Commonly Used Energy Parameter Sensors

Abstract

From the discussion in Chapter Two, we know that energy-related parameters include: liquid pressure, liquid level, liquid flow, temperature, torque, speed, rotational speed, voltage, current, power, etc. These parameters are measured by sensors, and turned into standard analog signals or digital signals, then output to the energy-saving control system. We will discuss these sensors in this chapter.

From the discussion in Chapter Two, we know that energy-related parameters include: liquid pressure, liquid level, liquid flow, temperature, torque, speed, rotational speed, voltage, current, power, etc. These parameters are measured by sensors, and turned into standard analog signals or digital signals, then output to the energy-saving control system. We will discuss these sensors in this chapter.

4.1 Liquid, Gas Pressure Sensor and Liquid Level Sensor

The pressure of liquids and gases represents the pressure energy they have.

Liquid and gas pressure sensors are among the most widely used sensors. Liquid and gas pressure sensors are used to measure the pressure in pipes and containers. Pressure sensors use resistance strain gauges, semiconductor strain gauges, piezoresistive, inductive, capacitive, magnetically controlled, ceramic piezoelectric, etc. The U-shaped manometer uses the height of the liquid to directly indicate the level of air pressure.

For the strain gauge type pressure sensor, the strain gauge is bonded to the measuring body. The pressure of the measuring body changes, and the strain gauge also deforms together, so that the resistance value of the strain gauge changes, the voltage on the strain changes. The bridge circuit extracts the differential signal, amplifies it, and gives the corresponding measurement signal. The principle of the strain gauge sensor is shown in Fig. 4.1.

Fig. 4.1

Strain gauge sensor

For the pressure sensor using the pressure measuring tube, the measured liquid will produce mechanical deformation after passing through the pressure measuring tube. The measuring elements such as the potentiometer are driven by the mechanical deformation to give an electrical signal proportional to the pressure of the measured liquid.

For corrosion resistance, impact resistance and high temperature resistance, there are diaphragm type, oil filled, shockproof and high temperature resistant pressure gauges. In order to meet different pressure measurement ranges, there are pressure gauges with positive pressure, negative pressure, differential pressure, positive and negative pressure and other ranges. In order to measure different media, there are pressure gauges for different purposes. A transmitter capable of measuring the pressure difference between two pipes is called a differential pressure transmitter. If one end of the sensor is connected to atmosphere, the other end can measure the gauge pressure of the pipe.

For continuous measurement pressure transducers, the simplest is a remote pressure gauge that outputs a signal of change in resistance, similar to a potentiometer. When the pressure of the pipe or container changes, the resistance value between the middle tap of the potentiometer and the fixed end also changes accordingly.

It should be noted that for the pressure gauge installed at the outlet of the well. If the check valve is not tight when the pump stops running, it will cause negative pressure. When the pump supplies water to a low place, negative pressure also occurs when the pump stops. For pressure sensors that only have a positive pressure display function, they are often damaged. You should choose a pressure gauge or pressure sensor with positive and negative ranges.

When the pressure sensor outputs standard 0–10 mA, 4–20 mA, 0–5 V, 1–5 V signals, it is called a pressure transmitter. There are two wiring methods for pressure transmitters: 2-wire and 4-wire. The main parameters of the pressure sensor include measurement range, output signal type, explosion-proof grade, protection grade, etc.

The shape of a common pressure transmitter is shown in Fig. 4.2. The shape of a common pressure gauge is shown in Fig. 4.3.

Fig. 4.2

Common pressure sensors

Fig. 4.3

Common pressure gauges

The height of the liquid surface represents the potential energy of the liquid.

Liquid level sensors are divided into switch type and continuous measurement type according to different signals, and are divided into contact type and non-contact liquid level sensors according to whether the sensor is in contact with the liquid to be measured.

