Measurement of �nock �ensors

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Keywords—Knock sensor, frequency response, input impedance

I. INTRODUCTION Detonation combustion sensors (knock sensors) are

used in the cars as a cheap accelerometers used to diagnose the combustion engine activity. It is a piezoelectric accelerometer without built-in amplifier. Mechanical construction of the sensors corresponds to their predetermination and from the price perspective, they might be appropriate for usage in various other industrial applications. Therefore it is necessary to know the parameters, especially the reference course for their sensitivity and frequency response.

II. THE KNOCK SENSORS - BASIC PARAMETERS PROVIDED BY THE MANUFACTURER

Depending on the type of knock sensor the manufacturers indicate following basic parameters (Ref. [1], [2]):

TABLE I. BASIC PARAMETERS OF KNOCK SENSORS

reference voltage sensitivity 25 ÷ 35 mV / g ( at f = 5 kHz)

resonant frequency > 25 kHz

usable frequency range 1 ÷ 20 kHz

(some manufacturers may indicate 30 Hz ÷ 30 kHz)

usable temperature range -40 ÷ +150 °C

The reference voltage sensitivity of the knock sensors is ten times lower than commonly used accelerometers. Given frequency range is not so optimistic. When we want to use the sensor for a diagnose of a machine which is powered by an electric motor, we require a lower frequency range from 50 Hz at 3000 rpm. The useful upper cutoff frequency is approximately 1/3 of the resonant frequency and it can be assumed that the sensors can be used up to frequency 6.6 kHz. In terms of usability, the upper frequency limit is quite high, but the lower cutoff frequency lies in an area where the application of the sensors is at the discretion. The temperature range should meet most of the potential industrial applications requirements.

III. THE EQUIVALENT CIRCUIT OF THE SENSOR AND ITS CONNECTION TO THE PREAMPLIFIER

The low frequency limit of the knock sensor given by the manufacturers is rather high, therefore it needs to be connected to a voltage amplifier with a suitable input impedance. This situation is caused by the fact that the sensors does not have a built-in emitter follower as other standard accelerometers. Fig. 1 shows a simplified equivalent electrical circuit of sensor for low frequencies.

It is a simplified model, where is excluded the impact of mounting the sensor to the examined object. The inductance Ls corresponds to seismic weight divided by the transformation constant kb of the sensor (the weight does not include the weight of the sensor case). The resistance Rm represents the mechanical resistance divided by the transformation constant. Cm represents the mechanical compliance of the piezoelectric element. The capacity C0 is the capacity of the piezoelectric element itself. R0 is the resistance between the terminals of the sensor.

Fig. 1. The equivalent circuit of the sensor and its connection to the amplifier.

C0 Ck CvR0 Rv

accelerometer cable preamplifierLs Rm Cm

G

125978-1-4673-1179-3/12/$31.00 ©2012 IEEE

Fig. 2: The scheme of the measurement of the frequency range of the knock sensors.

The cable connecting between the sensor and the

amplifier is represented only by the stray capacitance Ck, because the different impedances of the equivalent scheme of cable are for the considered frequency range and the cable length irerelevant. The input terminal is characterized by the input impedance of the voltage amplifier Rv and the stray capacity Cv. In this alternative scheme, there can be easily deduced the equation for the calculation of the lower limit frequency (1),

md RC

fπ2

1=

( 1 )

where R is the parallel combination of the amplifier

input resistance Rv and output resistance R0 of the sensor

0

0

RRRRR

v

v

+=

( 2 )

Because of the demand of the lower cut-off frequency

(50 Hz), it is necessary to select a suitable large input impedance, but large input impedance may have a negative influence on the level of electromagnetic interference.

IV. THE MEASUREMENT OF FREQUENCY RANGE OF THE KNOCK SENSORS WITH THE PREAMPLIFIERS AT DIFFERENT

SIZES OF THE IMPEDANCE For the measurements of the knock sensors it was

designed and realized a prototype of the preamplifier (preamplifier_2 in Fig. 2) with a differential input with

the variable input impedance from 200 k{ to 9.7 M{ and the gain G = 10 or 100. The industrial reference accelerometer was connected to a standard constant curent power preamplifier (preamplifier_1 in Fig. 2). The second reference accelerometer BK 4507 was directly connected to the analyzer BK 3560C, which is also used as a signal generator. All accelerometers were mounted on a shaker. The shaker is driven by the power amplifier. The signal generator provides the white noise or sine wave. The scheme of the measurement of the frequency range of the knock sensors is shown in Fig. 2.

The knock sensor parameters were obtained from comparison of two types of reference sensors mounted on the shaker as a vibration sourece. The measurement of the sensitivity was carried out with the harmonic excitation signal of frequency 300 Hz. The frequency response measurement was realised in 7 Hz to 12.8 kHz bandwidth using white noise signal. Both measurements were performed at 1 g acceleration value. The cable length to the preamplifier was 4 m.

In accordance with the theory in chapter III, the

frequency response was measured as a dependance of bandwidth on the input impedance of the preamplifier. As shown in Fig. 3, the low frequency limit decreased with the increasing of input impedance. It was measured using the FFT frequency spectrum, the shaker was driven by the white noise limited to 7 Hz to 12.8 kHz bandwidth.

