Study on the electromagnetic susceptibility of the speed sensor of an aircraft starter generator

Andreea Creosteanu Technical Military Academy

Bucharest, Romania, andreea.caian@gmail.com

Abstract-An electronic system is compatible with its electromagnetic environment if it satisfies the following two criteria: it does not emit (unintentional) electromagnetic energy above a certain minimum level, and it is not susceptible to malfunction if unintentional electromagnetic energy below a certain level is incident on it. This paper presents a study on the effect that electromagnetic interference has on the speed sensor an aircraft starter generator. An experimental setup was created to generate the radiated emissions, and results were plotted on the oscilloscope. Results obtained by measurement were also compared to the theoretical results.

Keywords- EMC, susceptibility, interference, radiated emissions, radiated susceptibility, RSIOl, speed sensor, starter generator.

I. INTRODUCTION

Electromagnetic compatibility (EMC) can be defined as "the capability of electromagnetic equipments or systems to be operated in the intended operational electromagnetic environment at designated levels of efficiency" [1].

In the figure bellow, Figure I are represented typical EMC requirements for a product.

RAOLATEORF

CONDUCTED RF (TELECOM PORTS)

�RT -:::;::==:::::7'/ CONDUCTED RF f"-----f'"

/ EFT TRANSIENT

(IF CABLE :>:3 m) I \ RADIATED RF ESO

HARMONICS

& fliCKER

ACVOLlAGE

SAGS & INTERRUPTIONS

HV TRANSIENT

EFT & SURGE

Figure I. Typical commercial EMC requirements [I]

In this paper we will focus on studying the immunity of a starter generator's speed sensor towards radiated emissions, both theoretically and experimentally.

978-1-4673-1717-7112/$31.00 ©2012 IEEE

Laurentiu Creosteanu Politehnica University

Bucharest, Romania

II. DEFINING THE STUDY

The equipment on which we performed the electromagnetic interference study was an aircraft starter generator. The starter generator has two operation modes:

• As a starter, operating as a series-wound DC motor ;

• As a generator, providing a DC voltage of 30 V and a power of 12 kW for driving speeds ranging from 7 200 to 12 000 rpm.

The starter generator operates in conjunction with a Generator Control Unit (GCU) and together with this it:

• Starts the aircraft engines;

• Provides a regulated DC power supply for the aircraft electrical systems.

During the starting phase, the GCU operates to:

• Control the engine starting phase;

• Maintain the torque produced by the starter­generator;

• Control the starting contactor relay;

• Provide differential protection and over-speed protection.

Also, when the starter-generator is operating as a generator, the GCU provides:

• Regulation of the aircraft power supply system voltage by regulating the generator excitation current;

• Control of the generator contactor relay;

• Protection with regard to overvoltage and under­voltage conditions;

• Protection against over-excitation of the generator;

• Differential protection;

• Over-load protection.

In our study, we will focus on the radiated emISSIOn response of a starter generator's element, the speed sensor. The

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speed sensor provides speed thresholds used in the start sequence and also for some oeu fault conditions.

The speed sensor output voltage that we want to perturb has, during the start operation, with no load and at 5000 rotations per minute (rpm) the shape plotted bellow, in figure 2:

T == lmsec

/\ v poak / � I � I "

"" / IV

Figure 2. Speed sensor characteristics [4]

The signal has a I ms period (though operating at a frequency of 1 KHz), and in normal functioning has a positive voltage between lAY and 2Y.

The radiated emission was obtained using a current driven inductor, operated at the same frequency as the speed sensor in order to maximize the obtained coupling perturbation results.

The measurement setup consisted in a signal generator connected to an audio amplifier supplied from a source, and a perturbation inductor on which the obtained signal was applied. A brief setup scheme is presented in the figure bellow, figure 3:

Signal generator

Aud io t amplifier � �

T � spe

Sei:�:�sor

� Perturbat ion � inductor

Figure 3. Measurements block-schematic

The signal generator provided a rectangular voltage, with 0.8Y amplitude, 1 KHz frequency and a 0.5 duty cycle. This signal was amplified by the audio amplifier, and then applied on the inductor positive terminal, having the other terminal connected to ground. The audio amplifier had a operating bandwidth between 20Hz-20KHz, and a gain Au =20dB.

