Methods for measuring power characteristics. Means and methods for measuring force

The definition of force is implicitly contained in Newton's three laws of motion.

1. Every body is in a state of rest or uniform and rectilinear motion, until some forces take it out of this state.

2. An unbalanced force imparts acceleration to the body in the direction in which it acts. This acceleration is proportional to the force and inversely proportional to the mass of the body.

3. If the body BUT acts with some force on the body AT, then the body AT acts with the same but oppositely directed force on the body BUT.

Based on Newton's second law, the unit of force is defined as the product of mass and acceleration (F = ma). There is another formulation of Newton's second law. The momentum of a body is equal to the product of its mass times its speed of motion, so that ma is the rate of change in momentum. The force acting on a body is equal to the rate of change of its momentum. There are different ways to measure strength. Sometimes it is enough to balance the force with a load or determine how much it stretches the spring. Sometimes forces can be calculated from other observable quantities, such as accelerations, when considering jumping or throwing projectiles. In other cases, it is best to use one of the many electrical devices known as mechanoelectric transducers. These devices, under the action of applied forces, generate electrical signals,

which can be amplified and registered in the form of any record and converted into force values.

The strength of a person's action depends on the state of a given person and his volitional efforts, that is, the desire to show one or another amount of strength, in particular, maximum strength, as well as on external conditions, in particular, on the parameters of motor tasks, for example, articular angles in the body's biocircuits .

Achievements in almost all sports depend on the level of development of strength qualities, and therefore the methods of control and

significant attention is paid to improving these characteristics.

Ways to measure force

Strength control methods have a long history.

The first mechanical devices designed to measure human strength were created back in the 18th century. When controlling strength qualities, three groups of indicators are usually taken into account.

1. Basic: a) instantaneous values ​​of force at any moment of movement (in particular, maximum force); b) average strength.

2. Integral, such as the momentum of a force.

3. Differential, such as force gradient.

Max Strength is very illustrative, but in fast movements it characterizes their final result relatively poorly (for example, the correlation of the maximum repulsive force and the height of the jump can be close to zero).

According to the laws of mechanics, the final effect of the action of a force, in

In particular, the effort achieved as a result of a change in the speed of a body is determined by the impulse of the force. If the force is constant, then pulse is the product of the force times its duration Si = F∆t). In other conditions, for example, with shock interactions, the calculations of the impulse of force are carried out by integration, therefore the indicator is called integral. Thus, the most informative force impulse at

control of shock movements (in boxing, on the ball, etc.).

Average strength- this is a conditional indicator equal to the quotient of dividing the impulse of the force by the time of its action. The introduction of an average force is equivalent to the assumption that a constant force (equal to the average) acted on the body during the same time.

There are two ways to register strength qualities:

1) without measuring equipment (in this case, the assessment of the level of strength training is carried out according to the maximum weight that the athlete is able to lift or hold);

2) using measuring devices - dynamometers

or dynamometers.

All measuring procedures are carried out with the obligatory

compliance with general physical fitness control

metrological requirements. It is also necessary to strictly

comply with the specific requirements for the measurement of force

1) define and standardize in repeated attempts

the position of the body (joint) in which the measurement is taken;

2) take into account the length of body segments when measuring moments

3) take into account the direction of the force vector.

Strength control without measuring devices. In mass sports, the level of development of strength qualities is often judged by the results of competitive or training exercises. There are two ways to control: direct and indirect. In the first case, the maximum strength corresponds to the greatest weight that an athlete can lift in a technically relatively simple movement (for example, a bench press). In the second case, not so much absolute strength is measured as speed-strength qualities or strength endurance. To do this, use exercises such as long and high jumps from a place, throwing stuffed balls, pull-ups, etc.

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Introduction

1. General information about the measured value

2. Overview of measurand methods

3. Description of the inductive transducer

3.1 Uncertainties of inductive transducers

3.2 Measuring circuits of inductive transducers

4. Calculation of the main parameters of the converter

5. Calculation of the bridge circuit

6. Determining the error of an inductive transducer

Conclusion

Bibliography

Introduction

Measuring transducers are technical devices that convert values ​​and form a channel for transmitting measurement information. When describing the principle of operation of a measuring device that includes a series of measuring transducers, it is often represented as a functional block diagram (measuring circuit), which reflects the functions of its individual parts in the form of interconnected symbolic blocks.

The main characteristics of the measuring transducer are the conversion function, sensitivity, error.

Measuring transducers can be divided into three classes: proportional, functional and operational.

Proportionals are designed to similarly reproduce the input signal in the output signal. The second - to calculate some function from the input signal; the third - to obtain an output signal, which is the solution of some differential equation. Operational converters are inertial, since their output signal value at any time depends not only on the input value at the same time. But also from its values ​​in the previous moments of time.

When designing a specialized non-standard measuring instrument, one should take into account the essential organizational and technical forms of control, the scale of production, the characteristics of the measured objects, the required measurement accuracy, and other technical and economic factors.

In our case, only the converter is being designed, and therefore some of these factors can be neglected. We only care about the required measurement accuracy of a given parameter. Any measurement task begins with the choice of a primary transducer - a “sensor” capable of converting the initial information (any type of deformation, kinematic motion parameter, temperature changes, etc.) into a signal that is subject to subsequent research. The primary converter is the initial link of the measuring system. The converter in this course work is an inductive converter.

1 . Generalintelligenceaboutmeasurablesize

Force is a vector physical quantity, which is a measure of the intensity of the impact on a given body of other bodies, as well as fields. The force applied to a massive body is the cause of a change in its speed or the occurrence of deformations and stresses in it.

Force as a vector quantity is characterized by its modulus, direction and point of application of the force. The concept of the line of action of a force is also used, denoting a straight line passing through the point of application of the force, along which the force is directed.

