ISO/DIS 15548-1:2025(en)

ISO/TC 135/SC4/WG 1

Secretariat: AFNOR

Date: 2025-04-05

Non-destructive testing — Equipment for eddy current examination — Part 1: Instrument characteristics and verification

© ISO 2025

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Contents

Foreword v

Introduction Erreur ! Signet non défini.

1 Scope 1

2 Normative references 1

3 Terms and definitions 1

4 Eddy current instrument characteristics 1

4.1 General characteristics 1

4.1.1 Type of instrument 1

4.1.2 Power supply 2

4.1.3 Safety 2

4.1.4 Technology 2

4.1.5 Physical presentation 2

4.1.6 Environmental effects 2

4.2 Electrical characteristics 2

4.2.1 General 2

4.2.2 Functional block diagram 3

4.2.3 Generator unit 3

4.2.4 Input stage characteristics 4

4.2.5 Balance 4

4.2.6 HF signal and demodulation 5

4.2.7 Demodulated signal processing 5

4.2.8 Signal output 6

4.2.9 Digital interface 7

4.2.10 Digitization and data resolution 7

5 Verification 8

5.1 General 8

5.2 Levels of verification 8

5.3 Verification procedure 9

5.4 Corrective actions 9

6 Measurement of electrical characteristics of instrument 10

6.1 Measuring requirements 10

6.2 Generator unit 10

6.2.1 Excitation frequency 10

6.2.2 Harmonic distortion 11

6.2.3 Harmonic distortion 12

6.2.4 Maximum output voltage 12

6.2.5 Maximum output current 13

6.2.6 Output 13

6.3 Input stage characteristics 13

6.3.1 Maximum allowable input voltage related to saturation and non-linearity 13

6.3.2 Input impedance 15

6.4 Balance 16

6.4.1 Excitation frequency 16

6.4.2 Residual output value at balance 16

6.5 Demodulation 16

6.5.1 Orthogonality of signal components 16

6.6 Demodulated signal processing 18

6.6.1 Gain accuracy and linearity 18

6.6.2 Phase-setting accuracy 19

6.6.3 Bandwidth 20

6.6.4 Cross-talk 23

6.6.5 Common-mode rejection 23

6.6.6 Maximum instruments noise 24

Annex A (informative) Principle of frequency beat method 26

Annex B (informative) Method of measurement of linearity range between output and input 27

Annex C (informative) Table of characteristics 28

Bibliography 30

Foreword

ISO (the International Organization for Standardization) is a worldwide federation of national standards bodies (ISO member bodies). The work of preparing International Standards is normally carried out through ISO technical committees. Each member body interested in a subject for which a technical committee has been established has the right to be represented on that committee. International organizations, governmental and non-governmental, in liaison with ISO, also take part in the work. ISO collaborates closely with the International Electrotechnical Commission (IEC) on all matters of electrotechnical standardization.

The procedures used to develop this document and those intended for its further maintenance are described in the ISO/IEC Directives, Part 1. In particular, the different approval criteria needed for the different types of ISO documents should be noted. This document was drafted in accordance with the editorial rules of the ISO/IEC Directives, Part 2 (see www.iso.org/directives).

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This document was prepared by Technical Committee ISO/TC 135 Non-destructive testing, Subcommittee SC 4, Eddy current testing.

This second edition cancels and replaces the first edition (ISO 15548-1:2013), which has been technically revised.

The main changes are as follows:

Clarification on scope: "If necessary, this standard may be supplemented by an application document specifying acceptance criteria for the characteristics of the eddy current instrument."

— Terms and definitions: References to ISO 18173 and ISO GUIDE 99 apply in addition to ISO 12718.

— Section 4.1.4: Clarification that "The instrument can be fully analog, mainly digital, or a combination of both analog and digital."

— Section 4.2.2: Clarification that "Each part of the eddy current instrument may be either analog or digital."

— Section 4.2.6: Complete revision of this section.

— Section 4.2.7: Addition of a subsection on crosstalk (4.2.7.4) and instrument noise (4.2.7.5).

— Section 4.2.9: Addition of a subsection on digital interface.

— Section 4.2.10: Revision of the digitization section.

— Section 6.1: Review and revision of this section.

— Section 6.2.1.1: Modification of the formula.

— Section 6.2.2.1: Modification of the formula.

— Section 6.2.3.1: Modification of Figure 2.

— Section 6.2.3.2: Modification of the formula.

— Section 6.2.6.1: Modification of the formula.

— Section 6.3.2.2: Complete revision of this section.

— Section 6.4: Full restructuring, including the addition of a subsection on balance, compared to the previous version.

— Section 6.5: Addition of a new section on demodulation.

— Section 6.6: Addition of a new section on demodulated signal processing.

— Annex C: Renamed from "Alternative measurement of the input impedance" to "Table of characteristics".

A list of all parts in the ISO ##### series can be found on the ISO website.

Any feedback or questions on this document should be directed to the user’s national standards body. A complete listing of these bodies can be found at www.iso.org/members.html.

