Since the 1950s, the current loop has been used to transmit data from transducers in monitoring and control processes. With low implementation costs, high noise immunity and the ability to transmit signals over long distances, the current loop has proven to be particularly suitable for industrial environments. This material is devoted to the description of the basic principles of the current loop, the basics of design, configuration.
Using current to transmit data from the converter
Industrial grade sensors often use a current signal to transmit data, unlike most other transducers such as thermocouples or strain gauges that use a voltage signal. Although converters that use voltage as a communication parameter are indeed effective in many industrial applications, there are a number of applications where the use of current characteristics is preferable. A significant disadvantage when using voltage for signal transmission in industrial conditions is the weakening of the signal when it is transmitted over long distances due to the presence of resistance in wired communication lines. You can, of course, use high input impedance devices to get around signal loss. However, such devices will be very sensitive to noise generated by nearby motors, drive belts, or broadcast transmitters.
According to Kirchhoff's first law, the sum of the currents flowing into a node is equal to the sum of the currents flowing out of the node.
In theory, the current flowing at the beginning of the circuit should reach its end in full,
as shown in Fig.1. one.
Fig.1. According to Kirchhoff's first law, the current at the beginning of the circuit is equal to the current at its end.
This is the basic principle on which the measurement loop operates. Measuring current anywhere in the current loop (measuring loop) gives the same result. By using current signals and data acquisition receivers with low input impedance, industrial applications can benefit greatly from improved noise immunity and increased link length.
Current loop components
The main components of the current loop include a DC source, a sensor, a data acquisition device, and wires connecting them in a row, as shown in Figure 2.
Fig.2. Functional diagram of the current loop.
A DC source provides power to the system. The transmitter regulates the current in the wires from 4 to 20 mA, where 4 mA is a live zero and 20 mA is the maximum signal.
0 mA (no current) means open circuit. The data acquisition device measures the regulated current. An efficient and accurate method of measuring current is to install a precision shunt resistor at the input of the measuring amplifier of the data acquisition device (in Fig. 2) to convert the current into a measuring voltage, in order to finally obtain a result that unambiguously reflects the signal at the output of the converter.
To help you better understand how the current loop works, consider as an example a system design with a converter that has the following specifications:
The transducer is used to measure pressure
The transmitter is located 2000 feet from the measuring device
The current measured by the data acquisition device provides the operator with information about the amount of pressure applied to the transducer
Considering the example, we begin with the selection of a suitable converter.
Current System Design
Converter selection
The first step in designing a current system is choosing a transducer. Regardless of the type of measured quantity (flow, pressure, temperature, etc.), an important factor in choosing a transmitter is its operating voltage. Only connecting the power supply to the converter allows you to adjust the amount of current in the communication line. The voltage value of the power supply must be within acceptable limits: more than the minimum required, less than the maximum value, which can damage the inverter.
For the example current system, the selected transducer measures pressure and has an operating voltage of 12 to 30 V. When the transducer is selected, the current signal must be correctly measured to provide an accurate representation of the pressure applied to the transmitter.
Selecting a Data Acquisition Device for Current Measurement
An important aspect to pay attention to when building a current system is to prevent the appearance of a current loop in the ground circuit. A common technique in such cases is isolation. By using insulation, you can avoid the influence of the ground loop, the occurrence of which is explained in Fig. 3.
Fig.3. Ground loop
Ground loops are formed when two terminals are connected in a circuit at different potential locations. This difference leads to the appearance of additional current in the communication line, which can lead to measurement errors.
Data Acquisition Isolation refers to the electrical separation of the signal source ground from the instrument input amplifier ground, as shown in Figure 4.
Since no current can flow through the isolation barrier, the ground points of the amplifier and signal source are at the same potential. This eliminates the possibility of inadvertently creating a ground loop.