The perturbation inductor was made of thin cooper wire coiled around isolating paper, with an air gap inside. We used 40m of cooper wire, and obtained a 3-layer inductor, having the total number of coils N=21O, and a diameter of

978-1-4673-1717-7112/$31.00 ©2012 IEEE

approximately 6cm. The measured inductor impedance was ZL=40.

The inductor used is pictured in the figure bellow, figure 4:

Figure 4. Experimental inductor used

The perturbation inductor was placed around the wire that drove the speed sensor signal to an oscilloscope used for visualizing the signal shape. The complete test setup environment is represented in figure 5:

Figure 5. Test setup environment

Also, the start generator connected in the test setup configuration is visible in the next figure, figure 6:

Figure 6. Start generator connection

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III. RESUL IS

The influence of the radiated emission obtained as described above over the speed sensor signal was analyzed both theoretically and In practice by experimental m easurem ents.

A. Theoretical results At the begging of the theoretical study, for better describing

the perturbation element, we calculated the inductance for the built multilayer air-core coil inductor. Using the next formula (3.1) we obtained a calculated 5.25mH inductance.

(3.l)

In the equation above, N represents the number of turns, r is the coil radius, I is the physical length of the coil, d is the depth of the coil, and the inductance result is expressed in IlH.

For out study, we first needed to calculate the current that flew through the inductor in our experimental setup. Since we generated a O.SV signal through the signal generator, and this signal entered the audio amplifier having a 20dB gain, the voltage applied on the positive inductor terminal was SV. Also, having the wire impedance ZL =40 and applying Ohm's law, we obtained a 2A current across the inductor.

Next, we analyzed the influence that the current-crossed inductor could have on a conductive wire similar to the real connection of the speed sensor to the oscilloscope. This influence results into parasitical voltage induced into the conductive wire (CW).

To determine the parasitical voltage induced into the speed sensor CW, we will start from the equation bellow, eq (3.2) [2]:

(3.2)

In this equation circuit 1 represents the perturbation inductor wire and circuit 2 the speed sensor wire. Up(t) defines the electromagnetic voltage induced by circuit 1 intro circuit 2, and L21 is the coupling inductance between the two circuits.

Since the current variation in time in the first circuit was known, in order to obtain the parasitical voltage induced by the first circuit into the second one, we needed to calculate the coupling inductance between the two circuits. This was done using the equation bellow [3]:

L21 = llo/2n' In (gL12/g(g2) (3.3)

In the equation above, glz is the medium geometrical distance between the inductor and the speed sensor wires, gl is the medium geometrical distance for the first CW cross-section SJ, and gz is the medium geometrical distance for the second CW cross-section Sz.

The medium geometrical distances gl and gz were calculated using eq (3.4) bellow :

(3.4) [3]

In our experiment, the inductor CW had the radius rl=O.lcm and the speed sensor cable rz=0.5cm.

978-1-4673-1717-7112/$31.00 ©2012 IEEE

Having a 3cm distance between the two CW, we calculated the coupling inductance between the two circuits, and obtained L21= I .2SIlH. Also, considering the current through the inductor of 2A in the 1ms time period, we obtained the perturbation electromagnetic voltage up = 2.56m V.

But this voltage would be obtained by having only one turn near the speed sensor CWo Considering the N=2 I 0 inductor turns we will obtain an actual perturbation voltage 210 times higher, of up= 53Sm V. So, we expect to obtain a perturbation voltage variation of around 500m V in our experimental setup too.

B. Experimental results The experiment was performed according to figures 5 and

6, by supplying the perturbation inductor, and placing it around the CW that drove the speed sensor signal to the oscilloscope, while having the starter generator running.

For connecting the speed sensor signal to the oscilloscope, we used a simple unshielded cable, in order to create a worst­case experiment that would correspond to a degraded aircraft shielding. Degradation effects appear on the electrical characteristics of the aircraft wiring shielding due to aircraft aging and also exposure to environmental conditions [5].

