Connecting the pressure sensor to the controller. Connecting sensors with current output to secondary devices

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 connection schemes to secondary devices. Modern sensors with low power consumption and a current output of 4-20 mA are most often connected using a two-wire circuit. That is, only one cable with two cores 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 a two-wire connection circuit have a passive output and require an external power source to operate. This power source can be built directly into the secondary device (into its input) and when the sensor is connected to such a device, 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 instruments and controllers have built-in power supplies to operate sensors with passive outputs.

If the secondary device has a passive input - essentially, 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 required for the sensor to operate. External unit In this case, the power supply is connected in series with the sensor and the secondary device in a break in the current loop.

Secondary devices are typically designed and manufactured to accept both two-wire 4-20 mA sensors and 0-5, 0-20 or 4-20 mA sensors 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 sensors, as mentioned above, may have not only a 4-20 mA output, but, for example, 0-5 or 0-20 mA, or they cannot be connected using a two-wire circuit due to their high power consumption (more than 3 mA) , then a three-wire connection diagram is used. In this case, the sensor power circuit and the output signal circuit are separated. Sensors with a three-wire connection usually have an active output. That is, if you apply a supply voltage to a sensor with an active output and connect a load resistor between its output terminals “output” and “common,” then a current proportional to the value of the measured parameter will flow 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 the 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 connecting three-wire sensors is usually used in the case when the voltage of the power supply 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 with a voltage from 10 to 16V.

Some secondary devices may have multiple input channels and a sufficiently powerful power supply to power external sensors. It must be remembered that the total power consumption of all sensors connected to such a multichannel device must be less than the power of the built-in power supply intended to power them. Moreover, by studying specifications When using the device, it is necessary to clearly distinguish the purpose of the power units (sources) built into it. One built-in source is used to power the secondary device itself - to operate the display and indicators, output relays, electronic circuit device, etc. This power source can have quite a large power. The second built-in source is used to power exclusively the input circuits - those connected to the sensor inputs.

Before connecting the sensor to a secondary device, you should carefully study the operating manuals for this equipment, determine the types of inputs and outputs (active/passive), check the compliance of the power consumed by the sensor and the power of the power source (built-in or external) and only then make the connection. Actual designations of input and output terminals for sensors and devices may differ from those shown above. So the terminals “In (+)” and “In (-)” can be designated +J and -J, +4-20 and -4-20, +In and -In, etc. The "+U power" 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 with a four-wire connection diagram have a similar connection diagram as two-wire sensors with the only difference being that the four-wire sensors are powered via a separate pair of wires. In addition, four-wire sensors can have both, which must be taken into account when choosing a connection diagram.

Here I separately brought out such an important practical question, like connecting inductive sensors with transistor output, which in modern industrial equipment– everywhere. In addition, given real instructions to sensors and links to examples.

The principle of activation (operation) of sensors can be anything - inductive (proximity), optical (photoelectric), etc.

The first part described possible options sensor outputs. There should be no problems connecting sensors with contacts (relay output). But with transistor ones and connecting to a 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, as an example, are diagrams for connecting sensors with a transistor output. Load – as a rule, this is the controller input.

Sensor. The load (Load) is constantly connected to “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. The load (Load) is constantly connected to the “plus” (+V). Here, the active level (discrete “1”) at the sensor output is low (0V), while the load is supplied with power through the opened transistor.

I urge everyone not to get confused; the operation of these schemes will be described in detail below.

The diagrams below show basically the same thing. Emphasis is placed on the differences in the PNP and NPN output circuits.

Connection diagrams for NPN and PNP sensor outputs

In the left picture there is 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 is the case with a transistor PNP at the exit. This case is the most common, since in modern electronics it is customary to make the negative wire of the power supply common, and activate the inputs of controllers and other recording devices with a positive potential.

How to check an inductive sensor?

To do this, you need to supply power to it, that is, connect it to the circuit. Then – activate (initiate) it. When activated, the indicator will light up. But the indication does not guarantee proper operation inductive sensor. You need to connect the load and measure the voltage on it to be 100% sure.

Replacing sensors

As I already wrote, there are fundamentally 4 types of sensors with transistor output, which are divided according to internal structure and connection diagram:

• 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 is mechanically changed.
• Changing the existing sensor connection circuit.
• Switching the type of sensor output (if there are such switches on the sensor body).
• Program reprogramming – changing the active level of a given input, changing the program algorithm.

Below is an example of how you can replace a PNP sensor with an NPN one by changing the connection 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 to understand the fact that a transistor is key element, which can be represented by ordinary relay contacts (examples are below in the notation).

So, here's the diagram 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 current flowing creates a voltage drop across the load.

The internal load is shown with a 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 “hangs 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, voltage (+V) is supplied to the controller input through an open transistor, and it is activated. How can we achieve the same with NPN output?

There are situations when the required sensor is not at hand, and the machine must work “right now”.

We look at the changes in the diagram on the right. First of all, the operating mode of the sensor output transistor is ensured. To do this, an additional resistor is added to the circuit; its resistance is usually about 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 of the additional resistor current passes through this transistor.

