The concept of control dates back to the time of the Greeks, in which different types of mechanisms were built, such as: a water clock, oil lamps, etc. As well as a mechanism to open and close the doors of one of the most popular temples at that time (Ktesibios), which generated a peculiar wind, with what people believed was an act of power created by the Olympian gods.
Man, knowing himself limited in his abilities, has created devices that allow him to expand the way of doing things, creating devices that can control some variables that are considered necessary to apply them in various industrial processes.
measurement-elements-in-automation-and-industrial-robotics-1There are those who, supported by the marked development of new technologies in this beginning century, consider that this can be easily achieved, with the help of robots that replace “the work of human beings”, since “more technology”, “ more quality, lower costs and, therefore, low price ”.
In recent decades "the European industry has invested most of its budget in machinery and robotics" and "it is estimated that in the coming years the number of robots in Europe and the US will increase significantly".
As for robots, their "capabilities and versatility" are expected to "continue to expand" and their prices will drop.
According to the experts' forecast, in the long term, the number of activities carried out by robots will reach 80% of all sectors of the economy.
Sensor-guided agricultural production programs, genetic engineering, molecular farms, computerized operators with voice recognition, ATMs, communication systems, office automation to the point of virtual office, show just a few from the sectors and areas of production, how far is the new technological advance.
The benefits of technology: low costs, faster processes, increased competitiveness and efficiency, have an impact on society that should be analyzed.
Where industrialization is most palpable, employment rates will be directly affected.
Measuring elements
In any automatic control system, the measurement of the variables to be controlled is necessary.
In addition to the variables to be controlled, it is common to measure other variables so that you have a better understanding of what is happening in the process.
The measurement of the magnitudes of the process (pressures, fluids, temperatures, pH, humidity, speed, etc.) is carried out by the primary elements, which, in most cases, transform them into magnitudes of another species (pneumatic pressures, electrical potentials, mechanical displacements, etc.) but easy to measure or transmit remotely.
The instruments that produce this transformation of variables are known as transducers. An analog relationship is intended to exist between the measured magnitude values and the output of the transducer.
There are cases in which it is not possible to directly measure the quantity to be controlled. Then we resort to the measure of another magnitude on which the first depends. For example, in a tempering furnace the quantity that is constantly required to be maintained is the temper of the steel. The difficulties of rapid, precise and continuous measurement of tempering require recourse to oven temperature control.
Automatic controller
An automatic controller compares the output signal with a reference (desired value), determines the error, and produces a control signal that tries to reduce the error to zero or less.
Static and dynamic characteristics
For the study of the automation of a process, it is interesting to know the relationships between the input and output variables (manipulated variable and controlled variable) when there are no variations in time, that is, under equilibrium conditions. The relationships between the variables, under equilibrium conditions, are the static characteristics. Thus, in the process represented in the following figure, each input cost Q1 corresponds, after reaching equilibrium, to a certain level h.
The relations h = f (Q1) express one of the static characteristics of the process.
The variable time does not enter the relationships that express the static characteristics.
The dynamic characteristics can be established in the simplest cases, analytically, by the sole knowledge of the physical laws and the constants of the process.
In addition to the dynamic characteristics of the measurement system, the following properties are very important:
- Accuracy: It expresses the degree of agreement between the value indicated by the measurement system and the actual value of the quantity. It is represented by the deviation, expressed as a percentage of the maximum value. The best way to know the precision is to determine the error curve, in the entire measurement band. Linearity: It means that the function that relates the output to the input variable is a linear function (geometrically represented by an inclined line). The deviations from the linearity are expressed as a percentage. Hysteresis: Difference between the values indicated by the system to pass the same value of measurement quantities, when the value has been reached by increasing or decreasing values. Sensitivity: Represents the relationship of the output signal and input signal. For the same input signal,the output is greater the higher the sensitivity.
Pressure measurement
Pressure measurements are very important in continuous processes in which you have to deal with fluids
The primary elements for measuring pressure belong to the following main categories:
- Liquid pressure gauges Bellows Diagrams Vacuometers of various types Piezoelectric or piezo resistive elements Strain Cages with capacitive detectors Reluctance elements
In Industrial Control three slightly different concepts of pressure are of interest. Gauge pressure is normally measured, representing the difference between the absolute pressure at the installation site and the atmospheric pressure.