Contact liquid level sensors include plug-in liquid level transmitters, static pressure liquid level transmitters, and differential pressure liquid level transmitters. Differential pressure transmitters measure the difference in pressure between the top of the liquid level and the bottom of the liquid. Float driven rotary encoder changes with liquid level. Capacitive liquid level gauges use the dielectric constant of a capacitor to measure liquid level as the liquid level changes. A liquid level sensor that utilizes a magnet on a float that changes as the liquid level rises and falls. Sensors that use the principle of magnetostriction and use electrode height to measure liquid level, etc.

Non-contact liquid level sensors include ultrasonic liquid level gauges that use ultrasonic echoes to measure liquid level heights, radar level gauges that use radar reflected waves to measure liquid levels, and ray level gauges that use the principle of nuclear radiation, etc.

The liquid level sensor for continuous measurement is also called liquid level gauge, which can be composed of strain gauge, capacitor plate, magnetic element, ultrasonic wave, radar, infrared ray, etc.

The measuring principle of the strain gauge type is similar to that of a pressure sensor. Four strain gauges connected to the measurement chamber form an electrical bridge. Due to the pressure generated by the liquid level, the sensor probe will deform the measuring chamber. The depth of the liquid is measured based on the deformation of the measuring chamber.

The principle of ultrasonic liquid level sensor and radar liquid level sensor is similar. In both measurement methods, the sensor and the liquid cannot come into contact, or they can be placed outside the pressure vessel.

For submersible liquid level sensors, in order to eliminate the influence of atmospheric pressure on the liquid level measurement, there is a gas guide tube on the sensor probe, which is placed in the liquid leading from the cable to keep the reference pressure chamber connected to the ambient pressure. Be careful not to block or disrupt this airway during installation.

For open containers, install a static pressure liquid level transmitter directly at the bottom of the container to measure the liquid level directly.

For a closed container with pressure, a differential pressure liquid level sensor is required to obtain the liquid level value by measuring the pressure difference between the liquid surface and the liquid bottom.

The main parameters of the liquid level sensor include measuring range, output signal type, etc., when the output signal of the liquid level sensor is a standard 0 ~ 10 mA, 4 ~ 20 mA, 0 ~ 5 V, 1 ~ 5 V signal, it is also called a liquid level transmitter.

If the transmitter uses only two wires to complete the power supply and measurement signal return at the same time, the transmitter is called a two-wire transmitter, and if the power line and signal line are separated, it is called a four-wire transmitter. A 4-wire sensor can also be 3-wire if the OV and signal negative terminals are shared.

Figure 4.4 shows the appearance of the liquid level sensor.

Fig. 4.4

Liquid level sensor

4.2 Temperature Sensor

The temperature sensor is also one of the most widely used sensors. The temperature sensor is used to measure the temperature of liquid, gas, solid or thermal radiation. Typical temperature sensor types include thermal resistance, thermocouple, semiconductor, bimetal, pressure, glass liquid (such as thermometers), optical (infrared), radiation, colorimetric thermometers, etc., of which temperature sensors (thermometers) of thermal resistance, thermocouple, semiconductor, bimetal, pressure, and glass liquid type are the measurement methods that contact the measured substance, optical, radiation, colorimetric, etc. are non-contact temperature sensors.

Thermal resistance and thermocouple temperature sensors are the most used temperature sensors. In thousands of households, glass liquid thermometers may be widely used. In stations and airports with dense floating populations, non-contact infrared thermometers are the best choice. Appears more convenient.

The main material of the thermal resistance is metal. The resistance value of metals with different components will change with the change of temperature. Using this principle, the temperature of the measured object can be calculated by measuring the change of metal resistance value. Its main measurement range –200–500 °C, the linearity and accuracy of platinum (Pt) resistance is better than that of copper resistance (Cu), thermal resistance temperature sensors include Pt10, Pt100, Pt1000, Cu50, Cu100, NTC, etc.

Two different conductors A and B, or two different semiconductors A and B are combined to form a thermocouple. If the temperature t1 at the junction is different, the electromotive force at both ends of the conductor or semiconductor will be different. This phenomenon is called Seebeck effect. Thermocouple temperature sensors have different divisions such as K, S, E, B, J, N, T, R, WRE, etc., and are used to measure different temperature ranges.

