A device for measuring the speed of a car on the road. What radars does the traffic police use and how to fool them? Variable differential pressure flowmeters
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Flight speed. One of the most important characteristics for any aircraft. We are all accustomed to the fact that an airplane necessarily means “fast.” All associations work only in this direction. Many people like speed. Almost anyone would like to have a blast in their car (unless, of course, the police interfere 🙂). And information about traffic here is easy to get. Just look at the speedometer, which is mechanically or electronically connected to the wheel. The speed of rotation of the wheel ultimately gives us the speed at which the car moves on the road.
But what about the plane? There are no roads in the air on which you could drive :-). The only medium with which the aircraft comes into direct contact is air. It is from him that he receives most of the information about his movement. Regarding flight speed specifically, it is quite clear that the faster the plane flies, the more pressure the oncoming air flow (velocity or dynamic pressure) puts on it. From here it would be logical to determine the flight speed depending on the magnitude of this pressure. The same as, by the way, with atmospheric pressure and altitude. After all, the higher the plane flies, the lower the atmospheric pressure. However, we will talk about height in one of the following articles, but for now on the agenda flight speed.
There are special systems for collecting and processing this kind of data on modern aircraft. One of the names for them is Air Signal System (ASS).
The operation of such a system's sensors, which collect data to determine flight speed, is based on two already venerable inventions. The first is pitot tube. It was invented in 1732 by the French scientist A. Pitot. He studied hydraulics, that is, he studied the flow of fluid in pipes. As you know, the laws of hydraulics, under certain conditions, are quite applicable to gases, that is, to air. We will keep this in mind in the future.
Schematic of a classic pitot tube
Pitot tube is an L-shaped tube, one end of which is placed in a high-speed (air:-)) flow. This flow in the tube is slowed down, creating excess pressure in it, the magnitude of which can be used to judge the flow speed, that is, in fact, the flight speed if this tube is installed on an aircraft. In general, the principle is quite simple :-).
However, here we must not forget about one more important thing. Everything that is inside the earth's atmosphere exists in it under constant atmospheric (static) pressure. We practically don’t feel it (unless, of course, everything is in order with our health :-)), but it is there and in one way or another affects almost all the physical processes occurring around us, that is, our entire life. Just like in the movie "DMB" :-):
- Do you see the gopher?
- No…
- And I don’t see... But he is there!
Seriously, the pressure that we get when the air flow in the pitot tube slows down is the so-called total pressure. It is actually equal to the sum of the other two pressures.
Total pressure = dynamic pressure (velocity head) + static pressure.
This is a simplified statement, by the way. Bernoulli equations, the same scientist whom we already mentioned in the article about. Everything is correct, because in both articles we are talking about gas flows, and this is the element of any aircraft :-).
Dynamic pressure, also called velocity head, this is the same pressure that gives us flight speed. Static pressure is our invisible (like a gopher :-)) pressure. And when measuring speed, it must be taken into account, because it can have different values at different points in space, especially with changes in flight altitude, and thereby influence the value of the measured flight speed.
Now, for ease of understanding, I will give a couple of formulas. Precisely for ease of understanding, although this is not in the traditions of the site :-). So let's call (as my physics teacher said) total pressure R , dynamic - P 1, static - P 0, flight speed (flow) – V. And we also need such a physical parameter as air density ρ . I think everyone still remembers from school what it is :-).
The velocity head is expressed by the following formula P 1 = ρV²/2.
As a result, we have the following equation: Р = Р 0 + Р 1 = Р 0 + ρV²/2
From it it is very simple to obtain the required flight speed: V = √((2(P - P 0))/ρ)
Based on this simple expression, all aviation air (aerodynamic) speed meters operate. As an example, we can give a fairly simple speed indicator for low-speed aircraft US-350.
Speed indicator US-350.
As you can see, in order to determine the flight speed, we need to measure the total flow pressure and the static pressure. Classical pitot tube gives only full pressure. Therefore, statics have to be measured separately. To avoid this inconvenience pitot tube has been improved.