The speed sensor signal was visualized on the oscilloscope, and its waveform was compared both in the presence and in the absence of radiated perturbation.

Fist, we note the speed sensor signal, in normal functioning, with the start generator running 5000 rpm, and without applying the radiated perturbation through the inductor around the CWo The shape of the signal corresponds to the typical one from figure 2, as it can be seen in figure 7 bellow:

Figure 7. Speed sensor signal without perturbation

Then, in the conditions discussed in chapter II, when generating a O.SV rectangular signal with 1 KHz frequency on the signal generator, and applying it through the audio amplifier to the inductor placed around the CW, the speed sensor signal had completely changed its form, as it is shown in the next figure, figure S:

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Figure 8. Speed sensor signal with perturbation

By comparing the two previous figures, 7 and 8, we can conclude that the speed sensor signal is deeply affected by the radiated emission obtained with our experimental setup. The signal amplitude is decreased by a voltage of approximately 0.5V, and also the entire shape of the signal has changed. In figure 8, on the top appears the zoomed signal for a better signal shape perturbation notice.

The voltage variation in the speed sensor signal amplitude corresponds to the theoretical results obtained earlier in section A of this chapter.

IV. EMC STANDARDS CONTEXT

The commercial avionics industry has its own set of EMC standards, which are similar to those of the military. These standards apply to the entire spectrum of commercial aircraft, which includes light general aviation aircraft, helicopters, and jumbo jets. [1]

Our intended study could be related to the radiated susceptibility RS101 magentic field standard between 30 Hz and 100 KHz . This test procedure is used to verify the ability of the EUT to withstand radiated magnetic fields.

The radiating loop has 20 turns, with 12cm diameter, and a magnetic flux density of 9.5x1 07 pTfAmpere of applied current at a distance of 5 cm from the plane of the 100p. [8]

In our study, having 210 turns and a diameter of 6cm, the radiation element has a magnetic flux density of 1.35xlO' pTf Ampere. [9]

978-1-4673-1717-7112/$31.00 ©2012 IEEE

In the future, we intend to conduct experiments on this matter also on setups according to international standards.

V. CONCLUSIONS

A study was performed to investigate the effect of electromagnetic interferences on the speed sensor of an aircraft starter generator.

The electromagnetic interferences were obtained using a perturbation inductor placed around the CW that drove the speed sensor signal to an oscilloscope, while having the start generator running. Theoretically results were compared to the ones obtained experimentally.

The speed sensor voltage variation obtained theoretically, by calculus, sustained the results obtained in practice on the experimental run.

ACKNOWLEDGMENT

We would like to thank Mr. C. Petrescu for all his support and guidance along the performing of the experiment, together with all the team from the Maintenance Department in Tarom Romanian Airline Company.

REFERENCES

[I] H. w. Ott, "Electromagnetic Compatibility Engineering," Wiley Publication, 2009.

[2] A. Sotir, "Electromagnetic perturbation interferences", "Interferente electromagnetice perturbatoare" (Romanian), Military Publication, 2005, pp.129-142.

[3] P. L. Kalantarov and L. A. Teitlin, "Inductances Calculations", Technical Publication, 1958, pp. 95-98, pp. 281-282.

[4] D. L. Sengupta, V. V. Liepa, "Applied Electromagnetics and Electromagnetic Compatibility", Wiley Publication, 2006.

[5] Federal Aviation Administartion, "Aircraft Wiring Harness Shield Degradation Study", Final report, August 2004.

[6] H.-C. Tsai, "Investigation of the electromagnetic interference effect on a conduction wire in a Schmitt trigger circuit", Journal of Taiwan Normal University, 2006.

[7] M. Young, The Technical Writer's Handbook. Mill Valley, CA: University Science, 1989.

[8] Department of defense United States of America, "Requirements for the control of electromagnetic interference characteristics of subsystems and equipment - MIL-STD-461 F", I 0 December 2007.

[9] T. Eichhorn, "Estimate inductor losses easily in power supply designs", Maxim integrated products, 2005.

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