Evaluación de eficiencia

METHODOLOGY

The literature review has shown that while some preliminary research has been undertaken, no reliable and easy-to-use method exists for accurately determining the dynamic characteristics (FRF) of road vehicles using only in-service response data. Furthermore, the current methods available to estimate the dynamic characteristics each have their own significant limitations and have not been comparatively evaluated.

To undertake the comparative evaluation and validate the in-service analysis techniques, an idealised vehicle must be developed. The broad approach to be taken in this research is to:

1) Develop and configure a physical idealised vehicle.

2) Experimentally establish and compare estimates of the transmissibility FRF of the idealised vehicle using existing laboratory techniques.

3) Drive the idealised vehicle over a range of selected roads under various conditions (namely speed) and use the measured response data to predict the true FRF of the vehicle and compare these estimates with the laboratory experiments.

5.1 IDEALISED VEHICLE DEVELOPMENT AND

CONFIGURATION

There are a number of reasons why undertaking the research with a standard road vehicle is not appropriate. These include: drive-train noise, wheel imbalance and Multiple-Input Single- Output (MISO) effects due to the excitation being applied via multiple wheels. In order to undertake a series of critical evaluation and in-service experiments, an idealised physical quarter vehicle must be developed. As a physical quarter car, the vehicle should be designed to have a single wheel-spring-shock and exhibit realistic dynamic characteristics.

• The vehicle is to be towed using any capable vehicle.

The tow bar must be sufficiently long enough to minimise the pitch motion and stiff enough to prevent torsion to ensure the vehicle remains upright. Conversely, the tow bar must not be too long such that it is difficult to maneuver with the towing vehicle through public streets.

• The suspension system of the vehicle must be interchangeable.

The components of the suspension system should be able to be changed with relative ease to allow for different vehicle configurations (i.e. different shock absorbers and springs).

• The vehicle should be able to support varied mass loading configurations.

A frame to support additional mass loading allows for another option to easily vary the dynamic characteristics of the vehicle.

5.2 LABORATORY-BASED EXPERIMENTAL METHODOLOGY

To critically evaluate and compare the various techniques currently available to estimate the dynamic characteristics of the sprung mass mode of the different configurations of the idealised vehicle, a series of laboratory experiments are to be undertaken.

1) Response-only (transient) methods (outlined by the CEU).

For the response-only (transient) experiments, the idealised vehicle is instrumented to measure the sprung mass acceleration and subjected to the three tests outlined by the CEU (1996). The measured response requires analysis to estimate the dynamic characteristics. It is expected that there will be noise and the effects of other modes (such as the motion of the unsprung mass) present in the measured response, therefore the response of the sprung mass will be isolated using appropriate filtering. The filtered response data is analysed using the FFT and the PP

method to estimate the damped natural frequency of the sprung mass mode. The damping ratio is estimated by computing the instantaneous magnitude envelope using the Hilbert Transform. Finally, the sprung mass natural frequency is estimated using the damped natural frequency and the damping ratio (Equation 3-6).

2) Excitation-response methods (random vibration excitation).

The excitation-response experiments require a suitable vibration table to subject the vehicle to a predefined excitation by inducing vertical motion at the wheels, while the sprung mass acceleration of the idealised vehicle and the excitation acceleration of the vibration table are simultaneously measured. While sinusoidal-based excitations were once used, a significantly improved representation of the distribution environment is provided by subjecting the vehicle to random vibration. Two spectral shapes will be used for the evaluation. One spectral shape is acceleration band-limited white noise, which excites the vehicle with equal level at all frequencies within the specified bandwidth. The other spectral shape is acceleration band- limited violet noise, which corresponds to the PSD model adopted under ISO standard 8608 (1995) to approximate the excitation of longitudinal pavement profiles (Equation 3-19). During the tests, the temperature of the shock absorber must be monitored to ensure the starting temperature is same for each test. Once the excitation and response data have been measured, the FRF of the vehicle is then established (Equation 3-44). To extract the sprung mass natural frequency and damping ratio from the FRF, a least-squares regression curve-fit of the transmissibility magnitude of an SDoF system is applied (Equation 3-53).

5.3 IN-SERVICE EXPERIMENTAL METHODOLOGY

The second stage of the research is focused on a series of in-service experiments, where the idealised vehicle is driven along a selected route at different nominally constant operating speeds. One of the most important considerations is to identify a suitable road to undertake the in-service experiments. The most significant criterion is to undertake the measurements only while the vehicle is travelling at the desired nominal constant speed. Maintaining constant speed is not only dependent on the driver, but also on the road being devoid of stop signs, traffic lights, large ascents or descents, sharp turns, roundabouts and other hazards that may require the driver to slow down or stop the vehicle. Furthermore, it is desirable to undertake the experiments while the traffic on the road is very light, as other vehicles on the road may force the driver to slow down or stop.

The speed limit of the road must remain constant, or at least the slowest permitted speed on the road must be the same as the selected operating speed. Another consideration is the length of the road; the longer the road is, the better chance the excitation spectrum will approach the spectral model outlined under ISO standard 8608 (1995). It is also important that the selected road is fairly uneven so as to induce a reasonable level of excitation into the vehicle across a broad range of frequencies.

Once an appropriate road has been identified, the vehicle is instrumented to measure the vertical vibration acceleration of the sprung mass (body) and driven over a series of roads at various nominally constant operating speeds. The vehicle is then driven out to the location near the beginning of the selected route. The driver begins driving along the route and once the vehicle reaches the desired operating speed the measurement of the sprung mass acceleration vibration may begin. Throughout the experiment, the driver must ensure that the nominally constant operating speed is maintained. Once the vehicle approaches the end of the route, the measurement is stopped and the driver may begin to slow the vehicle down. The driver then turns the vehicle around safely where possible and the process is repeated for the return length of the road at the same constant operating speed. This may be repeated for as many different operating speeds as is desired.

Once the various measurements have been undertaken, the measured vibration response of the vehicle is analysed using two experimental approaches. The first approach relies on the assumption that the excitation may be assumed using a predetermined spectral function, while the second approach is based on the use of the random decrement technique. Both of these experimental techniques are described in detail in Chapter 7. The first series of in-service experiments will be undertaken using the idealised vehicle, where it will be towed along the selected routes using a light transport vehicle. As a final phase of the in-service experiments, once the analytical techniques have been validated using the idealised vehicle, a series of in- service experiments using various road vehicles will be undertaken and their response analysed.