On the contrary, if different conductors A and B, or different semiconductors A and B, pass current, cooling and heating will occur, this phenomenon is called the Peltier effect. P-type and N-type semiconductors are connected to form a galvanic pair, and direct current is applied to both ends of the electric refrigeration sheet, so that one side of the electric refrigeration sheet becomes cold and the other side becomes hot. The cold surface can be used to make an electronic refrigerator or water chiller, can also be used for heat dissipation of other electronic devices.

The principle of the semiconductor cooler and the appearance of the actual product are shown in Fig. 4.5. The author has used this principle to invent a body temperature battery for watches and a physical battery for heat meters. Using the way of producing integrated circuits, a large number of PN junctions are connected in series. The temperature of the lower sides is high, and the temperature of the upper sides is low. This is a physical battery.

Fig. 4.5

Principle and appearance of semiconductor coolers

Temperature sensors vary greatly depending on the application occasions. The sensor for measuring room temperature and air temperature may be a small metal bead, which is thinner than a wire. When measuring the temperature in a pipeline, a protective shell is required, as well as accessories such as an oil cup that is easy to disassemble and maintain, so sometimes it is difficult to judge whether it is a temperature sensor based on its appearance.

Temperature sensors that can output standard signals of 0–10 mA, 4–20 mA, 0–5 V, and 1–5 V are called temperature transmitters. The main parameters of the temperature transmitter are measurement range, output signal type, signal accuracy, linearity, two-wire system or four-wire system, protection method and explosion-proof or not.

The appearance of the temperature sensor is shown in Fig. 4.6.

Fig. 4.6

Temperature sensor

The temperature transmission module and display transmission unit matched with the temperature sensor can be installed externally on site to match the temperature measuring element, or can be built into the temperature sensor, or can be centrally installed in the instrument cabinet with guide rails. The temperature transmission modules are shown in Fig. 4.7.

Fig. 4.7

Temperature Transmitting Module

The appearance of temperature sensors such as electric contact thermometers, colorimetric thermometers, and glass thermometers is shown in Fig. 4.8.

Fig. 4.8

Electric contact thermometer, colorimetric thermometer, glass thermometer

4.3 Flow Sensor

Flow sensors are mainly used to measure the flow of liquid or gas in pipelines, or the flow of liquid in open channels. Common flow sensors include orifice plate type, turbine type, electromagnetic type, ultrasonic type, vortex type, rotor type, open channel type, etc.

In addition, there is the target type, in which the target for measuring pressure is placed in the center of the pipe. Annubar type inserts a tube with a hole in the middle of the pipeline, and measure the pressure difference before and after the tube. Verabar types measure the differential pressure before and after the plate with holes. Venturi type has a tube that shrinks and then diffuses in the center of the pipeline to measure the pressure difference of different sections. Pitot tube type inserts a pitot tube with a hole in the tube head to measure differential pressure. Cone type places a cone inside the middle of the pipeline to measure differential pressure. Oval gear uses fluid to drive the gear to rotate, measure flow rate. Volumetric type, rotor type, and other different forms of flow sensors.

The simplest flow sensor is a flow switch. Its principle is to put a small baffle in the pipeline. When the fluid passes through, the baffle is pushed to make the baffle move. The baffle moves a micro switch to send a switch signal. For the flow rate switch, the action value of the flow rate can be set, when the flow rate is greater than the set value, the contact switch will act.

Turbine flow sensors exploit the effect of liquid in a pipe on a turbine or propeller placed in the pipe. The speed of the turbine or propeller is directly proportional to the flow rate or flow rate. The greater the flow, the faster the speed. The flow rate is calculated by measuring the speed of the turbine and the known pipe diameter. The common water meters found in thousands of households are basically turbine type.

