METHOD FOR OPERATING AN ELECTRIC MOTOR, CONTROL UNIT, PISTON PUMP

Description

[0001] The invention relates to a method for operating an electric motor, in particular a piston pump, wherein the electric motor has a rotor shaft and is controlled as a function of a power requirement with a desired speed and a desired direction of rotation for the rotor shaft, and wherein an actual speed of the rotor shaft is monitored.

[0002] The invention also relates to a control device for an electric motor and a piston pump. State of the art

[0003] A brake control system, in particular an anti-lock braking system (ABS) or a driving dynamics control system (electronic stability program ESP), as part of a motor vehicle's braking system, generally has at least one piston pump driven by an electric motor. This can be operated to generate hydraulic pressure in a brake circuit of the braking system. For this purpose, a cam or an eccentric disc is rotationally fixedly connected to a rotor shaft of the electric motor. The cam can be arranged directly on the rotor shaft or rotationally fixedly connected to the rotor shaft via a gear. The rotor shaft rotates about an axis of rotation. The cam converts the rotational movement of the rotor shaft into a translational movement or longitudinal movement of a piston of the piston pump. The cam rests against the piston in such a way that the piston is subjected to a compressive force by the cam, which causes the slidably mounted piston to be displaced longitudinally.This displacement occurs against a spring element and / or a pressure in the brake circuit. The electric motor must generate a drive torque in order to overcome a load torque caused by the displacement of the piston against the spring force dependent on a displacement path and / or the pressure in the brake circuit. The maximum drive torque that can be provided by the electric motor is normally greater than the maximum load torque. The load torque is at its maximum within one revolution of the cam, when the maximum displacement of the piston is reached, since this is when both the spring force dependent on the displacement path and the pressure in the brake circuit are at their maximum. If the rotor shaft is rotated further, the piston moves back in its original direction because the piston is pressed against the cam due to a preload from the spring element.The electric motor is usually controlled based on a power requirement to achieve a specific pressure or delivery volume of the piston pump in the brake circuit with a target speed and a target direction of rotation for the rotor shaft. However, the electric motor can block if the drive torque of the electric motor is not sufficient to overcome the load torque. This can occur in particular due to a low on-board voltage in the vehicle, which limits the power consumption of the electric motor and thus the maximum drive torque, or due to a high temperature of a motor winding of the electric motor, which leads to higher electrical resistance, or due to increased friction on the piston. With the electric motor blocked, the power requirement cannot be met and the piston of the piston pump cannot be actuated.In order to overcome such a blockage, it is known to design the electric motor with a greater maximum drive torque than would be necessary to overcome the assumed maximum load torque, so that no case occurs in which the electric motor blocks.

[0004] The published patent application US 2008 / 224533 A1 describes a method for operating an electric motor in which a predetermined target direction of rotation for a rotor shaft is temporarily changed in order to actively reduce an excessively high actual speed of the rotor shaft. Disclosure of the invention

[0005] The method according to the invention with the features of claim 1 is characterized in that, when the actual speed is zero and the target speed is not zero, the target direction of rotation is changed for a predetermined period of time and the electric motor is then controlled again at the target speed and the target direction of rotation. The electric motor is initially controlled, as known from the prior art, depending on a power requirement with a target speed and a target direction of rotation for the rotor shaft of the electric motor. The actual speed of the rotor shaft is also monitored. A blocked electric motor is detected by the fact that the actual speed is zero and the target speed is not zero.To release the blockage of the electric motor, the invention reverses the target rotational direction for a specified period of time when the blocked electric motor is detected, so that the rotor shaft briefly rotates in the opposite direction. After this specified period of time, the electric motor is again controlled at the target speed and the target rotational direction, so that the rotor shaft rotates in the originally specified direction again. The change in rotational direction is based on the assumption that the electric motor has to overcome a lower load torque when rotating in the opposite direction than when rotating in the originally specified direction, and thus the drive torque requirement of the electric motor decreases or is at least lower.This assumption is based on the arrangement of the piston on the cam described at the beginning, in which the load torque is at its maximum once within one revolution of the cam, when the maximum displacement of the piston is reached, since this is when both the spring force dependent on the displacement path and the pressure in the brake circuit are at their maximum. A change in the direction of rotation results in the piston being displaced back and the load torque and thus the demand on the drive torque of the electric motor decreasing. When the electric motor is controlled again in the target direction of rotation after the specified period of time, additional kinetic energy resulting from the acceleration is available to overcome the load torque occurring in the target direction of rotation from the rotation in the direction of the maximum load torque. By means of this kinetic energy the load torque can then be overcome without the target torque having to be increased.The method according to the invention thus has the advantage that a blocked electric motor is detected and, through appropriate control of the electric motor, the piston can be actuated again by briefly rotating the rotor shaft of the electric motor in the opposite direction and then returning it to the desired direction of rotation. This is achieved without increasing the desired torque of the electric motor. In particular, there is no need to use an electric motor with a higher maximum torque.