The SI unit of force is the newton (N). Newton is a force that gives a mass of 1 kg in the direction of this force an acceleration of 1 m / s 2.

Units of force are allowed in technical measurements:

1 kgf (kilogram-force) = 9.81 N;

1 tc (ton-force) = 9.81 x 103 N.

Force is measured by means of dynamometers, force-measuring machines and presses, as well as by loading with weights and weights.

Dynamometers - devices that measure the force of elasticity.

Dynamometers are of three types:

DP - spring,

DG - hydraulic,

· DE - electric.

According to the method of recording the measured forces, dynamometers are divided into:

pointing - they are used mainly for measuring static forces arising in structures installed on stands, when external forces are applied to them and for measuring traction force during smooth movement of the product;

Counting and writing dynamometers that record variable forces are most often used to determine the traction force of steam locomotives and tractors, since due to strong shaking and inevitable jerks when accelerating their movement, as well as uneven loading of the product, variable forces are created.

The most widespread are general purpose spring indicating dynamometers.

The main parameters and dimensions of general-purpose spring dynamometers with a scale reading device, designed to measure static tensile forces, are established by GOST 13837.

The limits of measurement and the error of the dynamometer must be determined in one of two ways:

· calculated,

according to tables OST 1 00380.

Working measuring instruments used in force measuring systems are given in OST 1 00380.

There are various types of forces: gravitational, electromagnetic, reactive, nuclear, weak interaction, inertia force, friction force and others. Forces must be measured in a wide range - from 10 -12 N (van der Waals forces) to 10N (impact, thrust). Small forces are dealt with in scientific research, when testing precise force sensors in control systems, etc. Forces from 1N to 1MN are typical for testing equipment and when determining forces in vehicles, rolling machines, and more. In some areas of mechanical engineering, steel rolling and aerospace engineering, it is necessary to measure forces up to 50-100 MN. The measurement errors of force and moments in technical measurements are 1--2%. The measurement of force is reduced to the measurement of such physical quantities as pressure, acceleration, mass, the measurement error of which in many cases should not exceed 0.001%.

2 . Reviewmethodsmeasurablequantities

In modern technology, measurements of non-electric quantities (temperature, pressure, forces, etc.) are widely used by electrical methods. In most cases, such measurements come down to the fact that a non-electric quantity is converted into an electrical quantity dependent on it (for example, resistance, current, voltage, inductance, capacitance, etc.), by measuring which, it becomes possible to determine the desired non-electric quantity.

A device that converts a non-electric quantity into an electrical one is called a sensor. Sensors are divided into two main groups: parametric and generator. In parametric sensors, a non-electrical quantity causes a change in some electrical or magnetic parameter: resistance, inductance, capacitance, magnetic permeability, etc. Depending on the principle of operation, these sensors are divided into resistance sensors, inductive, capacitive, etc.

Devices for measuring various non-electric quantities by electrical methods are widely used in eps. and locomotives. Such devices consist of sensors, some electrical measuring device (galvanometer, millivoltmeter, milliammeter, logometer, etc.) and an intermediate link, which may include an electric bridge, amplifier, rectifier, stabilizer, etc.

Force change by balancing method

The method is based on balancing the measured force with the force created by the inverse electromechanical transducer, most often magnetoelectric, as well as the reaction force that occurs in the dynamic system. Such forces include centripetal force, inertia force during oscillatory motion, gyroscopic moment.

A promising way to create high-precision instruments for measuring large forces (from 105 N and more) is the use of electrodynamic inverse force transducers with superconducting windings, which allow you to reproduce forces up to 107-108 N with an error of 0.02-0.05%.

The gyroscopic method for measuring forces is based on measuring the angular velocity of the gyroscope frame precession, which occurs under the influence of a gyroscopic moment that balances the measured moment or the moment created by the measured force. This method has found application in weighing technology.

The reaction force is uniquely determined by the geometry of the system, the masses of the wedges, and the frequency of their rotation. Thus, with the parameters of the measuring device unchanged, the measured force Fx is determined from the engine speed.

Force method

It is based on the dependence of the force or moment of forces developed by an inelastic or elastic sensitive element on the applied pressure. According to this method, two types of instruments and pressure sensors are built:

Power sensors of direct conversion, in which the force developed by the sensitive element is converted by means of an electrical converter into an electrical quantity

Instruments and sensors with force compensation, in which the force developed by the sensing element is balanced by the force generated by the compensating element. Depending on the type of compensating device, the output signal can be current, linear or angular displacement.

Measurement of force, mechanical stresses

Force sensors can be divided into two classes: quantitative and qualitative.

Quantitative sensors measure force and represent its value in electrical units. Examples of such sensors are torque cells and strain gauges.

Qualitative sensors are threshold devices whose function is not to quantify the value of the force, but to detect an excess of a given level of applied force. That is, in the first case, we are talking about the measurement, and in the second case, the control of force or mechanical stress. Examples of such devices are, for example, strain gauges and a computer keyboard. High-quality sensors are often used to detect the movement and position of objects.

Methods for measuring force can be divided into the following groups:

* balancing an unknown force by the gravity of a body of known mass;

* measuring the acceleration of a body of known mass, to which the force is applied;

* balancing unknown force by electromagnetic force;

* conversion of force into fluid pressure and measurement of this pressure;

* measurement of deformation of the elastic element of the system caused by an unknown force.

Most sensors do not directly convert the force into an electrical signal. This usually requires several intermediate steps. Therefore, as a rule, force sensors are composite devices. For example, a force sensor is often a combination of a force-to-displacement converter and a position (displacement) detector. The principles of construction of scales are reduced to the measurement of force. The applied force acts on the primary transducer (sensor) consisting of an elastic element and a deformation transducer mechanically connected to the elastic element and converting this deformation into an electrical signal.

