Theoretically speaking, the brake pedal feeling is mainly the relationship between the brake pedal force and the vehicle braking deceleration, as well as the relationship between the brake pedal stroke and the vehicle braking deceleration.
Taking a conventional hydraulic brake vehicle with a vacuum servo system as an example, the relationship between the brake pedal force Fp and the vehicle braking deceleration ab, and the relationship between the brake pedal stroke Sp and the vehicle braking deceleration ab are deduced.
Taking the current mainstream disc brake as an example, establish a braking force analysis diagram of a single wheel, as shown in the figure, where the tire rolling radius Re, when the brake caliper brakes, the ground braking force generated is Fxb, and the braking force is Fxb. The diameter of the piston of the dynamic caliper is d, and the distance from the center of the piston to the center of the wheel is the braking effective radius r.
Before the wheels are not locked, the braking deceleration ab generated by the vehicle is equal to the ratio of the ground braking force Fxb generated by the front and rear wheels to the mass m of the vehicle.
For conventional passenger cars, the ground braking force Fxb is the sum of the braking forces generated by the four brake calipers, that is, twice the sum of the front wheel braking force Fxb1 and the rear wheel braking force Fxb2
. The pipeline pressure p has the following relationship:
Fs, FB are respectively the resistance produced by the return spring on the brake pedal and the total reaction force Fp produced by the control valve, air valve and return spring in the vacuum booster
. The relational expression of the brake deceleration ab after the braking force generated by the brake caliper
According to the principle of advanced fluid mechanics, the liquid volume compression coefficient β of the brake fluid is introduced
Among them, dab is the braking deceleration variation caused by the braking deceleration ab with the volume compression dV of the brake fluid. The volume compression dV of the brake fluid is related to the stroke of the brake master cylinder and the area of the brake master cylinder, and is also related to the expansion of the brake pipeline, especially the brake hose, and also related to the brake caliper The amount of deformation during work is related, because the expansion of the brake hose and the deformation of the brake caliper will offset the volume compression of the brake fluid. The
unit movement of the brake master cylinder and the unit movement of the brake pedal are as follows The last term in the relationship
is the stroke that the brake master cylinder needs to move to eliminate the gap between the friction lining and the brake disc.
From these theoretical formulas, it can be seen that the fit clearances x0, x1, and x2 between the various parts of the brake system have a more intuitive impact on the direction of the initial segment of the two curves, because the brake pedal force and deceleration In terms of the relationship curve, the pedal needs to resist a series of spring reaction forces in the process of eliminating these gaps, but it does not generate braking force and cannot bring about an increase in deceleration; for the brake pedal travel and brake deceleration In terms of the speed relationship curve, eliminating these gaps requires a longer stroke of the brake pedal but does not bring about an increase in deceleration, which makes it impossible to produce a greater braking deceleration within the total travel range of the brake pedal. For It is very disadvantageous to control the total travel of the brake pedal.
For a good brake pedal feel, the following characteristics should be present:
- When there is a pedestrian crossing ahead or a vehicle braking in the distance, the driver needs to brake lightly to decelerate to a certain extent. By lightly stepping on the brake, the deceleration that the vehicle can perceive can be obtained to realize the so-called point braking;
- When there is a red light at the intersection ahead or a traffic jam needs to be stopped in time, the driver needs to brake in time and step on the brake pedal gradually, hoping to obtain a continuous, stable and controllable braking deceleration of the vehicle. For the latter, due to many working conditions, it is required not to cause fatigue during the braking process or to keep the vehicle pitch as small as possible to improve ride comfort.
- When the vehicle encounters an obstacle in front of the vehicle and needs emergency braking, the driver's reaction is how to stop the vehicle as soon as possible. At this time, ABS or ESP has already intervened in the braking process, and the driver no longer pays attention to the magnitude of the brake pedal force and braking force. The length of the stroke of the brake pedal is too short. Since such operating conditions are rare, the evaluation of the brake pedal feel is not carried out.