In this case, a rephasing of the sensor operation occurs. But the sensor works in mode, and the controller receives information. In most cases this is enough. For example, in the pulse counting mode - a tachometer, or the number of workpieces.

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 the 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 active state 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 shown below.

NO NC sensors. Schematic diagrams.

On top diagram– normally open (NO) contact (conventionally designated PNP transistor). The second circuit is normally closed, and the third circuit is both contacts in one housing.

Color coding of sensor leads

Exists standard system sensor markings. All manufacturers currently adhere to it.

However, before installation, it is a good idea to make sure that the connection is correct by referring to the connection manual (instructions). In addition, as a rule, the wire colors are indicated on the sensor itself, if its size allows.

This is the marking.

• Blue – Power minus
• Brown – Plus
• Black – Output
• White – second output, or control input, you need to look at the instructions.

Designation system for inductive sensors

The sensor type is indicated by a digital-alphabetic code, which encodes the main parameters of the sensor. Below is the labeling system for popular Autonics sensors.

Download instructions and manuals for some types of inductive sensors: I meet in my work.

Thank you all for your attention, I look forward to questions about connecting sensors in the comments!

In the process of automating technological processes to control mechanisms and units, one has to deal with measurements of various physical quantities. This can be temperature, pressure and flow of liquid or gas, rotation speed, light 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 an ordinary mechanical contact: the door is opened - the contact opens, closed - it closes. Such a simple sensor, as well as the given operating algorithm, often... For a mechanism with translational movement, which has two positions, for example a water valve, you will need two contacts: one contact is closed - the valve is closed, the other is closed - it is closed.

A more complex algorithm for translational movement has a mechanism for closing the thermoplastic mold of the automatic machine. Initially, the mold is open, this is the starting position. In this position, they are removed from the mold finished goods. Next, the worker closes the safety guard and the mold begins to close, and 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 speed of movement decreases significantly and the mold closes smoothly.

This algorithm allows you to avoid impact when closing the mold, otherwise it can simply be broken into small pieces. The same change in speed occurs when opening the mold. Here two contact sensors are no longer enough.

Thus, contact based sensors are discrete or binary, have two positions, closed - open or 1 and 0. In other words, we can say that an event has occurred or not. In the example above, several points are “caught” by the contacts: the beginning of movement, the point of speed reduction, the end of movement.

In geometry, a point has no dimensions, just a point and that's it. It can either be (on a piece of paper, in the trajectory of movement, as in our case) or it simply does not exist. Therefore, discrete sensors are used to detect points. Perhaps a comparison with a point is not very appropriate here, because for practical purposes they use the accuracy of the response of a discrete sensor, and this accuracy is much greater than the geometric point.

But mechanical contact itself is unreliable. Therefore, wherever possible, mechanical contacts are replaced by contactless sensors. The simplest option is reed switches: the magnet approaches, the contact closes. The accuracy of the reed switch leaves much to be desired; such sensors should only be used to determine the position of the doors.

A more complex and accurate option should be considered various contactless sensors. If the metal flag entered the slot, the sensor was triggered. An example of such sensors is the BVK sensors (Proximity Limit Switch) various series. The response accuracy (travel differential) of such sensors is 3 millimeters.

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 types BTP, KVP, PIP, KVD, PISH are also used. Each series has several types of sensors, designated 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 them all in one article. Various contact sensors are even more common and are still widely used.

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 in general, but in real time. More precisely the transformation 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 technical literature.

An analog sensor usually consists of several parts. The most important part is the sensor element - 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 - Wheatstone bridge.

Figure 2. Wheatstone bridge

The original purpose of a bridge circuit is to accurately measure resistance. The source is connected to the diagonal of the AD bridge direct current. 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, you should achieve equilibrium of the bridge and set the galvanometer needle to zero.

The deviation of the instrument arrow in one direction or another allows you to determine the direction of rotation of resistor R2. The value of the measured resistance is determined by the scale combined with the handle of resistor R2. The equilibrium condition for the bridge is the equality of the ratios R1/R2 and Rx/R3. In this case, a zero potential difference is obtained between points BC, and no current flows through the galvanometer V.

The resistance of resistors R1 and R3 is selected very precisely, 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 diagonal BC. 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 technique.

To use the signal received from the sensor, it requires further processing - amplification and conversion into an output signal suitable for transmission and processing by the control circuit - 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 built on the basis of current sources. This allows you to get rid of the influence of the resistance of connecting lines on the output signal and use long connecting lines.

Further conversion 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 on a resistor with a resistance of 100 Ohm, the voltage will be 10 * 100 = 1000 mV, as much as 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 at the measuring resistor can be easily converted into digital view, suitable for input to the controller. The conversion is done using analog-to-digital converters ADC.

Digital data is transmitted to the controller by serial or parallel code. It all depends on the specific switching circuit. A simplified connection diagram for an analog sensor is shown in Figure 3.

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 an overpressure sensor of the Zond-10 type.

Figure 4. Overpressure sensor Zond-10

At the bottom of the sensor you can see connecting thread for connection to the pipeline, and on the right under the black cover there is a connector for connecting the communication line with the controller.