Currently, the following types of converters have found application in weighing technology:

1. Rheostatic converters. Their work is based on a change in the resistance of the rheostat, the engine of which moves under the influence of force.

2. Wire converters (strain resistance). Their work is based on a change in the resistance of the wire during its deformation.

4. Inductive transducers. Change in the inductance of the converter from a change in the position of one of its parts under the action of the measured value. used to measure force, pressure, linear displacement of a part.

5. Capacitive transducers. Change in the capacitance of the transducer under the action of a measured non-electric quantity: force, pressure of linear or angular displacement, moisture content, etc.

According to the principle of operation, generator converters are divided into groups:

1. Induction converters. Their work is based on the conversion of a measured non-electric quantity, such as speed, linear or angular displacement, into an induced emf.

3. Piezoelectric transducers. Piezoelectric effect, i.e. emf occurrence. in some crystals under the influence of mechanical forces, is used to measure these forces, pressure and other quantities.

3 . Descriptioninductiveconverter

In technical and scientific measurements of non-electric quantities, inductive transducers belonging to the group of parametric sensors are widely used. They differ in constructive simplicity, reliability and low cost. In addition, they do not require complex secondary equipment for their work.

An inductive transducer is a choke whose inductance changes under the action of an input (measured) value. In measuring technology, transducer designs with a variable air gap and solenoid (or plunger) transducers are used, which are studied in this paper.

An inductive transducer with a variable air gap is shown schematically in fig. 1. It consists of a U-shaped magnetic circuit 1, on which a coil 2 is placed, and a movable armature 3. When the armature moves, the length of the air gap changes and, consequently, the magnetic resistance. This causes a change in the magnetic resistance and inductance of the converter L. Under certain assumptions, the inductance of the converter can be calculated using formula (1):

Rice. 1. The design of an inductive transducer with a variable air gap (1 - U-shaped magnetic circuit, 2 - coil, 3 - armature): a) single transducer; b) differential converter

where w is the number of turns of the coil, µ o = 4 10 7 H/m is the magnetic constant, µ is the magnetic constant of steel, is the cross-sectional area of ​​the magnetic flux in the air gap, is the average length of the magnetic field line in steel.

Single inductive converters have a number of disadvantages, in particular, their conversion function is non-linear, they can have a large additive error caused by a temperature change in the active resistance of the winding, and a number of others.

These shortcomings are devoid of differential converters, which are two single converters with a common armature. On fig. 1b shows a differential inductive transducer consisting of two transducers shown in fig. 1a.

When moving the armature, for example, to the left, the inductance L, increases, and the other inductance L2 decreases.

Rice. 2. The design of the inductive plunger transducer (1 - coil, 2 - plunger): a) single transducer; b) differential converter

Another type of inductive transducers are plunger transducers. On fig. 2a shows a single plunger converter, which is a coil 1 from which a ferrimagnetic core 2 (plunger) can be extended. At the middle position of the plunger, the inductance is maximum.

The differential converter, consisting of two single plunger-type converters, is schematically shown in fig. 2b. 3here also, when the plunger is moved, one inductance decreases and the other increases.

When using inductive converters, the output quantity is usually not the inductance as such, but the reactance of the converter Z, which, if we neglect the active component, is equal to Z = jwL.

3.1 Errorsinductiveconverters

The errors of inductive transducers are mainly due to a change in the active component of their resistances. This error is additive and decreases in the case of bridge circuits. In addition, when the temperature changes, the magnetic permeability of the steel changes, which leads to an additional change in the additive and multiplicative errors. Changes in the supply voltage and its frequency also cause changes in sensitivity and the appearance of multiplicative errors.

Among the errors of inductive sensors, the following can be distinguished:

1.1) Error due to temperature conditions. This error is random and must be evaluated before the sensor starts to work. The error occurs due to the fact that certain parameters of the component parts of the sensor depend on temperature, and with a fairly strong deviation from the norm in one direction or another, the error can be very impressive.

1.2) Error due to the action of the force of attraction of the armature

1.3) Linearity error of the transformation function

During the operation of inductive converters in bridge circuits, an error occurs due to the instability of the voltage and frequency of the power supply of the bridge, as well as a change in the shape of the supply voltage curve. To improve the properties of inductive MTs, differential transducers are used (their design is shown in Fig. 1b). Differential transducers can significantly reduce errors, increase sensitivity and increase the linear section of the characteristic.

3.2 Measuringchainsinductiveconverters

Bridges for measuring the inductance and quality factor of inductors. The inductor, the parameters of which are measured, is included in one of the arms of the four-arm bridge, for example, in the first arm:

In order for the bridge to be balanced, at least one of the remaining legs must contain reactance in the form of inductance or capacitance.

Preference is given to containers, because. inductors are inferior to capacitors in terms of manufacturing accuracy, but are much more expensive. A diagram of such a bridge is shown in Fig. 3

Rice. 3. Bridge for measuring the parameters of inductors

When the bridge is in equilibrium, according to the general equilibrium equation, it is true. Equating the real and imaginary parts separately, we obtain two equilibrium conditions:

Such a bridge is balanced by adjustment and. The value is proportional to the inductance, and - the quality factor of the measured coil. The disadvantage of the considered scheme is the poor convergence of the bridge when measuring the parameters of coils with a low quality factor. If Q = 1, the balancing process is already difficult, and when Q< 0,5 уравновешивание моста практически невозможно.

measuring force inductive transducer

4 . Calculationmajorparametersconverter

It is required to develop a sensor for which the following characteristics of the measuring instrument are given:

Measured value: force;

Value of the measured parameter: 70-120 kN;

Measurement error: 0.25%

Type of output signal: electrical signal

Transducer: inductive

For our course work, we choose a single inductive transducer with a variable air gap, since it is characterized by measurements ranging from 0.01 to 10 mm, which allows you to measure a given parameter.