When the driver needs to brake lightly
As long as you step on the brake lightly, you can obtain perceptible braking, which requires x0, x1 and x2 to be as small as possible, so that a small brake pedal stroke can produce perceivable braking deceleration, for x0, x1 It is to ensure that the vehicle will not cause the pedal to malfunction when the vehicle is in a bumpy state. They are values related to the spring stiffness Ks and the brake pedal lever ratio ip. Only the brake pedal should be as light as possible while ensuring the strength. Only by reducing ip can x0 and x1 be reduced to obtain sufficient safety margin. And x2 is a value related to the brake pedal lever ratio ip, the brake master cylinder area Am and the brake caliper diameters d1 and d2, but it cannot be reduced by changing these values, only by increasing the friction of the brake caliper The flatness of the disc, the return amount of the brake caliper friction plate and the flatness of the brake disc are reduced, which involves the processing technology and cost of parts. Braking reaction time is also affected by x0, x1 and x2, so their reduction is beneficial to improve braking sensitivity and reduce braking distance. However, the reduction of x2 means that the friction lining and the brake disc will always be in a close state. If the flatness of the friction lining and the brake disc is poorly manufactured or poorly assembled, the driving resistance of the vehicle will be relatively large and the fuel economy will be reduced.
With progressive depression of the driver's brake pedal
The required brake pedal force should increase gradually and be in a state that the driver can easily control without obvious exertion, so as not to make the vehicle pitch too large and affect the ride comfort. This just requires that the brake pedal stroke should not be too long, so as to avoid the excessive movement of the driver's legs and feet during the braking process and fatigue, and requires that the brake pedal force should not be too large so that the driver is strenuous and fatigued during continuous braking. This not only requires the brake pedal force Fp to be small, but also the brake pedal stroke sp to be small, and the brake pedal force Fp is inversely proportional to the brake pedal lever ratio ip and the area Am of the brake master cylinder, and the brake pedal stroke sp is inversely proportional to the brake pedal stroke sp. The pedal lever ratio ip is proportional to the brake master cylinder area Am. For a smaller brake pedal stroke sp, the brake pedal lever ratio ip should be as small as possible and the brake master cylinder area Am as large as possible should be selected to reduce the brake master cylinder stroke required to push the brake caliper. By increasing the vacuum The assist ratio iv of the booster and the spring reaction force are reduced to reduce the brake pedal force. The appropriate brake master cylinder area Am and brake caliper diameters d1 and d2 can reduce the brake pedal force Fp to a certain extent. This involves the reasonable matching of the parameters of each component while meeting the design requirements of vehicle braking deceleration and braking distance. In addition, the deformation of the brake caliper and the expansion and deformation of the brake hose are important factors affecting the brake pedal stroke sp. They will increase the volume of the brake fluid due to deformation, reducing the friction applied to the brake caliper. The brake fluid pressure P on the disc, and their deformation will increase with the increase of the brake fluid pressure, which requires more brake pedal travel to generate sufficient brake fluid pressure. Therefore, in the design, the shorter the length of the brake hose, the better, and the design of the brake caliper will also put forward stricter requirements for its stiffness.
Commonly Used Test Instruments
data processing
Each data collection starts from a vehicle speed greater than 100km/h, and the collection is terminated until the vehicle is completely stationary. Since the data collection frequency is 100Hz (collected once every 0.01s), anything higher than this frequency can be considered as noise, so Use filters to filter data. Data processing uses Matlab for low-pass filter design, and uses the filtfilt function (zero phase shift forward and backward digital filter) for processing, which can keep the phase characteristics of the original component of the input signal at zero, without distortion and the amplitude-frequency characteristic is greatly attenuated by the square of the filter amplitude-frequency characteristic. Both the initial and final values are chosen with small jumps to match the initial conditions. For the data starting point, the calculation is performed when the braking deceleration begins to increase significantly relative to the time history, and is terminated when the braking deceleration reaches the maximum value on the time history. However, in order to be able to evaluate the empty travel and ABS working conditions, on the basis of the above-mentioned interception interval, add 200 data points before and after (the data of 2 seconds before and after) for processing and analysis.
Objectively evaluate goal setting
The setting of evaluation goals should follow three principles:
1) Keep up with the trend, and the goal should be as close as possible to the development trend of the reference vehicle;
2) Higher than the existing capabilities, so as to promote the existing 3) can be achieved, the scope of the target should be able to be achieved after optimization, not out of reach.