Liquid level sensors in the tank allow you to both perform a current measurement of the amount of liquid filled and report when its limit values have been reached. Such devices consist of a sensitive sensor that responds to certain physical parameters, and a measurement, control and indication circuit. Depending on the area of application, devices are used that differ in their operating principles.
The information presented in the article will help you learn about the operating principles of different types of sensors and their areas of application. A brief overview of their advantages and disadvantages will be carried out, and the main manufacturers that have proven themselves on the market will be indicated.
Classification of devices
Liquid level sensors in a tank can be level meters or alarms. The first of them are designed to continuously measure the liquid level at the current time. They use sensors that operate on different physical principles. Further processing of the signals coming from them is carried out by analog or digital electronic circuits that are part of the level gauges. The obtained indicators are displayed on the display elements.
Alarms warn when a certain value of the liquid level in the container, preset by the setting elements, has been reached. Another name for them is water level sensors in the tank to shut off its further supply. Their output signal is discrete. The warning may be provided in the form of a light or sound alarm. In this case, the operation of the filling or draining systems is automatically blocked.
Sensors and microcontrollers. Part 1. Materiel
In the era of ready-made debugging boards and thousands of ready-made modules for them, where it is enough to take a couple of blocks, connect them together, and get the desired result, not everyone understands the basics of circuit design, why and how it works, and most importantly, what needs to be done if it works not this way. The Circuit Design hub just opened, so, as Buford Mad Dog Tannen said,
The courthouse is already being built, which means it’s time to hang someone.
In this series I will talk about sensors - as an important element of the control system of a certain object or equipment.
process. I will conduct my entire story regarding practical issues of implementing digital control systems based on microcontrollers.
The guide does not claim to be comprehensive in its coverage of the issue. Although, after my outline went beyond 20 pages of text, I decided to break the article into the following parts:
- Part 1. Mat.
Part. In it we will consider a sensor that is not tied to any specific measured parameter. Let's consider the transfer functions and dynamic characteristics of the sensor, and understand its possible connections. - Part 2. Climate control sensors.
In it I will consider the features of working with temperature, humidity, pressure and gas composition sensors - Part 3. Sensors of electrical quantities.
In it I will touch on the measurement of current and voltage
Introduction
In a process plant control system, taking current readings of a certain quantity - temperature, humidity, pressure, liquid level, voltage, current, etc.
carried out using sensors - devices and mechanisms designed to convert an external influence signal into a form understandable to the control system. For example, a humidity sensor generates an electrical signal proportional to the current air humidity value. As a rule, sensors are not used on their own, but are part of a control system, providing a feedback signal.
Figure 1. Typical diagram of a closed-loop control system
Figure 1 shows a typical diagram of the control system.
There is a task signal Xз
, which is compared with the output signal obtained using a sensor having a transfer function
Wд(p)
.
The control error is fed to the controller, which, in turn, generates a control signal for the actuator unit, which generates the output signal Y.
[1]
A simple example is a centrifugal engine speed controller, where the sensor is a platform with balls, which, when rotating, sets one or another position of the fuel rail. The damper, controlled by this rack, regulates the amount of fuel supplied to the engine. The reference signal will be the required speed value.
1.1 Classification of sensors
The classification of sensors is very diverse. I will give only part of it based on [4]. All sensors are divided into two main classes:
- Passive, which do not require an external source of electricity, and in response to input influence generate an electrical signal. Examples of such sensors are thermocouples, photodiodes and piezoelectric sensing elements.
- Active, which require an external signal for their operation, called an excitation signal. Since such sensors change their characteristics in response to changes in external signals, they are called parametric. Examples of active sensors are thermistors, the resistance of which can be calculated by passing an electric current through them.
It should be noted that in the literature there is also an alternative classification option, when Generator sensors are defined as Active, and Parametric sensors as Passive. Here and below I am guided by the option according to Fariden's reference book.
Another important criterion for us is the choice of data reference point. Thus, sensors are
- Absolute, the measured value of a physical quantity of which does not depend on the measurement conditions and the external environment.
- Relative, when the output signal of such a sensor is interpreted differently in each specific case.
A striking example is again the thermistor, the resistance of which directly depends only on the temperature of the object being measured, and the thermocouple, the output voltage of which depends on the temperature difference between the hot and cold ends.
When designing electronic equipment, an important factor in sensor performance is also the nature of the output signal.
- Analog sensors at the output have a continuous output signal, to remove which it is necessary to use an analog-to-digital converter, after which it is necessary to convert the ADC value into the format of the measured value.
- Digital sensors, information from which is collected using various digital interfaces. As a rule, information is available directly in the format of the measured quantity and does not require additional conversions.