Sometimes the measurement of absolute pressure is of interest, especially when measuring pressures below atmospheric.
When it comes to flow measurements, in ventilation systems, etc., the measurement of differential pressure is also very common.
These concepts are illustrated in the following figure:
Some of these types are just indicators and are not interested in automatic control. The following figure only shows the schematic operation of the indicator manometers.
On other types of pressure gauges, the output variable is a mechanical movement. They are suitable for acting on signal transmitters or input instruments in pneumatic controllers.
A float gauge is schematically shown in Figure R. The float movements are proportional to the pressure difference. They transmit abroad through a mechanical system and a shaft with a watertight packing, or in certain cases through a torsion tube. It can be used for differential pressure measurements up to 600 in. Of water and for static pressures up to 5000 psi.
The diagram of a ring gauge is shown in the figure. The angle of rotation depends, in this case, on the pressure difference. Ring movements can be applied as input to instruments or controllers. To finish this quick reference to liquid manometers, figure R '' illustrates the principle on which a bell manometer is based.
They are suitable for measuring small differential pressures. For example, it is used to measure the pressure in the combustion chambers-
In this device there is a scale that is divided into elements of elemental magnitude forming a grid, a sensor that is attached to the mobile carriage of the machine, translates each line into an electrical impulse and therefore originates an analog or digital indication of the displacement.
- Absolute:
They are used to achieve greater precision than with thermocouples, or for measurements of small temperature deviations (of the order of 0.02 ° C). When measuring temperatures close to room temperature, resistances are essential. The maximum error of industrial resistance thermometers is close to 0.5%.
This method takes advantage of the change in resistance of electrical conductors with temperature. The substances it uses are platinum metallic threads (they are the most used for precision and resistance to corrosion), copper or nickel, silver, etc.
The thermometric resistance metallic wire is wound on insulating supports, generally ceramic. Externally, the resistances are protected by thermometric covers of various substances (metal, ceramic, glass, etc.).
Flow Measurements
There are many basic methods of flow measurement. Some are quite generalized, others are applied in restricted cases.
For classification purposes, the primary flow measurement elements can be grouped into the following main groups:
- Differential Pressure Gauges Rotary Gauges (Meters and Turbines) Electromagnetic Flow Gauges Variable Area Gauges Discharge Gauges Mass Flow Gauges Solid Flow Gauges Vortex Gauges Ultrasonic Flow Gauges
This paper will only refer to the most important types of flow meters in the industry.
Differential pressure gauges
Differential pressure flow measurement elements, universally used in the measurement of fluid flow, are based on the universal theorem of hydrodynamics (Bernoulli's theorem).
P 1 + pgh 1 + pv 1 2 = P 2 + pgh 2 + pv 2 2
- two
The general expression that relates the flow of the incompressible fluids "q" with the differential pressure is as follows:
q = K Ö (P 1 - P 2)
Where:
q: Fluid flow
k: Spending coefficient
P 1: Pressure 1
P 2: Pressure 2
Despite the great theoretical advance in this aspect, the complexity of the phenomena in question is such that, in order to calculate the differential pressure elements, we use experimental data and empirically determined tables. This is the only way to achieve acceptable precision.
Differential pressure elements are restrictions or constrictions of various types inserted in the pipeline where the fluid to be measured circulates. The pressure drop that occurs in the restriction is a measure of flow.
Of the various types of restrictions, the most used are:
- Holes Venturi tubes Dall tubes
Holes:
This is the most used type of restriction. It has the shape of a circular plate, inserted into the pipe between two flanges, in which a hole is made with the dimensions indicated by the calculation. The orifice material must resist chemical and mechanical corrosion of the fluid. The various types of stainless steel are widely used.
Venturi tubes:
They are more elaborate restrictions than the hole. The Venturi tube allows greater precision than the orifice, in addition, the pressure drop is largely recovered. Another advantage is that we have greater consistency in the indications over time, that is, there is greater repeatability.
The Venturi Tube is particularly recommended for liquids with suspended solids. The only downside to the Venturi Tube is the high cost.