This is the second invention (or rather, improvement) of the two that I spoke about above. It was made by the German physicist Ludwig Prandtl, who is even sometimes called the father of modern aerodynamics. It combined the measurement of total flow pressure and static pressure in a single tube. To do this, it has one hole in the direction of flow for total pressure and a number of holes on the surface, usually located in a ring, for static pressure. Both of these pressures are usually diverted into sealed containers separated by a sensitive membrane, and its movement is transmitted to the flight speed dial. That's all. Everything ingenious is simple, as you know :-)… Such a device is called Prandtl tube or Pitot-Prandtl. In the figure: 1 - Prandtl tube, 2 - air ducts, 3 - speed indicator scale (SI), 4 - sensitive membrane.
Scheme of operation of a Prandtl tube (PVD).
The operation of the speed indicator is well demonstrated in this short video.
On modern aircraft these devices have received a new, simpler and more correct name: air pressure receivers (APR). They provide primary data to a complex air signal system. Pitot tubes in its pure form are now practically not used. Although in some places they are still found in small aviation. They are then supplied with static pressure receivers in the form of a plate with a number of holes on the skin of the aircraft.
A pitot tube under the wing of a Cessna 172.
The so-called combined PVDs are used more often. They are typical Prandtl tubes in design. These devices must be equipped with a powerful electrical heating system, since small holes for measuring pressure when the aircraft is icing may well be clogged with ice, which, of course, can interfere with their correct operation. When parked, air pressure receivers are closed with special plugs or covers to prevent foreign objects and dirt from getting into the holes.
Typical PVD of a modern aircraft.
Air pressure receiver on the SU-24M (numbers 1 and 2).
All data produced by the PVD, as I already said, is ultimately transmitted to the arrows of special devices - airspeed indicators. They are quite diverse, as are the definitions for aircraft flight speeds. After all, it moves not only relative to the earth, but also relative to the atmosphere, which itself is a very unstable environment.
So, aircraft speed.
Airspeed(most important:-)). It is divided into two types:
True airspeed(True Airspeed (TAS)) and Indicated airspeed(Indicated Airspeed (IAS))
Indicated speed is the speed that the pilot sees in his cockpit on the speed indicator. It is used to pilot an aircraft directly at a given moment in time.
True speed is actual flight speed aircraft relative to the air. It is used for navigation. Knowing it, for example, the time of arrival at the final destination of the route and possible deviations are calculated. It is usually impossible to measure this speed. It is calculated using indicated air speed, air pressure and air temperature. In this case, the errors of the instrument speed indicator are taken into account. They are always there, like any measuring device on our earth :-). These errors (or errors) are:
Instrumental. They arise due to imperfections and manufacturing features of the device itself.
Aerodynamic. These are errors that occur when measuring static pressure. They are determined by the design of the aircraft, the location of the sensors and the flight speed.
Methodical. These errors are due to the fact that each speed indicator is calculated and calibrated under certain conditions. In physics, such conditions are called normal. This is when the atmospheric pressure is 760 mmHg. , and the air temperature is 15° C. But in fact, with increasing altitude, these conditions change. The air density also changes and, consequently, the speed that the device shows, that is, the instrument speed. As you ascend to altitude, the indicated speed is always less than the true speed. They are equal only under normal atmospheric conditions. All these errors are taken into account in the form of corrections in navigation calculations.
Ground speed(Ground Speed (GS)). This is the speed of the aircraft relative to the ground. It is calculated based on true speed taking into account wind speed and is used to solve navigation problems.
Cruising speed. At this speed, the ratio of the required thrust to the flight speed is minimal. That is, the aircraft in this mode is as economical as possible while maintaining a speed sufficient to complete the task. Cruising speed is usually 0.7-0.8 of the maximum. It carries out long-term flights along routes.
