Source: Applus IDIADA
This is the third and final TBR Technical Corners by Narcis Molina, Project Manager, Braking Systems in Applus IDIADA, about finding the optimal braking system for a vehicle.
First article: TBR Technical Corner: Optimized Braking System Sizing by means of a Parametric 1D Brake Model (Part 1 out of 3)
Second Article: TBR Technical Corner: Optimized Braking System Sizing by Means of a Parametric 1D Brake Model (Part 2 out of 3)
This article presents a methodology that incorporates a 1D (non-geometric) brake model, together with conventional vehicle tests, into the solution of problems associated with the sizing of a braking system.
The first part introduced the problem and presented some results from Phase 1, whereas the second part of the article dealt with the results from Phase 2. This third and last part compares the newly-obtained evidence with the new components against the baseline experimental results, thus checking whether the targeted improvements are successfully met.
The selected off-the-shelf components are finally installed in the vehicle in order to determine objectively their impact on the overall braking performance. Therefore, the vehicle is instrumented with an array of sensors, as listed in Table 10:
The legislative tests are carried out as per the conditions stated in UNECE Reg. No. 13. The vehicle shall be driven at the prescribed test speed and then, in neutral, the brake pedal is to be rapidly applied without exceeding 700 N.
Regarding the pedal feeling test, the driver is asked to bring the vehicle to a halt by depressing the pedal at a constant force rate (N/s) until reaching about 4 m/s2 —then, the pedal is released. This ramp apply allows measuring a wide range of deceleration levels throughout a single stop. Note that the test is repeated several times to ensure the accuracy of the results. Actually, the curve that represents the experimental data is the mean of the three stops with the lowest deviation.
For the sake of simplicity, this essay only reports those attributes deemed as critical, i.e. the excessive length of the pedal and the unsuccessful attempt to achieve a deceleration not less than 2.2 m/s2 with the secondary braking system —using only the rear brakes.
As detailed in Table 11, all emergency braking stops achieve a mean fully developed deceleration higher than 2.2 m/s2, with a stopping distance less than 51 m. Hence, the vehicle finally succeeds in passing the minimum legislative requirements.
Figure 6 shows the predicted pedal feeling for both baseline and new configurations, along with the experimental results for the new configuration. Note that, due to time constraints, the new configuration is only tested in GVW condition (solid maroon line).
It is worth emphasizing that the abrupt trend change seen in the experimental curves occurs as soon as the driver releases the brake pedal —that is, when reaching the requested deceleration of 4 m/s2.
Results from vehicle testing confirm that the object of shortening the brake pedal, while maintaining the other characteristics, is satisfied. Furthermore, the stiffness —understood as the stroke / deceleration ratio— is also enhanced: at 2 m/s2, the travel is diminished by 17 mm; at 4 m/s2, by 26 mm.
The main conclusions that can be inferred from this paper are:
- The procedure that has been presented proves to be suitable for the optimization of troubleshooting activities concerning the sizing of a braking system. The number of vehicle test iterations can be considerably reduced, thus being more efficient in terms of time and economic investments.
- The prediction formulated by the 1D (non-geometric) brake calculation is positively validated through vehicle testing. It has been demonstrated that this tool is accurate enough to support engineers in understanding the impact that any change in the braking system might have on its overall performance.
- The methodology can be applied to an existing vehicle —in this particular case, to face a sizing problem—, but also to define a new vehicle platform. The 1D model can help in generating brake system recommendations during the initial development stages: it is an adequate estimate of the equipment that is most likely to achieve the required braking performance.
- Concerning the vehicle under study, selected countermeasures have succeeded in addressing the two main issues: a ‘spongy’ brake pedal feeling, and the inability to meet all legislative requirements —specifically the minimum MFDD for the primary circuit failure test.
About Applus IDIADA
With more than 25 years’ experience and 2,450 engineers specializing in vehicle development, Applus IDIADA is a leading engineering company providing design, testing, engineering, and homologation services to the automotive industry worldwide.
Applus IDIADA has locations in California and Michigan, with further presence in 25 other countries, mainly in Europe and Asia.