Dall tube:
This tube produces permanent pressure loss of about 15% and is cheaper than the Venturi Tube.
Rotary meters (meters and turbines)
Counters of various models are used (oscillating pistons, mutation disc, geared tooth rotors, gas counters, etc.) when it is required to measure fluid quantities with good precision (from 0.1 to 1%). Domestic gas and water meters are of this type.
Turbine type flowmeters allow higher pressures than Venturi orifices and tubes allow in the measurement of liquid flow. Physically, turbine meters are very small in size when compared to other types of primary elements. The main organ is a turbine that measures the average speed of the liquid.
In a coil mounted on the internal bottom of the meter, whose core is a permanent magnet, impulses are induced to the passage of each one of the blades of the turbine. These impulses are amplified and transformed into rectangular impulses. An electronic pulse counter allows digital indications of the flow and the amount of liquid. Turbine meter accuracy is typically better than 0.5% over a wide range of measurements.
Electromagnetic flow meters
Meters of this type are the only ones that do not present an obstruction to the flow of the liquid. The pressure drop they introduce is equal to that of a free pipe with the same size. For this reason they are the ideal primary elements for the measurement of flows in viscous liquids or with solids in suspension. The only condition will be that the liquid has an electrical conductivity above an established minimum.
The operation of these meters is based on the phenomenon of electromagnetic induction. An electrical conductor that moves with velocity perpendicular to an induction magnetic field, is the seat of an electromotive force, given by the relation:
e = (B) (l) (v)
Where: e: Electromotive force
B: Magnetic induction field
l: conductor length
v: Perpendicular velocity
The induced electromotive force, which is proportional to the flow of the liquid, will be amplified by an electronic amplifier. One of the difficulties of this measure lies in the low value of the emf (millivolts), and the appearance of various parts of the circuit, of emf induced by the magnetic fields existing in manufacturing facilities.
Another difficulty is related to variations in network voltage, which cause variations in magnetic induction. Variations in the conductivity of the liquid can also introduce errors.
It is very useful for measuring the flow in liquids with suspended, pasty or corrosive solids. There are currently electromagnetic primary elements whose electrodes do not have ohmic contact (resistance) with the liquid, but only capacitive.
Vortex meters
It is a primary flow element that offers superior precision than that of the orifices, has no moving parts and operates with a wide band of expenses. Pressure and temperature variations do not affect measurements.
Having no mechanical parts, its reliability is high. The instrument is based on the detection of the passage of vortices formed by an obstacle (vortex generating element), interspersed in the passage of the fluid.
The vortices are small eddies in localized areas. The vortex generator element runs diagonally through the metering pipe and divides the flow in half.
The vortices are alternately formed in each of the two halves. The geometry and the profile of the generating element are determined in order to obtain the following characteristics of the vortices:
- Stability Number of vortices proportional to expense
The number of vortices proportional to expense within a wide range of measurements. Therefore, there is a linear relationship between the flow and the number of vortices in a fixed interval of time.
Whenever a vortex occurs, a differential pressure is produced between the top and bottom sides of the generator element. The succession of pressure pulses is detected by a sensitive element inserted inside the generating element. The rate of the impulses sent by the detector is proportional to the number of vortices and therefore proportional to the cost.
This instrument is used with great success in common applications using orifices and in the measurement of liquids with suspended or corrosive solids.
Ultrasonic flow meters
A narrow beam of sound waves (in the acoustic or ultrasonic band) thrown through a moving fluid suffers a drag effect. The ultrasonic flow meter takes advantage of this effect.
In its simplest form, it consists of an ultra sound transmitter transducer (TT) and a receiver transducer (TR).
The ultrasonic wave, sent in pulses, goes through the fluid twice as it is reflected on the opposite wall. fluid.
This type of meter, even in its initial phase, has a better precision than that of the orifice plate and does not present any obstruction, as occurs with the electromagnetic meter. It is therefore used for viscous pasty or dangerous liquids (high pressure, corrosive, radioactive).
It needs an automatic temperature correction by means of a thermistor because the speed of sound is altered depending on the temperature present in the body.