That's probably all for now. However, in conclusion I will say one important detail. When speaking in this article about air flows and speeds, we meant speeds of up to 350-400 km/h. The fact is that starting from these speeds, a new effect of air flow appears - compressibility. It gives rise to a new methodological error in measuring speed, which also must be taken into account. The influence of compressibility increases with increasing altitude and flight speed, turning into supersonic effects. But flight speed at supersonic speed, pitot tube in this mode, other speed measuring devices are also used - this is the topic of the next article...
Until next time :-)…
P.S. In conclusion, I suggest you watch an additional video about Pitot and Prandtl tubes.
An anemometer is a meteorological instrument that measures the speed of air currents and winds. It was invented in 1667. Modern anemometers, in addition to the speed characteristics of air masses, measure air temperature.
Classification of anemometers and the principle of their operation
There are many types of anemometers, but the most commonly used for measurements are:
Cup anemometer
The cup anemometer has the simplest design: a moving element with four blades. As soon as the wind acts on them, the axis begins to rotate and transmit data to the measuring device. It records the number of rotations of the blades over a specific period of time. This type of anemometer is ideal for use in open areas and is therefore prized by meteorologists.
Vane anemometer
The vane anemometer is the most common among instruments that measure the speed of air masses. It consists of an impeller protected by a ring and connected directly or by a flexible wire to the measuring instrument. This design allows it to be used to record air speed in hard-to-reach places.
Ultrasonic anemometer
An ultrasonic anemometer is less commonly used to measure wind speed. As the name implies, it measures the speed of sound in a room, which changes depending on the direction of movement of air masses.
In addition to wind speed, two-component devices can determine where the wind is moving depending on parts of the world. The speed of sound in such equipment depends on the time it takes the ultrasonic pulses to cover the distance from the emitter to the ultrasonic microphone. Almost all anemometers are powered by rechargeable batteries or rechargeable batteries.
Scope of application of anemometers
Modern digital equipment is equipped with a liquid crystal display. The measurement result is displayed on it. You can choose in which units to display wind speed, and sometimes connect the device to a computer, collect data by synchronizing the anemometer with PC time, or upload the collected information to a separate file.
A vane anemometer is used in construction to determine the speed of movement of air masses in ventilation, pipes and shafts. This device is also used in agriculture to test air conditioning systems. Timely diagnosis of the speed of movement of air masses will help prevent various diseases in animals and stop or prevent the spread of infection. Most modern anemometer models calculate wind speed, air volume and even air humidity.
Anemometer - a device for measuring wind speed
An anemometer is a meteorological instrument that measures the speed of air currents and winds. It was invented in 1667. Modern anemometers, in addition to the speed characteristics of air masses, measure air temperature. Classification of anemometers and the principle of their operation There are many types of anemometers, but most often they are used for measurements:
- How to make your own device for measuring wind speed
- How to determine wind strength
- What is an anemometer
Making an anemometer with your own hands: nuances of work
To make a device that measures air flow speed, you will need available tools. For example, you can use halves of plastic Easter eggs as anemometer blades. You will also definitely need a compact brushless permanent magnet motor. The main thing is that the resistance of the bearings on the motor shaft is minimal. This requirement is due to the fact that the wind can be very weak, and then the engine shaft simply will not turn. To create an anemometer, a motor from an old hard drive will do.
The main difficulty in assembling an anemometer is making a balanced rotor. The engine will need to be installed on a massive base, and a thick plastic disk will be mounted on its rotor. Then you need to carefully cut out three identical hemispheres from plastic eggs. They are secured to the disk using pins or steel rods. In this case, the disk must first be divided into sectors of 120 degrees.
It is recommended to carry out balancing in a room where there is absolutely no wind movement. The anemometer axis must be in a horizontal position. Weight adjustment is usually done using needle files. The idea is for the rotor to stop in any position, and not in the same one.
Instrument calibration
A homemade device must be calibrated. It is best to use a vehicle for calibration. But you will need some kind of mast so that the anemometer does not fall into the zone of disturbed air created by the car. Otherwise, the readings will be greatly distorted.