The orifice plate flowmeter is to place a plate with a hole in the middle of the pipeline. When the fluid (gas or liquid) flows through the orifice plate, due to the throttling loss, a pressure difference is generated on both sides of the orifice plate. The square root value is proportional to the flow rate. The differential pressure transmitter is used to measure the pressure difference, and then the flow rate of the fluid in the pipeline can be obtained through calculation. The orifice flowmeter has a high degree of reliability because it has no moving parts. The disadvantage is Waste of energy.

The electromagnetic flow sensor is used to measure the flow of conductive liquid. The conductive liquid flows through the pipeline with the induction coil. According to the law of electromagnetic induction, the different flow speed of the conductive liquid induces a voltage perpendicular to the magnetic field and proportional to the flow speed. Using this parameter change, the flow rate of the conductive liquid is measured.

The ultrasonic flow sensor is composed of a transmitting probe and a receiving probe. The flow of the liquid in the pipeline affects the sound velocity of the ultrasonic wave in the pipeline. The liquid flow rate is measured by measuring the sound velocity change measured by the ultrasonic receiving probe. At the same time, the flow rate of the liquid in the pipe is based on the known pipe diameter. This is the time-of-flight ultrasonic flowmeter, used to measure liquids such as clear water; another ultrasonic flowmeter uses the particles in the fluid to reflect ultrasonic waves to form the Doppler effect, so that the ultrasonic frequency reflected by the particles change, uses the change in frequency to calculate the flow velocity of the fluid. This ultrasonic flowmeter is used to measure fluids such as sewage; the ultrasonic flowmeter is installed outside the pipeline without resistance loss, and the cost has little to do with the diameter of the pipeline to be measured.

The vortex street flow sensor is used to form a vortex behind the measuring rod when the fluid flows through the measuring rod. The flow rate and the formed vortex are in a certain proportional relationship.

The rotor type flow sensor is similar to putting a float (rotor) in a glass tube. The diameter of the tube is large and the bottom is small. When the flow rate is large, the float will rise. It is used for flow measurement in vertical pipelines or for direct observation. If magnetism or other objects that can be sensed by the outside are added inside the float, the flow signal can also be transmitted.

The main parameters of the flow sensor are the measurement range, the minimum flow velocity of the fluid, the output signal type, the protection level, the explosion-proof requirements, etc.

For engineers and technicians, special attention should be paid to the minimum flow velocity requirements, otherwise the flow results measured at low flow rates may appear larger error. Flow rate sensors that can output standard signals of 0–10 mA, 4–20 mA, 0–5 V, 1–5 V are called flow rate transmitters.

The shapes of flow sensors such as turbines, orifice plates, vortex streets, ultrasonic waves, and rotors are shown in Fig. 4.9. The appearance of target type, Annubar, gear and other flowmeters are shown in Fig. 4.10.

Fig. 4.9

Flow sensor

Fig. 4.10

Target, Annubar, and gear flowmeters

4.4 Force Sensor

Sensors used to measure tension, pressure, and gravity are all force sensors.

Force sensors include resistance strain gauge type, transformer type and silicon semiconductor type.

For strain gauge force sensors, the strain gauge resistors are pasted on the parts of the measuring body that are subject to tension and compression. The measuring body is deformed by the external force, resulting in changes in the resistance of the 4 sets of strain gauges on it, which makes the output of the bridge circuit unbalanced, generate a signal voltage output, and measure the change of the voltage to indirectly measure the magnitude of the force.

For a transformer-type force sensor, the iron core is connected to the measuring body, and the excitation coil and the measuring coil are coupled through the iron core. When an external force is applied to the measuring body, the deformation of the measuring body will cause the iron core to move, thereby changing the excitation coil to the measuring coil. The change of the electrical signal of the measuring coil reflects the magnitude of the action force.

A semiconductor-type force sensor measures force by using changes in the electrical properties of a semiconductor when it is deformed under pressure.

Load cells are often used in the fields of batching, material conveying, and machinery manufacturing. In the conveying and processing of strip materials, it is often necessary to measure the tension of the tape, which requires a tension sensor.

