The Fundamentals of Semiconductor Junction Thermometers

Temperature sensors can be fabricated with semiconductor processing technology by employing the temperature characteristics of the pn junction.  The batch processing and well-defined manufacturing processes associated with semiconductor technology can provide low cost and consistent quality temperature sensors.

Most semiconductor junction temperature sensors utilize a diode-connected bipolar transistor (short-circuited collector-base junction). A constant current passed through the base-emitter junction produces a junction voltage between the base and emitter (V_be) that is a linear function of the absolute temperature. The overall forward voltage drop has a temperature coefficient of approximately 2 mV °C^-1.

Fig: Bipolar transistor configured as a temperature sensor

In the above figure, the base of the transistor is shorted to the collector. A constant current flowing in the remaining pn (base to emitter) junction produces a forward voltage drop V_F proportional to temperature.

The temperature coefficient of a semiconductor sensor is larger but still quite small when compared to a thermocouple or resistive temperature device/detector (RTD). Furthermore, the semiconductor sensor’s forward voltage has an offset that varies significantly from unit to unit. Nonetheless, the semiconductor junction voltage versus temperature is much more linear than that of a thermocouple or RTD. Also, the temperature-sensing element, circuitry is easily integrated to produce a monolithic temperature sensor with an output that can be easily interfaced to a microcontroller and to provide features that are useful in particular applications. For instance, by using an embedded temperature sensor with additional circuitry, protection features can be added to integrated circuits (ICs). A temperature sensor becomes an embedded item in a semiconductor product when it has a secondary or supplemental purpose instead of the primary function.

The Operation of Linear Variable Differential Transformer (LVDT)

 Linear variable differential transformer (LVDT) is an inductive transducer that is commonly used to translate linear motion into electrical signals.

An illustration of LVDT circuit is shown below:

Fig 1.0 LVDT connection circuit

The transformer consists of a single primary winding P and two secondary windings S₁ and S₂ wound on a cylindrical former. A sinusoidal voltage of amplitude 3 to 15 volt and frequency 50 to 20 kHz is employed to excite the primary winding. The two secondary windings have equal number of turns and are identically placed on either side of the primary winding.

The primary winding is connected to an alternating current source. A movable soft-iron core is placed inside the former. The displacement to be measured is applied to the arm attached to the soft iron core. The core is usually made of high permeability, nickel iron. This is slotted longitudinally to reduce eddy current losses. The assembly is placed in a stainless steel housing to provide electrostatic and electromagnetic shielding. The frequency of ac signal applied to primary winding can be between 50 Hz and 20 kHz.

As the primary winding is excited by an alternating current source, it produces an alternating magnetic field which in turn induces alternating voltages in the two secondary windings.

The output voltage of secondary S₁ is ES₁ and that of secondary S₂ is E_S2. In order to convert the outputs from S₁ and S₂ into a single voltage, the two secondary S₁ and S₂ are connected in series opposition. The differential output voltage is:

E₀ = ES₁ – ES₂                                                                        

Operation of LVDT

When the core is at its normal (NULL) position, the flux linking with both the secondary windings is equal and hence equal voltages are induced in them. Therefore at null position: E_S1 = E_S2. Thus, the output voltage E₀ is zero at null position.

If the core is moved to the left of the null position, more flux links with S₁ and less with winding S₂. Correspondingly, output voltages E_S1 is greater than E_S2. The magnitude of output voltage is thus,

 E₀ = E_S1 – E_S2 and we can say, it is in phase with primary voltage.

In the same way, when the core is moved to the right of the null position E_S2 will be more than E_S1. Therefore the output voltage 

E₀ = E_S1 – E_S2 and 180° out of phase with primary voltage.

The amount of voltage change in either secondary winding is proportional to the amount of movement of the core. Thus, we have an indication of amount of linear motion. By noticing whether output voltage is increased or decreased, we can determine the direction of motion.

 Related: Transducers and Sensors

Merits of LVDT

• Output is quite high. Hence, immediate amplification is not necessary.
• Output voltage is step-less and hence the resolution is very good.
• The sensitivity is high (about 40 V/mm).
• It does not load the measured mechanically.
• Linearity is good up to 5 mm of displacement.
• It consumes low power and low hysteresis loss.

The Limitations of LVDT

• It is affected by stray electromagnetic fields. Thus, proper shielding of the device is required.
• LVDT has large threshold.
• The ac inputs generate noise.

