Method for determining the braking force in an electromechanical brake device having an electric brake motor

Abstract

In the case of a method for determining the braking force in an electromechanical brake device having a brake motor, the braking force is determined from a prevailing load current that is determined from the difference between the measured motor current and the calculated switch-on current.

Claims

1. A method for determining a braking force in an electromechanical brake device having an electric brake motor configured to urge a brake piston against a brake disc, the method comprising: determining a prevailing load current from a difference between a measured prevailing motor current of the brake motor and a calculated switch-on current; and determining a braking force from the prevailing load current, in response to a brake lining on the brake piston lying against the brake disc at a point in time in which the brake motor is switched on.

2. The method according to claim 1 further comprising: calculating the switch-on current as a function of a motor time constant.

3. The method according to claim 2 further comprising: calculating a motor time constant based on a motor resistance, motor constants, and an armature mass inertia using the equation: $\tau =\frac{R_M.Math.J}{K_M^2},$ where τ is the motor time constant, R.sub.M is the motor resistance, K.sub.M is the motor constants, and J is the armature mass inertia.

4. The method according to claim 2, the calculating the switch-on current further comprising: calculating the switch-on current using a recursive algorithm based on the motor time constant.

5. The method according to claim 4, the calculating the switch-on current further comprising: calculating the switch-on current based on the motor time constant using the equation: $I_{\mathrm{peak},n+1}=I_{\mathrm{peak},n}\frac{T_{\mathrm{cyc}}}{T_{\mathrm{cyc}}+\tau }.Math.\left(I_L-I_{\mathrm{peak},n}\right),$ wherein I.sub.peak is the switch-on current, n represents a point in time in a calculating cycle of the recursive algorithm, τ is the motor time constant, T.sub.cyc represents a cycle duration of the calculating cycle, and I.sub.L, represents an idling current.

6. The method according to claim 4 further comprising: determining an initial value of the switch-on current at the point in time zero in the recursive algorithm based on a falling edge of the switch-on current.

7. The method according to claim 1 further comprising: calculating a motor resistance and a motor constant during a preceding procedure of switching on the brake motor to create a movement of the brake piston toward the brake disc.

8. The method according to claim 1 further comprising: providing braking force with the electromechanical brake device according to the the determined braking force.

9. A control device for controlling adjustable components of an electromechanical brake device having an electric brake motor configured to urge a brake piston against a brake disc, the control device being configured to: determine a prevailing load current from a difference between a measured prevailing motor current of the brake motor and a calculated switch-on current; and determine a braking force from the prevailing load current, in response to a brake lining on the brake piston lying against the brake disc at a point in time in which the brake motor is switched on.

10. An electromechanical brake device for a vehicle, the electromechanical brake device comprising: an electric brake motor configured to move a brake piston in a direction toward a brake disc; and a control device for controlling adjustable components of the electromechanical brake device, the control device being configured to: determine a prevailing load current from a difference between a measured prevailing motor current of the brake motor and a calculated switch-on current; and determine a braking force from the prevailing load current, in response to a brake lining on the brake piston lying against the brake disc at a point in time in which the brake motor is switched on.

11. The electromechanical brake device according to claim 10, wherein the electromechanical brake device is a locking brake configured to hold the vehicle in a standstill.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Further advantages and expedient embodiments are apparent in the description of the figures and the drawings. In the drawings:

(2) FIG. 1 illustrates a schematic view of a vehicle hydraulic brake, wherein the wheel brake devices of the vehicle brake are equipped with electromechanical brake devices having in each case an electric brake motor,

(3) FIG. 2 illustrates a sectional view through an electromechanical brake device having an electric brake motor,

(4) FIG. 3 illustrates a flow diagram showing method steps for determining the braking force in an electromechanical brake device.

DETAILED DESCRIPTION

(5) In the figures, like components are provided with like reference numerals.

(6) The brake system illustrated in FIG. 1 for a vehicle comprises a dual-circuit vehicle hydraulic brake 1 having a first hydraulic brake circuit 2 and a second hydraulic brake circuit 3 so as to supply and control wheel brake devices 9 on each wheel of the vehicle with a brake fluid that is subjected to hydraulic pressure. The two brake circuits 2, 3 are connected to a common master brake cylinder 4 that is supplied with brake fluid via a brake fluid storage device 5. The master brake cylinder piston in the master brake cylinder 4 is actuated by the driver via the brake pedal 6, the pedal travel created by the drive is measured via a pedal travel sensor 7. A braking force booster 10, which comprises by way of example an electric motor that preferably actuates the master brake cylinder 4 (iBooster) via a gear, is located between the brake pedal 6 and the master brake cylinder 4.

