RU2077700C1 - Method for object position detection using foucault currents - Google Patents

Abstract

FIELD: instruments which measure movements and vibration of current conducting objects. SUBSTANCE: method involves supply of Foucault current detector by current of specific shape. Instantaneous values of current follow continuous periodic function which derivative with respect to time for specific function time slot jumps to zero and keeps this value during some time interval. Object movement is judged by average electromotive force which is inducted in detector coil by Foucault current field after jump of derivative when self-induction electromotive force in detector coil is equal to zero. EFFECT: increased precision, increased dynamic range, possibility to apply method for materials with different magnetic characteristics. 2 cl, 2 dwg

Description

 The method can be used in the field of measuring equipment, in particular for measuring distances, displacements, vibration displacements.

 There is a known eddy current method for determining the positions in which the sensor coil, fed by a high-frequency current, creates an alternating magnetic field in the surrounding space [1] In a metal object placed in this space, eddy currents arise, which tend to compensate for the cause that caused them, creating an opposite induction by the sign of induction of the coil, which leads to a decrease in its self-induction coefficient, and due to losses in the eddy current zone, the equivalent active resistance of the coil increases. By changing the inductance or resistance of the sensor coil, the position of the object relative to the sensor is determined.

 The disadvantages of the known method and devices that implement it are: significant non-linearity of the operating characteristic, low measurement accuracy, small dynamic measurement range, lack of universality of application for materials with different magnetic properties.

 The aim of the invention is to improve the linearity of the characteristics, increase accuracy and expand the dynamic range, as well as providing the possibility of use regardless of the magnetic properties of the material of the object.

 This goal is achieved by the fact that in the proposed method, the eddy current sensor coil is supplied with a current of complex shape, the values of which are determined by a continuous function, and the derivative of the current function in time at a certain point in time jumps from a constant value to zero (for example, a current in the form of a trapezoidal pulse). The position of the object is determined by the average value of the EMF induced in the sensor coil by the eddy current field for a fixed period of time after the instantaneous change in the derivative of the magnetic flux of the sensor coil when the self-induction EMF in this coil is zero.

 The position determination by the proposed method for the case when the current supplying the sensor coil has the shape of a trapezoidal pulse is as follows.

In fig. 1 shows the equivalent circuit of the sensor-object system, in which the metal object is represented by the circuit L ₂ -r ₂ with lumped parameters: L ₂ inductance of the eddy current circuit, r ₂ its active resistance; L ₁ , r ₁ - inductance and resistance of the sensor coil; M is the coefficient of mutual induction.

Временная диаграмма токов и напряжений в схеме рис.1 дана на рис.2. Катушка датчика питается от генератора тока током i₁ в форме трапецеидального импульса, причем: t₂-t₁= t₃-t₂= t₄-t₃= Δt..The circuit of Fig. 1 is described by the following equations:

The time diagram of currents and voltages in the circuit of Fig. 1 is given in Fig. 2. The sensor coil is powered by a current generator with current i ₁ in the form of a trapezoidal pulse, and: t ₂ -t ₁ = t ₃ -t ₂ = t ₄ -t ₃ = Δt ..

In the area (1-2) from t ₁ to t ₂ :
i ₁ A • t (3)
where A const.

Из (4) находим i₂:

где δ₂= r₂/L₂ величина, обратная постоянной времени контура вихревых токов.For the eddy current circuit in this section:

From (4) we find i ₂ :

where δ ₂ = r ₂ / L ₂ is the reciprocal of the time constant of the eddy current circuit.

ЭДС, наводимая в катушке датчика на участке (1-2):

На участке (2-3) i₁ const

Решая (8) с учетом начальных условий (6), найдем i₂ (2-3)

ЭДС, наводимая в катушке датчика полем вихревых токов на участке (2-3).Choosing Δt> 3L ₂ / r ₂ at time t ₂

EMF induced in the sensor coil in the area (1-2):

In the area (2-3) i ₁ const

Solving (8) taking into account the initial conditions (6), we find i ₂ (2-3)

EMF induced in the sensor coil by the eddy current field in the area (2-3).