Also read: Signal Converters commonly used in Industrial Instrumentation

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The Principle of Operation of an Electromagnetic Flowmeter

Electromagnetic flowmeters are widely used in industrial process flow measurement. These meters come with several features for example: they offer non-invasive flow measurement, they can measure reverse flows and are insensitive to viscosity, density and flow disturbances. Additionally, electromagnetic flowmeters can respond swiftly to flow changes and are linear devices for a wide range of measurements.

Working Principle of the Electromagnetic Flowmeter

Electromagnetic flowmeter operation is based on Faraday’s law of electromagnetic induction. The induced voltages in an electromagnetic flow meter are linearly proportional to the mean velocity of liquids or to the volumetric flow rates. As in the case in many applications, if the pipe walls are made from non-conducting elements, then the induced voltage is independent of the properties of the fluid.

Faraday’s law of induction states that if a conductor of length l (m) is moving with a velocity v (m/s) perpendicular to a magnetic field of flux density B (Tesla) then the induced voltage (e) across the ends of a conductor can be expressed by:

   e = Blv                                      

This is demonstrated in the figure below:

Figure 1.0 The Operational principle of electromagnetic flowmeter

In the above illustration, the magnetic field, the direction of the movement of the conductor, and the induced emf are all perpendicular to each other.

Let’s consider a simplified electromagnetic flowmeter construction below:

Figure 1.1 Construction of practical electromagnetic flowmeter

The externally located electromagnets create a homogenous magnetic field (B) passing through the pipe and the liquid inside it. When a conducting flowing liquid cuts through the magnetic field, voltage is generated along the liquid path between the two electrodes positioned on the opposite sides of the pipe.

The conductor is the liquid flowing through the pipe, and the length of the conductor is the distance between the two electrodes, which is equal to the tube diameter (D). The velocity of the conductor is proportional to the mean flow velocity (v) of the liquid. Hence, the induced voltage becomes:

  e = BDv

If the magnetic field is constant and the diameter of the pipe is fixed, the magnitude of the induced voltage will be proportional to the velocity of the liquid. If the ends of the conductor, in this case the sensors that are connected to an external circuit, the induced voltage causes a current, i to flow, which can be processed appropriately as a measure of the flow rate.

Electromagnetic flowmeters are often calibrated to determine the volumetric flow of the liquid. The volume of liquid flow Q can be related to the average fluid velocity as:

Q = Av

Where A is the area of the pipe, which can be written as:

That gives the induced voltage as a function of the flow rate:

We know that fluid velocity v = Q/A

We can derive the induced voltage as:

          

Equation 1.4 indicates that in a well-designed electromagnetic flowmeter, if all other parameters are kept constant, the induced voltage is linearly proportional to the liquid flow only.

Even though the induced voltage is directly proportional to the mean value of the liquid flow, the main problem in the use of electromagnetic flowmeters is that the amplitude of the induced voltage is small relative to extraneous voltages and noise. The noise sources include:

• Capacitive coupling between signal and power circuits.
• Stray voltage in the process liquid.
• Capacitive coupling in connection leads.
• Inductive coupling of the magnets within the flowmeter.
• Electromechanical emf induced in the electrodes and the process fluid.

Key Merits of Electromagnetic Flowmeters

The electromagnetic flowmeters have the following advantages:

• The output (voltage) is linearly proportional to the input (flow).
• There is no obstacle to the flow path which may cause reduction in pressure.
• The electromagnetic flowmeter can measure flow in pipes of any size provided a powerful magnetic field can be produced.
• The output is not affected by changes in the characteristics of the liquid such as pressure, viscosity, and temperature.

You can also read: Working Principle of Ultrasonic Flowmeter

Shortcomings of Electromagnetic Flowmeters

The electromagnetic flowmeters have the following limitations:

• The conductivity of the liquid being measured should not be less than 10 μꭥ/m.  It is important to note that most water based/aqueous solutions are adequately conductive while a majority of hydrocarbons solutions are not sufficiently conductive.
• The operating cost is usually very high in an electromagnetic flowmeter specifically if heavy slurries are handled.

Related resource: Ultimate guide to Industrial flow Instruments

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Turning Fork Level Switches

This level switch uses a metal tuning fork structure to detect the presence of a liquid or solid (powder or granules) in a vessel.