Calibration should only be carried out on a calm day. Then the process will not be delayed. If the wind blows, you will have to drive along the road for a long time and calculate the average wind speed. It must be taken into account that the speedometer speed is measured in km/h, and the wind speed is measured in m/s. The ratio between them is 3.6. This means that the speedometer reading will need to be divided by this number.
Some people use a voice recorder during the calibration process. You can simply dictate the speedometer and anemometer readings to an electronic device. At home, you can create a new scale for your homemade anemometer. Only with the help of a properly calibrated device can one obtain reliable data on the wind conditions in the required area.
Tip 1: How to make your own wind speed measuring device
👍, A device for measuring wind speed or air flow is called an anemometer.
Anemometer Benetech GM816. Main functionality: measurement of air flow speed and temperature, Wind speed measurement range: 0.3…30 m/s.0. 90 km/h, Wind speed measurement accuracy: ±5% in 0.1 m/s steps, Temperature measurement range
Detectable materials: air flow speed, Type: anemometer
DT-619 Air speed and temperature meter High sensitivity and accuracy when measuring air flow speed Convenient ergonomic design Large LCD display 2m cable Functions Data/Max/Min hold Sapphire sliding stops Indication of low capacity ba.
DT-618 Air speed and temperature meterFeatures: High sensitivity and accuracy when measuring air flow speed Convenient ergonomic design Large LCD display 2m cable Data/Max/Min hold functions Sapphire sliding stops Indication of bottom.
Anemometer Benetech GM816A. Main functionality: measurement of air flow speed and temperature, Wind speed measurement range: 0.3…30 m/s, Wind speed measurement accuracy: ±5% in steps of 0.1 m/s, Temperature measurement range: -10… +45
DT-620 Air Velocity and Temperature Meter High sensitivity and accuracy in measuring air velocity Comfortable ergonomic design Large LCD display Surface temperature range: -50ºC to 260ºC (pyrometer) Optical resolution: 8:1 Cable.
Anemometer Benetech GM8902. Main functionality: measurement of speed, temperature and air flow volume, Speed measurement range: 0.3…45 m/s, Speed measurement accuracy: ±3% in steps of 0.1 m/s, Temperature measurement range: -10… +45
In the process of checking and adjusting ventilation systems, it is necessary to perform quite a lot of measurements and calculations, which affects the efficiency of the technical staff. Using the Testo 416 anemometer, you can significantly increase the efficiency of your work, because...
The ADA AeroTemp anemometer is used to measure air speed and temperature. The device is ideal for use at environmental monitoring stations, for testing ventilation, air conditioning, in sailing, aviation and parachuting. Anemometer AD.
ADA brand store, official service center
Anemometer-thermometer ADA AeroTemp Anemometer-thermometer ADA AeroTemp A00406 is a compact and lightweight device designed to determine air flow speed and temperature. It is widely used for testing ventilation, air conditioning, and monitoring stations.
Instruments for measuring wind speed in Sevastopol
Large catalog of products: instruments for measuring wind speed in Sevastopol▼ – comparison of prices in online stores, descriptions and characteristics of products, reviews
We measure the strength of the wind. Home weather station
15 comments:
Grandiose!))) And simple at the same time! We'll go to the seaside and I'll try to bring this project to life!))
Oh, what a good new laptop! Beauty!))
I’m terribly glad, Lena, that you liked it and that it will be useful to Lyovushka! I wish you a good and useful rest during the holidays!
By the way, I keep forgetting to write to you! It’s been a long time since I received your prize for Findus. Thank you very much. Both the bookmark and the card are so cute – I just loved them!
))) I’m very glad that you liked it)))
Thank you! Finally something interesting has been invented! Otherwise, it was usually boring with these observations.
Oh, I just hated filling out an observation diary at school! And then it turned out that observing the weather is so interesting!))) There is so much you can do, measure and check!
Cool! And you can do this with your daughter!
I will be glad if your daughter likes it
Thank you Tatiana. for the sorcerer! My children and I saw this at the flying club, then the idea was born to create a smaller copy, I also thought about garbage bags), now, thanks to your experience, I have a rough idea of how to do this))).
