## Monthly Archives: March 2014

## Modern energy consumption simulation software

As mentioned before, the simulation software depends on the considered node architecture. In this section we present two different instruction level simulation software, developed

Figure 8. Currentconsumptionmeasurementsfora WSNnodeduring transmission withacknowledgement |

respectivelyforthe AVR microcontroller, producedbyAtmel, andfor the MSP430 microcontroller, produced by Texas Instruments.

The first considered simulator, known as Avrora, is a set of simulation andanalysistools developed theUCLACompUois Group [y5t. n^particular, the simulator can handle up to a few thousands nodes, by taking advantage of the processing power of modern computers. Avrora is not only a simulator to test program execution on the node but it also allows online monitoring of the code execution on the WSN, profiling utilities to study the program’s behavior, source level debugging, a control flow graph, providing a graphical representation of program’s instructions, and an energy analysis tool, capable of analyzing the energy consumption of a specific application.

The simulator has been enhanced by Haas et al., after evaluating the performance of the analysis tools of Avrora, comparing the simulation results with experimental measurements carried out with the SANDbed platform [36]. The test application, run over TinyOS and involving four nodes with fixed routing path. Using the collected data Haas et al. developed and released an enhanced version of Avrora, called Avrora+, improving the calibration of energy model, modeling transition state cost, and taking into account the effect of manufacturing tolerance on the energy consumption.

The experimental verifications showed that the Avrora+ is very accurate, reducing the difference between measurements consumption measurements and simulation results to less than 5%.

The Worldsens simulation Framework is another WSNs simulator that support MSP430 based node [37]. This open source platform, released under the CeCILL and GNU GPL license agreements, includes three simulation tools, often used in conjunction:

• WSim: this is the platform simulator. It performs a full instruction simulation of the node, driven by the microprocessor internal clock.

• WSNet: an event driven wireless network simulator that can be used in conjunction with WSim to simulate a whole sensor network with high accuracy.

• eSimu: a software module that implements platform specific energy consumption models, and provideann ettim ation of the current absorbed by a node [38].

For estimating the power consumption, WSim and eSimu are usually being jointly used, interacting anshoevnin Fipcire9.ta particular, the WSim tool, compiled with eSimu support, receives the binary file that would be executed by a real microprocessor for a given application and providen a traca iile, Oetcribine sdate irsneitionsofthenc deanditsperipheralo. Notice that, when mmdelin. aradia transmiasion, WSNet is used as well, in conjunction with WSim. Using the trate file end aaalibrationliie iopoatcng the cuteena eOeotbedbythenode inits various stateo, Site overallccrrent ocnsgmption cf anode idateeacuien a giaan thtk oan be estimated anOprofilndagatniitheexocution time. Sioce thnnodaispowerod Fya consiant voltage source, °hepower tonsumptionaaneasily bederivedfrom .hecuwhemt abeotptionTar an exhaustive overeaew et extstingtaalr tor simulations, modeHdgandmeashrementsof WSNs refer to [09].

Figure 9. Wsim, WSNet and eSimu simulation process |

## Attitude Sensors

As mentioned earlier, an attitude control system requires information about the body orientation. This information is provided by attitude sensors. An attitude sensor actually provides the relative orientation of the spacecraft with respect to a reference vector (e. g., a unit vector in the direction of the Sun, a known star, the Earth, or the Earth’s magnetic field). Therefore, three-axis attitude determination requires two or more sensors. The definitive reference for a more in-depth discussion of attitude sensors and actuators and their principles

of operation is Wertz (1). This reference also includes a fairly detailed overview of hardware implementation issues.

Sun Sensors. Sun sensors are the most common type of attitude sensor. Their field of view ranges from a few square arc – min (10-7 rad2) to approximately n rad2 and their resolution ranges from several degrees to less than 1 arc-sec.

Horizon Sensors. Horizon sensors can be used when the spacecraft is in a close orbit around a celestial body. For low – Earth orbiting satellites, for instance, the difference in brightness of Earth’s disk from the background darkness of space can be easily detected by a horizon sensor and provides a coarse attitude measurement.