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Fig. 3: Example of the relative frequency responthe preamplifier for different input impedances,

V. THE MEASUREMENT OF THE SENSITCHARACTERISTICS OF THE SE

For the comparison of the characteristics, the six different sensorsNo.6) were used.

The sensors were connected by the scwhich was also screwed with industrKD 37V and reference accelerometersensitivity of the sensors was comreference sensors at the frequency of 30the input impedance of the preamplifiereference acceleration was 1g. The ssensors are in the range from 13.6 toTable II.

TABLE II. THE VALUES OF THE SENSITIVITY

sensor No. 1 2 3

Voltage sensitivity for f = 300 Hz [mV/g]

17,9 15,9 16

Fig. 4a: The relative frequency characteristics of t1 - No. 3.

nse of the sensor with , knock sensor No. 2.

TIVITY AND THE ENSORS

sensitivity and s (marked as No.1-

crew to the shaker, rial accelerometer r BK 4507. The

mpared with two 00 Hz (sine wave), er was 2 M{ and ensitivities of the

o 17.9 mV/g , see

Y OF THE SENSORS

3 4 5 6

,2 13,7 16,1 13,6

the knock sensors No.

Fig. 4b: The relative frequency charact- No.6

The frequency characteristics winput resistance of the preampthere are shown the charaimpedance 2 M{.

VI. THE MEASUREMENT OF THE OF THE KNOCK

To determine the directional support was built to allow thethe both excitation axis and pethe excitation. Due to the attashaker, vibrations in the perpenbe ruled out. Therefore, it frequency spectra of the accperpendicular to the axis measurement conditions wermeasurement of the sensitivity 5b).

Fig. 5a: The influence of knock sereference accelerometer in the axis an

excitatio

teristics of the knock sensors No.4 6.

were measured for various plifier. In Fig. 4a and 4b, acteristics for the input

DIRECTIONAL PROPERTIES K SENSORS

properties, the aluminum e attachment of sensors in erpendicular to the axis of chment of the plant to the ndicular direction e can not were compared only the

celeration in the axis and of the excitation. The

re the same as for the of the sensors (Fig. 5a and

ensor No. 2 mounting and the nd perpendicular to the axis of the on.

127

Fig. 5b: Influence of knock sensor No. 2 mountaccelerometer in the axis and perpendicular to the

VII. THE MEASUREMENT OF THE LELECTROMAGNETIC INTERFE

Due to the high input impedanc

measuring scheme of the susceptibility tinterference can be assumed, so the mout with considerable vibration electromagnetic interference. As a moalternator was driven by the chosen enpower of 11 kW was supplied from tsensor, reference sensor and powintentionally placed on the generator engine wiring.

Measurements were carried for pimpedance range from 200 k{ to 9.4 Mwere also carried out for the disconnethe power cable was terminated by impnF, which replaced the knock sensor. Ois one of the measurement results - outpreamplifier and the output voltage accelerometer BK 4507. Significant inin knock sensor usage only at the frequit will be necessary to accept this factmeasurement method using these sensor

Fig. 6: Output voltage of knock sensor and refe

ting and the reference e axis of the excitation.

LEVELS OF ERENCE

ce, the complete to electromagnetic

model was carried measurement of

odel situation, the ngine. The engine the driver. Knock

wer cables were and close to the

preamplifier input M{. Measurements ected sensor when pedance 2 M{ / 1

On the Fig. 6, there tput voltage of the

of the reference nterference occurs uency of 50 Hz, so t for the design of rs.

ference accelerometr

VIII. CONCLU

From the measurements, we be used as a cheap vibration sensor is almost ten times accelerometer. The advantage linearity of the frequency depethe temperature range. Thesensitivity, which can also be cthe declared data by manufactua preamplifier with high inpsensors measured at least 1 M{electromagnetic interference industrial accelerometer. Interfat the frequency of 50 Hz. problem in many applications,of machines with known rotatisensors requires low electromThe lower frequency limit depfor 1M{ it is possible to use Hz. In the worst case (sensor limit was 5 kHz.

ACKNOWLED

The work was financially suppoFR-TI1/159 of Ministry o"Compressed air generation system".

REFEREN

[1] Robert Bosch, Piezoelectric Vibr

http://www.bosch.com.au/contennock.pdf

[2] Delphi Technologies Inc., Delphihttp://delphi.com/shared/pdf/ppd/

USIONS suggest that the sensor can knock sensor. The knock cheaper than industrial

is the mechanical design, endence on sensitivity and e disadvantage is lower considerably different from urer. The necessity of using put impedance (for most {) causes less resistance to

in comparison with ference is most significant This fact could not be a for example in diagnostic ion speed. Using of knock

magnetic interference area. pends on input impedance, the knock sensor from 30 No. 1) the high frequency

DGEMENT orted by project

of Industry and Trade, and treatment integrated

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ation Sensor, t/language1/downloads/sensors_k

i Flat Response Knock Sensor, /sensors/et_flat_knock.pdf

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