DE4038080A1 - Motor vehicle brake temp. monitoring system - Google Patents

Abstract

The temp. monitoring system allows a warning signal to be supplied and/or reduction of the torque delivered by the engine and/or switching out of the anti-skid braking regulation upon a max, threshold temp. being reached. The temp. increase during braking and anti-slip braking regulation and the temp. reduction during cooling phases are accounted for during the determination fo the brake temp.

Description

In ASR systems, long-term brake intervention can cause a thermal Overloading the brake disc may occur without the driver experiencing this condition is reported.

In the known systems, therefore, the drive becomes the engine intervention Reduced torque supplied to wheels, or the ASR function to a certain Time span reduced. In certain ASR systems, the Brake load with ASR only specified a limited slip differential effect, however cannot prevent the two drive wheels from spinning together and thus ensures no ASR function.

Proposals are presented here that also ensure a full ASR function as long as the brake disc temperatures are below a permissible maxi painting temperature, the driver is informed if this maximum temperature is exceeded however warned and / or the ASR function switched off. This monitoring of Disk temperature is now not due to a complex and prone to failure Measurement, but by observing the temperature behavior in the control tech niche sense.  

In some ASR systems, the pressure for the ASR function is determined by itself priming return pump generated, the maximum pressure by a pressure limiter control valve is limited. When the ASR control has started, a constant pre pressure for ASR can be assumed.

The following now approximately applies to the heat balance on the brake disc Approach for services N:

- Power absorbed by pressure in the wheel brake cylinder:
N _on = K _wheel · P _wheel · n _wheel

- output by convection:
N _Konv = K _Konv · f₁ (n _{Rad, Δ} )

- Radiated power output:
N _ray = K _ray · T⁴ ≈ K _ray · (Δ + 273)

and for the heat quantity A stored in the brake disc:
A _Sch = K _C · Δ,

which can be combined into the following balance equation:
(N _on - N _conv - N _beam ) t = A _Sch (1)

With:
t: duration
P _wheel : pressure in the wheel brake cylinder
n _Wheel : wheel speed
T: absolute temperature
Δ: temperature difference to ambient temperature
K _wheel , K _conv , K _beam , K _C : disc and FZ-specific sizes
f₁: disk and FC-specific function that must be determined and can be stored in the control unit ROM.

To ensure the ASR function, a certain pressure must now be present in the brake disc, which is generated by actuating the control valves. Since the pressure-volume characteristic curve of the brake circuit is now known and is also constant according to the pre-suspension of the ASR pre-pressure, each pressure build-up and dismantling time t _up , t _down can be assigned to a change in volume or pressure, which can be achieved by integration or Sum of the individual amounts allows an approximate specification of the absolute pressure on the brake caliper. Multiplying this pressure value by the wheel speed known in the control unit and the constant K _wheel gives the current power introduced into the brake disc.

Thus, equation (1) represents an implied determination equation for the pane temperature A at all times, since all other variables are known and stored in the control unit. If this pane temperature Δ now reaches a limit value Δ _max , an action can be derived from it.

By solving equation (1) for each time interval, e.g. B. cycle time of the µC, and updating or storage of the last result is obtained Temperature curve of the brake disc at discrete time marks. The one to trigger Equation (1) required parameters have to be in different test series are measured or calculated and stored in the control unit ROM.

However, there is one main problem: when the control unit is switched off (e.g. after "Stalling" the engine), the control unit "forgets" the history and begins at t = 0, although the brake discs are already heated up.

Frequent ASR function with the ignition key removed in the meantime can occur (eg "engine stall", taxi operation in northern countries etc.) and thus the brake discs can be considerably overheated, must also in the phases with the ignition key removed or while driving without ASR function the temperature can be "observed" further.  

The following options are available:

• a) The heat capacity of the brake disk is associated with a capacitor (or Akkumu lator), the proportional with _on to N, N N _Conv and _beam currents loaded or is unloaded. When a voltage proportional to Δ _max is reached, a warning is given and / or the ASR function is switched off. The control unit is also switched off when the ignition key is removed. In this state, one or more such resistors (possibly non-linear as PTC, NTC, varistor, diodes, transistors or circuits thereof) are connected in parallel to the capacitor in such a way that the discharge behavior of the capacitor / resistor system corresponds to the cooling mechanism of the brake disc when the wheels are stationary. When the control unit is switched on again, the "observation" of the temperature is continued at the correct value.
• b) The heat capacity of the disk is assigned an electrical resistance (possibly non-linear as NTC, PTC, varistor, diodes, transistors or interconnections thereof), which is thermally packaged so that its cooling curve corresponds to the disk standstill caloric. By feeding with a current that corresponds to the power _balance N _on - N _conv - N _beam from (1), the resistance is heated and the voltage changes. When a voltage equivalent to Δ _max is reached, a warning is given and / or the ASR function is switched off. When the control unit is switched off and switched on again, the system behaves as described under a).
• c) The disk caloric is simulated by a digital "observer" in the RAM / ROM of the control unit. After removing the ignition key, the RAM and the µC remain switched on via an internal control until the observer has calculated a Δ _min and then switches off the control unit completely.
• d) In the ASR control unit there is a switching time clock in addition to the digital observer and a battery-buffered RAM available for the switch-off time. With Ab pulling the ignition key switches off the control unit, but the Switch-off time saved in RAM. When the control unit is switched on again the time elapsed since the last switch-off is calculated and it becomes then with the cooling characteristic when the wheels are stationary in the acti again  ob observers the cooling achieved until the switch-on time "recalculated". Thus, when the ASR function is reinstated, the correct pane temperature.
• e) as d), instead of the switch-on time clock in the control unit, the time of the vehicle clock - which of course must have corresponding outputs - is read in and stored in RAM. Other "temperature" calculation as under d).
  The cases outlined under a) to e) do not take the brake disc heating due to brake / ABS operation into account. Since there is no information about the braking area in the partial braking area or the HZ pressure is not known in the ABS case, only approximate information can be given. Prerequisite: disengaged, deceleration only by braking (BLS signal).
• f) Disc heating by braking: "observation" by FC delay.
  • f1) FZ with rear-wheel drive: Based on the full-load characteristic of the braking force distribution (or the statistical characteristic), the assigned rear-axle braking force B _H (a _F ) can be determined from the current FZ deceleration a _F (which can be calculated from the speed sensor signals), from which the thermal output per pane is calculated to: (1/2 because of two rear wheels).
    Temperature observation then as described under a) -e)).
  • f2) FZ with front-wheel drive: The procedure is the same as under f1), only for the calculation of the front axle braking force (B _V (a _F ), the empty characteristic curve of the braking force distribution diagram must be assumed: K ′ _wheel , K ′ ′ _wheel are FZ-specific parameters, B _H (a _F ), B _V (a _F ) are deceleration and FZ-specific parameters that must be measured or calculated in the experiment and stored in the ROM are.
• Since the actual brake force distribution is not known for f1), f2), the worst case is assumed, d. H. in the subsequent ASR case, in usually warned when the pane temperature is too "low" or switched off.
• g) Brakes with electronic brake force distribution, "observation" by FZ deceleration:
  g1), g2): The procedure is the same as in f1), f2), except that the actual brake force distribution that is known for EBKV algorithms is to be assumed here. The current pane temperature can thus be determined more precisely.
  When driving down a pass, there may now be heating of the window, although there is no or only a slight FZ deceleration or even a slight FZ acceleration. The temperature can be observed here with another auxiliary signal:
• h) FZ delay sensor (a _F sensor). When driving downhill, the a _F signal is independent of whether the FZ is decelerating or traveling at a constant speed when disengaged (worst case) is a measure of the FZ deceleration due to braking intervention, ie it is the method described under f) and g) can be used accordingly when engaged.

The advantage of the invention is that in ASR systems the brake disc temperature, especially the drive wheels, can be "observed" and thus next to the front given limited slip differential action also a full drive slip regulation with brake intervention is possible, d. H. Starting from a standing start at high engine excess torque. When a maximum pane temperature is reached the ASR function is switched off. Hach expiry of a "calculated" Ab The ASR function is the cooling time and falling below a minimum temperature admitted again. With the exception of case h), the "observation" of the Window temperatures only through software or hardware measures in the control unit and not through additional sensors in the FC. Of course, the "Monitoring" the brake disc temperature by using additional sensors, preferably by pressure sensors on the drive wheels, because this provides a better statement of the frictional force on the brake disc and thus of the performance brought in can be reported regardless of whether  is engaged or disengaged or there is an ascent or descent. It will be the Convection and radiation share as well as heat storage in the brake disc taken into account as indicated. These additional parameters can also be used can also be used advantageously for braking, ABS, ASR and EBKV.

Claims (1)

Traction control system, characterized in that the temperature of the brake discs is calculated by means of the pressure build-up and pressure reduction times in the ASR operation and by means of determinable vehicle-specific values and by taking into account the cooling in phases without brake actuation, and a warning and / or or shutdown signal is generated.