- Discrete sensors that have only two signal options at the output of the sensor channel - log 0. and log. 1. An example of such a sensor is a limit switch that has the closed and open states. A discrete sensor can have several output channels, each of which is in one of two states. For example, a 12-bit absolute encoder.
- Pulse sensors that generate output signal pulses, the amplitude or duration of which depends on the measured value. For example, an incremental position encoder generates a Gray code at its output. In this case, the higher the rotation speed of the sensor shaft, the higher the frequency of the signal will be at the output, which will make it possible to determine the shaft rotation speed with high accuracy.
2 Sensor characteristics
Most sensors have a complex procedure for converting the measured value into an electrical signal. For example, in a strain gauge pressure sensor, the measured value acts on the sensing element, changing its resistance. After applying the excitation signal, the voltage drop across the resistor will allow you to indirectly determine its resistance and, based on the dependence of the resistance on pressure, calculate the measured value.
To the designer, the sensor is a black box with known signal relationships between inputs and outputs.
2.1 Range of measured and output values
The range of measured values
shows what maximum value of the input signal the sensor can convert into an output electrical signal without going beyond the established errors. These figures are always given in the specifications for the sensor, at the same time displaying the possible measurement accuracy in a particular range.
It should be understood that some sensors, when applying an input signal greater than the maximum values, will simply go into saturation and return incorrect data. Other sensors (for example temperature sensors) may fail. In the future, recommendations will be given for each type of sensor.
Sensor output range
- this is the minimum and maximum voltage that the sensor is capable of producing under minimum and maximum external influences. Since we are considering sensors that convert an input signal into an electrical signal, the range of output values of the sensor will be determined by the voltage it generates, or the current passed through it. One of our tasks when connecting a sensor will be to match the output range of the sensor with the input range of the measuring path.
2.2 Transfer function - static and dynamic characteristics
When working with a sensor, you need to know the ratio of signal levels at the input and output.
The ratio Wд(p) = Y(p)/X(p) in operator form is the transfer function of the sensor and uniquely determines the characteristics of the sensor in statics and dynamics. Equation Y(p) = Wд(p)*X(p) in the real plane, i.e. function Y = f(x) will be a static characteristic. The static characteristic can be linear and will be defined as: (1) Where a is the slope of the straight line, determined by the sensitivity of the sensor and b is the constant component (i.e. the level of the output signal in the absence of a signal on entrance)
Figure 2. Linear relationship
In addition to sensors with a linear dependence, there may be sensors with a logarithmic dependence, with an equation of the form (2) Exponential: (3) Or power law: (4) Where k
– constant number. There are sensors with more complex characteristics. But there is documentation for that.
However, the transfer function also reveals what properties the sensor has in dynamics, i.e., how quickly and accurately the sensor processes the output signal when the input value changes quickly. Almost every real sensor has an energy storage device - a capacitor, mass, etc. Let's consider the behavior of a sensor whose dynamic characteristics are described by a first-order equation:
(5) In automatic control theory, there are two test input signals. This is a unit function - supplying a unit at the zero moment of time, and a delta function - supplying a signal of infinite amplitude and infinitesimal duration.
Figure 3. Unit and delta functions
An inertia-free, that is, an ideal sensor will exactly repeat the shape of the input signal. A real sensor described by formula (5) will give the following response:
Figure 4. First order aperiodic link response to test signals
It should be noted that the value at the output of the sensor will correspond to the value supplied at the input only after the completion of the transient process, which will last 3-4 τ
, where
τ
is the time constant of our link.
At t = 1τ
, the output value will reach It is easy to calculate that at
t = 2τ
the output value will be 86%, and at
t = 3τ
- 95% and the transition process will be considered complete.
Thus, you need to understand that, for example, the same temperature sensor will respond to changes in ambient temperature with some delay due to the fact that there is a housing between the sensor and the environment, which must absorb heat and heat up. This takes time.
Of course, inertial sensors can be described by more complex equations, for example, represented by aperiodic links of the second order, have a response delay, etc. The behavior of such links is described in detail in [1].
2.3 Accuracy, nonlinearity
One of the important characteristics of the sensor is its accuracy
in the range of measured values. The output signal of the sensor corresponds to the value of the measured quantity with some reliability, called error.
For example, a temperature sensor has an accuracy of ±2 degrees. This means that if the actual temperature of the measured object is 100 degrees, the permissible readings of this temperature sensor are within the range of 98 - 102 degrees. The sensor error varies.
There are additive and multiplicative errors.
The additive error is constant over the entire measurement range.
Figure 5. Additive error
The multiplicative linearly depends on the level of the measured quantity:
Figure 6. Multiplicative error
In addition, there is a nonlinearity of the sensor in the measured range. Depending on the current measurement range, the slope of the transfer function varies within certain limits. At the same time, the specification indicates either accuracy change curves over the range, or the worst nonlinearity indicators in a particular range.
