Research and /or Engineering Questions/Objective:
The braking module of a conventional vehicle is a complex coupled system which includes mechanical, pneumatic, hydraulic and electrical components. In addition to that, it is important to remark that the main command entered comes from the driver by means of the leg. This has two important consequences: first, the human driver is responsible for closing the loop for tracking a trajectory reference; and second, the leg is a compliant element with a certain dynamics affecting the force transmission to the vehicle pedal. The identification of the behaviour of the latter is the objective of the present paper.
In the present article, the effective mechanical impedance of a typical human leg during braking actuations is experimentally characterized and mathematically modelled by using general spring and damping elements. For obtaining the model, we have designed a test bench which permits to measure the response of the leg at different load levels with a variable frequency rate of an input displacements signal. The analysis of the obtained results permits us to obtain the equivalent transfer function of the leg and to represent it in terms of simple mechanical elements, like springs and dampers.
The present analysis is interesting from three different perspectives: firstly, it develops a testing and characterizing methodology for representing a human leg in typical braking conditions; secondly, the derived representation gives interesting insights in the behaviour of the leg, especially in its frequency dependent response; and thirdly, the derived transfer function can be used for evaluating the effect of the human actuation in the operating conditions, for example during ABS actuations.
Limitations of this study:
The study is mainly focused on the frequency response of the leg during braking actuations. In consequence, the derived model does not include other phenomena like saturation limits in the force response of the leg, or the force control loop arranged by the driver. Although interesting, these two aspects are out of the scope of the present work, and could be further introduced in the same methodological and representation framework.
What does the paper offer that is new in the field in comparison to other works of the author:
Compared with other representations found in the bibliography, the proposed approach is focused on the response of the leg during a braking actuation. In contrast to more general models, the number of required parameters to take into consideration is mainly reduced, and therefore also the characterization and testing effort. As a consequence of the application scope, the methodology and designed test bench represents a specific and simple response to the problem of leg actuation effects during braking manoeuvres.
The proposed methodology succeeds in obtaining a simple representation of a leg during a braking actuation. This representation has been applied to the analysis of complete braking systems during ABS actuations, and it permits to analyze interesting phenomena due to the mechanical effect of the leg. For example, resonant problems affected by the compliant nature of the leg would be directly related to noise effects, and the changing stiffness of the leg could be linked to load levels in the hydraulic circuit.
José Carlos Peña, José Manuel Rodríguez, Raúl Monge - ITAINNOVA; Thomas Pütz - TRW