Transmission of Signals
The classic definition of a transmitter tells us that it is an instrument that captures the variable in process and transmits it remotely to an indicator or controller instrument; but in realities that and much more, the primary function of this device is to take any signal to convert it into a standard signal suitable for the receiving instrument, this is how a transmitter captures signals from both a sensor and a transducer, always clarifying that everything transmitter is transducer but not a transducer can be a transmitter; as we already know the standard signals can be pneumatic whose values are between 3 and 15 Psi, the electronic ones that are 4 to 20 mA or 0 to 5 volts
Analog and digital data transmission
ANALOG TRANSMISSION
- Analog data takes continuous values An analog signal is a continuous signal that is propagated by certain means. Analog transmission is a way of transmitting analog signals (which may contain analog data or digital data). The problem with analog transmission is that the signal weakens with distance, so signal amplifiers have to be used every certain distance.
DIGITAL TRANSMISSION
- digital take discrete values discrete values Digital data is usually represented by a series of voltage pulses that represent the binary values of the signal. Digital transmission has the problem that the signal attenuates and distortes with distance, so each a certain distance you have to introduce signal repeaters.
Lately, digital transmission is widely used because:
Digital technology has become much cheaper.
- By using repeaters instead of amplifiers, noise and other distortions are not cumulative. Broadband utilization is more exploited by digital technology. Transported data can be encrypted and therefore there is more information security. By digitally processing all signals, analog data services (voice, video, etc.) can be integrated with digital as text and others.
Transmission disturbances
Attenuation
The energy of a signal decays with distance, so it must be ensured that it arrives with enough energy to be captured by the receiver circuitry and, in addition, the noise must be significantly less than the original signal (to maintain the energy of the signal amplifiers or repeaters are used).
Because the attenuation varies as a function of frequency, the analog signals arrive distorted, so systems must be used that restore the initial characteristics to the signal (using coils that change the electrical characteristics or amplify the higher frequencies more).
Delay distortion
Because in guided media, the propagation speed of a signal varies with frequency, there are frequencies that arrive before others within the same signal and therefore the different frequency components of the signal arrive at different times at the receiver. Equalization techniques are used to mitigate this problem.
Noise
Noise is any signal that is inserted between the emitter and receiver of a given signal. There are different types of noise: thermal noise due to thermal agitation of electrons within the conductor, intermodulation noise when different frequencies share the same transmission medium, crosstalk occurs when there is a coupling between the lines that carry the signals and impulsive noise These are discontinuous pulses of short duration and of great amplitude that affect the signal.
Channel capacity
Channel capacity is called the rate at which data can be transmitted on a data communication channel.
Data rate is the rate in bits per second at which data can be transmitted
Bandwidth is that bandwidth of the transmitted signal that is limited by the transmitter and by the nature of the transmission medium (in hertz).
The error rate is the rate at which errors occur.
For a given bandwidth, the highest possible transmission speed is recommended, but in such a way that the recommended error rate is not exceeded. To achieve this, the biggest drawback is noise.
For a given bandwidth W, the highest possible transmission speed is 2W, but if it is allowed (with digital signals) to encode more than one bit in each cycle, it is possible to transmit more information.
Nyquist's formulation tells us that by increasing the differentiable voltage levels in the signal, it is possible to increase the amount of information transmitted.
C = 2W log 2 M
The problem with this technique is that the receiver must be able to differentiate more voltage levels in the received signal, which is made difficult by noise.
The higher the transmission speed, the greater the damage that noise can cause.
Shannon proposed the formula that relates the signal strength (S), the noise strength (N), the channel capacity (C) and the bandwidth (W).
C = W log 2 (1 + S / N)
This capacity is the theoretical maximum transmission quantity capacity, but in reality it is less because nothing other than thermal noise has been taken into account.
Transmitter Types
Pneumatic transmitters: They are based on the principle that complies with the shutter nozzle system consisting of a tube with a constant supply of pressure not exceeding 25 Psi that passes through a restriction that reduces the diameter by around 0.1 mm and that at its other end it becomes a nozzle with a diameter of 0.25 - 0.5 mm that is exposed to the atmosphere causing an exhaust that is regulated by a shutter which fulfills the mission of controlling the leakage proportional to the separation between it and the nozzle.