Figure 7. Sensor non-linearity
In addition, some sensors have a hysteresis effect, when for the same input signal, after increasing and decreasing, the output signal values are different. A typical cause of hysteresis is friction and structural changes in materials. Sensors based on ferromagnetic materials are subject to the greatest hysteresis effect.
To improve accuracy and compensate for additive and multiplicative errors, a sensor calibration process can be performed. For example, for a linear sensor it is necessary to determine with known accuracy the readings at two points located at different ends of the operating range. For some sensors, calibration data may be provided in the passport for each specific instance. To carry out the calibration procedure, you can use more accurate equipment, you can use a standard (for example, a black body, a standard kilogram, etc.). The accuracy after calibration naturally cannot exceed the accuracy of the standard.
2.4 Sensor sensitivity, resolution and dead zone
The dead zone
of a sensor is the insensitivity of the sensor over a certain range of input signals. Within this zone, the output readings are incorrect.
For the example in Figure 2, the output value readings for all values from 0 to x0 are not defined. This feature is common, for example, in some current sensors that have zero output voltage at currents lower than, for example, 10 mA.
Throughout the rest of the range there is a certain sensitivity
sensor, i.e. how strong is the increase in the output signal due to a change in the input signal. i.e. the sensitivity is determined by the following formula: For a linear sensor, the sensitivity will be constant over the entire measured range.
Resolution
shows how small a change in the measured quantity can cause a change in the output signal. For example, some incremental position sensor has a resolution of 1 degree. Analog sensors have an infinitely high resolution, since it is impossible to determine individual stages of its change in their output signal.
3 Method of connecting sensors
Depending on the type of sensor, it is connected to the measuring path in different ways.
Connecting a passive sensor
Since a passive sensor, without outside help, in response to external influences independently produces an electrical signal for us, we need to read this signal.
Depending on whether our sensor is a current source or a voltage source, the connection method will be different. For example, a thermocouple is a voltage source - the output voltage does not depend on the output current (within reasonable limits, of course). Our task is to measure the generated EMF. Since the measuring path will have some finite resistance, the connection diagram will be as follows: Figure 8. Connecting a voltage source to the ADC
If Radc
will be much greater than the internal resistance
r
, then the voltage drop across it will tend to zero and the voltage at the ADC input will tend to the EMF value. In the second part, I will take a closer look at the thermocouple, as one of the most accurate and fastest sensors. Another case is if our sensor is a current source, that is, the voltage it generates depends on the current passed through the load.
Connecting the sensor is similar: Figure 9. Connecting the current source to the ADC
However, the load resistance of the current source should now tend to zero.
To do this, the sensor is shunted with a resistor of the required resistance, thereby turning the current source into a voltage source: Figure 10. Correct connection of the current source to the ADC
Resistor resistance Rsh
is calculated as the quotient of dividing the maximum voltage supplied to the input of the ADC by the maximum current that the sensor is capable of producing. The most striking example of such a sensor is a current sensor.
ATTENTION: sensors that have an equivalent circuit in the form of a current source must be shunted with a resistance and the shunt circuit must not be allowed to break in the presence of any small input influence. Otherwise, the same current sensor generates a voltage of kilovolts at the free terminals of the secondary winding until the measurement circuit or the sensor itself breaks down. Modern current sensors are tested at voltages of 1 kV or more, so getting 2-3 kV at the output and also hitting them with your finger is not the most difficult task.
Connecting an active sensor
Let's consider active sensors that represent a variable resistance.
In particular, these are thermistors, strain gauges and other similar sensors. To measure the resistance of the sensor, it must be connected to a current source and the voltage drop across it must be determined: Figure 11. Connecting the sensor to an unregulated current source
The current source produces a constant current of a known magnitude. Then, the output voltage will be determined by the formula: (7)
For example, let's calculate the output voltage value at a source current of 10 mA if our sensor changes resistance from 0.1 kOhm to 1 kOhm. Then the maximum output voltage will be equal to (8) Which is quite consistent with the required voltage value for an analog control system based on operational amplifiers.
Where can I get a power source? It happens that it is built into the microcontroller itself. For example, the ADuCM360/361 microcontrollers have two built-in current sources of 0.01-1mA. True, they have a diagnostic task - by supplying a small current through the sensor circuits, they can be convinced of its presence and serviceability.
Of course, we are more accustomed to using a voltage source with a divider: Figure 12. Connecting the sensor to a voltage source with a divider
To be honest, the U-R1 chain forms the same current source, only its parameters depend on the load - Rd
. The output voltage will be determined by the following formula:
(9)
And here the main problem of this method emerges - there is no way to get rid of the resistance of our sensor in the denominator and the readings become nonlinear, unlike, by the way, the first option.