Non-destructive testing — Equipment for eddy current examination — Part 1: Instrument characteristics and verification

ISO 15548-1 specifies the characteristics of general-purpose eddy current instruments and provides methods for their measurement and verification.

The evaluation of these characteristics permits a well-defined description and comparability of eddy current instruments.

By careful choice of the characteristics, a consistent and effective eddy current examination system can be designed for a specific application.

If necessary, this standard may be completed by an application document specifying acceptance criteria for the characteristics of the eddy current instrument.

Where accessories are used, these are characterized using the principles of this part of ISO 15548-1 (e.d. additional external amplifiers).

The following documents are referred to in the text in such a way that some or all of their content constitutes requirements of this document. For dated references, only the edition cited applies. For undated references, the latest edition of the referenced document (including any amendments) applies.

ISO 9712, Non-destructive testing — Qualification and certification of NDT personnel

ISO 12718, Non-destructive testing — Eddy current testing — Vocabulary

ISO 15549, Non-destructive testing — Eddy current testing — General principles

ISO 18173, Non-destructive testing — General terms and definitions

ISO GUIDE 99, International vocabulary of metrology — Basic and general concepts and associated terms (VIM)

ISO #####‑##:20##, General title — Part ##: Title of part

For the purposes of this document, the terms and definitions given in ISO 12718, ISO 18173 and ISO GUIDE 99 apply.

a) An instrument has a general-purpose application (e.g. crack detection) when the relationship between the measured quantity and the output of the instrument is established by the user. A range of probes can be connected to the instrument. The instrument may have a display that should be configurable by the user. The instrument manufacturer shall provide a list of adjustable parameters., in order that the user can design the examination system. The examination system shall be in accordance with ISO 15549. The user shall be able to vary the excitation frequency, gain, balance, phase and filters (unless an automatically setting is used).

b) An instrument is of specific application (such as coating thickness measurement, magnetic permeability, or electrical conductivity measurement) when the relationship between the measured quantity and the output is explicitly defined in the range of application. The probe is specific to the instrument. For this type of instrument, ISO 15548 may be partially applied.

The instrument can be powered by internal batteries or by an external AC or DC power supply. The nominal values of voltage, frequency and power consumption shall be stated, together with the tolerance for correct operation.

The instrument and its accessories shall meet the applicable safety regulations, for example, electrical hazard, surface temperature, explosion, etc.

The instrument can be completely analogue or mainly digital or partly digital and analogue.

The excitation can be single frequency, multi-frequency, swept frequency or pulsed.

The instrument can be single or multichannel.

The instrument settings can be manual, remote controlled, stored or preset.

The instrument shall provide the eddy current signal at an analogue or digital interface.

The instrument can be with or without a built-in display.

The instrument can be portable, cased or rack mounted, with the component parts integrated or modular.

The weight and size shall be specified for the instrument and its accessories.

The plugs and sockets shall be specified regarding type and pin interconnections.

The instrument manufacturer, manufacturer’s address, model number, serial number, year of manufacturing, relevant technical data (power requirements, IP class), used standards (if any) and markings (e. g. CE) shall be clearly readable and located in a readily accessible place.

The warm-up time necessary for the instrument to reach stable operating conditions within specified limits shall be stated.

The temperature, humidity and vibration ranges for normal use, storage and transport shall be specified for the instrument and its accessories.

The instrument shall conform to relevant electromagnetic compatibility (EMC) regulations.

The electrical characteristics of an instrument shall be evaluated after the warm-up time has elapsed.

The electrical characteristics are only valid for the stated operating conditions.

The electrical characteristics apply to various items of the functional block diagram of the instrument. Where applicable, they are provided by the manufacturer. Some of these characteristics can be verified according to the methodology described in Clause 6.

The functional block diagram of a typical general-purpose eddy current instrument is shown in Figure 1.

Each part of the eddy current instrument may be analogue or digital.

Figure 1 — Functional block diagram of eddy current instrument

The source of excitation is the generator unit.

The characteristics to be defined are as follows;

— type of generator: current or voltage;

— wave shape of the excitation signal;

— type of excitation: single or multi-frequency;

— frequency setting: range, step size, deviation from nominal value;

— differential source resistance;

— maximum output voltage and current;

— amplitude setting, if available: range, step size, deviation from nominal value.

In the case of sinusoidal alternating excitation, the additional characteristic to be defined is:

— harmonic distortion

In the case of non-sinusoidal alternating excitation (triangular, rectangular, etc.), additional characteristics to be defined are:

— duty cycle;

— rise and fall time;

— linearity;

— overshoot.

In the case of multi-frequency excitation, it shall be stated whether frequencies are injected simultaneously or multiplexed, independent or related, and the multiplexing sequence shall be specified, when relevant.

The input stage interfaces the probe to the instrument. It provides impedance matching and amplification, as required.