An electronic circuit continuously excites the tuning fork, causing it to mechanically vibrate. When the prongs of the fork contact anything with substantial mass, the resonant frequency of the structure dramatically decreases. The circuit detects this change and indicates the presence of material contacting the fork. The fork’s vibrating motion tends to shake off any accumulated material such that this style of level switch tends to be resistant to fouling.

You can also read: Ultrasonic Level Switches

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Types of Signal Converters Commonly used in Industrial Control

Signal converters change a signal from one form to another. In most cases, we have standard inputs and output ranges.

Most signal converters have two adjustments-zero and range.

Examples of Signal converters

Nozzle-flapper and differential pressure cells

The nozzle-flapper system is widely used in D.P. Cells. An example is shown below, that converts differential pressure (e.g. from a differential Pressure flow meter into a standard pneumatic signal). This is widely used in the control of air operated pipeline valves.

The bellows respond to the differential pressure and moves the lever. This moves the flapper towards or a way from the nozzle. The air supply passes through a restrictor and leaks out of the nozzle. The output pressure hence depends on how close the flapper is to the end of the nozzle. The range of the instrument is adjusted by moving the pivot and zero position is adjusted by moving the relative position of the flapper and nozzle. This system is used in a variety of forms. Instead of bellows, a bourdon tube might be used and this is operated by an expansion type temperature sensor to produce a temperature-pneumatic signal converter.

You can also read: Types of Proximity Sensors used in Industrial Control

Current/Pressure conversion 

The figures below show typical units for converting 4-20 mA into 0.2-1 bar or 3-15 psi. They contain adjustments for range and zero. They are widely used for converting the standard pneumatic and electric signals back and forth. They can also be adjusted to work with non standard inputs to convert them into a standard form.

Current to Pressure Converter

Pressure to Current Converter

Electric D.P. Cells

They provide the same functions as the pneumatic versions but they are given an output of 4-20 mA using electrical pressure transducers. They are typically used with D.P. Flow meters.

Differential Pressure Transmitter

Analogue -Digital Converters

Analogue to digital conversion is a process of turning an analogue voltage or current into a digital pattern which can read by a computer and processed, this is done by analogue -digital converters.  Lets look at the Binary Numbers, a number may be represented in digital form by simply setting a pattern of voltages on a line high or low. It is normal to use 4, 8, 16 or 32 lines. An 8 bit binary pattern is shown in the below figure:

The total pattern is called a word and the one above is an 8 bit word. The pattern may be stored in an 8 bit register. A register is a temporary store where the word may be manipulated. The Bit zero is called the least significant bit (LSB) and the bit with highest value is called the most significant bit (MSB). Each bit has a value of zero when off (LOW) or the value shown when on (HIGH). The maximum value for an 8 bit word is 255.

Digital to Analogue Converters

These are devices for converting a binary number into an analogue voltage. The change in the binary value from zero to a maximum corresponds with a change in the analogue value from 0 to maximum.

One of the manufacturers of signal converters is Omega.

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The Inputs and Outputs of Process Measurement Instruments Commonly used in Control Systems

Basically when you are doing the troubleshooting of any instrumentation system you assume that every instrument has at least one input and at least one output and that the output(s) should accurately correspond to the input (s). In normal circumstances, if the instrument’s output is not corresponding to its input according to the instrument’s design function, then there could be something wrong with the instrument. Lets consider the inputs of the following examples of  instruments that are commonly used in process control systems:

• Differential Pressure transmitter
• Temperature Transmitter
• Controller

Each of the above instruments takes in (input) data and generates the (output) data.  In an instrumentation loop, the output of one instrument feeds into the input of the next. Such information is passed from one instrument to another.

You can also read:

• The Basics of Industrial Instrumentation and Control Systems
• How to Troubleshoot a 4-20 mA Loop Current with Test Diodes

By intercepting the data communicated between components of an instrument system, we are able to locate and isolate faults. For us to able to properly understand the intercepted data, we must understand the inputs and outputs of the respective instruments and the basic functions of those instruments. From the above diagrams, we  are able to highlight the kind of inputs and outputs for each of the instruments indicated.

To be able to check the right correspondence between the instrument inputs and outputs, we must therefore use appropriate test equipment to intercept the signals into and out of these instruments e. g. in case of analogue instruments using 4-20 mA signals we can use the electrical meters capable of measuring the current and voltage.