Oh, how wonderful that you will do too! I hope you’ll show it later in “Katya’s Collection”? ,)
We'll definitely show you))). Now I am collecting a collection of activities and games with wind and air. I like to do everything in bulk))).
Oh, what a rich topic - there are so many interesting things there!
It turned out to be a wonderful weather vane!
Thank you. It’s a shame there are no stripes on it – but maybe we’ll do it again and then we’ll add it.
Interesting idea. I liked it very much. Thank you.
I'll be glad if it's useful :)
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The new school year will begin very soon. And many girls and boys will sit at school desks. It's no secret that for many of them the exit is...
Measuring the wind force
How to make a windsock (sorcerer) with your own hands. We engage with children in observing the weather and natural phenomena.
Speedometers
The speedometer informs the driver about the vehicle's speed and distance traveled, and combines two measuring devices - a speed indicator and a trip meter called an odometer.
The speedometer is an important monitoring and measuring device, as it informs the driver about safe driving conditions, therefore, operating a car with a faulty speedometer is prohibited by traffic rules.
It is believed that the speedometer (from the English “speed” - speed) was invented in 1801 by our compatriot, a self-taught mechanic serf Egor Kuznetsov. He adapted a counter of his own design to the horse-drawn carriage, which allowed not only to count the number of fathoms and miles traveled, but also the speed of movement.
The curiosity, which was called a “verstometer,” was shown to Emperor Alexander I and amused the courtiers for some time.
Then, as often happened in Russia, the “verstometer” was forgotten for a long time.
And only two hundred years later, employees of the St. Petersburg Hermitage discovered this unique device in one of the repositories of the famous museum. It was restored and put on display in the museum.
The first speed measuring device was installed on a car in 1901. Until 1910, the speedometer was considered an outlandish thing and was installed as an optional option; only years later, car factories began to include it as a mandatory feature of cars.
The speedometer design, invented in 1916 by Nikola Tesla, has survived to this day without undergoing any changes.
Speedometers are driven by an electric drive or a flexible shaft (mechanical drive, usually called a “speedometer cable”). The type of speedometer drive depends on the distance of the device and the location of its connection to the vehicle transmission.
It is recommended to install flexible drive shafts if the length of the route does not exceed 3.55 meters. For longer routes, an electric drive is recommended.
The speedometer is driven from the driven shaft of the gearbox or transfer case. To do this, a gearbox is installed in the unit from which the drive is carried out, the gear ratio of which is selected depending on the gear ratio of the main drive and the rolling radius of the car wheel.
The gearbox is connected to the speedometer either mechanically (flexible shaft) or electrically (using a special sensor). The signal from the gearbox (or a sensor driven from the gearbox) is sent to the speedometer, where it is converted into the corresponding information.
Additional information about car speedometers and their drives can be obtained.
Speedometers with mechanical drive (from a flexible shaft)
All speedometers driven by a flexible shaft have the same principle of operation and differ only in the design features of the speed and counting units and in their external design.
On rice. 1 The speedometer is mechanically driven (from a flexible shaft), which is driven by the input roller 1 with a square socket into which the square tip of the flexible shaft is inserted. A permanent magnet is attached to the other end of the input roller 5 and thermal compensation washer (magnetic core) 4 . Magnet 5 magnetized so that its poles are directed towards the edges of the disk.
Rice. 1. Flexible shaft driven speedometer: 1 - input roller; 2 - felt wick; 3 - plug; 4 - washer; 5 - magnet; 6 - coil; 7 - screen; 8 - axis; 9 - lever; 10 - spiral spring; 11 - arrow; 12, 13 - rollers
On axis 8
, freely rotating in two bearings, an arrow is fixed on one side 11
, and on the other – a coil 6
. The coil is most often made in the form of a bowl, which covers the magnet with some clearance 5
. The coil is made of a non-magnetic material, such as aluminum. Outside coil 6
covered with a screen 7
made of soft magnetic material that concentrates the magnetic field of the magnet 5
in the coil area.
