Características del desarrollo psicomotor en niños

Because for MARINER model the potentiometer readings were recorded on a portable computer system, they were immediately available for analysis and provided information on the tow angles of the tow line (relative to tow post and model heading). Some preliminary study was conducted on these data. As a first step the computer simulation was run to get the conditions for some of the tests. This showed encouragingly good correlation with the time traces of the angles taken during the test. In particular the frequency and phasing were in close agreement but there were some apparently small differences in amplitude (see Figures 7.15 - 7.18). This gave some confidence in further analysis of the data. However on a closer look at the raw recordings it was found that they suffered a high level of noise as shown in Figure 7.19. This was originally caused by the elastic fishing line used and worsened by the mounting design of the potentiometers. Apart from this there still remained the difficulty of an unreliable and inconsistent sample rate of recordings as described above in Section 7.5. It was concluded, therefore, that the potentiometer measurements were insufficiently accurate and unsuitable for reliable results later in the derivative analysis of the tests. This method is still usable but it would require higher accuracy potentiometers, a more powerful data logger and a means of accurately aligning the axis of the potentiometer with the tow point.

The measurements by the Glasgow SELSPOT tracking system took a while to compile and to find a suitable form of transfer to the UCL analysis system. When this data acquired from the tests was received at UCL, some difficulty was encountered in loading the data as the Glasgow system had adopted a higher rate of data acquisition and this led to an overload in memory storage available. When this was overcome some better traces were obtained due to the higher order of accuracy of the facility, though they suffered from the similar problems of ‘springing’ noise especially for MARINER tests. These SELSPOT records were utilised for both M ARINER and BRITISH BOMBARDIER model tests in the calculation for the variables ( p) required in the analysis. The details of the processing of the raw records are presented in the following sections.

Chapter 7 Model Tests At GLASGOW University

7.6.1 Tests on MARINER Model

A series of experiments were performed for six tow point positions and four trim angles at two carriage speeds to obtain the model's motion and the towing forces.

The signals picked up by the force gauge and two LEDs were recorded in five individual data channels and experimental data are sampled at a rate of ten samples per second per channel for forty five seconds. The contents of measuring signals recorded in each channel are described as follows:

Channel 1 Channel 2 Channel 3 Channel 4 Channel 5

Displacement in Y-Direction of hole No. 1 ( y^) Displacement in Y-Direction of hole No. 21 (y^y) Displacement in X-Direction of hole No. 1 (%y) Displacement in X-Direction of hole No. 21 (x^y) Tension in tow post load cell ( T)

From the tow force signals recorded in Channel 5, it was noted that during the runs that the model exhibited a high frequency axial oscillation due to the elastic springing of the tow line. Therefore the data was filtered by the Fast Fourier Transform technique (FFT) and the line force was assumed as a constant, during a run, with the mean value of the filtered transient data given by Equation 7.3.

The heading angle of the model was determined directly from the measuring data:

Here the value of 1.016 (m) is the distance between the two LEDs and X j can be calculated if the tow channel bar was fixed through the nth hole (counted from behind tow point) on the hull screw at station 1 (see Figure 7.6):

X j = LCG+ 1.099+ (2X 0.0254)x n (7.5)

Here the value of 7.099(m)was the distance between station 1 and midship (Station 10). In order to calculate the lateral displacement at LCG, the distance between LCG and either LED 1 (hole 1) or LED 2 (hole 21) can be evaluated as follows:

dj = LCG+ 1.0 9 9 + (2 x 0 .0 2 5 4 )x ( n - I) = LCG+ 1.099- (2 x 0.0254) x ( 2 x 1 - n )

and the lateral displacement of LCG given by:

y c c = V 2 , - ^ ^ ‘j j f x ‘i2, (7 .7 )

or

The tow line angle relative to the central axis of the tank a is determined by:

a = s in - '( ^ ) = s i n - ' ( ^ + (7.9)

Typical outputs from five channels of records are given in Figure 7.20. Some examples of processed results ( P, for runs of test Group 1 and 2 runs were plotted as shown in Figures 7.21 - 7.28. These curves showed much better traces than the potentiometer records, especially for y^G* There still remained some higher frequency noise superimposed on the slow motion oscillations of the model. During the tests it had been observed that the fishing line was slightly elastic and that a form of 'springing' was occurring. This related to this discrete frequency noise on the recording and triggered worse noise in the potentiometer readings as explained earlier. If the calculation of the velocities of the motion were to be incorporated in the later stage of derivative analysis using certain SI models, they would have given large variations in the numerical differentiation, which could have resulted in poor correlation for control derivative values. This led to the ‘filtering’ or ‘curve fitting’ analysis being adopted before the application of SI approaches as is presented in Chapter 8.