(7) The vehicle hydraulic brake 1 may comprise in addition or as an alternative to the iBooster an integrated electro-hydraulic brake unit having a plunger that is driven by an electric motor. The vehicle brake 1 is advantageously configured as a brake-by-wire system, wherein actuation of the brake pedal causes the hydraulic volume to be displaced into a pedal travel simulator. The brake pressure accordingly builds up following the brake pedal actuation as a result of the plunger being actuated by means of an electric motor. In the event of a failure of the electro-hydraulic brake unit, separating valves that connect the brake circuit to the master brake cylinder 4 are opened with the result that as the driver actuates the brake pedal a direct hydraulic influence is exerted on the wheel brake devices 9.

(8) The adjustment movement of the brake pedal 6 measured by the pedal travel sensor 7 is transmitted as a sensor signal to a control device 11 in which control signals for controlling the braking force booster 10 are generated. The wheel brake devices 9 are supplied with brake fluid in each brake circuit 2, 3 via different switching valves that together with other components are part of a hydraulic brake system 8. Furthermore, the hydraulic brake system 8 comprises a hydraulic pump that is a component of an electronic stability program (ESP).

(9) The two hydraulic brake circuits 2 and 3 of the dual-circuit vehicle brake 1 are by way of example allocated diagonally with the result that the first brake circuit 2 supplies brake fluid by way of example to the two wheel brake devices 9 on the left-hand front wheel and on the right-hand rear wheel and the second brake circuit 3 supplies brake fluid to the two wheel brake devices 9 on the right-hand front wheel and on the left-hand rear wheel. Alternatively, it is also possible to allocate the two hydraulic brake circuits 2 and 3 of the dual-circuit vehicle brake 1 to the wheel brake devices on the front axle and the wheel brake devices on the rear axle.

(10) FIG. 2 illustrates in detail the wheel brake device 9 that is arranged on a wheel on the rear axle of the vehicle. The wheel brake device 9 is part of the vehicle hydraulic brake 1 and is supplied with brake fluid 22 from a brake circuit 2, 3. The wheel brake device 9 comprises moreover an electromechanical brake device that as a locking brake holds the vehicle at a standstill but also may be used to decelerate the vehicle during a movement of the vehicle.

(11) The electromechanical brake device comprises a brake caliper 12 having a claw 19 that engages over a brake disc 20. The brake device comprises as an actuator a DC electric motor as a brake motor 13, the rotor shaft of which drives a spindle 14 in a rotary manner and a spindle nut 15 is mounted on said spindle in such a manner as not to rotate. As the spindle 14 rotates, the spindle nut 15 is moved in an axial manner. The spindle nut 15 moves within a brake piston 16 that is a carrier of a brake lining 17, said brake lining being urged by the brake piston 16 against the brake disc 20. A further brake lining 18 that is held on the claw 19 in a location-fixed manner is located on the opposite side of the brake disc 20. The brake piston 16 is sealed on its outer side via a circumferential sealing ring 23 in a pressure-tight manner with respect to the receiving housing.

(12) As the spindle 14 rotates, the spindle nut 15 is able to move axially forward within the brake piston 16 in the direction toward the brake disc 20 and accordingly as the spindle 14 rotates in the opposite direction said spindle nut moves axially backward until it arrives at a stop 21. In order to generate a clamping force, the spindle nut 15 impinges against the inner end face of the brake piston 16 as a result of which the brake piston 16 that is mounted in an axially displaceable manner in the brake device and comprises the brake lining 17 is urged against the facing end surface of the brake disc 20.