Let us depict the block diagram of this device in Figure 4. The output signal is obtained in the form of an alternating voltage taken from the load resistance R H included in the circuit of the winding 2 placed on the core 1. Power is supplied by an alternating voltage U. Under the action of the input signal, the armature 3 moves and changes the gap:

Rice. 4 - Single inductive transducer with variable air gap

Let us calculate the main parameters of the frame of the developed sensor:

Material - precision alloy 55 VTYu;

Poisson's ratio - 0.295;

Modulus of elasticity - 11 * N / \u003d 1.1209 * kgf /;

Let the radius of the membrane;

24.77 MPa = 2.43 kgf;

42.46 MPa = 4.17 kgf.

Calculate the membrane thickness using the formula (2)

h = 0.0408 cm;

Using formula (3), we determine the minimum and maximum deflection of the membrane

P = 0.044 cm;

P = 0.076 cm;

Using formula (4), we calculate the inductance at the maximum deflection of the membrane.

Sectional area of ​​the air gap;

Air magnetic permeability;

Variable air gap area.

The obtained data will be presented in Table 1 and displayed on the graph dependence (Р) (Figure 5) and dependence L(Р) (Figure 6):

Table 1

Calculation of an inductive transducer

Rice. 5 - Dependency (P)

Rice. 6 - Dependence L(P)

5 . Calculationpavementscheme

Maxwell Bridge - Guilt is shown in the figure (3)

Let's take = 800 ohms;

Calculate at the minimum and maximum value of the inductance.

6 . Definitionerrorsinductiveconverter

The informative ability of an inductive sensor is largely determined by its error in the conversion of the measured parameter. The total error of an inductive sensor consists of a large number of component errors, such as the error from the non-linearity of the characteristic, temperature error, the error from the influence of external electromagnetic fields, the error from the magnetoelastic effect, the error from the connecting cable, and others.

According to reference data, the error of the ammeter is 0.1%, the error of the bridge is 0.02%.

0,25 - (0,02 + 0,1) = 0,13%;

The error of the inductive sensor is determined by the formula (1):

Let's find the necessary variables.

0.065*24.77=1.61 MPa;

169.982 mH.

We substitute the obtained data into expression (6) and find the error of the inductive sensor:

Let us compare the obtained error with the given one

0,23% < 0,25%

Thus, the resulting error is not greater than the specified one, therefore, we conclude that the developed system meets the requirements.

Conclusion

Course work was devoted to the development of a method for measuring force using an inductive transducer that meets the requirements of the terms of reference. During the design, various methods for measuring force were studied, on the basis of which the resulting method for measuring this parameter was developed.

A review of methods for measuring force was made, an appropriate method was selected in the measured range, the main parameters of the transducer were calculated, and the error of the obtained method for measuring force was calculated.

Thus, in the process of completing the course work, all the points of the technical assignment were completed and a method for measuring the corresponding parameter that meets the requirements presented to it was developed.

Listliterature

1. Meizda F. Electronic measuring instruments and methods of measurement: Per. from eng. M.: Mir, 1990. - 535 p.

2. Brindley K.D. Measuring transducers. M.: Electr, 1991. - 353 p.

3. Spector S.A. Electrical measurements of physical quantities: Methods of measurement: Textbook for universities. L.: Energoatomizdat, 1987. - 320 p.

4. Levshina E.S. Electrical measurements of physical quantities. M.: Mir, 1983 - 105 p.

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Force is called a quantitative characteristic of the process of interaction of objects (for example, friction force).

The concept of "mass" characterizes inertia objects and their gravitational ability.

In measurements, they usually do not distinguish between mass (the amount of matter) and weight - the force of attraction of the body by the Earth (gravitational force), therefore, the same measurement methods are used to measure force and mass-weight.

Devices for measuring mass by the gravitational ability of an object are called scales. Force measurement is carried out by means of dynamometers. The division of force measuring instruments into scales and dynamometers is due to the fact that the direction of the gravitational force vector is strictly defined in space. This circumstance is taken into account when designing instruments for measuring the gravitational force, as well as when preparing the balance for work. In particular, the design of the scales provides for levels and plumb lines that allow you to set them in a horizontal position with the required accuracy. The working position of the dynamometers can be any - the main thing is that the measurement line coincides with the direction of the force vector. Under this condition, balances can be used to measure non-gravitational force, and dynamometers can be used to determine weight. Thus, the division of force measuring instruments into scales and dynamometers is determined by their purpose.

Strength measurement. In the general case, dynamometers consist of a force transducer - an elastically deformable element, a strain transducer, if necessary, and an indicating device.

Dynamometers (dynamometer from the Greek dynamis - force and meter) are made of three types: DP - spring, DG - hydraulic, DE - electric.

The variety of designs of elastic elements can be classified depending on the type of realized deformation: using compressive or tensile deformations, bending deformation, shear deformation and mixed deformation (Fig. 61)

Dynamometric tension or compression springs are usually made in the form of a solid or hollow cylinder, sometimes in the form of a rectangular rod (from 10 kN to 1 MN).

Fig.61. Force converters into deformation: a) compression, b) bending, c) shear, d) mixed

Bending deformation is also realized in elastic elements made in the form of a system of radially placed beams, rings, membranes, frames, etc. (from 10 N to 10 kN - working tools). For ring elements up to 2 MN.

Dynamometers with a complex elastic element (Fig. 3d) are designed to bring the conversion characteristic closer to a linear one and are widely used as working and reference measuring instruments.