The question arises - what should the resistance R1
?
It should provide the maximum output voltage range. i.e., with known values of the minimum and maximum sensor resistance Rд1
and
Rд2
,
abs(Uout1 - Uout2) ->max
On the other hand, the maximum output voltage is limited by the input circuits of the measuring device. For example, a voltage of, for example, no more than 2.5V must be applied to the input of a microcontroller with a 5V power supply. I note that if the maximum possible voltage supplied to the ADC input is less than the supply voltage, then we will be able to supply it there.
If our sensor changes resistance from 0.1 kOhm to 1 kOhm, then we take the resistance of resistor R1 equal to the upper limit of the sensor resistance. Then Uout can vary from 1/11Uin to 1/2Uin. In absolute numbers for this example - from 0.45 to 2.5V. And with these values we use (2.5-0.45)/2.5 = 82% of the entire ADC range, which is pretty good.
Another sensor can be plugged into the measuring bridge and measure the voltage difference in its arms:
Figure 13. Sensor as part of a measuring bridge
In this case, we work with a differential ADC, measuring the potential difference
Uab
. It will be equal to: (10)
Moreover, the resistance of resistor R1
may be such that
Uab
may be negative. There are sensors whose internal circuit is already a balanced bridge with the necessary characteristics. I will look at examples of such sensors later.
There are sensors that are more convenient to use. They produce the necessary analog signal without dancing with resistors. For example, an analog humidity sensor HIH-4010-004 - three-pin case, 5V power supply, linear output. This miracle is connected like this:
Figure 14. Connecting the humidity sensor HIH-4010-004
Two wires to the reference voltage source, the output to the ADC of the microcontroller.
Connecting digital sensors using the 1-Wire standard
1-Wire is a bidirectional low-speed digital data bus that requires only two wires - the data wire and ground. The bus is quite easy to use, supports parasitic power supply of devices from the line and allows you to connect in parallel many similar devices such as temperature sensors (the beloved DS18B20), or identification chips (iButton). Parasitic nutrition is organized as follows:
Figure 15. Parasitic power supply to 1-Wire bus devices
And this is the usual active power supply of the device, when the source is just a stone's throw away.
Figure 16. Powering a 1-Wire device from an external source
The number of sensors connected in parallel is actually limited only by the line parameters. Hot plugging and on-the-go identification possible. Moreover, the computational complexity of the identification algorithm is O(log n)
We will work with this protocol in more detail in the second part. In the meantime, you can read about the protocol itself using the classic link: https://datasheets.maximintegrated.com/en/ds/DS18B20.pdf
Connecting digital sensors using the I2C(Twi)/SMBus standard
If 1-Wire required one data wire, then this bus, based on the name Two-Wire Bus, requires two. One of the wires - SCL will be clocked, the second - SDA, data will be transmitted in half duplex. The bus is an open collector, therefore both lines must be connected to power. The sensor will be connected as follows:
Figure 17. Connecting sensors via I2C
The total number of devices that can be connected to the I2C bus is 112 devices with 7-bit addressing. Each device is actually allocated two consecutive addresses; the low-order bit sets the mode - read or write. There is a strict requirement for bus capacitance - no more than 400pF.
Commonly used speeds are 100 kbit/sec and 10 kbit/sec, although the latest standards also allow speed modes of 400 kbit/sec and 3.4 Mbit/sec.
The bus can work both with an irreplaceable master and with flag transmission. A huge amount of information on the protocol can be found at this link: https://www.esacademy.com/en/library/technical-articles-and-documents/miscellaneous/i2c-bus.html
Connecting digital sensors using the SPI standard
Requires at least three wires, operates in full duplex mode - i.e. organizes simultaneous data transfer in both directions. Communication lines:
- CLK - clock signal line.
- MOSI - master output, slave input
- MISO - master input, slave output
- CS—chip selection (optional).
One of the devices is selected by the master. It will be responsible for bus clocking. The connection is made crosswise:
Figure 18. SPI connection and the essence of the transfer
Each device in the circuit contains its own data shift register. Using clock signals, after 8 clock cycles the contents of the registers are swapped, thereby exchanging data.
SPI - The fastest data transfer interface available. Depending on the maximum possible clock frequencies, the data transfer rate can be 20, 40, 75 Mbit/s and higher.
The SPI bus allows devices to be connected in parallel, but a problem arises here - each device requires its own CS line to the processor. This limits the total number of devices on a single interface. The main difficulty in setting up SPI is setting the polarity of the clock signal. Seriously. Setting up SPI is not easy, but very simple.
Briefly and clearly about SPI with a description of peripheral SPI modules for AVR and MSP430 can be read here https://www.gaw.ru/html.cgi/txt/interface/spi/index.htm
4 Taking readings from sensors
It's time to read at least some information from our sensors.