The characteristics to be defined are as follows:

— the maximum allowable input voltage related to saturation and non-linearity;

— input impedance;

— input configuration (single ended, differential);

— number of inputs (parallel, multiplexed)

In the case of multi-channel instruments, additional characteristics to be defined is:

— cross-talk.

Balance is the compensation of an offset of the signal to achieve a predetermined operating point. The compensation may be performed manually or automatically. When available the compensation, shall include both the imbalance of the sensor and provide sufficient residual dynamic for the acquisition of the desired signals.

Conversely, the instrument with a maximum dynamic range less than 90 dB should be balanced accordingly through the following characteristics:

— residual value at balance (expressed as a percentage of a specified range, e.g. full-scale output).

— maximum compensable input voltage

High Frequency (HF) input filter

Filters reduce the signal frequency content which can have an undesirable effect on the test result.

When applicable, the filters used before demodulation are referred to as carrier frequency filters (HF filters). These are usually band-pass filters which suppress any signal frequencies which do not correspond to the excitation frequency.

The characteristics to be defined are as follows:

— Type of filter;

— Bandwidth at -3 dB;

— Attenuation rate;

HF amplification

The characteristics to be defined are as follows:

— gain-setting range;

— step-size.

Demodulation

Demodulation shall be a synchronous demodulation that extracts the low-frequency amplitude and phase variations from the HF signal.

For positive polarity of demodulation, a delay in the signal will cause the signal vector to rotate clockwise. The polarity of demodulation shall be positive and shall be confirmed.

The characteristic to be defined is:

— orthogonality of signal components (X and Y).

Vector amplification

Vector amplification generally consists of two transmission channels of identical design. These channels amplify the vector components produced by synchronous demodulation. In some instruments, these components can be amplified with different gains.

The characteristics to be defined are as follows:

— common gain setting range, step size, deviation from nominal value for both vector components;

— individual gain setting range, step size, deviation from nominal value for both vector components.

Phase setting

Phase setting permits rotation of the demodulated signal vector on the complex plane.
If a phase setting is available for the instrument.

The characteristics to be defined are as follows:

— phase rotation setting range, step size, deviation from nominal value;

— amplitude variation of the signal vector with phase setting.

Low Frequency (LF) filtering

The filters used after demodulation are referred to as low-frequency filters (LF filters). The bandwidth of the filter is chosen to suit the application, e.g. wobble, surface speed, etc.

The characteristics to be defined for each filter are as follows:

— cut-off frequency setting at 3 dB attenuation: range, step size, deviation from nominal value;

— rate of attenuation;

— ripple, if present (e.g Chebyshev filter).

LF filters may have a variable cut-off frequency synchronized with the testing speed by an external encoder. In this case the additional characteristics to be defined are as follows:

— type of the encoder signal;

— frequency range of encoder signal;

— relation between cut-off frequency of the filter and frequency of the encoder signal.

NOTE: Devices displaying spatial dimension filters can also be stated in spatial frequency

Crosstalk

Crosstalk is related to multi-channel instruments only. It is the variation of the output of a channel in relation to the variation of the input of another channel.

The characteristics to be defined are as follows:

— variation of the output of a channel versus input variation of any other channel.

Instrument noise

Instrument noise is the stochastic variation of the output at constant input. The maximum noise occurs usually at maximum amplification and is influenced by the filter settings.

The characteristic to be defined is:

— maximum peak-to-peak amplitude of the output at constant input.

The type of output can be a display, a hard-copy device, analogue outputs or digital interface.

The type of presentation can be, for example, complex plane, strip chart, imaging or threshold signal.

The characteristics of a display shall include at least the following:

— type of presentation;

— size and resolution (number of pixels) for digital displays;

— grid divisions if present;

— full-scale-display voltage range or time range;

— linearity;

— bandwidth for analogue display or sampling rate for digital displays.

If the analogue output is generated by a digital to analogue converter (DAC), additional characteristics shall include at least the following:

— sampling rate per output;

— D/A resolution: number of bits and voltage per digit;

If a threshold output is available, it should be characterized by:

— Type (x-,y-amplitude, box, circle…);

— Adjustment range;

— Hysteresis (if available);

— D/A resolution: number of bits and voltage per digit;

The characteristics of digital interfaces shall include at least the type of the interface (e. g. USB, LAN, RS232, CAN, IEEE, …) and could also provide following:

— data protocol and format;

— serial or parallel;

— voltage and current levels;

— data rate and format;

— sampling rate;

— analogue/digital (A/D) resolution, range and linearity.

The characteristics of logical inputs and outputs shall include at least the following:

— functionality (e.g trigger input, encoder input, gate output);

— voltage and current levels;

— setting delay;

— hysteresis;

— active high or low;

— galvanic isolation, if present;

— external power, if required (e.g if galvanic isolated);

General

Whenever a digitization is performed, the following characteristics shall be defined as a minimum:

— Location of the digitization stage in the signal chain (before or after demodulation);

— A/D resolution;

— Sampling rate (total sampling rate and sampling rate per channel for multichannel instruments);

The definition of the digitization technique and the triggering are optional.