Mechanical dynamometers are used only to measure static forces. The deformation of the sensing element (0.1 - 2 mm) is measured with a dial indicator or an indicator head. Mechanical dynamometers are commercially available for loads up to 10 MN. The accuracy class reaches 0.1 - 2%.

For elastic elements of high rigidity (rods), strain-resistive and string converters of deformation into an electrical signal are used. With low rigidity (ring, elastic beam elements), capacitive, inductive and other transducers are applicable.

Among electric dynamometers, strain gauges are of the greatest importance. The range of their application is from 5 N to 10 MN and more. The sensitive element of such dynamometers is made in the form of a rod, a pipe, a radially loaded ring, a double beam, a cantilever torsion beam, etc. A strain gauge glued to the sensitive element registers tensile strains - compression, bending, torsion, shear. Strain gauge dynamometers are suitable for both static and dynamic measurements.

In string dynamometers, a string strain gauge is used. The sensitive element is a ferromagnetic string located along the axis of the elastic hollow cylinder and connected with it by two planes. When a load is applied to the cylinder due to its deformation, the tension of the string and the frequency of its vibrations excited by the electromagnet simultaneously change. The natural oscillation frequency affects the voltage value at the terminals of the measuring coil and is a measure of the load. Force range from 200 N to 5 MN. Accuracy class 1%.

When measuring large loads (up to 50 MN), magnetoelastic transducers are used.

Magnetoelastic dynamometers are based on ferromagnetic materials (for example, iron-nickel alloys), which change their magnetic permeability in the direction of exposure to a tensile or compressive force. The magnetoelastic dynamometer can be made in the form of a coil with a closed core made of a soft magnetic material. The change in inductance that occurs when loading can be measured by electrical methods (Fig. 62). The accuracy class of magnetoelastic dynamometers is from 0.1 to 2%.

Rice. 62. Scheme of inclusion of a magnetoelastic dynamometer

Piezoelectric dynamometers are used to measure dynamic and quasi-static forces (unsuitable for static forces). Accuracy class 1%.

The action of a force can be converted into a change in pressure (hydraulic dynamometers). The hydraulic force measurement system includes a sensing device with a completely closed chamber and an indicating device. The force acting on the piston creates pressure. In principle, all pressure gauges (pressure gauges) can be used as an indicating instrument. Most often, mechanical devices are used. Rated forces from 200 N to 20 MN. Accuracy class 1 - 2%.

Dynamometer errors are due to the following reasons: non-linearity of the conversion characteristic, its reproducibility, hysteresis, temperature dependence of sensitivity and zero position, creep (elastic aftereffect).

Main parameters and dimensions general purpose dynamometers, spring with a scale and digital reading device, designed to measure static tensile forces, establishes GOST 13837 “General purpose dynamometers. Specifications".

Limits of measurement of dynamometers provided by the standard: the largest from 0.10 to 500 kN, the smallest - 0.1 from the largest limit.

GOST 13837-79 provides for the manufacture of dynamometers of accuracy classes 0.5, 1 and 2. The accuracy class is determined by the maximum permissible basic error of the dynamometer, presented as a reduced error. The normalizing value in this case is equal to the largest measurement limit.

The limits of the additional error of dynamometers caused by changes in ambient temperature in the operating temperature range different from the temperature of normal conditions are: no more than 0.5 of the main error for every 10 ° C - for dynamometers of the 1st class; no more than 0.25 of the basic error for every 10 ° C - for dynamometers of the 2nd class.

For calibration, verification and calibration of force transducers, force-measuring machines / installations are used, as well as measuring instruments, which include reference dynamometers and force-setting devices (presses). According to their functional purpose, the listed devices are referred to as measures of force.

Force-measuring machines / installations allow you to reproduce any force values ​​​​in the established range or a number of discrete values.

Depending on the constructive implementation, there are direct loading machines, force-multiplier installations (lever, hydraulic and wedge-shaped) and force division installations.

Direct loading is realized with the help of weights and the gravitational force of the Earth.

The creation of force-multiplier installations is due to the fact that at high values ​​of force, direct loading leads to an increase in errors and metal consumption, and high economic costs. However, even in force-multiplier installations, the value of the force is initially set with the help of weights, which then increases with the help of unequal levers ( up to 1MN), piston pairs of different effective areas ( up to 10 MN) or wedge effect (up to 5 MN?).

To reduce the force, the same design solutions can be used as for increasing it, but with a gear ratio less than 1. However, such a solution is not economically viable and has limited functionality. The most acceptable solution for dividing the force is a device with a change in the angle of inclination of the axis of a cylindrical mass suspended in an aerostatic suspension (Fig. 63).

Screw, lever, hydraulic, electromechanical, etc. are used as force-setting devices. presses. One of the main requirements for force-setting means is the constancy of the set value of force over time.

Mass measurement. When weighing, the gravitational force is compared with a known force created in the following ways:

By a load of known mass (classical method);

Spring tension/compression (spring balance)

Deformation of rigid elastic elements (deformations are measured by electrical methods (electromechanical scales);

Pneumatic or hydraulic device (measure air or liquid pressure);

Electrodynamically with the help of a solenoid winding in a constant magnetic field (the measured value is the current);

Immersion of the body in a liquid (the depth of immersion depends on the mass of the body).

In this connection distinguish scales mechanical (lever, spring, piston), electromechanical (with capacitive, strain-resistive, inductive and piezoelectric displacement or deformation transducers), optical-mechanical (with a mirror or interference pointing device), radioisotope (absorption and scattered radiation). The main applications are mechanical and electromechanical scales.

Requirements for scales for static weighing are established by GOST 29329 - 92.