Depending on the method of connecting the sensor and its type, various ways of taking readings are possible. It should be noted that some sensors, such as digital sensors or gas sensors, require a preliminary start of the measurement mode, which may take some time.
Thus, the measurement process consists of two cycles - a data measurement cycle and a data acquisition cycle. When organizing the program, you can choose one of the following options:
Figure 19. Process of reading readings from the sensor
Let's consider each option separately and sketch out the skeletons: Option 1.
started the measurement mode, waited, and counted. The option is attractive in its simplicity, but behind it lies a problem - while waiting for measurements to be completed, the microcontroller is blatantly idle, not performing tasks. In most automation systems, such a mode is an unaffordable luxury.
In code it would look like this:
Sensor.Start();//start the measurement process delay(MINIMAL_SENSOR_DELAY_TIME);//wait for the process to complete int var = Sensor.Read();//read the data
Option 2
. started the measurement mode, returned to other tasks, after a while the interrupt triggered, and counted the data. One of the best options. But the most complex one: void Setup(){ TimerIsr.Setup(MINIMAL_SENSOR_DELAY_TIME);//set up a timer interrupt with the required frequency int mode = START;//state variable Sensor.Start();//start the measurement process for the first time } TimerIsr .Vector(){//timer interrupt handler if (mode == START{ mode = READ; var = Sensor.Read();//if the sensor was in measurement mode, read the data } else { mode = START; Sensor. Start();///if the sensor was in data reading mode, start a new measurement cycle } } Looks good. It allows you to vary the time between measurement cycles and reading cycles. For example, the gas composition sensor must have time to cool down after previous measurements, or have time to heat up during measurement time These are different time periods.
Option 3:
We counted the data and launched a new round.
If the sensor allows you to start a new measurement cycle after reading the data, then why not - let’s do it the other way around. void Setup(){ TimerIsr.Setup(MINIMAL_SENSOR_DELAY_TIME);//set up a timer interrupt with the required frequency Sensor.Start();//start the measurement process for the first time } TimerIsr.Vector(){//timer interrupt handler var = Sensor.Read();//read the data Sensor.Start();///start a new measurement cycle
A great way to save time. and you know what - this method works great without interruptions. Digital sensors store the calculated value until the power is turned off. And taking into account the fact that it is often not necessary to read signals from a humidity sensor due to its inertia of 15 seconds, you can do this:
void Setup(){ Sensor.Start();//start the measurement process for the first time while(1){ //a lot of other routine var = Sensor.Read();//read the data Sensor.Start();// /start a new measurement cycle } } It may also be that our sensor independently starts a new measurement cycle and then, using an external interrupt, it reports the completion of measurements. For example, an ADC can be configured to automatically read data at a frequency of N Hz. On the one hand, in the interrupt handler it will be enough to implement only the process of reading new data. On the other hand, you can use the ADC interrupt with Direct Memory Access (DMA) mode. In this case, upon an interrupt signal, the peripheral ADC module at the hardware level will independently copy the data to a specific memory cell in RAM, thereby ensuring maximum data processing speed and minimal impact on the working program (no need to go into an interrupt, call a handler, etc.).
But the use of DMA is far beyond the scope of this cycle.
Unfortunately, the first method is widely used in libraries and examples for Arduino; it does not allow this platform to correctly use the resources of the microcontroller. But it is easier to write and debug.
4.1 Working with ADC
When dealing with analog sensors, we are dealing with an ADC.
In this case, we consider an ADC built into the microcontroller. Since the ADC is essentially the same sensor - it converts an electrical signal into an information one - everything that is described above in section 2 is true for it. The main characteristics of an ADC for us are its effective bit capacity, sensitivity, reference voltage and speed. In this case, the output value of the ADC conversion will be a certain number in the output register, which must be converted into an absolute value in units of the measured value. In the future, examples of such calculations will be considered for individual sensors. 4.1.1 Reference voltage
The ADC reference voltage is the voltage that will correspond to the maximum output value of the ADC. The reference voltage is supplied from a voltage source, either built into the microcontroller or external. The accuracy of the ADC readings depends on the accuracy of this source. A typical on-chip reference voltage is equal to the supply voltage or half the microcontroller supply voltage. There may be other meanings.
For example, a table of possible reference voltage values for the Atmega1280 microcontroller:
Figure 20. Selecting the reference voltage for the ADC of the Atmega1280 microcontroller
4.1.2 ADC capacity and sensitivity
The ADC width determines the maximum and minimum values in the output register at the minimum and maximum input effect of the electrical signal.