[0006] According to a preferred embodiment of the invention, it is provided that a motor winding for controlling the electric motor is energized depending on the angular position of the rotor shaft, and that an actual angular position of the rotor shaft is detected and provided with an offset for a predetermined period of time in order to determine the angular position therefrom, wherein the offset is selected such that the direction of rotation of the rotor shaft is changed. Preferably, to control the electric motor, the motor windings are energized depending on the angular position of the rotor shaft if the electric motor is an electrically commutated or brushless electric motor. For this purpose, the actual angular position of the rotor shaft is first detected. In particular, a permanent magnet is attached to the rotor shaft for this purpose, so that a sensor aligned therewith, in particular a Hall-effect-based sensor, detects the orientation of a magnetic field of the permanent magnet and the actual angular position is determined therefrom.The angular position used to control the electric motor is determined by adding an offset to the actual angular position for the specified period of time, i.e. after a blocked electric motor has been detected. The angular position therefore corresponds to the sum of the actual angular position and the offset. This offset is selected in such a way that the direction of rotation of the rotor shaft is changed because the motor windings are energized differently than would be the case without the offset. The offset therefore energizes the motor windings in such a way that, at the actual angular position, this causes the rotor shaft to briefly rotate in the opposite direction. The rotor shaft is thus brought into a new actual angular position, from which the electric motor has to overcome a lower load torque when the direction of rotation changes again than when rotating in the originally specified direction.After the specified time period, the angular position is no longer provided with the offset, so that the angular position corresponds to the actual angular position. Providing the angular position with the offset therefore has the advantage that, in particular, a control unit designed to control the electric motor does not need to be given a change in the desired direction of rotation as a parameter. Instead, the change results directly from the manipulated angular position.

[0007] According to a further preferred embodiment, the predetermined time period is between 2 and 5 milliseconds. By setting the predetermined time period in an interval between 2 and 5 milliseconds, it is advantageously ensured that the electric motor is controlled for a sufficiently long period of time to enable actual rotation of the rotor shaft opposite to the original desired direction of rotation. Depending on the speed of the rotor shaft, the time period is also advantageously sufficiently short so that the rotor shaft rotates in the changed desired direction of rotation, in particular only for a certain number of revolutions or a certain part of a revolution. This is particularly advantageous if the load torque to be overcome by the electric motor has at least a maximum within one revolution of the rotor shaft, as is particularly the case when the cam is arranged directly on the rotor shaft.

[0008] The control unit for an electric motor according to the invention is characterized by the features of claim 4 in that the control unit is specifically designed to carry out the method according to the invention. This results in the aforementioned advantages. Further preferred features and combinations of features emerge from the above description and the claims.

[0009] The piston pump according to the invention with the features of claim 5 has an electric motor, the electric motor having a rotor shaft, the rotor shaft having a cam or an eccentric disc, a piston of the piston pump resting against the cam or the eccentric disc in such a way that rotation of the rotor shaft causes a longitudinal displacement of the piston in the axial direction. The piston pump is characterized by the control unit according to the invention. This also results in the aforementioned advantages. Further preferred features and combinations of features emerge from the above description and from the claims.

[0010] The invention is explained in more detail below with reference to the drawings. These show: Figure 1 shows an electric motor with a control unit and an inverter, Figure 2 shows part of a piston pump and Figure 3 shows a method for operating the electric motor.