Scales for static weighing are classified according to the following criteria.

By area of ​​application(operational purpose) scales are divided into: wagon; trolley; automotive; monorail; crane; commodity; for weighing livestock; for weighing people; elevator; for weighing milk; luggage; trading; medical; postage.

By weighing accuracy Accuracy scales are divided into 4 classes: Class 1 - scales of special accuracy; 2 class - high accuracy; Grade 3 - medium accuracy; Grade 4 - normal accuracy. Standard GOST 29329 - 92 applies to non-automatic scales of medium and conventional accuracy classes.

By installation method at the place of operation, the scales are divided into: built-in, mortise (mortise scales - mobile scales, the platform of which is on the same level with the floor of the room), floor, table, mobile, hanging, stationary.

Type of balancing device scales are distinguished: mechanical, electromechanical (electronic - the term "Electronic scales" is applicable to desktop scales).

Mechanical scales - scales in which the balancing of gravity is carried out using various mechanisms. There are weight scales, spring, hydraulic, pneumatic. Scales in which the transmission device is a lever or a system of levers are called lever scales.

Electromechanical scales - scales with a balancing device in the form of a transducer, in which gravity is converted into an electrical signal.

By type of load receiving device There are scales: bunker, monorail, bucket, conveyor, hook, platform.

According to the method of reaching the equilibrium position balances are distinguished: with automatic balancing, with semi-automatic balancing, with non-automatic balancing.

Depending on the type of reading device There are scales: with an analog reading device (dial and scale), with a discrete reading device (digital).

The GOST 29329-92 standard provides for the following main characteristics of scales.

Verification scale interval e- conditional value, expressed in units of mass and characterizing the accuracy of the scales.

Verification division price for accuracy class "medium" 0.1 g ≤ e≤ 2 g at the number of verification divisions n= 100…10000 and e≥5 g at n= 500…10000; for accuracy class "normal" e≥5 g at n= 100…1000. (n- the number of verification divisions, defined as the ratio the largest limit of weighing scales to the price of verification division).

Values ​​of the verification division value ( e), scale intervals ( d) and sampling discreteness ( d d) in units of mass is chosen from the range: 1×10 a; 2×10 a and 5×10 a, where a is a positive integer, a negative integer, or zero. The value of the calibration division of scales without an auxiliary reading device must correspond to the scale division value for scales with an analog reading device and the readout resolution for scales with digital indication.

The value of the division value or the resolution of the mass reading, as well as the value of the calibration division value are indicated on the scales or in the operational documentation for them.

largest(NIP) and smallest(NmPV) weighing limits- the largest and smallest values ​​of the mass, at which the compliance of the scales with the requirements of regulatory documents is ensured.

The maximum weighing limit (LEL) provided by GOST 29329-92 is from 200 g to 500 tons (the range of LEL values ​​does not correspond to the series of preferred numbers).

The smallest weighing limit - for the accuracy class, the average is taken equal to 20 e; for accuracy class ordinary - 10 e. Where e- the price of verification division.

Limits of error weights are normalized depending on the NmPV and the accuracy class and range from 0.5∙e to 1.5∙e during the initial verification at the enterprises: manufacturer and repair. During operation and after repair at the operating enterprise - from 1.0∙е to 2.5∙е. Limits of error zero setting devices-±0.25 e.

There are the following types balance scales for measuring mass: laboratory (analytical, quadrant, electronic, equal-arm), desktop dial, counting rocker, platform mobile (scale, dial, mail).

The principle of operation of a lever balance is to balance the moment created by the gravitational force from the measured mass, the moment of gravity of the weight or load.

The following transducer options are implemented in the balance scales:

With variable balancing mass: lever with scale and weights; lever with overhead weights;

With variable lever length: lever with movable weights; lever with roller weight;

Variable Angle: Quadrant; counterweight.

Requirements for the parameters of general-purpose lever scales are established by GOST 14004.

Depending on the maximum weighing limit, general purpose scales are divided into three groups: - desktop (up to 50 kg); - mobile and mortise (50 - 6000 kg); - stationary (wagon, automobile, elevator) (from 5000 to 200000 kg).

The smallest weighing limit is 20 d (d-scale division price) for desktop scales and 5% of P max for the rest.

Lever scales are used in conjunction with weights, which, depending on the purpose, are divided into general-purpose, reference and special-purpose weights. The last group includes reference weights (used to improve the reading accuracy of laboratory balances), conditional weights (designed to complete scales and other devices with a ratio of the arms of the lever system of 1:100), weights built into balances, and weights used in technological scales and dispensers.

Structurally, general purpose weights are made in the form of a wire, a polygonal plate (triangular, square or pentagonal), a cylinder with a head, a parallelepiped. The nominal value of the mass of the weight is taken from a range of values ​​1·10 n , 2·10 n , 5·10 n (n is any positive or negative integer). Standard GOST 7328 - 2001 “Weights. General Specifications" provides for the release of weights weighing from 1 mg to 5000 kg. Depending on the manufacturing tolerance, weights are assigned accuracy classes: E 1, E 2, F 1, F 2, M 1, M 2, M 3 (in descending order of accuracy). Weights can be supplied in the form of sets, the composition of which is formed in accordance with the recommendations of GOST 7328 - 2001.

An example of a symbol in the documentation of a 500 g weight of accuracy class F 1: Weight 500 g F 1 GOST 7328-2001. Weights set: Set (1 mg - 1 kg) E 2 GOST 7328 - 2001.

In spring balances, the sensitive element is a spring (compression, tension, spiral, etc.), the deformation of which is proportional to the force of gravity. The strain value is measured directly or subjected to an additional transformation.

In electronic scales, two main types of sensors are used as a primary converter: piezoquartz and strain-resistive.