It should be noted that the maximum bit capacity of the ADC may not correspond to its effective bit capacity. Some of the low-order bits can be lost to noise. Let's turn to the datasheet for the ADuCM360 microcontroller, which has a 24-bit ADC with an effective bit width of 14 bits:
Figure 21. ADC data register bit assignments
As can be seen from the figure, in a 32-bit register, part is allocated to the sign, part to zeros and part to noise. And only 14 bits contain data that has the specified accuracy. In any case, this data is always indicated in the documentation.
Its sensitivity depends on the effective bit capacity of the ADC. The more intermediate stages of the output voltage, the higher the sensitivity will be.
Let's say the ADC reference voltage Uop
. Then, an N-bit ADC, having 2N possible values, has sensitivity (11)
Thus, for a 12-bit ADC and a reference voltage of 3.3V, its sensitivity will be 3.3/4096 = 0.8mV
Since our sensor also has a certain sensitivity and accuracy, it will be nice if the ADC has better performance
4.1.3 ADC performance
The speed of the ADC determines how quickly the readings are read. A SAR ADC requires a certain number of clock cycles to digitize the input voltage level. The larger the bit depth, the more time is required; accordingly, if by the end of the measurement the signal level has time to change, this will affect the accuracy of the measurement.
ADC performance is measured in the number of data samples per second. It is defined as the frequency of the ADC clock signal divided by the number of clocks required for the measurement. For example, having an ADC clock frequency of 1 MHz and 13 clock cycles for taking readings, the ADC speed will be 77 kilosamples per second. For each bit depth option, it is possible to calculate its performance. The technical documentation usually indicates the maximum possible clock frequency of the ADC and its maximum performance at a particular bit depth.
4.2 Digital sensors
The main advantage of digital sensors over analog ones is that they provide information about the measured value in a ready-made form.
A digital humidity sensor will return the absolute humidity value in percent, a digital temperature sensor will return the temperature value in degrees. The sensor is controlled using the register in it in the question-answer form. The questions are:
- Write the value of B to register A
- Return the value stored in register C
In response, the sensor, accordingly, either writes the necessary data into the register, setting parameters or starting some mode, or transfers the measured data to the controller in finished form.
With this I will finish the general material. In the next part we will look at HVAC sensors with examples. After the sensors, we will consider the actuators - there is quite a lot of interesting stuff there from the point of view of the significance of the theory of automatic control, and then we will get to the synthesis and optimization of the controller of all this disgrace.
UPD: I express my gratitude to amartology, Arastas and Stross for their fair comments on the article. I added material on sections 2 and 4 and clarified some controversial issues.
List of useful literature:
- Bessekersky V.A., Popov E.P. Theory of automatic control systems / V.A. Bessekersky, E.P. Popov - Ed. 4th, revised And additional - St. Petersburg, Profession, 2007. - 752 p.
- Sensors: Reference manual / V.M. Sharapov, E.S. Polishchuk, N.D. Koshevoy, G.G. Ishanin, I.G. Minaev, A.S. Sovlukov. - Moscow: Tekhnosphere, 2012. - 624 p.
- G. Wigleb. Sensors Device and application. Moscow. Publishing house "Mir", 1989
- Modern sensors. Directory. J. FRIEDEN Translation from English by Yu. A. Zabolotnaya, edited by E. L. Svintsov TECHNOSPHERE Moscow Technosphere-2005
Level measurement methods
Depending on the properties of the liquid whose level in the tank needs to be determined, the following measurement methods are used:
- contact, in which there is direct interaction of the liquid level sensor in the tank or its part with the measured medium;
- non-contact, allowing to avoid direct interaction of the sensor with the liquid (due to its aggressive properties or high viscosity).
Contact devices are located in a container directly on the surface of the liquid being measured (floats), in its depth (hydrostatic pressure gauges), or on the tank wall at a certain height (plate capacitors). For non-contact meters (radar, ultrasonic), it is necessary to provide a zone of direct visibility of the surface of the liquid being measured and the absence of direct contact with it.
Directional wave radar
Directional wave radar uses what is called TDR or Time Domain Reflectometry technology. In this technology, pulses propagate forward from the sensor at high speeds to the surface of the medium and when they reach the medium, part of the energy is reflected back to the sensor. The time between signal transmission and reception is used to determine the liquid level. The advantages of using a radar are the vertical mounting at the top of the tank and the fact that the radar can be used with liquids with floating solids or coating liquids. Another feature of this technology is its insensitivity to emulsification, dust, foam or vapors. This technology has a programmable output range that is insensitive to changes in fluid dielectric constant and specific gravity and has no actual minimum sensor length requirements. Some disadvantage of this technology is that the length of the sensor must be the full length of the sensing range and because of this there is usually a higher cost of the sensor. Care must be taken to select the correct sensor type in applications with bypass chambers or distillation settlers.