From the arrow side to the axis 8
a spiral spring is attached at one end 10
. The other end of the spring is attached to the lever 9
, by turning which you can adjust the tension of the spiral spring.
When the car moves, the input roller rotates from the flexible shaft 1
and with it a magnet 5
. At the same time, its magnetic flux, penetrating the coil 6
, induces eddy currents in it, which cause the formation of a magnetic field in the coil.
Two magnetic fields (magnet and coil) interact with each other in such a way that a torque is applied to the coil, the direction of which is opposite to the moment created by the spring. As a result, the coil, together with the axis and the pointer, will rotate at an angle at which the increasing moment of the elastic forces of the spring becomes equal to the moment of the magnetic forces acting on the coil.
Since the torque of the coil is proportional to the speed of rotation of the magnet, and, consequently, to the speed of the car, the angle of rotation of the coil and arrow increases with increasing speed.
Thermal compensation washer 4 , installed together with a magnet 5 , neutralizes the effect of changes in ambient temperature on the coil resistance. An increase in the resistance of the coil leads to a decrease in the currents induced in it and the magnetic flux they cause. Washer 4 at the same time, it provides an increase in the magnetic flux penetrating the coil by changing the magnetic permeability.
Roller 1 Most speedometers are equipped with an oil nipple installed at the rear of the speedometer. It consists of a stub 3 with a hole and a felt wick located underneath it 2 , which is soaked in oil and lubricates the roller.
The counting unit is driven from the input roller 1 through the rollers 12 And 13 through three reduction worm gears connected in series. Worm gears provide gear ratio 624 or 1000 .
By design, counting units come with external and internal engagement of counting drums. Typically, the counting unit contains six drums, which are loosely mounted on one axis.
With external gearing ( rice. 2) every drum 7
on the one hand it has 20
teeth 4
, which are in constant engagement with the teeth of the tribs 8
, also freely rotating on its axis.
On the side opposite the toothed one, the drums, except for the leftmost one, have two teeth 5
with a cavity between them. Each tribka has six teeth. Three teeth of the trib from the side of two teeth 5
drums are shortened in width every other.
Rice. 2. Counting unit with external gearing: 1, 3 - long trib teeth; 2 - trib tooth shortened in width; 4 - drum teeth; 5 - two drum teeth; 6 - a notch that shortens the tooth of the trib; 7 - drum; 8 - trib
The rightmost drum is constantly driven by a worm gear. When two teeth 5
approach the shortened tooth of the trib, they grab it and turn it 1/3
turnover. In this case, the next drum turns to 1/10
turnover.
The rotated tribka after rotation is installed so that the next time the teeth pass 5
they will again capture the shortened tooth.
The drum cannot stop in a different position, since this is prevented by long teeth sliding along the cylindrical part of the drum.
This ensures that each drum rotates 1/10 with a full turn of the previous one. With this design, every 100 thousand revolutions the initial (right) drum, a full revolution of which corresponds to 1 km mileage of the car, all the drums return to their original position, and the reading starts from zero.
On rice. 2 The device of the speedometer 16.3802 installed on UAZ cars is shown. Speedometer 16.3802 is mechanical, driven by a flexible shaft from the transfer case. It consists of a dial indicator of the vehicle speed and a total counter of the distance traveled. Equipped with a headlight high beam indicator.
Rice. 2. UAZ car speedometer: 1 - drive roller; 2 - felt with a supply of lubricant; 3 - hole for lubrication; 4 - permanent magnet; 5 - coil; 6 - return spring arrows; 7 - spring tension adjusting plate; 8 - arrow axis bearing; 9 - drum bracket; 10 - arrow; 11 - arrow axis; 12 - drum axis; 13 - counting drum gear; 14 - mechanism body; 15 - intermediate worm shaft; 16 - horizontal worm roller; 17 - screen; 18 - arrow stand; 19 - trib bracket; 20 - trib; 21 - counting drum; 22 - locking plate
Main characteristics of speedometer 16.3802:
- Speed indication range, km/h: 0-120;
- Value of division, km/h: 5;
- Trip meter capacity, km: 99999.9;
- Drive shaft speed corresponding to 1 km mileage: 624 ;
- Housing diameter ( mm): 100 ;
- Connecting dimensions with flexible shaft, mm: M18×1.5 square 2,67 ;
- Weight, kg: 0.54.