The function of the nozzle - shutter is that as the shutter foil decreases or increases the distance to the nozzle, it will cause an inversely proportional effect on the internal pressure that is intermediate between atmospheric and supply pressure and equal to the output signal. of the transmitter which for the totally closed nozzle is equivalent to 15 Psi and totally open to 3 Psi.
To obtain an efficient output and because of the small volumes of air that are obtained from the system, a pilot valve that amplifies is attached to it, forming a two-stage amplifier.
The pilot valve consists of a shutter that allows the passage of two air flows which determine the output through pressure differentials between surfaces one and two, managing to overcome the spring that seeks to hold the valve closed, although there is really a minimum opening that what determines the 3 Psi as minimum output. The valve functions are:
- Increase of the supplied flow rate or the exhaust flow rate to achieve response times of less than a second. Pressure amplification (gain), from four to five to obtain the pneumatic signal of 3 - 15 Psi.
Pneumatic transmitters have the following characteristics:
- Lower air consumption for zero output flow, Higher output flow to receiver, Dead zone of outlet pressures, Force balancing, Direct acting.
Electronic transmitters: Generally using balance of forces, the unbalance results in a relative position variation, exciting a displacement transducer such as an inductance detector or a differential transformer. An oscillator circuit associated with any of these detectors supplies a magnetic unit and this is how a feedback circuit is complemented by varying the output current proportional to the interval of the variable in process. Its accuracy is 0.5 - 1% on a standard 4-20mA output. They are characterized by the sensor input range.
Intelligent transmitters: They are those instruments capable of performing additional functions to that of transmitting the process signal thanks to a built-in microprocessor. There are also two basic models of smart transmitters:
- The capacitive that consists of a condenser composed of an internal diaphragm that separates the plates and that when the plates are opened is because a pressure is made this diaphragm fills with oil which makes the distance between plates vary by no more than 0.1 mm. This signal is then amplified by an oscillator and a demodulator that deliver an analog signal to be converted to digital and thus be taken by the microprocessor. Its semiconductor qualities allow a weaston bridge to be incorporated into which the microprocessor linearizes the signals and delivers the 4-20mA output.
Smart transmitters allow you to read values, configure the transmitter, change its measurement range and diagnose faults, calibration and change of measurement range. Some transmitters have self-calibration, self-diagnosis of electronic elements; its precision is 0.075%. It monitors temperatures, stability, wide measurement fields, has low maintenance costs but has disadvantages such as its slowness, when faced with fast variables it can present problems and for communication performance it does not present universal devices, that is, not interchangeable with other brands.
How to calibrate a transmitter:
1) Preliminary Check and Adjustments:
- Observe the physical condition of the equipment, wear of parts, cleanliness and response of the equipment. Determine the errors of indication of the equipment compared to a suitable pattern (according to the range and precision). Take adjustments of zero, multiplication, angularity and other additional to the recommended margins for the process or that allow their adjustment in both directions (not in extremes) preliminary scaling. This will minimize the angularity error.
2) Zero adjustment:
- Place the variable in a low value from zero to 10% of the range or in the first representative division, except for the equipment that has zero or live zero suppression, for this the variable must be simulated with an appropriate mechanism, according to range and precision the same as a suitable standard. If the instrument being calibrated does not indicate the value set above, the zero mechanism must be adjusted (a pointer, a spring, a rheostat, a micrometer screw, etc.). If the equipment has additional adjustments with Variable zero, with elevations or deletions must be done after the previous zero adjustment point.
3) Multiplication adjustment:
- Set the variable to a high value of 70 to 100%. If the instrument does not indicate the set value, the multiplication or span mechanism (one arm, lever, rheostat or gain) must be adjusted.
4) Repeat the last two steps until obtaining the correct calibration for the values
high and low.
5) Angularity adjustment:
- Set the variable to 50% of the span. If the increase does not indicate the value of 50%, adjust the angularity mechanism according to the equipment.
6) Repeat the last two steps 4 and 5 until obtaining the correct calibration, in the three points.
Note: After completing the procedure, a calibration report must be made, approximately in four points: Theoretical values against real values (as exactly as possible), both ascending and descending to determine if you have hysteresis.