Location of the digitization stage in the signal chain

Digitization may be performed at the input stage, at demodulation or at signal processing of the X and Y signal components after demodulation.

Triggering on digitization

Digitization can be triggered by using an internal clock (fix rate or synchronized to the test frequency) or an external encoder, depending on the digitization stage

Digitization technique

Digitization can be performed by direct conversion, successive approximation or similar techniques.

A/D resolution

In this context A/D resolution is defined as number of digitization bits. The input voltage corresponding to one bit can be calculated by dividing the input voltage range by 2N-1, where N is the number of digitization bits.

Sampling rate

Number of conversions per second of the A/D converter.

Data rate and resolution at the output

The data rate of the signal data at a digital output of the instrument may differ to the sampling rate of the A/D converter. It shall be specified at which speed (samples per second) and in which resolution (number of significant bits per sample) this data is provided for each vector component.

If the instrument has parallel or time multiplexed channels, the information shall be provided per multiplexed input.

For a consistent and effective eddy current examination, it is necessary to verify that the performance of the eddy current test instrument is maintained within acceptable limits.

The physical condition of reference blocks used for verification shall be within acceptable limits.

The end-user shall be informed on initial results (before any corrective actions).

The list of characteristics is available in Annex C.

For a better understanding, the verification procedure is described identically in all three parts of ISO 15548.

There are three levels of verification. Each level defines the time intervals between verification and the complexity of the verification (see Annex C).

It is understood that initial type testing has already been carried out by the manufacturer or under his control.

a) Level 1: Global functional check

A verification is performed at regular intervals of time on the eddy current test system, using reference blocks to verify that the performance is within specified limits.

The verification is usually performed by the user during standard usage.

The time interval and the reference blocks are defined in the verification procedure.

b) Level 2: Detailed functional check

A verification on an extended time scale is performed to ensure the stability of selected characteristics of the eddy current instrument, probe, accessories and reference blocks.

c) Level 3: Characterization

A verification is performed on the eddy current instrument, probe accessories and reference blocks to ensure conformity with the characteristics supplied by the manufacturer.

The organization requiring the verification shall specify the characteristics to be verified, in accordance with ANNEX C, as a minimum

In case of hardware repair of the instrument, a characterization (Level 2 detailed function check) is required.

In case of upgrade (hardware and/or firmware impacting the parameters verified under the current standard) of the instrument, a characterization (Level 3 verification) is required.

In case of adjustment and calibration, the end-user shall be informed on the detailed results. Then, a valid detailed functional check is required.

The main features of verification are shown in Table 1.

Table 1 — Verification Levels (see ANNEX C for the list of characteristics)

The characteristics to be verified are dependent on the application. The essential characteristics and the level of verification shall be specified in a verification procedure.

The examination procedure for the application shall refer to the verification procedure. This can restrict the number of characteristics to be verified for a defined application.

Sufficient data on the characteristics featured in an instrument, probe and reference block shall be provided, in order that verification can be performed within the scope of this part of ISO 15548.

Level 1: When the performance is not within the specified limits, a decision shall be made concerning the components examined since the previous successful verification. Corrective actions shall be made to bring the performance within the acceptable limits.

Level 2: When the deviation of the characteristic is greater than the acceptable limits specified by the manufacturer, a decision shall be made concerning the instrument, the probe or the accessory being verified.

Level 3: When the characteristic is out of the acceptable range specified by the manufacturer, a decision shall be made concerning the instrument, the probe or the accessory being verified.

All measurements described in the following subclauses are made at the inputs and outputs of the instrument. These measurements do not require opening the instrument (black-box concept).

Keeping the black-box concept, any alternative method, the equivalence of which shall be demonstrated, may be used.

Shielded, low inductive resistors (e.g. BNC type feed-through terminators) shall be used as loads. The resistors shall have a value of 50 Ω. Additional measurements may be made with other values of the resistor.

NOTE: The characteristics of an instrument can be significantly altered if a load is applied that is not in the range specified by the manufacturer or the application document. If a different load is required for the instrument or the application, the load used shall be noted in the test report.

The equipment used for measurements shall be in a valid state of calibration.

The measuring instruments shall have a bandwidth compatible with the frequency range of the eddy current instrument. Typically, the maximum usable frequency of the measuring instrument shall be at least twice the maximum frequency of the eddy current instrument.

Equipment measuring voltages (e.g. oscilloscope, voltmeter) shall have a high input impedance ≥1 MΩ.

Measured AC voltages and AC currents can be reported as peak, peak-to-peak or RMS values. The type of the value shall be denoted.

The measurements described hereafter shall be made at the minimum and maximum excitation frequency available by the instrument and:

— for detailed functional check (level 2 verification), at least one frequency per decade in the end-user range and the used frequencies;

— for characterization (level 3 verification), at least one, preferably two or three frequencies per decade on a logarithmic scale (e. g. 10 Hz, 100 Hz, 1 kHz, … or 10 Hz, 30 Hz, 100 Hz, … or 10 Hz, 20 Hz, 50 Hz, 100 Hz, …) between the minimum and maximum excitation frequency available by the instrument.

