Article by: Narcís Molina, Project Manager, Braking Systems in Applus IDIADA
In the first part of the article, the investigation has been introduced: it is aimed at analyzing the interaction between the chassis dynamics and the triggering, local and low-level stick-slip motion found in the friction interface. The vehicle under study and the methodology have been presented. For the sake of simplicity, the previously-explained array of tests is again summarized:
This work differentiates three phases throughout the development of creep groan:
- Triggering phase (Δt1 to Δt3): a transient, broadband and incipient stick-slip motion.
- Charging phase (Δt4): also transient, the noise is the sum of the broadband signal with clusters of pure frequencies.
- Unstable phase (Δt5): a tonal response of exponentially-increased amplitude that is caused by a self-sustaining excitation of vehicle resonance.
In the color-map, a resonance at 68 Hz and its harmonics are clearly detected:
In order to assess the overall impact of the creep groan excitation on the vehicle, the vibration energy is calculated as the average mean square of the acceleration levels; thus, a single value can be associated with each representative point. The following figure depicts the logarithmic increase in vibration energy, as measured in the slack-adjuster and the leading edge of the primary shoe throughout the development of creep groan. The inflection points identify the transition between each creep groan phase.
Acceleration results are also used to explore, by double integration, relative movements in the time domain between the different components of the brake assembly. This is the case of the figure below (left), which shows the measured relative displacements between the brake chamber and the slack-adjuster. It is observed that an axial relative displacement of about ±0.35 mm can be expected. The slack-adjuster transmits this oscillation to the S-cam shaft, thus forcing the S-cam to rotate by ±0.25 degrees approximately (right).
As previously mentioned, different modal analyzes are conducted in order to examine the origin of the resonance that justifies the fundamental frequency of 68 Hz. The image below indicated the test set-up for the isolated brake assembly —either fixed on a rigid bench and suspended under free-free condition. The slack-adjuster is impacted in the X-direction (green arrow), whilst the response is measured at the brake chamber’s bracket, the push-rod and the end of the S-cam shaft. Both tests are repeated with and without brake pressure; when actuated, shoes are forced to contact the drum with an air pressure of 2 bar, thus simulating the operational creep groan conditions.
The graphs below show the FRFs of the response at the brake chamber’s bracket when the brake system is actuated with an air pressure of 2 bar —no significant results are found for the condition of null pressure. Note that a resonance is clearly exhibited at 68 Hz as the assembly is fixed to a rigid bench (left), whilst no coupling is detected —regardless of the pressure— under free-free conditions (right).
The on-vehicle modal analysis is carried out by impacting the stub axle in X-, Y, and Z-directions, as depicted in the figure below (left) with green arrows. This type of excitation induces comparable inputs to those produced during braking. The analysis involves the front left corner, composed by the brake system (and its actuation), the suspension and the steering system; for the sake of simplicity, the location of the response points is schematically represented (right):
The following screenshots depict a lateral view of the global vehicle mode at 68 Hz; it highlights the deflection between the brake chamber (a) and the slack-adjuster (b). This three-dimensional relative displacement suggests that the natural mode at that given frequency produces a longitudinal movement of the push-rod, which at the same time causes the S-cam to rotate. This conclusion agrees with the actual movements observed under operational creep groan conditions.
Another mode shape characteristic is represented below, where again (a) and (b) make reference to the brake chamber and the slack-adjuster, respectively. The front left wheel and its brake system oscillates simultaneously about the stub axle and the king pin axis; this is clarified by means of the dotted line and the red arrow.
The running modes elucidate the harmonic movement at the frequency of interest, 68 Hz, using the actual creep groan event as excitation of the whole vehicle. An RMA of all 95 measured points is carried out to animate the most important modes being triggered by the creep groan mechanism and, thus, understand the frequency response of the vehicle. The animation includes from the in-brake stick-slip excitation to the response in the receiver points of the cabin, including the different structural transmission paths —thus allowing the detection of vibration amplification along the different components.
Due to the complexity of the vehicle vibration response, and the unavailability of enclosing the complete animation, two screenshots of the resulting RMA —in terms of displacements— of the brake system are shown below. It represents the rotation of the shoes about the stub axle, as well as the uncontrolled deflection of the actuation parts. Note that, in this case, the animation of the brake shoes does not include the out-of-plane (Y-direction) component of the vibrations (since this direction was not measured): the accelerometers glued within the drum are bi-axial.
The vehicle response to the creep groan-induced vibration is dominated by a harmonic motion. Therefore, the animations extracted from both time (RTA) and frequency (RMA) domains do coincide; thus, RTA results are omitted.
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