This application claims priority under 35 U.S.C. § 119 to application no. DE 10 2018 210 511.1, filed on Jun. 27, 2018 in Germany, the disclosure of which is incorporated herein by reference in its entirety.

TECHNICAL FIELD

The disclosure relates to a method for determining the braking force in an electromechanical brake device having an electric brake motor.

BACKGROUND

A method for estimating the force development in an electromechanical brake device is described in DE 10 2006 052 810 A1. In this case, an electric brake motor of the brake device is actuated, as a result of which a brake piston that is a carrier of a brake lining is urged against a brake disc.

In the case of the method described in DE 10 2006 052 810 A1, in order to achieve a precise as possible estimation of the force development, said force being exerted as the electric brake motor is actuated, a total electrical resistance and the motor constant of the electric motor are determined as characteristic variables and their prevailing temperature is taken into consideration.

SUMMARY

The method in accordance with the disclosure relates to determining the braking force in an electromechanical brake device in a vehicle, wherein the electromechanical brake device comprises an electric brake motor that influences a brake piston that is a carrier of a brake lining against a brake disc in order to generate braking force.

The electromechanical brake device may be integrated into a wheel brake unit that is also part of the vehicle hydraulic brake. In this case, the brake piston is influenced by braking pressure as the vehicle hydraulic brake is actuated and said brake piston is urged against the brake disc. The vehicle hydraulic brake and the electromechanical brake device form a common brake system.

The braking force in the electromechanical brake device may be determined with a greater degree of accuracy by means of the method in accordance with the disclosure even in the event that at the point in time in which the brake motor is switched on the brake lining on the brake piston already lies against the brake disc and is urged against the brake disc. If the brake motor is actuated in this situation in order to move the brake piston further against the brake disc so as to generate braking force, then usually initially in the first split seconds a clearance in the operative chain between the electric brake motor and the brake piston is eliminated and a force is built up following this. At the same time, a current peak in the switch-on current of the electric motor is still effective; in this situation the electric motor has not yet achieved its maximum idling rotational speed. The current that builds up the force is superimposed on the switch-on current with the result that it is not possible to evaluate the prevailing motor current directly for determining the braking force.

It is possible by means of the method in accordance with the disclosure for the current peaks that occur in the switch-on current to be filtered out from the prevailing motor current with the result that a current portion that is described as a load current is available and may be used to determine the prevailing braking force with a greater degree of accuracy. For this purpose, the switch-on current is calculated and subtracted from the prevailing motor current that is measured. The difference between the motor current and the switch-on current represents the prevailing load current that corresponds with the braking force that is generated as the electrical brake motor is actuated and the braking force may be determined from said load current.

The knowledge of the prevailing braking force that is generated by the electric brake motor of the electromechanical brake device may be used advantageously in different situations. If by way of example the electromechanical brake device is used to generate additional braking force in the drive mode, for example in the event of a failure of a system that boosts the braking force in the vehicle hydraulic brake, then it is possible via controlling the electromechanical brake device to ensure that an anti-lock braking system or another control system still functions. Where appropriate, an upper limit is specified for the electromechanically generated braking force.

In accordance with one advantageous embodiment, the switch-on current is calculated as a function of a motor time constant that depends in particular upon the motor resistance, the motor constants and the armature mass inertia of the electrical brake motor. The switch-on current may be determined as function of the motor time constants using a recursive algorithm in which the prevailing switch-on current is determined as a function of the switch-on current that is determined in a preceding calculation step by taking into consideration the motor time constants. The time difference between two successive calculation steps represents a cycle duration within a calculation cycle from calculation step to calculation step. The switch-on current is determined recursively in this manner.

The initial value of the switch-on current at the point in time 0 of the recursive algorithm may be determined in accordance with a further advantageous embodiment from a measured current value from the falling edge of the switch-on current progression. Consequently, all the information for the recursive algorithm is available for determining the switch-on current, wherein the switch-on current is continuously repeatedly calculated in discrete time steps. The switch-on current that is determined in the recursive algorithm is a current value that is determined within the framework of a model calculation.

The load current may thus be determined during continuous measurements of the current from the prevailing measured motor current of the brake motor by subtracting the calculated switch-on current. The motor torque may be determined from the load current by means of multiplying said load current by the motor constant and the braking force may be calculated from said motor torque from a kinematic equation in the knowledge of the radial spacing between the motor longitudinal axis and the engagement point of the brake lining on the brake disc.

In accordance with a further advantageous embodiment, the motor resistance and the motor constant are also calculated. As a consequence, it is possible to take into consideration factors such as in particular temperature that influence the motor resistance and the motor constant. The motor resistance and the motor constants are preferably calculated during a preceding procedure of switching on the brake motor so as to move the brake piston toward the brake disc. This approach has the advantage that the motor resistance and the motor constant are determined with a high degree of accuracy and they are available for the calculation of the braking force.

The disclosure relates furthermore to a method for providing a braking force in an above-described electromechanical brake device having an electric brake motor, wherein the described method for determining the braking force is used. In the case of the method for providing the braking force, the electric brake motor is controlled and moved in the direction toward the brake disc so as to generate a greater braking force or is moved in the release direction, in other words in the opposite direction thereto, so as to reduce the braking force in order to set a desired braking force level.

The disclosure relates furthermore to a control device for controlling the adjustable components of the electromechanical brake device with which the above-described methods are performed. Where appropriate, the control device may be part of a brake system in the vehicle. Control signals for controlling the adjustable components of the brake device, in particular of the electric brake motor, are generated in the control device.

The disclosure relates in addition to an electromechanical brake device for a vehicle that is used advantageously for holding the vehicle at a standstill but where appropriate it may also be used to reduce the speed of the vehicle from higher speeds. The electromechanical brake device comprises as described above an electric brake motor, a brake piston, which is a carrier for a brake lining, and also a brake disc against which the brake piston is moved together with the brake lining, and also an above-described control device for controlling the adjustable components.

The electromechanical brake device may where appropriate together with a vehicle hydraulic brake form a brake system in the vehicle.

The method for determining the braking force renders it possible to determine with a greater degree of accuracy the braking force in the event that at the point in time in which the brake motor is switched on and in particular as the brake piston is moved in the direction toward the brake disc the brake lining is already in contact with the brake disc so as to increase the braking force. However, the method may also be used in the event that at the point in time in which the brake motor is switched on the brake lining is at a distance from the brake disc.