NOTE: The filter settings used for a specific application will modify the characteristics, for example, bandwidth, gain setting accuracy and phase-setting accuracy. In this case, the measurement conditions for verification shall be specified in the application document.

Definition and measurement conditions

The frequency shall be measured at the generator output of the instrument loaded in accordance with 6.1.

The percentage deviation from the target value is:

(1)

where

f_t is the target frequency value;

f_m is the measured frequency value.

The maximum absolute percentage of the deviation in the total range of the frequencies measured shall be reported.

Measurement method

The frequency shall be measured using a frequency counter or digital oscilloscope.

In the case of simultaneous non-multiplexed multi-frequency instruments spectrum analysis shall be used.

Acceptance criteria

The maximum deviation shall not exceed ±3 % for each excitation frequency value.

Definition and measurement conditions

For a generator producing a sinusoidal waveform, the harmonic content is used as a measure of the deviation from a pure sinusoid.

The harmonic distortion is described by the Total Harmonic Distortion, k_THD

k_THD is defined as the ratio of the equivalent root mean square (RMS) voltage of all the harmonic frequencies (from the 2nd harmonic ) over the RMS voltage of the fundamental frequency:

(2)

where

V₁ is the RMS value of the first harmonic (fundamental);

V_n is the RMS value of the nth harmonic.

The distortion factor shall be measured at the generator output of the instrument loaded in accordance with 6.1.

In the case of multi-frequency instruments, sufficient instrumentation shall be used, e.g. spectrum analyser.

Measurement method

The distortion factor may be measured using a distortion-factor bridge, a spectrum analyser or a high-pass filter.

Acceptance criteria

The maximum distortion factor should be less than -40 dB (1 %)

Definition and measurement conditions

The differential source impedance Z_s is the internal impedance of the generator unit (see Figure 2), measured at different load resistances. The differential source impedance shall be measured for each independent output.

Key

V_s output voltage

I output current

Z_s source impedance

Figure 2 — Internal impedance of generator unit

Measurement method

The method proposed is based on the assumption that the capacity and inductivity of the complex source impedance Z_S can be neglected and the impedance can be considered as a resistor.

The generator output is loaded with a resistor R₁ (normally 50 Ω) and the voltage V₁ is measured with an oscilloscope or an adequate voltmeter.

Repeat the measurement with a resistor R₂ (normally R₂ = 0,5 R₁ or R₂ = 2 R₁) and measure V₂.Z_s, expressed in ohms, is:

(3)

I₁ and I₂ can be measured using a current probe or calculated by I_x = V_x/R_x.

NOTE 1: R₂ can be achieved by two resistors with the same value as R₁ in parallel (R₂ = 0,5 R₁) or in series (R₂ = 2 R₁).

NOTE 2: Verify that the values of V₁ and V₂ are less than the maximum output voltage and the currents I₁ and I₂ are less than the maximum output current.

Definition and measurement conditions

The maximum output voltage V_O,max is the voltage at the generator terminals with no load applied and the generator set to give its maximum output.

Measurement method

The maximum output voltage is measured using an oscilloscope or an adequate voltmeter.

Definition and measurement conditions

The maximum output current I_O,max is the current measured at the generator terminals when terminated with the lowest allowed resistive load, as defined by the manufacturer. The generator is set to give its maximum output.

Measurement method

The maximum output current is measured with a current probe connected to an oscilloscope or with an ammeter.

Definition and measurement conditions

In case of adjustable output amplitude, the output voltage shall be measured at the generator output of the instrument loaded in accordance with 6.1.

The percentage deviation from the target value is:

(4)

where

V_t is the target value;

V_m is the measured value.

Depending on the eddy current instrument, the unit of the target value may be volt, ampere or percentage. If the unit of the target value is ampere or percentage, the measured voltage shall be transformed to the unit of the target value before calculating the deviation using formula 5.

(5a)

(5b)

The maximum absolute value of the deviations measured shall be reported.

Measurement method

The output voltage can be measured using an oscilloscope or an adequate voltmeter.

Measurements shall be done at 10, 25, 75, 95, 100 % of the maximum output amplitude.

In the case of non-multiplexed multi-frequency instruments appropriate instrumentation shall be used, e.g. spectrum analyser.

Acceptance criteria

The maximum deviation shall not exceed ±5 % for each output voltage value.

Definition and measurement conditions

The maximum allowable input voltage is related to safety, saturation and non-linearity.

It is respectively the peak input voltage at minimum gain, corresponding to the following:

a) the maximum value given by the manufacturer; this is the safe input voltage such that the instrument is not damaged; it includes common-mode operating limits when relevant;

b) 90 % of the output at saturation;

c) the non-linearity exceeding a given value. The maximum allowable deviation from linearity should be defined by the user.