There are a huge number of ways to measure liquid levels using sensors, and each level measurement technology is original. To avoid confusion and sometimes repeated installation costs, contact our technicians by phone for more information on which level measurement technology is most suitable for your system.
Operating principles
Both level gauges and alarms use different operating principles to perform their functions. The most common types of devices are:
- float sensors for liquid level in the tank;
- capacitive;
- hydrostatic liquid level sensors;
- radar type devices;
- ultrasonic sensors.
Float valves, in turn, can be mechanical, discrete and magnetostrictive. The first three groups of sensors include devices using the contact measurement method, the other two belong to non-contact devices.
Displacer level gauges
Level gauges are adjusted to specified measurement limits using weights by simulating a hydrostatic buoyant force corresponding to the upper measurement limit.
The calculated pressure value corresponding to the upper measurement limit,
Calculation of mass of loads for displacer level gauges:
for liquid
for phase separation
where d is the diameter of the displacer of the level gauge being tested, cm; Hmax – upper limit of liquid level measurement, cm; ρl – density of the measured liquid, g/cm3; ρ l.l, ρ v.l - densities, respectively, of the lower and upper measured liquid in the case of measuring the phase interface level, g/cm3.
Mechanical float sensors
A light float, constantly located on the surface of the liquid in the tank, is connected by a system of mechanical levers to the middle terminal of the potentiometer, which is the arm of the resistance bridge. With a minimum amount of liquid in the container, the bridge is considered balanced. There is no voltage in its measuring diagonal.
As the reservoir fills, the float monitors the position of the liquid level by moving the movable contact of the potentiometer through a system of levers. Changing the resistance of the potentiometer leads to an imbalance in the bridge. The voltage that appears in its measuring diagonal is used by the electronic circuit of the display system. Its analog or digital readings correspond to the amount of liquid in the tank at the current time.
Hydrostatic level gauge
Hydrostatic level gauges will need to be installed near the bottom of the tank.
The pressure that the device will display at a constant liquid density will be proportional to the liquid level.
To measure the level of aggressive liquid, contact of the sensing element is prohibited. The liquid can be separated using gas, which will be supplied to the connecting line. Thanks to this, the sensitive element of the pressure gauge will no longer come into contact with the aggressive liquid. If you study the hydrostatic level transducer, then we can say that it will consist of the following elements: A special tube through which compressed air will pass. A valve. A glass. If the density of the liquid is constant, then the pressure gauge readings will be proportional to the level of the liquid.
Discrete float sensors
A discrete signal in the form of closing or opening the reed relay contacts is used by the electronic indication and signaling circuit to notify that the liquid level in the container has reached a certain value. Metal contacts, made of a material with low contact resistance when they are closed, are placed in a hollow, insulated glass bulb.
A water level sensor in a tank with a discrete output includes a guide in the form of a hollow tube into which liquid from the tank does not enter. The contacts of one or more reed relays are fixed inside the guide. Their location depends on the case in which it is necessary to receive an alarm when the liquid level reaches a given value.
The sensor float with a small permanent magnet built into it moves along the guide when the liquid level in the container changes. The contact group is triggered when it enters the magnetic field of the permanent magnet of the float. The signal through the wires connected to the contacts of the water level sensor in the reed switch tank is sent to the alarm circuit.
Types of sensors
All level gauges are classified according to the principle of their operation. Main types of measuring devices:
- Float _ This is the simplest option for measuring the water level in the tank. The design of the float level gauge includes 2 reed switches, a magnet and a float. As the fluid level increases, the float rises to the first reed switch, which turns off the motor relay. If the reservoir is empty, the float drops to the second reed switch, which triggers the relay and turns on the pump that pumps liquid from the well. You can make a reed limit liquid level sensor with your own hands. Moreover, it will work even if there is a voluminous layer of foam in the tank.
- Ultrasonic . This type of measuring device is used for both dry and liquid media. Ultrasonic sensors can have a discrete or analog output. That is, the device can constantly monitor the water level or limit the filling of the container when it reaches a specific point. Such a level gauge consists of a receiver, an ultrasonic emitter and a controller responsible for signal processing. Ultrasonic type alarms are wireless and non-contact, so they can be installed even in explosive and corrosive liquids.
- Electrode (conductometric). Such level meters are not suitable for containers with distilled water. The standard design is equipped with a three-level alarm, in which the filling of the tank is controlled by a pair of electrodes, and the third is intended for emergency situations, to start the active pumping mode.
- Capacitive . Using such level gauges, the maximum filling level of a tank can be accurately identified. They are suitable for both liquids and bulk substances. Capacitive level gauges operate on the same principle as capacitors: the measurement is performed between the plates of the sensing element. When the peak value is reached, a corresponding signal is sent to the controller. Sometimes capacitive alarms operate on the “dry contact” principle, in which the device is triggered through the wall of the tank. These devices can operate effectively over a very wide temperature range and their operation is not affected by electromagnetic radiation. Such performance properties expand the scope of use of capacitive level meters.