So what are some of the key considerations when using milliameters to measure loop current?

For you to measure the loop current, you have to break the circuit to connect the milliameter, in series with the current, and which means the current will fall to 0 mA until the meter is connected. Interrupting the current means interrupting the flow of information that is conveyed by that current, be it a process measurement or a command signal to a final control element. This can have adverse effects on the control system unless certain preparations are made before hand. The preparations can be in form of:

• Informing the personal in charge that signal will be interrupted - state the number of times you intend to do the interruption.
• For case, where the signal is coming from a process transmitter to a controller, the controller should be placed in manual mode, so that it will not cause an upset in the process. 
• If the current drives process shutdown alarms, these should be disabled on temporarily basis, so that nothing shuts down upon the interruption of the signal.
• All process alarms should be temporarily disables so that they do not cause panic.
• If the current signal to be interrupted is a command signal from a controller to a final control element, the final control element either needs to be manually overridden so as to hold a fixed setting while the signal varies or it needs to be bypassed completely by some other devices (s)

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Ultrasonic Level Switches

This type of level switch uses ultrasonic sound waves to detect the presence of process material either solid or liquid at one point.

The Working Principle

Sound waves pass back and forth within the gap of the probe, sent and received by piezoelectric transducers. The presence of any substance other than gas within the gap detects the received audio power, thus signalling to the electronic circuit within the bulkier portion of the device that process level has reached the detection point.

Fig 1

Fig 2

In Fig 1, the sound passes through the gap when filled with liquid (wet gap).

In Fig 2, the sound cannot pass through an air-filled gap (dry gap).

The lack of moving parts makes this probe quite reliable although it may be become fooled by heavy fouling.

You can also read: The Working Principle of Ultrasonic Flow Meters

Some of the industrial applications of Ultrasonic level  switches include:

• Leak detection
• As an overfill alarm
• Detection of presence of liquids in pipeline
• Pump Protection
• Engine oil level control

The Components that make up a 4-20 mA Current Loop

The 4-20 mA current  loop is an important aspect of process measurement and control circuits. The important components that make up this loop are:

• The Sensor
• The Transmitter
• The Power Source
• The Loop itself
• The Receiver

The following diagram shows each component with its function:

This is a simple illustration of a 4-20 mA current Loop System.

 You can also read:

• How to wire a 4-20 mA current loop with a PLC

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Key Applications of Thermal Mass Flowmeters

Thermal mass flowmeters work on the principle that when you place a heated object in the midst of a fluid flow stream and measure how much heat the fluid transfers away from the heated object, then you can be able to determine the mass flow rate. Industrial thermal mass flowmeters consists of a specially designed flow tube with two temperature sensors inside: One that is heated and one that is unheated. The heated sensor acts as the mass flow sensor (cooling down as flow rate increases) while the unheated sensor serves to compensate for the “ambient” temperature of the process fluid.  The following diagram shows an example of a thermal mass flowmeter from Magnetrol.

Magnetrol Thermal Flowmeter

An important factor in the calibration of a thermal mass flowmeter is the specific heat of the process fluid. Fluid with high specific heat values make good coolants because they are able to remove much heat energy from hot objects without experiencing great increases in temperature. Since thermal mass flowmeters work on the principle of convective cooling, this means a fluid having a high specific heat value will elicit a greater response from thermal mass flowmeter than exact same mass flow rate of a fluid having a lesser specific heat value. Therefore it is paramount that you know the specific heat value of the fluid you plan to measure with a thermal mass flowmeter and be assured that its specific heat value will remain constant. For this reason, thermal mass flowmeters are not suitable for measuring the flow rates of fluid streams whose chemical composition is likely to change over time.

Another potential limitation of thermal flowmeters is the sensitivity of some designs to changes in flow regime. Since the measurement principle is based on heat transfer by fluid convection, any factor influencing the convective heat-transfer efficiency will translate into a perceived difference in mass flow rate. Turbulent flows are more efficient at heat convection than laminar flows. Therefore, a change in flow regime from turbulent to laminar will cause a calibration shift for this design of thermal mass flowmeters.

So what are some of the applications where thermal flowmeters are used?

Generally thermal flowmeters are used in applications where the composition of the fluid is known especially in purified gases. Having said that, lets look at some of the areas where thermal flowmeters are commonly used:

Natural Gas

Flow measurement of natural gas fuel usage is important for combustion efficiency as well as general energy management projects for both industrial and commercial facilities. Thermal mass flowmeters will monitor the flow to individual combustion sources.