In all cases, the input voltage applied shall not exceed that given a).

Measurement method

Related to saturation

The frequency beat method is used (see principle in Annex A). The input voltage is provided by a sine-wave generator. The difference between the frequency of the signal generator and the selected frequency of the instrument shall not be greater than 10 % of the stated bandwidth of the instrument.

The gain of the instrument is set to minimum and the filters are set to have a minimum effect. The input and each output loaded with a pure resistor.

Ensure that the instrument is balanced. The input signal is measured using an oscilloscope or voltmeter.

The input voltage is increased from zero to the safe input voltage given by the manufacturer, and the positive and negative peak values of each component of the output voltage (V_x+, V_x−, V_y+, V_y−) are recorded. The first value of the four variables (i.e. that corresponding to the smallest value of the input), which ceases to increase when reaching a steady value V_s, provides the saturation output level V_s. The input value V_is thus obtained then decreased until the component being monitored reaches an output value of 90 % V_s.

The input voltage obtained corresponds to the maximum allowable input voltage, related to saturation, defined as V_ilim in Figure 3.

Related to non-linearity

For any input voltage varying in the given range, the non-linearity could be defined as the maximum admissible deviation. The measurement approach of the linearity range and the maximum allowable deviation of linearity are extensively explained in Annex B. It is related to the performance of linear regression between the input voltage and output levels. The extend of non-linearity might vary according to the application. Therefore, in case of specific application where a non-linearity must be verified, the maximum allowable deviation should be provided by the user.

For this specific case, substitute in Annex B the following:

Key

X input voltage (V_s saturation output level)

Y output voltage

NOTE The relative amplitudes of each output are for example only.

Figure 3 — Measurement of maximum allowable input voltage related to saturation

Definition and measurement conditions

The input impedance is the apparent impedance of the input stage. The equivalent circuit is the parallel combination of a resistor and a capacitor.

No voltage greater than the maximum input voltage shall be applied

Measurement method

A network analyser or an impedance meter can be used to measure the input impedance directly. Alternatively an oscilloscope with at least two channels and a current probe can be used to calculate the impedance from voltage, current and phase measurements.

Using an oscilloscope, connect the internal generator of the instrument or an external sin wave generator to the input of the eddy current instrument, set the frequency to the lowest excitation frequency of the instrument and measure the voltage V, current I and phase shift φ between voltage and current.

The input resistance is:

(6)

Then set the frequency to the highest excitation frequency of the instrument and measure the voltage V, current I and phase shift φ between voltage and current again.

The input capacitance is:

(7)

R_i and C_i shall be determined for all possible input circuitry options.

Maximum compensable input voltage

Maximum compensable input voltage is the maximum value of the input voltage that may be compensated electrically while maintaining the instrument specification relative to input saturation.

With the test conditions defined in section 6.3.1.2.1 to which an output derived from the instrument generator is added to the input signal.

For instrument with differential input available, this can be done by setting apparatus to one input (ex. Positive) and the derived generator to the other input (ex. Negative).

Evaluation method

The input voltage derived from the instrument generator is gradually increased until saturation is reached at V_ilim (i.e. 90 % V_s when measured with 0 drive voltage) or until the instrument communicate that its balancing limit has been reached. The maximum voltage for which this test passed is the reported maximum compensable input voltage.

Evaluated quantity and conditions

The absolute value of the output that is obtained following the balancing operation. The value shall be described as a percentage of a specified range, e.g. full-scale value.

This evaluation shall be done re-using the configuration defined in 6.4.1.1, with the input voltage derived from the instrument generator at 50 % of the maximum compensable input voltage and with a gain value (in dB) to 60 % of the maximum gain value of the instrument.

Evaluation method

Following the balancing operation, the output value of each component has to be evaluated. Each output value being the average of at least 100 measurements.

The maximum value of several balancing operations (a minimum of five) should be taken at various frequencies covering the instrument frequency range.

Acceptance criteria

The value of the output signal for each component should be ≤ 1 % of the selected full-scale value for the considered application.

Definition and evaluating conditions

Orthogonality of signal components is the capability of the instrument to output quadrature-demodulated components.

It is characterised by the deviation from orthogonality, or the deviation between 90° and the actual phase shift between channel X and channel Y.

The frequency beat method is used (see principle in Annex A: Measurement method).

Evaluation method

There are two different but equivalent methods of evaluating orthogonality.

1) Plot the X output vs. Y output. To have X and Y orthogonal the result shall be a circle, otherwise the result will be elliptical (Figure 4) and not orthogonal. The manufacturer shall provide the obtainable level of orthogonality as a percentage.

Key

X normalized X component of eddy current signal

Y normalized Y component of eddy current signal

Figure 4 — dashed: orthogonality; solid: non-orthogonality

2) Evaluate the phase shift between the X and Y component of the eddy current signal. The phase shift φ shall be 90° (Figure 5). The manufacturer shall provide the obtainable level of orthogonality as a percentage.