Scales form a separate group for weighing vehicles in motion . General technical requirements for them are given in GOST 30414-96.

The standard applies to scales designed for weighing in motion or for static weighing and weighing in motion of the following vehicles: railway cars (including tanks), trolleys, their trains, cars, trailers, semi-trailers (including tanks), road trains.

Table 7. Mechanical balances

Depending on the design of the load receiving device, it can determine the load immediately from the entire car (trolley, car, trailer, semi-trailer) or autonomously - simultaneously or in turn - from each bogie, wheel pair (axle) or from each wheel.

Depending on the normalized values ​​of metrological characteristics, the scales are divided into four accuracy classes: 0.2; 0.5; one; 2. The designation of the accuracy class corresponds to the error allowed during operation. At the same time, in the range from LmLL to 35% LEL inclusive, this is the reduced error, the normalizing value for which is 35% LEL. In the range above 35% LEL to LEL, the accuracy class determines the relative measurement error.

During the initial verification or calibration, the permissible errors are reduced by 2 times.

Flow measurement

The flow rate is the amount of substance flowing through a given section of the pipeline per unit of time. Distinguish between volume and mass costs. Flow measuring instruments are called flow meters. The variety of flowmeters is determined not only by constructive solutions, but also by the operating principles that are implemented in them. Consider the most used options.

Volume counters. The principle of operation of volumetric counters is based on the direct measurement of the volumes of the measured medium using measuring chambers of a known volume and counting the number of portions that have passed through the counter. The most common volumetric counter of liquid substances is a counter with oval gears (Fig. 64) Oval gears 1 and 2, placed in housing 3, rotate due to the pressure difference P 1 and P 2. For one revolution of the gears, the measuring cavities, the volume of which is precisely known V 1 and V 2 , are filled twice and emptied twice. The axis of one of the gears rotates the counting mechanism located outside the housing 3. Counter characterized high measurement accuracy (error 0.5 ... 1%), low pressure loss, independence of indications from viscosity, significant torque. The disadvantage of these meters is the need for good filtering of the measured medium, as well as a high level of acoustic noise.

Rice. 64. Diagram of a counter with oval gears

To measure gas flows, rotary gas meters are used, the principle of operation of which is similar to that of meters with oval gears. They are used to measure flows from 40 to 40,000 m/h and have accuracy classes 2 and 3.

Volume meters for measuring liquid flow include paddle counters, characterized by an upper measurement limit of 100 ... 300 m/h and accuracy classes of 0.25 and 0.5.

Speed ​​counters allow you to set the flow rate according to the dependence of the speed of rotation of the axial or tangential impeller on the volumetric flow rate. If a tachogenerator and a voltmeter are connected in series to the impeller (Fig. 65), then the flow rate can be judged from the reading of the voltmeter. And you can connect a rev counter and measure the consumption for a certain period of time. Instrument accuracy classes 1; 1.5; 2 at flow rates 3…1300 m/h.

Figure 65 also shows a high-speed meter with a tangential turbine 1. (The number 2 indicates a filter.) Such meters are used at a flow rate of up to 3 ... 20 m3 / h and have an accuracy class of 2 and 3.

Throttle flowmeters. One of the most common principles for measuring the flow of liquids, gas and steam is the principle of variable pressure drop across the orifice.

The advantages of this method are: simplicity and reliability, no moving parts, low cost, the ability to measure almost any flow rate, the possibility of obtaining the calibration characteristics of flowmeters by calculation.

Rice. 65. Scheme of a high-speed counter with axial and tangential impellers.

1 - jet straightener, 2 - transmission mechanism, 3 - counting device, 4 - chamber, 5 - worm pair, 6 - impeller.

In accordance with the above principle, a narrowing device is installed in the pipeline. The flow velocity through the orifice of the orifice is higher than before it, as a result of which a pressure drop is created on the orifice, measured by a differential pressure gauge. The reading of the differential pressure gauge depends on the flow velocity in the restriction or on the flow rate. Schemes of standard narrowing devices and the connection points of the branches of the differential pressure gauge are shown in Figure 66.

Rice. 66 Schemes of narrowing devices: a) diaphragm, b) standard nozzle, c) Venturi nozzle, d) Venturi tube

Flow meters around (rotameters). In these flowmeters, the streamlined body (float, piston, valve, rotating plate, ball, etc., examples in Figures 67 and 68) perceives a force effect from the oncoming flow, which increases with increasing flow velocity and moves the streamlined body. The weight of the streamlined body or the force of the spring serves as the counteracting force. Flowmeters are designed in such a way that the movement of the streamlined body is accompanied by a change in the flow area for the passage of liquid or gas. In this case, an increase in the flow rate leads to an increase in the flow area. As a result, the flow rate decreases. Such negative feedback leads to stabilization of the position of the streamlined body. The output signal of the considered flow transducers is the displacement of the streamlined body.

Rice. 67. Schemes of converting elements of flow meters a) float, b) valve, c) piston

Rice. 68. Schemes of flow meters around: a), b) - float type; c), d) - valve type; e) - piston type.

Designations in the figures.

Figure a: 1 - glass conical tube, 2 - float, 3 - float stop, 4 - scale.

Figure b: 1 - a cylindrical float with a hole in the middle, 2 - a fixed rod of a conical section, 3 - a glass cylindrical tube.

Figure c: 1 - valve, 2 - annular diaphragm, 3 - metal case, 4 - stem, 5 - core of the differential transducer element 7, 6 - non-magnetic steel tube.

Figure d: 1 - air throttle, 2 - pneumatic nozzle, 3 - magnet, 4 - tube made of non-magnetic material, 5 - core, 6 - valve, 7 - bellows.