Fig.4. Common-mode voltage and signal voltage in an isolated circuit
The isolation also prevents damage to the DAQ device in the presence of high common-mode voltages. Common mode is a voltage of the same polarity that is present at both inputs of an instrumentation amplifier. For example, in Fig.4. both the positive (+) and negative (-) inputs of the amplifier have +14 V common mode voltage. Many data acquisition devices have a maximum input range of ±10 V. If the data acquisition device is not isolated and the common mode voltage is outside the maximum input range, you could damage the device. Although the normal (signal) voltage at the input of the amplifier in Fig. 4 is only +2 V, adding +14 V can result in a voltage of +16 V
(The signal voltage is the voltage between the “+” and “-” of the amplifier, the operating voltage is the sum of normal and common mode voltage), which is a dangerous voltage level for devices with lower operating voltage.
With isolation, the common point of the amplifier is electrically separated from ground zero. In the circuit in Figure 4, the potential at the common point of the amplifier is "raised" to +14 V. This technique causes the input voltage value to drop from 16 to 2 V. Now that data is being collected, the device is no longer at risk of overvoltage damage. (Note that insulators have a maximum common mode voltage they can reject.)
Once the data collector is isolated and secured, the last step in configuring the current loop is to select an appropriate power source.
Power Supply Selection
Determining which power supply best suits your needs is easy. When operating in a current loop, the power supply must provide a voltage equal to or greater than the sum of the voltage drops across all elements of the system.
The data acquisition device in our example uses a precision shunt to measure current.
It is necessary to calculate the voltage drop across this resistor. A typical shunt resistor has a resistance of 249 Ω. Basic calculations for current loop current range 4 .. 20 mA
show the following:
I*R=U
0.004A*249Ω=0.996V
0.02A*249Ω=4.98V
With a 249 Ω shunt, we can remove the voltage in the range from 1 to 5 V by linking the voltage value at the input of the data collector with the value of the output signal of the pressure transducer.
As already mentioned, the pressure transmitter requires a minimum operating voltage of 12 V with a maximum of 30 V. Adding the voltage drop across the precision shunt resistor to the operating voltage of the transmitter gives the following:
12V+ 5V=17V
At first glance, a voltage of 17V is enough. However, it is necessary to take into account the additional load on the power supply, which is created by wires that have electrical resistance.
In cases where the sensor is located far from the measuring instruments, you must take into account the wire resistance factor when calculating the current loop. Copper wires have DC resistance that is directly proportional to their length. With the pressure transmitter in this example, you need to account for 2000 feet of line length when determining the operating voltage of the power supply. Linear resistance of a single-core copper cable is 2.62 Ω/100 ft. Accounting for this resistance gives the following:
The resistance of one strand 2000 feet long will be 2000 * 2.62 / 100 = 52.4 m.
The voltage drop on one core will be 0.02 * 52.4 = 1.048 V.
To complete the circuit, two wires are needed, then the length of the communication line is doubled, and
the total voltage drop would be 2.096 volts. The total would be about 2.1 volts due to the converter being 2000 feet away from the secondary. Summing up the voltage drops on all elements of the circuit, we get:
2.096V + 12V+ 5V=19.096V
If you used 17 V to power the circuit in question, then the voltage applied to the pressure transmitter will be below the minimum operating voltage due to the drop in wire resistance and shunt resistor. Selecting a typical 24V power supply will satisfy the power requirements of the inverter. Additionally, there is a voltage margin in order to place the pressure sensor at a greater distance.
With the right choice of transducer, data acquisition device, cable lengths and power supply, the design of a simple current loop is complete. For more complex applications, you can include additional measurement channels in the system.
Here I separately took out such an important practical issue as the connection of inductive sensors with a transistor output, which are ubiquitous in modern industrial equipment. In addition, there are real instructions for the sensors and links to examples.
The principle of activation (operation) of sensors in this case can be any - inductive (approximation), optical (photoelectric), etc.
In the first part, possible options for sensor outputs were described. There should be no problems with connecting sensors with contacts (relay output). And with transistors and with connecting to the controller, not everything is so simple.
Connection diagrams for PNP and NPN sensors
The difference between PNP and NPN sensors is that they switch different poles of the power source. PNP (from the word “Positive”) switches the positive output of the power supply, NPN - negative.