(13) In order to generate the hydraulic braking force, the hydraulic pressure of the brake fluid 22 from the vehicle hydraulic brake 1 acts on the brake piston 16. The hydraulic pressure may also be effective in a supporting role when the vehicle is at a standstill as the electromechanical brake device is actuated with the result that the entire braking force is composed of the portion that is provided by the electric motor and the portion that is provided hydraulically. When the vehicle is travelling, so as to perform a braking procedure either only the vehicle hydraulic brake is active or both the vehicle hydraulic brake and also the electromechanical brake device are active or only the electromechanical brake device is active in order to generate the braking force. The control signals for controlling both the adjustable components of the vehicle hydraulic brake 1 and also of the electromechanical wheel brake device 9 are generated in the control device 11.

(14) The wheel brake device 9 that is illustrated in FIG. 2 and is equipped in addition with the electromechanical brake device having the brake motor 13 is preferably located on the rear axle of the vehicle. The electric brake motors 13 may be controlled independently of one another in the two wheel brake devices 9 on the rear axle.

(15) FIG. 3 illustrates a flow diagram showing the method steps for determining the braking force in an electromechanical brake device 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.

(16) Initially, in a first method step 30 during a first procedure of switching on the electric brake motor during which the brake piston is moved toward the brake disc, the motor resistance R.sub.M and the motor constant K.sub.M are determined during the current peak of the switch-on current. The subsequent second step 31 characterizes a second sequential procedure of switching on the brake motor, wherein during this switch-on procedure the current progression of the switch-on current initially experiences a current peak again in a manner known per se. In the next step 32, it is queried whether at the point in time of the switch-on procedure (step 31) the brake lining on the end face of the brake piston is in contact with the brake disc. If this is not the case, the method follows the NO-branch (“N”) to the step 33 and the method is terminated.

(17) If on the other hand the condition is fulfilled and the brake lining on the end face of the brake piston is consequently in contact with the brake disc, the method follows the YES-branch (“Y”) to the next step 34. In this case, the switch-on peak and build-up of force are superimposed on one another in the current progression of the motor current. In order to determine the magnitude of the braking force, it is necessary to determine the engine torque from which it is possible to determine the braking force in accordance with a kinematic equation. The motor torque is calculated by multiplying the motor constants K.sub.M by a load current I.sub.F:
M.sub.Mot=K.sub.M.Math.I.sub.F

(18) The load current I.sub.F is the particular portion of the entire current consumption of the electric brake motor that is responsible for the brake lining being pressed against the brake disc. The load current I.sub.F is calculated from the difference between the measured, prevailing motor current I.sub.Mot of the brake motor and a calculated switch-on current I.sub.peak in accordance with:
I.sub.F=I.sub.Mot−I.sub.peak

(19) The prevailing motor current I.sub.Mot is available from measurements. The switch-on current I.sub.peak that occurs directly after the electric brake motor is switched on may be calculated in a recursive manner in accordance with:

(20) $I_{\mathrm{peak},n+1}=I_{\mathrm{peak},n}\frac{T_{\mathrm{cyc}}}{T_{\mathrm{cyc}}+\tau }.Math.\left(I_L-I_{\mathrm{peak},n}\right)$

(21) The index n therein represents a discrete point in time within a calculating cycle, wherein the cycle duration between two sequential discrete points in time n, n+1 is represented by T.sub.cyc.

(22) The motor time constant τ may be calculated from the equation:

(23) $\tau =\frac{R_M.Math.J}{K_M^2}$

(24) in dependence upon the motor resistance R.sub.M, the armature mass inertia J and the motor constants K.sub.M.

(25) If the query in the method step 32 confirms that the brake lining is in contact with the brake disc, the YES-branch is followed and in step 34 the prevailing motor current I.sub.Mot is measured. Subsequently, as described above, in the next step 35 the switch-on current I.sub.peak may be calculated in the recursive algorithm. The switch-on current I.sub.peak,n+1 at the point in time=I is calculated on the basis of the switch-on current I.sub.peak,n at the preceding point in time n.

(26) The initial value I.sub.peak,0 of the switch-on current is available at the point in time 0 for the start of the recursive algorithm, wherein the initial value I.sub.peak,0 may be determined from the falling edge of the switch-on current from a measurement.

(27) Following this, as described above, the load current I.sub.F is calculated from the difference between the measured motor current I.sub.Mot and the calculated switch-on current I.sub.peak, following which the motor torque may be calculated by multiplying the load current I.sub.F by the motor constant. The motor torque M.sub.Mot is the basis for determining the braking force by taking into consideration a kinematic equation.