Investigation of the electromagnetic interference effect on 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, interference, radiated emissions, speed sensor, starter generator.

I. INTRODUCTION

The IEEE Standard Dictionary of Electrical and Electronics Terms defines electromagnetic interference as "impairment of the reception of a wanted electromagnetic signal caused by an electromagnetic disturbance."

Electromagnetic compatibility (EMC) can though 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.

CONDUCTED AF (TELECOM PORTS)

RADIATEORF

--;;' ;;;;�7'/ �AT � CONDUCTED RF ,c------r

/ EFT TRANSIENT {IF CABLE;> 3 m}

ESO

HARMONICS & FliCKER

ACVOLTAGE SAGS &

INTERRUPTIONS

HV TRANSIENT EFT & SURGE

Figure 1. Typical commercial EMC requirements [1]

978-1-4673-1372-8/12/$31.00 ©2012 IEEE

Dr. Laurentiu Creosteanu Politehnica University

Bucharest, Romania

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

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 studying the radiated emission response for a starter generator element, the speed sensor. The speed sensor is used to obtain speed thresholds for the starter generator, which the start sequence and some oeu fault conditions use.

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

T = lm.eo

JI\ V peak J � 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 it has a positive voltage between I.4V and 2V.

The radiated emission was obtained using a current driven inductor, operated on 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 ------7 Audio ------7

generator amplifier

{ T Speed sensor

signal

E Perturbation inductor

Figure 3. Measurements schematic setup

The signal generator provided a rectangular voltage, with 0.8V 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 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 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 applied 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 connection in the test setup configuration is visible in the next figure, figure 6:

Figure 6. Start generator connection

III. RESULTS

The influence of the radiated emission obtained as

described above over the speed sensor signal was analyzed

both theoretically and in practice by experimental

measurements.

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.

L 6r+ 91 + 10d

(3.1 )

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 flH.

Then we wanted to obtain the current that flows through the inductor in our experimental setup. Since we generate a 0.8V signal through the signal generator, and this signal enters the audio amplifier having a 20dB gain, the voltage applied on the positive inductor terminal will be 8V. Also, considering the wire impedance ZL=40 and applying Ohm's law, we will have a 2A current across the inductor.

Next, we calculated 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 considered the equation bellow, eq (3.2) [2]:

(3.2)

In the equation above we considered circuit 1 to be the "IprtllrIYl ' inductor wire and circuit 2 to be the speed sensor

Up(t) represents the electromagnetic voltage induced by 1 intro circuit 2, and L21 represents the coupling

between the two circuits.

Since the current variation in time in the first circuit is , in order to obtain the parasitical voltage induced by the

circuit into the second one, we have to calculate the inductance between the two circuits. This was done

the equation bellow [3]:

L21 = J.lol2n 'In (g2dg(g2) (3.3)

In this equation, g12 is the medium geometrical distance ">""""','n the inductor and the speed sensor cable cross-section

Sl and S2, gl is the medium geometrical distance for first CW cross-section Sj, and g2 is the medium geometrical

J��'.uw�v for the second CW cross-section S2.

The medium geometrical distances gl and g2 were calculated by the next equation, eq (3.4)[3]:

gi = 0.778 . fi (3.4)

Where ri represents the radius for the two wires: the inductor wire and the speed sensor cable. In our calculations, we used for the two CW the radius rl=O.lcm and r2=0.5cm.

Using the areas for the two round wire sections, and having a 3cm distance between the two CW, we were able to calculate the coupling inductance between the two circuits, and though obtained L21=1.28flH. Also, considering the current through the inductor of 2A in the 1 ms time period, we obtained the perturbation electromagnetic voltage up = 2.56mV.

But this voltage would be obtained by having only one turn near the speed sensor CWo Considering the N=21O inductor turns we will obtain an actual perturbation voltage 210 times higher, of up= 538mV. So, we expect to obtain a voltage variation in the speed sensor signal of around 500mV, determined in the perturbation setup experimented by us.

B. Experimental results

The experiment was developed as visible in figures 5 and 6, by supplying the perturbation inductor, and using 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 simulate the shielding degradation of the aircraft. 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 results from figure 7 bellow:

Figure 7. Speed sensor signal without perturbation

Then, in the conditions presented in chapter II, when generating a O.8V rectangular signal with 1 KHz frequency on the signal generator, and applying it through the audio amplifier to the inductor located around the CW, the speed sensor signal had completely changed its form, as visible in the next figure, figure 8:

Figure 8. Speed sensor signal with perturbation

By comparing the last two 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 O.5V, and also the entire shape of the signal has changed. On the top in figure 8 appears the zoomed signal for a better signal change notice.

The voltage variation in the speed sensor signal amplitude is in concordance with the theoretical results obtained above in section A of this chapter.

IV. CONCLUSIONS

A study has been 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 around the CW that drove the speed sensor signal to the oscilloscope, while having the start generator running. Effects were analyzed both theoretically and experimentally, and results were compared.

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.