Electric speedometers
Electrically driven speedometers have the same magnetic induction and counting units as mechanically driven speedometers.
The speedometer electric drive consists of a sensor that is installed on the gearbox, an electric motor that rotates the drive roller of the magnetic induction pointer assembly, and an electronic motor control device. The electric motor and control device are mounted in the same housing with a magnetic induction unit.
The electric drive sensor is a three-phase alternating current generator, the rotor of which is a permanent four-pole magnet. Like a flexible shaft, the sensor rotor is driven by the transmission driven shaft.
When the rotor rotates in each phase of the stator connected by a “star” ( rice. 4), an alternating sinusoidal EMF is generated, the frequency of which is proportional to the speed of rotation of the gearbox shaft, and therefore to the speed of the vehicle. The signal from each stator phase drives the transistors VT1, VT2 And VT3 operating in electric key mode.
The collector-emitter circuits of the transistors are included in the circuits of the phase windings of a three-phase synchronous motor. The rotor of the electric motor is a four-pole permanent magnet. When a positive half-wave of the EMF arrives from the phase winding of the sensor to the base of the corresponding transistor, it opens, and current will flow through the corresponding phase winding of the electric motor.
Since the phase windings of the sensor are shifted by 120
˚, then the opening of the transistors will also be shifted in time. Therefore, the magnetic field of the stator of an electric motor, created by its windings, which are also shifted by 120
˚, will rotate at the speed of the sensor rotor.
The rotating magnetic field of the stator, acting on the permanent magnet of the rotor, causes it to rotate at the same frequency.
Resistors R1–R6 in the electronic switch circuit, the switching conditions for transistors are improved.
Tachometers
Instruments that measure crankshaft rotation speed are divided into tachometers, which record the number of revolutions per minute at a given moment, and tachoscopes, counters that show the number of shaft revolutions at a certain point in time. Tachoscopes are used when testing engines after major overhauls, and are not installed on cars.
Tachometers are used on cars if there is a need to control the engine crankshaft speed. According to the principle of operation, pressure gauges are centrifugal, electric, electronic (pulse), magnetic (induction), stoboscopic, etc. Electric tachometers are most widely used in cars, providing remote measurement of the crankshaft rotation speed.
On diesel engines, the tachometer is driven from the engine camshaft using a flexible shaft or an electric drive. Magnetic induction type tachometers, installed to monitor the speed of rotation of the diesel crankshaft, have an electric drive. Their design is similar to that of an electrically driven speedometer. They differ in the absence of a counting node.
On carburetor engines, to monitor the crankshaft speed, electronic tachometers are usually installed, the operating principle of which is based on measuring the frequency of pulses that occur in the primary circuit of the ignition system when the primary circuit is opened.
Electronic tachometer circuit ( rice. 5) provides measurements of the frequency of current interruption in the primary circuit of the ignition system.
Rice. 5. Electronic tachometer circuit
The circuit consists of three nodes: a node for generating triggering pulses, a node for generating measuring pulses and a pointer magnetoelectric device.
The tachometer input receives an input signal I from the primary circuit of the ignition system. Trigger pulse generation unit, consisting of resistors R1, R2, capacitors C1, C2, C3, C4 and zener diode VD1, extracts from a signal shaped like a damped sinusoid I signal II, having the form of a single pulse that arrives at the base of the transistor VT1 unit for generating measuring pulses.
In the initial state, the transistor VT2 open because through resistors R11, R10 And R5 the base current flows through it, and the capacitor C5 charged.
Transistor VT1 at this time is closed, since the potential of its emitter, caused by a significant voltage drop across the resistor R5, more base potential.