Below, for example, are the connection diagrams for sensors with a transistor output. Load - as a rule, this is the input of the controller.
sensor. The load (Load) is constantly connected to the “minus” (0V), the supply of discrete “1” (+V) is switched by a transistor. NO or NC sensor - depends on the control circuit (Main circuit)
sensor. Load (Load) is constantly connected to the "plus" (+V). Here, the active level (discrete “1”) at the output of the sensor is low (0V), while the load is powered through the opened transistor.
I urge everyone not to get confused, the work of these schemes will be described in detail later.
The diagrams below show basically the same thing. The emphasis is on the differences in the circuits of PNP and NPN outputs.
Connection diagrams for NPN and PNP sensor outputs
On the left figure - a sensor with an output transistor NPN. The common wire is switched, which in this case is the negative wire of the power source.
On the right - the case with a transistor PNP at the exit. This case is the most frequent, since in modern electronics it is customary to make the negative wire of the power source common, and activate the inputs of controllers and other recording devices with a positive potential.
How to test an inductive sensor?
To do this, you need to apply power to it, that is, connect it to the circuit. Then - activate (initiate) it. When activated, the indicator will light up. However, the indication does not guarantee the correct operation of the inductive sensor. You need to connect the load, and measure the voltage on it to be 100% sure.
Replacement of sensors
As I already wrote, there are basically 4 types of sensors with a transistor output, which are divided according to their internal structure and switching circuit:
- PNP NO
- PNP NC
- NPN NO
- NPN NC
All these types of sensors can be replaced with each other, i.e. they are interchangeable.
This is implemented in the following ways:
- Alteration of the initiation device - the design changes mechanically.
- Changing the existing scheme for switching on the sensor.
- Switching the type of sensor output (if there are such switches on the sensor body).
- Program reprogramming - changing the active level of this input, changing the program algorithm.
Below is an example of how you can replace a PNP sensor with an NPN one by changing the wiring diagram:
PNP-NPN interchangeability schemes. On the left is the original diagram, on the right is the modified one.
Understanding the operation of these circuits will help the realization of the fact that the transistor is a key element that can be represented by ordinary relay contacts (examples are below, in the notation).
So the diagram is on the left. Let's assume that the sensor type is NO. Then (regardless of the type of transistor at the output), when the sensor is not active, its output “contacts” are open, and no current flows through them. When the sensor is active, the contacts are closed, with all the ensuing consequences. More precisely, with current flowing through these contacts)). The flowing current creates a voltage drop across the load.
The internal load is shown by the dotted line for a reason. This resistor exists, but its presence does not guarantee stable operation of the sensor, the sensor must be connected to the controller input or other load. The resistance of this input is the main load.
If there is no internal load in the sensor, and the collector is “hanging in the air”, then this is called an “open collector circuit”. This circuit ONLY works with a connected load.
So, in a circuit with a PNP output, when activated, the voltage (+V) through the open transistor enters the controller input, and it is activated. How to achieve the same with the release of NPN?
There are situations when the required sensor is not at hand, and the machine should work “right now”.
We look at the changes in the scheme on the right. First of all, the mode of operation of the output transistor of the sensor is provided. For this, an additional resistor is added to the circuit, its resistance is usually of the order of 5.1 - 10 kOhm. Now, when the sensor is not active, voltage (+V) is supplied to the controller input through an additional resistor, and the controller input is activated. When the sensor is active, there is a discrete “0” at the controller input, since the controller input is shunted by an open NPN transistor, and almost all the current of the additional resistor passes through this transistor.
In this case, there is a rephasing of the sensor operation. But the sensor works in the mode, and the controller receives information. In most cases, this is sufficient. For example, in the pulse counting mode - a tachometer, or the number of blanks.
Yes, not exactly what we wanted, and interchangeability schemes for npn and pnp sensors are not always acceptable.
How to achieve full functionality? Method 1 - mechanically move or remake a metal plate (activator). Or the light gap, if we are talking about an optical sensor. Method 2 - reprogram the controller input so that discrete "0" is the active state of the controller, and "1" is passive. If you have a laptop at hand, then the second method is both faster and easier.