Figure e: 1 - weights, 2 - piston, 3 - core, 4 - induction coil, 5 - channel for supplying output pressure to the over-piston space, 6 - rectangular outlet from the under-piston space.

Rotameters with an output pneumatic signal of 0.02 ..0.1 MPa produce accuracy classes 1.5 and 2.5.

In addition to the listed types, variable-level flow meters, electromagnetic, thermal (calorimetric) and other flow meters are used for flow measurements.

Literature

1.Rannev G.G., Tarasenko A.P. Methods and means of measurement. - 2004.

2. Brindley K. Measuring converters. Reference manual. - 1991.

3. Kozlov M.G. Metrology and standardization. Study guide. - 2004.

4. Bolton. Metrology engineer's pocket guide. - 2002.

5. Hart Z. Introduction to measuring technology. - 1998.

6. Dimov Yu.V. Metrology, standardization and certification. Textbook. - 2010.

1.Methods and means of measuring electrical quantities…………………………..1

1.1.Measures of electrical quantities………………………………………………..1

1.2.Electrical measuring instruments…………………………………………….4

1.3 Oscilloscopes. Digital Instruments………………………………………..10

1.4.Analog measuring transducers……………………………..14

1.5.Measurement of electrical quantities……………………………………………17

2.Measurements of magnetic quantities…………………………………………………….....25

3.Measurement of non-electric quantities………………………………………………...28

3.1.Measuring transducers………………………………………… ...28

3.2. Measurements of lengths and angles……………………………………………………..35

3.3.Temperature measurement……………………………………………………..39

3.4.Pressure measurement…………………………………………………….…46

3.5. Measurement of force and mass………………………………………………………..50

3.6. Flow measurement………………………………………………………… .55

by means of an accelerometer; measuring the amplitude and frequency of vibrations

2. Comparison of an unknown force with gravity P \u003d mg: direct loading with exemplary weights;

by means of hydraulic transmission and exemplary weights;

by means of levers and exemplary weights;

by means of levers and a pendulum

3. Measurement of elastic deformation

body interacting with the unknown

known force F= with |; by strain gauges; by means of displacement sensors 4. Comparison of the unknown force with the force of the interaction of the current with the magnetic field F= / In I sin a by means of an electrodynamic exciter. The measurement of a variable harmonic force by determining the amplitude and frequency of vibrations of a body with a known mass can be carried out with high accuracy. Mass can be measured with an error not exceeding a few thousandths of a percent. The frequency of oscillations can also be measured with the same accuracy. The oscillation amplitude of a body with a known mass can be measured with an error not exceeding a few tenths of a percent, which, in essence, will determine the error in measuring the force by this method.

The method of measuring force by comparing an unknown force with gravity is used

are used for precise measurements and reproduction of static and quasi-static forces.

The method of direct loading is used to create State primary standards of the unit of force, reproducing it with the highest accuracy.

The method of comparing an unknown force with gravity by means of levers and reference weights is used to create exemplary means of the second category for measuring force, ensuring its measurement with an error not exceeding 0.2% of the measured value, as well as in force meters of testing machines that provide force measurement with an error , not exceeding 1% of the measured force in the range of 0.04 - 1 from the upper limit of the force meter.

The method of comparing an unknown force with gravity by means of hydraulic transmission and exemplary weights is also used in exemplary means of the second category for measuring force and in force meters of testing machines. For is-

Friction switches in hydraulic transmission use a piston-cylinder pair, in which one of the elements rotates relative to the other.

The method of comparing an unknown force with gravity by means of levers and a pendulum is used in force meters of testing machines.

All means of force measurement based on methods of comparing an unknown force with gravity are usually fixed installations. The process of comparison of forces in these installations is mechanized.

Measuring force by measuring the elastic deformation of a body interacting with an unknown force is the most common method used in both stationary and portable means to measure static and time-varying forces. This method is used in exemplary dynamometers of the first category, which ensure the transfer of a unit of force from the State standard to exemplary means of the second category with an error not exceeding 0.1% of the measured force. In addition, this method is used in working tools for measuring static and time-varying forces.

The method makes it possible to create stationary and portable means of measuring tensile and compressive forces - dynamometers, which contain an elastic element equipped with grips or supports for its inclusion in the power circuit. In the elastic element, a reaction force arises that opposes the measured force. The elastic element can be electrically inactive or electrically active, i.e. it is also a sensitive element.

The elastic electrically inactive element performs purely mechanical functions. The resulting deformation of the elastic element is perceived by a sensitive element, which can be either a strain sensor or

a displacement sensor that converts it into an output value.

An elastic, electrically active element reacts to the field of mechanical stresses or deformations created by the measured force by changing its electrical or magnetic characteristics. Elastic, electrically active elements include, for example, piezoelectric and magnetoanisotropic.

To achieve optimal metrological performance of a dynamometer, several principles must be observed.

The principle of structural integrity. The measured force must be transmitted in a dynamometer through a continuous medium of one material. Violation of the continuity of the design of the elastic element is the cause of friction between the mating elements. Associated with this friction are force measurement errors that can be significant.

The principle of integration. The dynamometer is more accurate, the better the sensitive element is distributed over the cross section of the elastic element. For this purpose, averaging is used - integration of stress or deformation of an elastic element, which can be characterized either as imaginary or as real.

With imaginary integration, the entire stress or strain field, and hence the measured force, is judged by the state at one point of this field. In this case, it is assumed that inside the limited area of ​​the elastic element there is a certain mechanical field, which does not depend on the point of application of the force. This makes it possible to use one sensing element. Structural solutions that provide imaginary integration are the removal of the force-receiving parts of the elastic element from the location of the sensitive element, limiting the area of ​​​​possible points of force application.