Air Efficiency

In combustion applications, thermal mass flow meters can ensure repeatability of air flow measurements to obtain an efficient air-to-fuel ratio. On compressed air, it is common to measure for plant allocation or to determine leaks.

Tank Blanketing

Nitrogen is frequently used to maintain an inert environment in the vapor space of a tank. Thermal meters are ideally suited for measuring the flow of nitrogen in such applications because they support mass measurement, they are easily installed into the pipe & are excellent at measuring low flow rates.

You can also read: 

• The Ultimate Guide to Industrial Flow Instruments
• How Pitot Tubes are used In Flow Measurement

Flare & Vent Gas

Thermal mass meters are a particularly good fit for flare measurement. Flares can range from vent gases at atmospheric pressure to high flow applications needing extended turn down. Oil and Gas is a common industry here.

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Biogas

Both the landfills and anaerobic digesters at wastewater plants produce a mixture primarily composed of methane and carbon dioxide. Excellent low flow sensitivity, hot tap capabilities and mixed gas calibration make thermal mass flow measurement a popular technology.

Source: Magnetrol

Types of Proximity Sensors used in Industrial Control

Almost every automated manufacturing operation has sensors that ensure that the system is working correctly.

Examples of Sensors that are used in industrial control are:

Non-Contact Presence Sensors (Proximity Sensors)

Contact sensors are often avoided in automated systems because wherever parts touch there is wear and a potential for eventual failure of the sensor. Automated systems are increasingly being designed with non-contact sensors. The three most common types of non-contact sensors in use today are:

• Inductive proximity sensor
• Capacitive proximity sensor
• Optical proximity sensor

The above sensors are actually transducers, but they include control circuitry that allows them to be used as switches. The circuitry changes an internal switch when the transducer output reaches a certain value.

Hall Effect Limit Switch

The inductive Sensor

This is the most widely used non-contact sensor due to its small size, robustness, and low-cost. This type of sensor can only detect the presence of electrically conductive materials.

The DC power supplied is used to generate AC in an internal coil, which in turn causes an alternating magnetic field. If no conductive materials are near the face of the sensor, the only impedance to the internal AC is due to the inductance of the coil. If however, a conductive material enters the changing magnetic field, eddy currents are generated in that conductive material, and there is a resultant increase in the impedance to the AC in the proximity sensor. A current sensor, also built into the proximity sensor detects when there is a drop in the internal AC current due to increased impedance. The current controls a switch providing the output.

Inductive Sensor

 

Capacitive proximity Sensors

These sensors sense the target objects due to the target’s ability to be electrically charged. This works both on conductors and non-conductors.

Inside the sensor is a circuit that uses the supplied DC power to generate AC, to measure the current in the internal AC circuit, and to switch the output circuit when the amount of AC current changes. Unlike the inductive sensor, the AC does not drive a coil, but instead tries to charge a capacitor. The AC can move current into and out of this plate only if there is another plate nearby that can hold the opposite charge. The target being sensed acts as the other plate.

Capacitive Proximity Sensor

If this object is near enough to the face of the capacitive sensor to be affected by the charge in the sensor’s internal capacitor plate, it will respond by becoming oppositely charged near the sensor, and the sensor will then be able to move significant into and out of its internal plate.

Optical Proximity Sensors

These are widely used in automated systems because they have been available longer and some can fit into small locations. They are commonly known as light beam sensors of the thru-beam type or of the retro-reflective type. A complete optical proximity sensor includes a light source, and a sensor that detects the light. The light source is supplied because it is usually critical that the light be tailored for the light sensor system. The light source generates a light of a particular frequency which is able to be detected by the light sensor in use. Infra-red light is used in most optical sensors. To make the light sensing system more foolproof, most optical proximity sensor light sources pulse the infra-red light on and off at a fixed frequency. The light sensor circuit is designed so that light that is not pulsing at this frequency is rejected.

Optical Proximity Sensors

The light sensor is a semiconductor device such as a photo diode which generates a small current when light energy strikes it or more commonly a photo transistor or a photodarlington that allows current to flow if light strikes it.

You can also read: Use of Smart Sensors in Industrial Measurement and Control Systems

Some of the manufacturers of proximity sensors include:

• Siemens
• Omron

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