Magnetometers. Magnetometers use Earth’s magnetic field to locate the body’s orientation. They have poor resolution due to uncertainty of Earth’s magnetic field. They work better at low-Earth orbits, where the magnetic field is stronger and better modeled.

Star Sensors. Star sensors provide attitude information of very high accuracy, but they are heavy and expensive. They are usually the choice for deep-space spacecraft where attitude measurements from Sun sensors or near-by celestial objects are either unavailable or inaccurate.

Gyroscopes. Gyroscopes (or simply gyros) use a rapidly spinning mass on a gimbal to sense changes in the spacecraft orientation. The principle of their operation is based on the fact that any change in the angular momentum vector about a certain axis will be sensed as a resulting movement about a perpendicular axis. They are very accurate for limited time intervals, but their measurements may become inaccurate over long periods of time due to drift. In this case, they have to be combined with some other attitude sensor to reset them periodically. Gyroscopes can also be used to get angular velocity measurements; in this case they are called rate gyros. Apart from their use in spacecraft, gyroscopes are also used as attitude or angular velocity sensors in aircraft, missiles, or marine vehicles (e. g., submarines).

## SHIELDING

The minimization of the external fringing field is becoming increasingly important for the siting of MRI systems, so the active shielding of MRI magnets with center fields up to 2 T is now almost universal. (Active shielding of MRI magnets with center fields above 2 T is uneconomical and is not generally attempted.) Active shielding is generally achieved by the inclusion in the coil array of two reverse polarity coils at diameters typically twice that of the main coils. Because of the large dipole moment of an MRI magnet, the unshielded fringing field will extend several meters from the boundary of the cryostat. Consequently, active shielding is applied to many MRI magnets with central fields of over 0.5 T (12). The effect of the shielding on the harmonics of the center field must, of course, be included in the design of the compensation coils.

## Automated Measurements and Analysis

Once the ECG signals are digitized, there are many forms of measurement and analysis that are automatically performed to aid the medical professional in interpreting the clinical information contained in the ECG. The section on applications, covers several of these, but one of the most common functions performed in each application is detecting each beat (10). In this case it is assumed that each beat means every ventricular contraction or every QRS-complex. There are times when atrial and ventricular activity are not synchronous and automated analysis requires detecting both atrial (P-wave) and ventricular activity (QRS-complex). Generally the first step in automated analysis is detecting each QRS-complex. It is the most prominent deflection of the ECG has the largest amplitude (—1.0 mV), and the most rapid change of potential. This rapid change in potential can be detected by taking the derivative (dV/dt) of the ECG and searching for the largest value of the derivative. Of particular concern in this approach is that noise which may contaminate the ECG recording also produces large derivative values, but the noise is not usually larger in overall amplitude than the QRS-complex. Once each possible beat is detected by searching for the largest first derivative, other algorithms can be used to examine the shape of the QRS-complexes and to classify them as normal or abnormal. By measuring intervals, amplitudes, and other wave characteristics, a number of ECG applications can be automated. The following section describes several of these applications where computer based algorithms are used to replace a human operator, and also to create new forms of analysis not amenable to human measurement.

## THE THIN LINEAR ANTENNA

This section deals with the analysis and properties of a finite – length dipole. The wire is considered to be thin such that tangential currents can be neglected and the current can be considered as only linear. The thin linear antenna and its geometry are shown in Fig. 2. The boundary conditions of the current are that the currents are zero at the two ends and maximum at the center. There is experimental evidence that the current distribution is sinusoidal. The current distribu-

exp (-jkr) 4:71 Г |

Ee = jn0k0I0 dl |

(26a) (26b) (26c) |

sin 0 |

Ег=Еф= Hr=Hg= 0 |

exp (-jkr) 4:71 Г |

Нф = jV0dl sin 0 |

The intrinsic impedance Zm of the medium is defined as the ratio of the tangential electric and magnetic fields and is given by

(b)

Figure 2. (a) Thin linear antenna and (b) its coordinate system. This figure geometrically shows how the field at any observation point can be formulated using the basic building block, namely the infinitesimal dipole.