Key

X normalized X component of eddy current signal

Y normalized Y component of eddy current signal

t time

t₁ time shift between X and Y

T period duration

Figure 5 — Orthogonality

φ = 180° t₁ / T

In the case of a multi frequency instrument each channel can be assessed individually or alternatively the frequency for each channel set to the same value to table simultaneous measurement.

Definition and evaluating conditions

Gain-setting accuracy is the capability of the instrument to set the amplification of a signal in a linear way. It is characterised by the maximum deviation from linearity in decibels (dB) between a set gain and the evaluated gain. It shall be evaluated for each component.

The gain accuracy and linearity will be required only if the instrument allows gain setting.

Evaluation method

If the sine wave generator does not include an attenuator, a calibrated attenuator shall be fitted between the signal generator and the instrument.

With the initial condition of minimum gain, the output values of each component are measured and taken as the references, X_ref and Y_ref.

The gain range of the instrument shall be divided into at least five equidistant spaced intervals, e.g. 6 dB or 10 dB.

The gain of the instrument is increased by this interval and the output of the generator reduced by the same interval. The two components of the output are measured for each interval.

The deviation in gain at each interval value, in decibels, is given by:

(8)

(9)

The maximum deviation is the largest value of the deviation in gain.

Acceptance criterion

The maximum deviation in gain linearity is ±0,2 dB.

Definition and evaluating conditions

Phase-setting accuracy is the difference between the expected and the actual change in the value of the phase of the output vector when a phase shift is made using the phase control. If phase is adjustable, the amplitude deviation due to the phase setting shall be reported.

Evaluation method

The instrument generator output is feed through an attenuator to the input.

Adjust the input voltage to half the maximum input voltage related to non-linearity.

Set the phase setting to 0°(Φ₀), evaluate the output of each component, X₀ X₀ and Y₀. Calculate the amplitude and the phase angle of the output vector.

(10)

(11)

(12)

Change the phase setting in i steps not exceeding 10° (iΦ_e) and repeat the evaluation for a total of 360°.

The phase deviation, in degrees, is:

(13)

The amplitude deviation, in percentage, is:

(14)

The maximum values of Φ_d and V_d shall be reported.

Acceptance criteria

The phase deviation shall be less than ±5°.

The amplitude deviation shall be less than ±1 %.

Definitions and evaluating conditions

The bandwidth is the frequency range of input variations that can be observed at the output with an attenuation of less then -3 dB.

At most eddy current instruments, the lower limit of the bandwidth can be set by a high pass filter and the upper limit can be set by a low pass filter. In this case, both limits shall be verified independently. Set the low pass filter to its maximum value to measure the characteristics of the high pass filter and set the high pass filter to its minimum value or switch the high pass filter off to measure the characteristics of the low pass filter.

Since the evaluated value is the output value, no distinction can be made between the filtering effect of the input filter, the demodulator and the filtering after demodulation. If the input filter is adjustable, it should be set to the value recommended by the manufacturer for the selected test frequency.

Using the frequency beat method (see Annex A), adjust the output voltage of an external signal generator, connected to the input, to half of the maximum allowable input voltage of the instrument related to linearity.

Generally, the maximum difference f_d between the frequency of the signal generator and the selected frequency of the instrument shall be at least one decade greater than the expected bandwidth of the instrument, as specified by the manufacturer.

Evaluation method

The difference f_d between the signal generator frequency and the excitation frequency of the instrument shall take at least 10 values equally spaced on a logarithmic scale. The lowest value depends on the excitation frequency setting range of the instrument, and the highest value should be at least ten times the upper limit of the bandwidth of the instrument specified by the manufacturer.

The magnitude of the eddy current signal () and the frequency difference f_d is recorded. Results may be stated in the form of a table of frequency and amplitude for each component of the output signal.

The reference value is obtained at frequencies where the magnitude remains almost constant at a high level. For a low pass this is usually at frequencies at least one decade lower than the cutoff frequency, for a high pass at least one decade higher than the cutoff frequency, and for a bandpass in the middle between both cutoff frequencies. The median value calculated from the values at the flat area can be used to improve accuracy.

The frequency differences f_D where the magnitude of the eddy current signal drops by -3 dB are the cutoff frequencies of the instrument.

The maximum magnitude relative to the reference value in dB is the ripple of the frequency response.

Figure 4 shows three different examples of frequency response.

Example 1: low pass, attenuation = 20 dB per decade, f_LP = 10 kHz, bandwidth = f_LP.

Example 2: band pass, attenuation = 40 dB per decade, f_HP = 100 Hz, f_LP = 100 kHz, ripple = 2 dB, bandwidth = f_LP - f_HP = 99,9 kHz.

Example 3: low pass, attenuation = 80 dB per decade, f_LP = 10 kHz, ripple = 1 dB, bandwidth = f_LP,

Key

Y normalized output (arbitrary unit)

G frequency response (gain)

f frequency

fLP -3 dB cutoff frequency of low pass filter

Figure 4 — Examples of frequency responses

Definitions and evaluating conditions

Crosstalk characterises the mutual interference between inputs of a multichannel instrument.