Proximity sensor symbol
On circuit diagrams, inductive sensors (proximity sensors) are designated differently. But the main thing is that there is a square rotated by 45 ° and two vertical lines in it. As in the diagrams below.
NO NC sensors. Principal schemes.
On the top diagram there is a normally open (NO) contact (conditionally marked as a PNP transistor). The second circuit is normally closed, and the third circuit is both contacts in one housing.
Color coding of sensor outputs
There is a standard sensor marking system. All manufacturers currently adhere to it.
However, it is useful to make sure that the connection is correct before installation by referring to the connection manual (instructions). In addition, as a rule, the colors of the wires are indicated on the sensor itself, if its size allows.
Here is the marking.
- Blue (Blue) - Minus power
- Brown (Brown) - Plus
- Black (Black) - Exit
- White (White) - the second output, or control input, you have to look at the instructions.
Designation system for inductive sensors
The sensor type is indicated by an alphanumeric code that encodes the main parameters of the sensor. Below is the labeling system for popular Autonics gauges.
Download instructions and manuals for some types of inductive sensors: I meet in my work.
Thank you all for your attention, I'm waiting for questions on connecting sensors in the comments!
Discrete sensors
Such an algorithm avoids impact when the mold is closed, otherwise it can simply be split into small pieces. The same change in speed occurs when the mold is opened. Here, two contact sensors are indispensable.
Application of analog sensors
Figure 2. Wheatstone bridge
Connecting analog sensors
Analog Sensor Outputs
But the matter, as a rule, is not enough with a single sensor. Some of the most popular measurements are temperature and pressure measurements. The number of such points in modern production can reach several tens of thousands. Accordingly, the number of sensors is also large. Therefore, several analog sensors are most often connected to one controller at once. Of course, not several thousand at once, it’s good if a dozen is different. Such a connection is shown in Figure 7.
Figure 7. Connecting multiple analog sensors to the controller
This figure shows how a voltage is obtained from a current signal, suitable for conversion into a digital code. If there are several such signals, then they are not processed all at once, but are separated in time, multiplexed, otherwise a separate ADC would have to be installed on each channel.
For this purpose, the controller has a circuit switching circuit. The functional diagram of the switch is shown in Figure 8.
Figure 8. Analog sensor channel switch (clickable image)
The current loop signals converted into voltage across the measuring resistor (UR1…URn) are fed to the input of the analog switch. The control signals alternately pass to the output one of the signals UR1…URn, which are amplified by the amplifier, and are alternately fed to the input of the ADC. The voltage converted into a digital code is supplied to the controller.
The scheme, of course, is very simplified, but it is quite possible to consider the principle of multiplexing in it. Approximately this is how the module for input of analog signals of MCTS controllers (microprocessor system of technical means) produced by the Smolensk PC "Prolog" is built.
The release of such controllers has long been discontinued, although in some places, far from the best, these controllers are still in use. These museum exhibits are being replaced by controllers of new models, mainly imported (Chinese) production.
If the controller is mounted in a metal cabinet, it is recommended to connect the braided shields to the cabinet earth point. The length of connecting lines can reach more than two kilometers, which is calculated using the appropriate formulas. We will not count anything here, but believe that this is so.
New sensors, new controllers
With the advent of new controllers, new analog sensors have also appeared that operate using the HART (Highway Addressable Remote Transducer) protocol, which translates as "Measuring transducer addressed remotely via the trunk."
The output signal of the sensor (field device) is an analog current signal in the range of 4 ... 20 mA, on which a frequency-modulated (FSK - Frequency Shift Keying) digital communication signal is superimposed.
It is known that the average value of the sinusoidal signal is equal to zero, therefore, the transmission of digital information does not affect the output current of the sensor 4 ... 20mA. This mode is used when configuring sensors.
HART communication takes place in two ways. In the first case, the standard one, only two devices can exchange information over a two-wire line, while the output analog signal 4 ... 20mA depends on the measured value. This mode is used when configuring field devices (sensors).
In the second case, up to 15 sensors can be connected to a two-wire line, the number of which is determined by the parameters of the communication line and the power of the power supply. This is the multipoint mode. In this mode, each sensor has its own address in the range 1…15, by which the control device accesses it.