The appearance of common sensors for measuring force and weight is shown in Fig. 4.11. There are several types of common tension sensors, such as pedestal type, cantilever type and shaft-through type, and their appearance is shown in Fig. 4.12.

Fig. 4.11

Force and load cell

Fig. 4.12

Appearance of common tension sensors

4.5 Speed Sensor

Rotational speed is also a common parameter of energy. The sensors for measuring the signal of rotational speed are mainly rotary encoders, resolvers and tachogenerators.

Rotary encoders can be divided into absolute type and incremental type. The rotation angle position of the absolute type encoder corresponds to the output rotation angle position one by one. The incremental type encoder sends out a pulse every time a certain angle is increased, and is measured by measuring the number of pulses. Corresponding to the angle value, the output of the incremental encoder is mostly two-phase A and B plus a zero-phase Z, or two-phase output of A and B, as shown in Fig. 4.13. The absolute encoder needs to be subdivided at different positions, there are many grating gaps on the inner disk grating from the center of the circle to the outermost circle that are staggered according to certain rules. The number of output lines varies with the accuracy. Generally speaking, the number of lines is more than that of incremental encoders. The output lines of the absolute value encoder output by serial communication can be only 3. The encoder measuring 16 absolute positions is shown in Fig. 4.13. The input shaft of the rotary encoder can be a hollow shaft or a solid shaft. The encoder of the hollow input shaft has a spring leaf for fixed installation. When the output shaft of the load is not concentric with the hole of the encoder, the spring leaf provides cushioning. There are fixed holes on the shaft encoder for installation, and the rotary encoder is a common sensor for measuring angles, especially in servo motors.

Fig. 4.13

Principle of rotary encoder

There are main scale and zero scale on the dial. There is only one zero scale for one circle. The main scale is evenly distributed along the circumference according to the required resolution, such as 2500. There are three gaps A, B, Z on the dial, where the distance between A and B is half the distance between the two scales on the main scale. The input shaft drives the dial to rotate. The light emitted by the light-emitting element passes through the scale and index plate on the dial and is received by the photosensitive element. It receives a Z signal for each revolution. The A and B signals each have 2500 pulses in one revolution. Since the signals between A and B have a 90° difference, the actual resolution of the encoder is 4 times the number of 2500 scales in one circle, 10,000 pulses/axis.

Resolver (also known as synchronous resolver) is based on the principle of transformer, by applying an excitation signal to the stator winding (primary side), generally an AC voltage signal above 400 Hz, and detecting the phase angle of the secondary winding side on the rotor with the rotation angle periodic changes occur to determine the angle and speed.

The self-aligning machine (also called self-synchronizing machine) is similar to the rotary transformer. It is equivalent to a wound AC motor, and the angle can be measured by measuring the phase change of the winding voltage signal.

The tachogenerator is used to measure the speed of the rotating object, also known as the speed sensor. There are AC tachogenerator and DC tachogenerator. The output voltage is proportional to the speed. The structure of the DC tachogenerator is similar to that of the DC motor. The rotor of the machine is in the shape of a hollow cup, one phase winding on the stator is excited, and the other phase winding outputs an AC voltage with a constant frequency.

The main parameters of the speed sensor are resolution, range, accuracy and linearity, etc., and the main parameters of the encoder are the number of pulses per revolution, the number of output phases, and the signal type.

The appearance of the speed sensor is shown in Fig. 4.14.

Fig. 4.14

Speed sensor

4.6 Torque and Speed Torque Compound Sensor

Torque is a commonly used energy parameter. In the case of driving equipment such as electric motors or internal combustion engines, in order to calculate the input and output power, in addition to measuring the rotational speed, the output torque of these equipment must also be measured. The product of torque and rotational speed is proportional to the output power of these equipment.

Common torque sensors and torque speed composite sensors, as shown in Fig. 4.15. There are an input shaft and an output shaft on both sides of the torque sensor, or an input flange and an output flange. The input shaft transmits the torque to the output shaft through the torque sensor in the middle, which is the same as the force sensor mentioned above. Similarly, the torque sensor is deformed by the torque, and the torque signal is measured according to the deformation. If the rotational speed of the shaft is also measured, the power is also measured. Based on such convenience, in addition to sensors measuring static torque, most dynamic torque sensors are torque-speed composite.