Pressure and temperature transmitters for:
• plastic extrusion
• plastic molding
• industry in general
Mass flow transmitters for gases, level and flow switches under the principle of thermal dispersion
Pneumatic and electronic pressure and level transmitters.
• For the paper industry
• For the industry in general
Level transmitters and switches by the
radio frequency principle.
Signal transmitters
• Alarms
• I / P, P / I converters
• Temperature transmitters
• Data communication systems
Review Questions
1.- What is needed a control system?
2.- What do the primary elements do?
3.- What is the name of the instrument that changes the magnitudes of the Transducer process?
4.- Mention at least eight primary elements to measure the pressure
5.- What is the difference between absolute pressure, differential pressure and relative pressure?
6.- What is the function of the bellows and diaphragms?
7.- Mention the three devices for measuring forces.
8.- Describe the device that uses the incremental method to measure displacements in a NC machine.
9.- What are the two types of electrodes that are used in pH measurement?
10.- Mention some devices for level measurement and tell which is the most used.
11.- Say which are the most suitable temperature measurement elements for automatic process control.
12.- What are the reasons why thermocouples are used more as temperature measurement elements?
13.- Mention the main primary elements of flow measurement.
14.- What are the primary measurement elements that do not present obstruction to the passage of the fluid?
15.- What primary measurement elements allow us to measure fluid quantities?
16.- What is the difference between analog transmission and digital transmission?
17.- What are Transmitters?
18.- How many types of transmitters are there in automation?
Author Ing. Iván Escalona
Logistics Consultant, Mobile Phone: 044 55 18 25 40 61 (Mexico)
Industrial Engineer
[email protected], [email protected]
Note: If you want to add a comment or if you have any doubts or complaints about any published work (s), you can write me to the emails indicated, indicating which work was the one you reviewed by writing the title of the work (s), also where are you from since you are dedicated (if you study or work) Being specific, also age, if you do not indicate them in the mail, I will delete the mail and I will not be able to help you, thank you.
- University Studies: Interdisciplinary Professional Unit of Engineering and Social and Administrative Sciences (UPIICSA) of the National Polytechnic Institute (IPN)
- Patoyac School Center, (Incorporated at UNAM)
Origin: Mexico
Bibliography
- Industrial Automation Techniques.
José J. Horta Santos.
Edit. Limousine
Mexico, 1982.
47-102 pp.
- Robotics: An Introduction
Mc cloy
1st. Edition.
Edit. Limousine
Mexico, 1993
22-27 pp.
Recommended reading
Introduction to Control System Technology (7th Edition),
Robert N., PE Bateson, Robert N. Bateson, Prentice Hall; 7th edition, 706 Pp.
Control Systems Engineering
Norman S. Nise
John Wiley & Sons; 3rd edition
950 pages
Conclusions
Automatic control is a concept that since its appearance has prevailed in our lives and will continue to do so due to its great importance and application to industrial processes.
In this sense, it is also important to know what type of elements or devices are available in order to control variables, such as displacement, pressure, temperature, hydrogen potential, speed, weight, flow, among others..
The devices used for the measurement of variables save work and provide accuracy in the process of some process or product.
The control of the processes, the level of liquids and solids contained in tanks and reactors, hoppers, etc., is an important variable in the industry in general. The devices for level measurement are very varied, for example Ultrasonic and nuclear are very complex devices in the industry.
For the study of the automation of a process, it is interesting to know the relationships between the input and output variables, normally in various industrial processes it is necessary to control the weight of the materials to be transformed, or adjust the magnitude of the forces acting.
We learned that for the measurement of forces, transducers are produced that convert these magnitudes into others that are easier to measure. The measurement of the variables mentioned above must be done with a direct measurement instrument, or the so-called transducers are used, which are elements that change the variable to another to facilitate the measurement, in this case there is no modification of the value of the same Since they are equivalent, the change is only made due to the ease of measuring said variable.
In industry, the concept of transmitter must be managed, which is an instrument that captures the variable in process and transmits it remotely to an indicator or controller instrument; but in realities that and much more, the primary function in the instrumentation and automation industry, for test analysis on instrument quality.
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