When a positive impulse II goes to the base of the transistor VT1, it opens. Capacitor C5 discharges through an open transistor VT1, creating on the basis of a transistor VT2 a negative bias that locks it.
Transistor VT1 maintained by open base current flowing through resistors R11, R9, R8 And R5. Open transistor VT1 ensures current flows through the measuring instrument through resistors R11, R7, R3 And R5.
Pulse duration III the current flowing through the measuring device is determined by the discharge time of the capacitor C5.
After capacitor C5 discharges, transistor VT2 opens because the negative bias at its base disappears, and the transistor VT1 closes.
Pulse frequency III current is equal to the opening frequency of the primary circuit of the ignition system. Effective value of current pulses I eff, proportional to their frequency, is shown by the device.
Variable resistor R7 When setting up, the amplitude of the pulse current is adjusted.
Thermistor R3 compensates for the temperature error of the device.
Diode VD2 serves to protect the transistor VT1.
Zener diode VD3 provides stabilization of the device supply voltage.
For aircraft, a distinction is made between true airspeed, airspeed, indicated airspeed and ground speed.
True airspeed is the speed at which the aircraft moves relative to the air.
Indicated (or indicated) airspeed is the true airspeed reduced to the normal (mass) air density. This speed characterizes the magnitude of the aerodynamic forces acting on the aircraft.
Ground speed is the speed of the aircraft relative to the Earth. It is equal to the geometric sum of true airspeed and wind speed.
In addition to speeds, a pilot in flight also needs information about the relative flight speed, i.e., the Mach number.
Airplanes and helicopters have corresponding sensors and indicators for the above-mentioned speeds.
To measure air speeds, the most widely used method is the aerodynamic method, based on measuring the total and static pressure of the oncoming air flow.
Measurement of ground speed of flight is carried out by radio engineering, inertial and other systems.
Air pressure receivers (APR) Fig. 167. It has a total pressure tube 1 and a static pressure cavity 2. The full pressure tube is open at the front and is installed in the direction of flight.
The static pressure cavity has side openings connecting it to the atmosphere. These holes should be located
where a is the speed of sound. 6*
The calibration of the true airspeed meter scale is determined by the following expression:
V = "I / , (2.23)
where y l is the air density at flight altitude H.
Or when dividing formula (2.23) by (2.21) we get
V = Vnp V~Tn (2’24)
Because the? = , then instead of formula (2.24) we can write
Consequently, the true speed is obtained from the indicated speed after making corrections for the static pressure pH and temperature Tn at a given flight altitude H, i.e., corrections for changes in air density when the flight altitude changes.
All the above expressions are taken into account when creating the design of the device. In Fig. 168 shows a schematic diagram of an instrument and airspeed meter. When the flight speed increases under the influence of the pressure difference pfull - Pst, the membrane box 1 turns the arrow 2 of the instrument speed indicator through the rod. At the same time, the center of the box 1 moves the rod 3 and, consequently, the arrow 5 of the true speed indicator.
If the flight altitude increases, then the aneroid box 4 expands and also turns the rod 3, overcoming the force of the spring R. In this case, the length of the arm I of the arrow 5 decreases, and it rotates at an additional angle, taking into account the change in air density.
In Fig. 169 shows a design diagram of a combined speed meter with a measurement range of up to 2,000 km/h (KUS-2,000). The movement of the center of the pressure gauge box 6 through the axles, drivers 7 and 8, sector 3 and tube 9 is transmitted to the wide needle 2 of the instrument speed and at the same time through a number of drivers, axes and sector 10 is transmitted to the narrow needle 1 of the true speed. With a change in flight altitude, the position of the center of the aneroid box 5 changes, which causes a displacement of the leash 4 and a change in the gear ratio between the M and A axes. The M axis is connected to the pressure box, and the A axis is connected to the true airspeed arrow.
To take into account changes in air temperature with flight altitude (it is assumed that the temperature changes in accordance with the standard atmosphere), the characteristics of the aneroid box 5 are selected accordingly.