The sensor with address 0 is disconnected from the communication line. Data exchange between the sensor and the control device in multipoint mode is carried out only by a frequency signal. The current signal of the sensor is fixed at the required level and does not change.
Data in the case of multipoint communication means not only the results of measurements of the controlled parameter, but also a whole set of all kinds of service information.
First of all, these are the addresses of sensors, control commands, settings. And all this information is transmitted over two-wire communication lines. Is it possible to get rid of them too? True, this must be done carefully, only in cases where the wireless connection cannot affect the security of the controlled process.
These are the technologies that have replaced the old analog current loop. But it does not give up its positions either, it is widely used wherever possible.
In the process of automation of technological processes for the control of mechanisms and units, one has to deal with measurements of various physical quantities. This can be temperature, pressure and flow of liquid or gas, rotational speed, luminous intensity, information about the position of parts of mechanisms, and much more. This information is obtained using sensors. Here, first, about the position of the parts of the mechanisms.
Discrete sensors
The simplest sensor is a conventional mechanical contact: the door is opened - the contact opens, closed - it closes. Such a simple sensor, as well as the above algorithm of operation, is often used in burglar alarms. For a mechanism with translational movement, which has two positions, for example, a water valve, you will need two contacts already: one contact is closed - the valve is closed, the other is closed - it is closed.
A more complex translational motion algorithm has a mechanism for closing the mold of an injection molding machine. Initially, the mold is open, this is the starting position. In this position, finished products are removed from the mold. Next, the worker closes the protective fence and the mold begins to close, a new work cycle begins.
The distance between the halves of the mold is quite large. Therefore, at first the mold moves quickly, and at some distance before the halves close, the limit switch is triggered, the movement speed decreases significantly and the mold closes smoothly.
Thus, contact-based sensors are discrete or binary, have two positions, closed - open or 1 and 0. In other words, you can say that an event has occurred or not. In the example above, several points are "caught" by the contacts: the beginning of the movement, the point of deceleration, the end of the movement.
In geometry, a point has no dimensions, just a point and that's it. It can either be (on a sheet of paper, in the trajectory, as in our case) or it simply does not exist. Therefore, discrete sensors are used to detect points. It may be that a comparison with a point is not very appropriate here, because for practical purposes they use the value of the accuracy of a discrete sensor, and this accuracy is much greater than a geometric point.
But in itself, mechanical contact is an unreliable thing. Therefore, wherever possible, mechanical contacts are replaced by non-contact sensors. The simplest option is reed switches: the magnet approaches, the contact closes. The accuracy of the reed switch operation leaves much to be desired; such sensors are used just to determine the position of the doors.
A more complex and accurate option should be considered various non-contact sensors. If the metal flag entered the slot, then the sensor worked. BVK sensors (Proximity Limit Switch) of various series can be cited as an example of such sensors. The response accuracy (stroke differential) of such sensors is 3 millimeters.
BVK series sensor
Figure 1. BVK series sensor
The supply voltage of the BVK sensors is 24V, the load current is 200mA, which is quite enough to connect intermediate relays for further coordination with the control circuit. This is how BVK sensors are used in various equipment.
In addition to BVK sensors, sensors of the BTP, KVP, PIP, KVD, PISCH types are also used. Each series has several types of sensors, indicated by numbers, for example, BTP-101, BTP-102, BTP-103, BTP-211.
All mentioned sensors are non-contact discrete, their main purpose is to determine the position of parts of mechanisms and assemblies. Naturally, there are many more of these sensors; it is impossible to write about all of them in one article. Even more common and still widely used are various contact sensors.
Application of analog sensors
In addition to discrete sensors, analog sensors are widely used in automation systems. Their purpose is to obtain information about various physical quantities, and not just like that in general, but in real time. More precisely, the conversion of a physical quantity (pressure, temperature, illumination, flow, voltage, current) into an electrical signal suitable for transmission via communication lines to the controller and its further processing.
Analog sensors are usually located quite far from the controller, which is why they are often called field devices. This term is often used in the technical literature.