- Radar . This type of alarm is universal, as it works with any type of process media, including explosive and corrosive liquids. In this case, the readings will not change under the influence of temperature and pressure. The device emits radio waves in a certain frequency range. The receiver picks up the reflected radio signal and determines how full the tank is, based on the signal delay period. The measuring sensor is not affected by temperature and pressure. The dustiness of the process environment also does not affect the readings. Experts note that radar devices have maximum accuracy, since their error does not exceed 1 mm.
- Hydrostatic . This type of alarm allows you to measure both the current and maximum filling of containers. The operating principle of a hydrostatic device is based on measuring the pressure of a liquid column. The popularity of such sensors is due to their low price and sufficient accuracy.
There are other types of devices, but they have specific purposes.
Magnetostrictive float sensors
Sensors of this type produce a constant signal depending on the level of liquid in the tank. The main element, as in the previous case, is a float with a permanent magnet inside, which takes its position on the surface of the liquid and moves in a vertical plane along the guide.
The inner cavity of the guide, isolated from the liquid, is occupied by a waveguide. It is made of magnetostrictive material. At the bottom of the element there is a source of current pulses that propagate along it.
When the emitted pulse reaches the location of the float with the magnet, the interaction of two magnetic fields occurs. The result of this interaction is the occurrence of mechanical vibrations that propagate back along the waveguide.
A piezoelectric element is attached next to the pulse generator, which records mechanical vibrations. An external electronic circuit analyzes the time delay between the emitted and received pulses and calculates the distance to the float, which is constantly located on the surface of the liquid. The display circuit constantly reports the liquid level in the tank.
Capacitive sensors
The operation of sensors of this type is based on the properties of a capacitor to change its electrical capacitance when the dielectric constant of the material filling the space between its plates changes. Coaxial type capacitors are used, which are a pair of coaxial hollow metal cylinders of different diameters.
The latter are the plates of the capacitor, between which liquid can freely penetrate. The dielectric constants of air and liquid media have different values. Filling the reservoir leads to a change in the value of the total dielectric constant of the coaxial capacitor and, accordingly, its electrical capacitance.
The frequency of the oscillatory circuit, in the circuit of which the capacitor is connected, changes in proportion to the change in its capacitance. An electronic frequency/voltage converter monitors this change and displays a value proportional to the degree of filling of the tank.
Hydrostatic sensors
Another name for such a device is a detector, or pressure transducer. They can be stationary, fixed at the bottom of the container filled with liquid, or portable. In the latter case, the pressure transducers are equipped with a cable of considerable length. This allows them to be used for tanks of different geometric sizes.
The sensing element of a hydrostatic sensor is a membrane that senses the pressure of a liquid column above it. It is configured in such a way that atmospheric pressure does not lead to deformation of the membrane. Based on the pressure at the measurement point, you can determine the height of the liquid column or the degree of filling of the tank.
The amount of membrane deformation is converted into a proportional electrical indicator, which is then used to display the level of liquid in the reservoir. Corrections are applied that take into account the density of the measured medium and the acceleration of gravity at the measurement point.
Hydrostatic level gauge
Hydrostatic level gauges will need to be installed near the bottom of the tank.
The pressure that the device will display at a constant liquid density will be proportional to the liquid level. To measure the level of aggressive liquid, contact of the sensing element is prohibited. The liquid can be separated using gas, which will be supplied to the connecting line.
Thanks to this, the sensitive element of the pressure gauge will no longer come into contact with the aggressive liquid. If we study the hydrostatic level converter, then we can say that it will consist of the following elements:
- A special tube through which compressed air will pass. A valve. A glass.
If the density of the liquid is constant, then the pressure gauge readings will be proportional to the liquid level.
Radar type sensors
The container liquid level sensor uses a non-contact measurement method based on the properties of this medium of any density and viscosity to reflect an electrical signal. The frequency of the emitted signal from a radar located above the surface of the measured liquid level changes according to a linear law.
Reflected from the surface, it arrives at the receiving device with a delay determined by the length of the path traveled. Thus, there is a difference between the frequencies of the two signals. Based on the frequency shift, the radar analyzing device determines the path traveled by the signal or the level of the reflecting liquid relative to the location of the radar.
Ultrasonic level sensors
The measurement scheme used for sensors of this type corresponds to that discussed in the previous section of the article. The location measurement method is used in the ultrasonic wavelength range.
The received data determines the time difference between the signals emitted by the transmitter and the signals received by the receiver. Using data on the speed of propagation of ultrasound in the space above the surface of the liquid, the analyzing device determines the distance traveled by the signal or the level of liquid in the reservoir.