Sealing threaded connection is made using a washer made of annealed copper (included in the delivery package of the sensor), and not by winding from fum tape or flax. 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 are their 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 sensor output. Although sometimes inverse switching is used: a larger output current corresponds to minimum value measured value at the sensor output. It all depends on the type of controller used. Some sensors even have a switch 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 while the value of the measured parameter remains unchanged, then there is a recommendation to install a capacitor with a capacity of 0.1...1 μF in parallel with the sensor output. The current signal in the range 0...20mA is more stable.

But both of these ranges are bad because zero at the beginning of the scale does not allow us to unambiguously determine what happened. Or did the measured signal actually reach zero level, which is possible in principle, or did the communication line simply break? Therefore, if possible, they try to avoid using these ranges.

The signal from analog sensors with an output current in the range of 4...20 mA is considered more reliable. Its noise immunity is quite high, and lower limit, even if the measured signal has a zero level, there will be 4mA, which suggests that the communication line is not broken.

Another good feature of the 4...20mA range is that sensors can be connected using only two wires, since this is the current that powers the sensor itself. This is its current consumption and at the same time a measuring signal.

The power supply for sensors in the 4...20mA range is turned on, as shown in Figure 5. At the same time, Zond-10 sensors, like many others, according to their data sheet have a wide supply voltage range of 10...38V, although they are most often used with a voltage of 24V.

Figure 5. Connecting an analog sensor with an external power supply

This diagram contains the following elements and symbols. Rsh is the measuring shunt resistor, Rl1 and Rl2 are the resistance of the communication lines. To increase the measurement accuracy, a precision measuring resistor should be used as Rsh. The flow of current from the power source 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 output contact of the sensor - AO2, connecting line Rl2, the resistor Rsh returns to the -24V power supply terminal. That's it, the circuit is closed, the current flows.

If the controller contains a 24V power supply, then connecting a sensor or measuring transducer is possible according to the diagram shown in Figure 6.

Figure 6. Connecting an analog sensor to a controller with internal power supply

This diagram shows one more element - the ballast resistor Rb. Its purpose is to protect the measuring resistor in the event of a short circuit in the communication line or a malfunction of the analog sensor. Installation of resistor Rb is optional, although desirable.

In addition to various sensors, measuring transducers also have a current output, which are used quite often in automation systems.

Transducer- a device for converting voltage levels, for example, 220V or a 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, as a rule, a single sensor is not enough. Some of the most popular measurements are temperature and pressure measurements. The number of such points per 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 are different. Such a connection is shown in Figure 7.

Figure 7. Connecting multiple analog sensors to the controller

This figure shows how a current signal produces a voltage suitable for conversion to digital code. If there are several such signals, then they are not all processed at once, but are separated in time and 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 (picture clickable)

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 alternately arrive at 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. This is approximately how the module for inputting analog signals of MSTS controllers (microprocessor system) is built technical means) produced by the Smolensk PC "Prolog". Appearance MCTS controller is shown in Figure 9.

Figure 9. MSTS controller

The production of such controllers has long been discontinued, although in some places, far from the best, these controllers still serve. These museum exhibits are being replaced by controllers of new models, mostly imported (Chinese).

If the controller is mounted in a metal cabinet, it is recommended to connect the shielding braids to the cabinet grounding point. The length of connecting lines can reach more than two kilometers, which is calculated using the appropriate formulas. We won’t count anything here, but believe me, it’s true.

New sensors, new controllers

With the arrival of new controllers, new analog sensors using the HART protocol(Highway Addressable Remote Transducer), which translates as “Measuring transducer addressed remotely via a highway.”

The output signal of the sensor (field device) is an analog current signal in the range 4...20 mA, on which a frequency-modulated (FSK - Frequency Shift Keying) digital communication signal is superimposed.

Figure 10. Analog Sensor Output via HART Protocol

The picture shows analog signal, and around it, like a snake, a sinusoid wriggles. This is a frequency modulated signal. But this is not yet digital signal, it has yet to be recognized. It is noticeable in the figure that the frequency of the sinusoid when transmitting a logical zero is higher (2.2 KHz) than when transmitting a unit (1.2 KHz). The transmission of these signals is carried out by a current with an amplitude of ±0.5 mA of a sinusoidal shape.

It is known that the average value of the sinusoidal signal is zero, therefore, the transmission of digital information does not affect the output current of the 4...20 mA sensor. This mode is used when configuring sensors.

HART communication is accomplished 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...20 mA 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 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.

In the case of multipoint communication, data means not only the actual measurement results of the monitored parameter, but also a whole set of all kinds of service information.

First of all, these are sensor addresses, control commands, and configuration parameters. 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 safety of the controlled process.

It turns out that you can get rid of the wires. Already in 2007, the WirelessHART Standard was published; the transmission medium is the unlicensed 2.4 GHz frequency, on which many computer wireless devices operate, including wireless local networks. Therefore, WirelessHART devices can also be used without any restrictions. Figure 11 shows the WirelessHART wireless network.

Figure 11. WirelessHART network

These technologies have replaced the old analog current loop. But it also does not give up its position; it is widely used wherever possible.