An analog sensor usually consists of several parts. The most important part is the sensitive element - the sensor. Its purpose is to convert the measured value into an electrical signal. But the signal received from the sensor is usually small. To obtain a signal suitable for amplification, the sensor is most often included in a bridge circuit - a Wheatstone bridge.
Wheatstone bridge
Figure 2. Wheatstone bridge
The original purpose of the bridge circuit is to accurately measure resistance. A DC source is connected to the diagonal of the AD bridge. A sensitive galvanometer with a midpoint, with zero in the middle of the scale, is connected to the other diagonal. To measure the resistance of the resistor Rx by rotating the tuning resistor R2, the bridge should be balanced, the galvanometer needle should be set to zero.
The deviation of the arrow of the device in one direction or another allows you to determine the direction of rotation of the resistor R2. The value of the measured resistance is determined by the scale, combined with the handle of the resistor R2. The equilibrium condition for the bridge is the equality of the ratios R1/R2 and Rx/R3. In this case, zero potential difference is obtained between the points BC, and no current flows through the galvanometer V.
The resistance of resistors R1 and R3 is selected very accurately, their spread should be minimal. Only in this case, even a small imbalance of the bridge causes a fairly noticeable change in the voltage of the BC diagonal. It is this property of the bridge that is used to connect sensitive elements (sensors) of various analog sensors. Well, then everything is simple, a matter of technology.
To use the signal received from the sensor, its further processing is required - amplification and conversion into an output signal suitable for transmission and processing by the control circuit - the controller. Most often, the output signal of analog sensors is current (analog current loop), less often voltage.
Why current? The fact is that the output stages of analog sensors are based on current sources. This allows you to get rid of the influence of the resistance of the connecting lines on the output signal, to use connecting lines of great length.
Further transformation is quite simple. The current signal is converted into voltage, for which it is enough to pass the current through a resistor of known resistance. The voltage drop across the measuring resistor is obtained according to Ohm's law U=I*R.
For example, for a current of 10 mA across a 100 Ohm resistor, the voltage will be 10 * 100 = 1000 mV, as much as a whole 1 volt! In this case, the output current of the sensor does not depend on the resistance of the connecting wires. Within reasonable limits, of course.
Connecting analog sensors
The voltage obtained on the measuring resistor is easily converted into a digital form suitable for input into the controller. The conversion is performed using ADC analog-to-digital converters.
Digital data is transmitted to the controller in serial or parallel code. It all depends on the specific switching scheme. A simplified analog sensor connection diagram is shown in Figure 3.
Connecting an analog sensor
Figure 3. Connecting an analog sensor (click on the picture to enlarge)
Actuators are connected to the controller, or the controller itself is connected to a computer included in the automation system.
Naturally, analog sensors have a complete design, one of the elements of which is a housing with connecting elements. As an example, Figure 4 shows the appearance of the overpressure sensor of the Zond-10 type.
Overpressure sensor Zond-10
Figure 4. Overpressure sensor Zond-10
At the bottom of the sensor, you can see the connecting thread for connecting to the pipeline, and on the right, under the black cover, there is a connector for connecting the communication line with the controller.
The threaded connection is sealed with an annealed copper washer (supplied with the sensor), and by no means with fum-tape or linen. This is done so that when installing the sensor, the sensor element located inside is not deformed.
Analog Sensor Outputs
According to the standards, there are three ranges of current signals: 0…5mA, 0…20mA and 4…20mA. What is their difference, and what features?
Most often, the dependence of the output current is directly proportional to the measured value, for example, the higher the pressure in the pipe, the greater the current at the output of the sensor. Although sometimes an inverse connection is used: a larger value of the output current corresponds to the minimum value of the measured value at the output of the sensor. It all depends on the type of controller used. Some sensors even have switching from direct to inverse signal.
The output signal in the 0...5mA range is very small and therefore susceptible to interference. If the signal of such a sensor fluctuates with a constant value of the measured parameter, then there is a recommendation to install a capacitor with a capacity of 0.1 ... 1 μF in parallel with the sensor output. More stable is the current signal in the range of 0…20mA.