Figure 7.29 shows the test records for the variation in trim. It was noted that there was a change in the behaviour of the model from damped motion with stern trim to increasing sheer off when the model was bow down. With 2° of stern trim the oscillations were of small amplitude and fairly strongly damped. At 1° stern trim the amplitude is greater and less damped. In level trim the oscillations are almost steady. At

1°

— bow trim the oscillations are slower and the model wanders towards the side and at 1°

2

bow trim the model sheers off. This is consistent with the normal tendency for a ship form to be unstable when down by the bow and distinctly more stable when trimmed stern down. Since the trim is closely associated with straight line stability of the model

Chapter 7 Model Tests At GLASGOW University

without analysis it can provide a good indication of the straight line stability of the hull form.

7.6.2 Tests on BRITISH BOMBARDIER Model

A second round of experiments were performed for six tow point positions and level trim angle at two carriage speeds to measure the model's motion and the towing forces.

The signals picked up by the force gauge, two LEDs, LVDT and potentiometer were recorded in seven individual data channels and experimental data are sampled at a rate of sixty four samples per second per channel for sixty seconds. The contents of measuring signals recorded in each channel were described as follows:

Channel 1 Channel 2 Channel 3 Channel 4 Channel 5 Channel 6 Channel 7

Angle in potentiometer ( ap)

Tension in towing post load cell ( T)

LVDT value in cm(L^)

Displacement in Y-Direction of LED No. 1 (at hole 1) (i/;) Displacement in Y-Direction of LED No. 2 (at LCG) (y^) Displacement in X-Direction of LED No. 1 (at hole I) (Xj)

Displacement in X-Direction of LED No. 2 (at LCG) (xy)

The towing force signal in channel No. 2 ( T) was improved by replacing the fishing line with a light wound wire and the mean value was also calculated and adopted as the line force instead of using the transient data. This time the lateral displacement of LCG was recorded, in Channel 4, directly from the LED sensor as - V i ' On the other hand the heading angle of the model (p ) was complicated due to the distance between the two LEDs ( Dj2) was no longer being constant (unlike the case of MARINER tests), but instead varied depending on which hole (nth) of the tow channel bar was connected with the hull screw at station 9.75 according to the following:

Dj2 = LCG+ 1.1875+ (2x 0.0254) x ( n - 1) (7.10)

Here the value of 1.1875(m) was the distance between station 9.75 and midship (Station 5) and Dj2 also stood for the length from LED 1 (hole position 1) to LCG, which was equivalent to di in case of MARINER tests. The tow point was one hole (i.e. 2 inches) ahead and the Xj- could be obtained accordingly:

Xj = 0,2 + 2x0.0254 (7.11)

The heading angle p could then be derived from the difference of the lateral displacements of two LEDs, which was recorded in Channels 4 and 5 respectively, divided by the responding distance between them i.e. D,^. There were two methods to attain the value of the tow line angle relative to the central axis of the tank. One was to use the calculated data of p and by the equation 7.9 of the MARINER tests; another was to deal with the readings from the LVDT sensor as follows:

^ _ 3 6 0 x ^ (in degree) (7.12)

or

K X ( ( )

(X = (in radius) (7.13)

here (p (=4.56c) was the diameter of the pulley on rotating tow system.

Example of typical outputs from seven channels of records are given in Figure 7.30. The processed results of both the positional variables (p, y^c) the towing force

T for test runs in group 1 are plotted in Figures 7.31-7.36. These show much clearer traces without the 'springing', that gave rise to a discrete frequency noise in the MARINER results, and with the oscillations of heading angle p well centred about the initial zero. During the steady speed recorded runs, there was little evidence of damping and barely two oscillations occuring. Based on these data, the control derivatives for the model (BRITISH BOMBARDIER) could then be analysed. This is described in Chapter

8.