7 – h.-

m — TT — tfn HФ

(27) |

Intermediate Field Region

For expressions for field components in the intermediate region (kr > 1), the reader is referred to any standard text on antennas (1).

Directivity

The radiation intensity U is given by

U = r2W: |

(28) |

av |

where |

Wav = – Re(£ x H )

(29) |

tions are for a dipole l and for length varying from A/2 to A. Thick dipoles will be treated in a subsequent section. |

T X/4 X X/4 L |

T X/4 f X/4 і |

T X/4 f X/4 L |

I |

0.707I, |

& |

I |

-►I |

The Current Distribution The current distribution on the thin dipole is given by |

t = T/8 (b) |

t = T/8 (c) T X/4 |

t = 0 (a) |

Ix (x’ = 0, y = 0, z1) |

T X/4 |

zI0sin[k(l/2 — z)], 0 < z < l/2 zI0sin[k(l/2 — z)], —1/2 < z < 0 |

(32) |

1° I —f – |

0 |

I |

I |

X/4 і t = 3T/8 (d) |

X/4 t = T/2 (e) |

The current distributions on the linear dipoles for different lengths are shown in Fig. 3, and Fig. 4 shows the current distributions on a half-wave dipole at different times. |

Figure 4. Current distribution on a A/2 wire antenna for different times. The current, which is alternating, changes with time. This figure shows the current changes on a half-wavelength wire antenna at different time instants. |

Fields and Radiation Patterns. To determine the field due to the dipole, it can be subdivided into small segments. The field at any point is a superposition of the contributions from each of the segments. Since the wire is very thin, we have x’ = 0 and y’ = 0. The electric and magnetic field components due to the elementary infinitesimal dipole segment of length dz’ |

at an arbitrary point are given by dE0 = jr]0k0Iz(x’, У, z’)—^—jkR) fa’ |

(33a) (33b) (33c) |

4n R |

dEr = dEф = dHr = dH9 = 0 |

exp(—jkR) . |

(b) |

dHф = jk0Iz (x’, y’, z) |

sin 9 dz1 |

4n R |

where |

-lIl |

l Il |

R = /x2 +y2 + (z — Z )1 = / (r2 — 2rZ cos в + Z2) |

l<< X (a) |

with |

r2 = x2 + y2 + z2, |

z = r cos 9 The expression for R can be expanded binomially as R = r — z cos 9 + higher-order terms decaying very fast with r ^ z’ |

Figure 3. Current distribution on dipoles of different lengths. Different physical lengths of the dipole support different current distributions with varying number of half sinusoids. This is because at the two ends of the wire, the boundary condition that the current must be zero has to be satisfied. |

(34) |

I |

The total field due to the dipole is obtained by integrating over the whole length. Omitting the straightforward steps, it turns out that the field components Eg and Hф are given by |

-lIl |

jr]0I0 exp (-jkr) 2n r |

cos(kl/2cos9) — cos(kl/2) sin 9 |

Eg = |

(35a) (35b) |

Hф = E9 /n |

вІ’І0 |

where is the intrinsic impedance of free space. To save space we will not describe the derivation of power radiated which involves Ci(x), Si(x), and Cin(x) functions. The Radiation Resistance. The resistance can be shown to be given by Rr = — {C + In (kl) — C(kl) + і sin(&Z) • S(2kl) — 2 S(kl)] 2n 2 + і cos(^Z)[C + ln(^Z/2) +C(2kl) — 2 C(klj] |

(c) |

(d) |

where C(X) and S(X) are well-known functions constituting Fresnel integrals (see Appendix IV in Ref. 1).

4. Define a set of testing or weighting functions wm, m = 1, 2, . . ., N, in the range. Taking the inner product of Eq. (40) with each wm and obtain

(42) |

Wi, L( Л) W2, L( f2) |

(36) |

(43b) |