But both of these ranges are not good because zero at the beginning of the scale does not allow you to unambiguously determine what happened. Or did the measured signal actually take on a zero level, which is possible in principle, or did the communication line simply break? Therefore, they try to refuse the use of these ranges, if possible.
The signal of analog sensors with an output current in the range of 4 ... 20 mA is considered more reliable. Its noise immunity is quite high, and the lower limit, even if the measured signal has a zero level, will be 4mA, which allows us to say that the communication line is not broken.
Another good feature of the 4 ... 20mA range is that the sensors can be connected with just two wires, since the sensor itself is powered by this current. This is its consumption current and at the same time a measuring signal.
The power supply for sensors in the 4 ... 20 mA range is turned on, as shown in Figure 5. At the same time, Zond-10 sensors, like many others, according to the passport, have a wide supply voltage range of 10 ... 38V, although stabilized sources with a voltage of 24V are most often used.
Connecting an analog sensor with an external power supply
Figure 5. Connecting an analog sensor with an external power supply
This diagram contains the following elements and symbols. Rsh - measuring shunt resistor, Rl1 and Rl2 - communication line resistances. To improve measurement accuracy, a precision measuring resistor should be used as Rsh. The passage of current from the power supply is shown by arrows.
It is easy to see that the output current of the power supply passes from the +24V terminal, through the line Rl1 reaches the sensor terminal +AO2, passes through the sensor and through the sensor output contact - AO2, the connecting line Rl2, the resistor Rsh returns to the -24V power supply terminal. Everything, the circuit is closed, the current flows.
If the controller contains a 24V power supply, then the connection of a sensor or measuring transducer is possible according to the scheme shown in Figure 6.
Connecting an analog sensor to a controller with an internal power supply
Figure 6. Connecting an analog sensor to a controller with an internal power supply
This diagram shows another element - a ballast resistor Rb. Its purpose is to protect the measuring resistor in case of a short circuit in the communication line or a malfunction of the analog sensor. Installing a resistor Rb is optional, although desirable.
In addition to various sensors, the current output also has measuring transducers, which are used quite often in automation systems.
A measuring transducer is a device for converting voltage levels, for example, 220V or current of several tens or hundreds of amperes, into a current signal of 4 ... 20mA. Here, the level of the electrical signal is simply converted, and not the representation of some physical quantity (speed, flow, pressure) in electrical form.
But the matter, as a rule, is not enough with a single sensor. Some of the most popular measurements are temperature and pressure measurements. The number of such points in modern production can reach several ten
Read also
- Types of wall lamps and features of their use
- About potential difference, electromotive force and voltage
- What can be determined by the meter, except for electricity consumption
- On the criteria for assessing the quality of electrical products
- What is better for a private house - single-phase or three-phase input?
- How to choose a voltage stabilizer for a country house
- Peltier effect: the magical effect of electric current
- The practice of wiring and connecting a TV cable in an apartment - process features
- Wiring problems: what to do and how to fix them?
- Fluorescent lamps T5: prospects and problems of application
- Retractable socket blocks: practice of use and connection
- Electronic amplifiers. Part 2. Audio frequency amplifiers
- Proper operation of electrical equipment and wiring in a country house
- The main points of using safe voltage in everyday life
- Necessary tools and devices for beginners to study electronics
- Capacitors: purpose, device, principle of operation
- What is transient contact resistance and how to deal with it
- Voltage relay: what are there, how to choose and connect?
- What is better for a private house - single-phase or three-phase input?
- Capacitors in electronic circuits. Part 2. Interstage communication, filters, generators
- How to ensure comfort with insufficient power supply
- How to make sure when buying a machine in a store that it is working?
- How to choose a wire cross-section for 12 volt lighting networks
- Method of connecting a water heater and a pump with insufficient network power
- Inductors and magnetic fields. Part 2. Electromagnetic induction and inductance
- operational amplifiers. Part 2. Ideal operational amplifier
- What are microcontrollers (purpose, device, software)
- Extending the life of a compact fluorescent lamp (housekeeper)
- Operational amplifier circuits without feedback
- Replacing the electrical switchboard of the apartment
- Why can't copper and aluminum be combined in electrical wiring?