Evaluation method

All channels are set to the same frequency.

The input of each channel in turn is connected to an external signal generator. The generator output shall be the maximum input voltage related to linearity. Unused inputs shall be shorted.

With the gain of the other channels set to maximum, the value of the component outputs of each of the other channels is measured.

For n channels where j = 1, n, the output of the jth channel is:

(15)

When feeding the ith channel, the cross-talk factor, t _i, for the channel is:

(16)

The cross-talk factor for the instrument is defined as t = max(t_i)

Acceptance criteria

The cross-talk factor of the instrument shall be below -40 dB.

Definition and evaluating conditions

This evaluation characterises the ability of the instrument to suppress the common-mode signal. This verification only applies to differential inputs.

Evaluation method

Using the frequency beat method, connect an external sine-wave generator and two matched resistors to the input as shown in figure 6.

Apply the maximum input voltage related to linearity and place the switch in open position.

V_X and V_Y become V_X1 and V_Y1.

The switch is then closed and the input voltage is divided by 2.

V_X and V_Y become V_X2 and V_Y2.

  i = 1 or 2 (17)

The common mode rejection q_r is:

(18)

Key

R resistor

S switch

Figure 6 — Arrangement for measurement of common-mode rejection

Acceptance criteria

The common mode rejection shall be higher than 20 dB.

Definition and evaluating conditions

The maximum instrument noise is the input voltage equivalent of the maximum residual output signal with shorted input.

The instrument noise may be measured under other operating conditions specified in an application document.

In all cases, the bandwidth shall be reported (see paragraph 7.1.2).

Evaluation method

With shorted input or output of the internal generator directly connected to the input, evaluate the output variation due to instrument noise V_noise out with the highest gain possible without saturation G_max.

The equivalent noise at the input is:

(19)

The peak-to-peak value of V_noise,out can be calculated by:

(20)

NOTE: The noise at the output can be different with shortened input and generator connected to the input depending on demodulation technique and stage of digitization.

Acceptance criteria

The equivalent noise at the input shall be less than the initial characterization (manufacturer value) + 3 dB.

1. 
   (informative)
   
   Principle of frequency beat method

Type text. The principle is described using, as an example, an eddy current instrument with multiplier and associated filter demodulation.

The method consists in applying at the input of the instrument a sinusoidal voltage, the frequency of which slightly differs from that of the frequency on the instrument: f_s = f₁ + f_d. At the level of the internal demodulation circuit (see Figure A.1), a beat is thus produced with the frequency f₁ of the generator.

Figure A.1—Demodulation circuit

In this example, demodulation consists in determining the real and imaginary components of the signal.

S_x(t) = Asin[2π(f₁ + f_d)t] sin 2πf₁·t

Rewritten as

S_x(t) = A/2[cos2πf_d·t − cos2π(2f₁ + f_d)·t] (A.1)

S_y(t) = Asin[2π(f₁ + f_d) t] cos2πf₁·t

Rewritten as

S_y(t) = A/2[sin2πf_d·t + sin2π(2f₁ + f_d)·t] (A.2)

The low-pass filters included in the circuit suppress the second terms in Formulae (A.1) and (A.2) which contain the frequency (2f₁ + f_d).

At the outputs of the instrument, two signals X and Y, the amplitude of which is proportional to A, modulated respectively by a cosine and a sine at the frequency f_d are available.

In the case of an ideal instrument, these two voltages, applied to an oscilloscope, display on the screen a circle, the radius of which is proportional to A, the spot rotating at f_d.

Generally, the difference f_d between the frequency of the signal generator and the selected frequency of the instrument shall not be greater than 10 % of the bandwidth of the instrument, as specified by the manufacturer.

To measure the frequency of the instrument generator, the frequency of the input signal f_s is adjusted, in order that the spot stops rotating on the screen.

1. 
   (informative)
   
   Method of measurement of linearity range between output and input

The extreme values of the parameter input, I, are Imin and Imax.

The parameter I varies in constant steps between Imin and Imax. For each value of I, the corresponding value of the parameter output, O, is recorded.

A linear regression is performed between these values of I and O. A relationship OIin(I) is thus obtained.

The deviation, ΔO(I) is defined as follows

The linear input range LIR is an interval defined by :

|∆O(I)| < ε

where ε is the maximum allowable deviation from linearity. It is given as a percentage of the total output swing for the corresponding input range

Key

1 linearity range

NOTE Such a measurement applies to amplitude linearity. For phase linearity, deviations are measured in degrees (0° to 360° scale instead of 0 to 100 %).

Figure B.1 — Determination of linearity range

1. 
   (informative)
   
   Table of characteristics

Bibliography

[1] ISO #####‑#, General title — Part #: Title of part

[2] ISO #####‑##:20##, General title — Part ##: Title of part