Fig. 4.15

Torque, speed and power integrated sensor

The main parameters of torque, speed and power sensors are measurement range, output signal size and type, explosion-proof grade, protection grade, etc.

4.7 Voltage Transmitter

The calculation of the power supply is inseparable from the measurement of the power supply voltage.

In electrical systems and automation systems such as power transmission, power distribution, and power equipment protection, it is necessary to measure the voltage, current, power factor, and power of the power supply line in order to perform power distribution or take other protective measures. The measurement of these parameters is used for single-phase, three-phase three-wire, three-phase four-wire, etc. Most of the currently used voltage, current, power factor, and power transmitters are integrated.

Voltage transmitter: convert the voltage signal of the line into a linear and isolated 4–20 mA (or 0–10 mA, 0–20 mA, 1–5 V, 0–5 V, 0–10 V, etc.) or digital signal. If the voltage signal of the AC line is measured, and the voltage is very high, it is necessary to use a voltage transformer to convert the high voltage into a low voltage signal, and then input it into the voltage transmitter. The voltage input has 0–220 V, 0–380 V direct input mode and PT secondary input mode. Fuses are added to the primary side of the voltage transformer to avoid the influence of the primary circuit on the secondary side when the secondary short circuit is too large. So, the secondary side is not allowed to be short-circuited and one end is grounded. The shape of the voltage transmitter is shown in Fig. 4.16.

Fig. 4.16

Voltage Transmitter

4.8 Current Transducer

The calculation of the supply power is inseparable from the measurement of the supply current.

Current transmitter: convert the current signal of the line into a linear and isolated 4–20 mA (or 0–10 mA, 0–20 mA, 1–5 V, 0–5 V, 0–10 V, etc.) or digital communication signal. If the current of the AC line is large, it is necessary to use a current transformer to convert the large current into a small current signal, and then input it into the current transmitter. The current input has 0–5A direct input mode and CT secondary input. The secondary side of the current transformer is not allowed to be open to avoid high voltage on the secondary side, so the secondary side is not allowed to install a fuse and must be grounded at one end. The shape of the current transmitter is shown in Figs. 4.3, 4.4, 4.5, 4.6, 4.7, 4.8, 4.9, 4.10, 4.11, 4.12, 4.13, 4.14, 4.15, 4.16 and 4.17.

Fig. 4.17

Current transmitter

4.9 Power Factor Transmitter and Supply Power Transmitter

Power factor transmitter: change the power factor of the AC line (that is, the cosine function of the phase angle between the voltage and the current, with a value of 0–1) into analog standard signal or digital communication signal output, such as 4–20 mA, 0–10 mA, 0–20 mA, 1–5 V, 0–5 V, 0–10 V, etc.

If the current and voltage of the circuit are large, it is necessary to use a current transformer or voltage transformer to reduce the large current and large voltage. The current and voltage signals are input to the power factor transmitter. The power factor represents the functional relationship between the electric energy used in the line and the electric energy occupied, and its meaning can refer to the reactive power compensation part of Chap. 24.

Power supply power transmitter: Input the voltage and current of the line, calculate the power and turn it into analog standard signal or Digital communication signal output, such as 4–20 mA, 0–10 mA, 0–20 mA, 1–5 V, 0–5 V, 0–10 V, etc.

If the current and voltage of the AC circuit are large, it is necessary to use a current transformer or a voltage transformer to convert the large current and large voltage into a smaller current and voltage signal. Then input the power transmitter, AC line active power transmitter and reactive power transmitter, the concept of active power and reactive power can refer to Chap. 24 of reactive power compensation.

The appearance of power factor transmitter and power transmitter is shown in Fig. 4.18.

Fig. 4.18

Power Factor Transmitter and Power Transmitter