensors first. Starting with capacitance level sensors. A capacitance sensor is a proximity sensor that gives off an electrical field and detects a level by the effect on its electrical field. These sensors are small, less expensive than other sensors, invasive to the product, accurate, and have no moving parts. However, they do have to be calibrated and will only detect certain liquids.

Capacitance sensors can be used in liquid storage tanks. A water treatment facility that has storage tanks would be an ideal use for a capacitance sensor.

The next point level sensor we will talk about is an optical level sensor. Optical sensors work by converting light rays into electrical signals which measure a physical quantity of light then translate it into a measurement. These sensors have no moving parts, they are not affected by high pressure or temperature, are small in size, and can be used in liquid applications. However, if the lens gets coated or dirty they would require cleaning. They can be used as low-level indicators to prevent run dry conditions when using oil, coolant, or hydraulics.

Another style of point level sensor is conductivity or resistance. A conductivity or resistance sensor uses a probe to read conductivity. The probe has a pair of electrodes and applies alternating current to them. When a liquid covers the probe its electrodes form a part on an electric circuit, causing current to flow which signals a high or low level. The advantages of using a conductivity level sensor are there are no moving parts, they are low cost, and fairly easy to use. The disadvantages are they are invasive, meaning they must touch the product being sensed, they only sense conductive liquids, and the probe will erode over time. Appropriate use for these sensors would be for signaling high or low levels.

Vibrating or tuning forks is another type of point level sensor. They use a fork-shaped sensing element with two tines. The fork vibrates at its natural resonant frequency. As the level changes, the frequency of the fork will change detecting the level. These sensors are cost effective and are also compact. They are invasive to the product, meaning they have to touch the material to sense the level. These sensors are easy to install and are essentially maintenance-free. They have unlimited uses based on the material that they can sense. Mining, food and beverage, and chemical processing industries use these sensors for their applications.

The last point level sensor that we will talk about is a float switch. Float switches use a float, a device that will raise or lower when a product is applied or removed, which will open or close a circuit as the level raises or lowers moving the float. The advantages of a float switch are that it is a non powered device, it provides a direct indication, and they are inexpensive. The disadvantages are they are invasive to the product, they have moving parts, and can be large in size. Float switches will only give an indication for a high or low level they cannot measure a variable level. A great use for float switches is in liquid storage tanks for high or low-level indication.

Now, let’s talk about continuous level measurement sensors. We will start with ultrasonic sensors. They work by emitting and receiving ultrasonic waves. The time it takes for the waves to reflect back is how distance is measured. These sensors have no moving parts, are compact, and reliable. The disadvantage of using this type of sensor is that they can be expensive and in some situations, the environment can have a negative effect on them. The benefits of ultrasonic sensors are that they are non-invasive, or non-contact, they are unaffected by the properties of the material they are sensing, and they are self-cleaning because of the vibrations they give off. An example application is a hot asphalt tank in a shingle manufacturing plant. The ultrasonic sensor would be placed in the top of the tank away from the hot asphalt and used to sense the level in order to send a fill request for the tank.

Radar or microwave is also a continuous level sensor. These sensors transmit microwaves by an antenna on the radar sensor. The product that is being sensed reflects these microwaves back to the antenna and the time from emission to reception of the signal is proportional to the level of the product. Radar sensors are not affected by temperature, pressure or dust. They can also measure liquids, pastes, powders, and solids. They are very accurate and require no calibration. This type of level sensor is also non-invasive because it does not have to touch the product that it is sensing. The disadvantages of radar sensors are that they are expensive as well as have a limited detection range. If we go back to our shingle manufacturing plant example a Radar level sensor could be an ideal solution. Much like the ultrasonic sensor, radar sensors are ideal for hot liquid storage tanks.

Her, we talked about seven different types of level switches and their applications. Some of their applications can overlap and when deciding on a sensor it is important to identify the product that you are sensing and the type of feedback that your application requires. Sensors like conductivity or resistance, capacitance, float switches, and optical level sensors can be used to indicate a high or low level. While ultrasonic, and radar level sensors can measure your level to give specific feedback to how much of product is in a tank.

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.