Sensors with a unified current output of 4-20, 0-50 or 0-20 mA, which are most widely used in the field of industrial automation, can have different schemes for connecting to secondary devices. Modern sensors with low power consumption and a current output of 4-20 mA are most often connected in a two-wire circuit. That is, only one cable with two wires is connected to such a sensor, through which this sensor is powered, and transmission is carried out through the same two wires.
Typically, sensors with a 4-20 mA output and two-wire connection have a passive output and require an external power supply to operate. This power supply can be built directly into the secondary device (into its input) and when the sensor is connected to such a device, a current immediately appears in the signal circuit. Devices that have a power supply for the sensor built into the input are said to be devices with an active input.
Most modern secondary devices and controllers have built-in power supplies to work with sensors with passive outputs.
If the secondary device has a passive input - in fact, just a resistor from which the measuring circuit of the device "reads" the voltage drop proportional to the current flowing in the circuit, then an additional one is needed for the sensor to work. The external power supply in this case is connected in series with the sensor and the secondary device to break the current loop.
Secondary instruments are usually designed and manufactured in such a way that they can be connected to both two-wire 4-20 mA sensors and sensors 0-5, 0-20 or 4-20 mA connected in a three-wire circuit. To connect a two-wire sensor to the input of a secondary device with three input terminals (+U, input and common), the "+U" and "input" terminals are used, the "common" terminal remains free.
Since the sensors, as mentioned above, can have not only an output of 4-20 mA, but, for example, 0-5 or 0-20 mA, or they cannot be connected in a two-wire circuit due to their large own power consumption (more than 3 mA) , then a three-wire connection scheme is used. In this case, the sensor supply circuits and the output signal circuits are separated. Sensors with a three-wire connection usually have an active output. That is, if a sensor with an active output is supplied with a supply voltage and a load resistance is connected between its output terminals "output" and "common", then a current proportional to the value of the measured parameter will run in the output circuit.
Secondary devices usually have a fairly low-power built-in power supply to power the sensors. The maximum output current of built-in power supplies is usually in the range of 22-50 mA, which is not always enough to power sensors with high power consumption: electromagnetic flow meters, infrared gas analyzers, etc. In this case, to power a three-wire sensor, you have to use an external, more powerful power supply that provides the necessary power. The power supply built into the secondary device is not used.
A similar circuit for switching on three-wire sensors is also usually used when the voltage of the power source built into the device does not correspond to the supply voltage that can be supplied to this sensor. For example, the built-in power supply has an output voltage of 24V, and the sensor can be powered from 10 to 16V.
Some secondary devices may have multiple input channels and a powerful enough power supply to power external sensors. It must be remembered that the total power consumption of all sensors connected to such a multi-channel device must be less than the power of the built-in power source designed to power them. In addition, studying the technical characteristics of the device, it is necessary to clearly distinguish the purpose of the power supplies (sources) built into it. One built-in source is used to power the secondary device itself - for the operation of the display and indicators, output relays, the electronic circuit of the device, etc. This power supply can have quite a lot of power. The second built-in source is used to power only the input circuits - connected to the sensor inputs.
Before connecting the sensor to the secondary device, you should carefully study the operating manuals for this equipment, determine the types of inputs and outputs (active / passive), check the correspondence between the power consumed by the sensor and the power of the power source (built-in or external) and only after that make the connection. The actual designations of the input and output terminals of sensors and devices may differ from those given above. So the terminals "In (+)" and "In (-)" can be designated +J and -J, +4-20 and -4-20, +In and -In, etc. The "+U supply" terminal can be designated as +V, Supply, +24V, etc., the "Output" terminal - Out, Sign, Jout, 4-20 mA, etc., the "common" terminal - GND , -24V, 0V, etc., but this does not change the meaning.
Sensors with a current output having a four-wire connection scheme have a similar connection scheme as two-wire sensors, with the only difference that four-wire sensors are powered by a separate pair of wires. In addition, four-wire sensors may have both, which must be taken into account when choosing a connection scheme.
Loading...