Среднее значение ЭДС в катушке датчика на участке (2-3):

Разлагаем

в ряд, принимая во внимание первые члены ряда:

Подставляя (12) в (11), получим для среднего значения E_1cp(2-3):

Индуктивность и активное сопротивление контура вихревых токов зависят от расстояния h между источником поля и объектом [1]

где σ проводимость материала объекта;
d- эквивалентная глубина проникновения тока в объект;
μ- магнитная проницаемость;
В константа.

The average value of the EMF in the sensor coil in the area (2-3):

We decompose

in a row, taking into account the first members of the series:

Substituting (12) in (11), we obtain for the average value of E _{1cp (2-3)} :

Inductance and active resistance of the eddy current circuit depend on the distance h between the field source and the object [1]

where σ is the conductivity of the material of the object;
d is the equivalent depth of current penetration into the object;
μ - magnetic permeability;
In constant.

Подставляя (14) в (13), и учитывая, что коэффициент связи между катушкой и контуром вихревых токов

получаем среднее значение E_1cp(2-3) за интервал t₂-t₃:

Таким образом, формула (15), по которой определяется положение объекта через измерение E_1cp(2-3), содержит линеаризирующий сомножитель

,
возрастающий с увеличением расстояния между датчиком и объектом. Для измерения E_1cp(2-3) не требуется применять дифференциальных схем, так как на участке (2-3) из-за постоянства тока i_1(2-3) const ЭДС самоиндукции катушки датчика

что повышает точность измерения и расширяет динамический диапазон. Отсутствие на интервале измерения t₂-t₃ в катушке датчика переменной составляющей питающего тока, подверженной изменению при внесении в магнитное поле катушки материалов с различными магнитными свойствами, упрощает электромагнитное взаимодействие катушки датчика с помещенным в ее поле объектом и делает способ универсальным для работы с объектами из токопроводящих материалов с различными магнитными свойствами.Therefore:

Substituting (14) into (13), and taking into account that the coupling coefficient between the coil and the eddy current circuit

we get the average value of E _{1cp (2-3)} for the interval t ₂ -t ₃ :

Thus, formula (15), which determines the position of an object through the measurement of E _{1cp (2-3)} , contains a linearizing factor

,
increasing with increasing distance between the sensor and the object. To measure E _{1cp (2-3),} it is not necessary to use differential circuits, since in the area (2-3) due to the constancy of the current i _{1 (2-3)} const the emf of the self-induction of the sensor coil

which increases the accuracy of measurements and extends the dynamic range. The absence on the measurement interval t ₂ -t ₃ in the sensor coil of a variable component of the supply current, subject to change when materials with different magnetic properties are introduced into the magnetic field of the coil, simplifies the electromagnetic interaction of the sensor coil with an object placed in its field and makes the method universal for working with objects from conductive materials with various magnetic properties.

The method is discrete, because at the end of the measurement cycle (time t ₃ ) we obtain the value of E _{1cp (2-3)} , which is uniquely related to the position of the object placed in the magnetic field of the sensor coil.

Claims (1)

1 1. The method of eddy current determination of the position of the object, which consists in the location of the conductive object in the variable magnetic field of the sensor coil and the assessment of the position of the conductive object relative to the sensor according to changes in the parameters associated with the coil, which are caused by the action of the eddy current field in the object, characterized in that the variable the magnetic field of the sensor coil is formed when a supply current is supplied as a continuous function of time with abrupt changes in the derivative of a given value d zero at certain points in time, and the position of the conductive object is determined by the average value of the EMF induced in the sensor coil by the eddy current field of the object, for a fixed time interval after the instantaneous change in the derivative of the magnetic flux of the coil, when the self-induction EMF in this coil is zero at a constant supply voltage coil current 2 2. The method according to claim 1, characterized in that the supply current is formed in the form of a sequence of trapezoidal pulses.

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