[0011] Figure 1shows an electric motor 1, with a motor winding 2 and a rotor shaft 3, on the front of which a permanent magnet 4 is arranged. The electric motor 1 is an electrically commutated or brushless electric motor. The control of the electric motor 1 is usually dependent on a power requirement with a target speed n target and a target direction of rotation for the rotor shaft 3. To control the electric motor 1, in the present case the orientation of a magnetic field 5 of the permanent magnet 4 is detected by a sensor 6 aligned therewith, and from this an actual angular position φ actual of the rotor shaft 3 is determined. A control unit 7 controls an inverter 8, which energizes the motor winding 2 depending on the power requirement and an angular position φ. As a rule, the angular position φ corresponds to the actual angular position φ actual .

[0012] Figure 2shows part of a piston pump 9 with a piston 10 and a spring element 11. The piston pump 9 can be operated in particular to generate a hydraulic pressure in a brake circuit of a brake system. The piston 10 of the piston pump 9 rests on a cam 12. The cam 12 is connected in a rotationally fixed manner to the rotor shaft 3 of the electric motor 1. In the present case, the cam 12 is attached directly to the rotor shaft 3 of the electric motor 1. The cam 12 is designed as an eccentric disk, i.e. it is circular and attached eccentrically to the rotor shaft 3. However, it is also possible for the cam 12 not to be attached directly to the rotor shaft 3, but to be connected in a rotationally fixed manner to the rotor shaft 3 via a gear. The rotor shaft 3 rotates about an axis of rotation. The cam 12 converts the rotational movement of the rotor shaft 3 into a translational movement or

[0013] The cam 12 is converted into a longitudinal movement of the piston 10 of the piston pump 9. The cam 12 bears against the piston 10 in such a way that the piston 10 is subjected to a compressive force by the cam 12, causing the displaceably mounted piston 10 to be displaced longitudinally. This displacement occurs counter to the spring element 11 and / or the pressure in the brake circuit. If the rotor shaft 3 is rotated further after the piston 10 has been displaced to its maximum, the piston 10 moves back to its original direction, since the piston 10 is pressed against the cam 12 due to a preload by the spring element 11.

[0014] In the following, with reference to Figure 3 An advantageous method for operating the electric motor 1 of the piston pump 9 is described. Figure 3The method is illustrated by a flowchart. In particular, the method ensures that a blocked electric motor 1 is detected and that the actuation of the piston 10 is enabled again by appropriately controlling the electric motor 1.

[0015] In a step S1, the control unit 7 determines a target speed n Soll and a target direction of rotation for the rotor shaft 3 as a function of a power requirement, in particular to achieve a specific pressure or delivery volume of the piston pump 9 in the brake circuit. At the same time, the sensor 6 determines the actual angular position φ Ist of the rotor shaft 3. In a step S2, the control unit 7 determines an actual speed n Ist and compares it with the target speed n Soll . Steps S1 and S2 are performed continuously.

[0016] If the actual speed n Ist and the target speed n Soll are the same, the method continues with step S5. If, however, the actual speed n Ist is zero and the target speed n Soll is not zero, the electric motor 1 is blocked. Then, in step S3, an offset φ Offset is determined. This offset φ Offset is selected such that the direction of rotation of the rotor shaft 3 is changed because the motor winding 2 is energized differently than would be the case without the offset φ Offset. The offset φ Offset therefore energizes the motor windings 2 in such a way that, at the actual angular position φ Ist, this causes the rotor shaft 3 to rotate in the opposite direction. The motor windings 2 are thus energized in such a way that the permanent magnet 4 attached to the rotor shaft 3 generates a torque in the changed direction of rotation.At the actual angular position φ Ist , which is determined in particular from the orientation of the magnetic field 5, the offset φ Offset is thus determined in such a way that the angular position φ as the sum of the actual angular position φ Ist and the offset φ Offset leads to the advantageous current supply to the motor windings 2.

[0017] In a step S4, the control unit 7 controls the inverter 8 depending on the angular position φ. The angular position φ corresponds to the sum of the actual angular position φ Ist and the offset φ Offset for a predetermined time period t. The motor winding 2 is energized according to the power requirement, so that the rotor shaft 3 of the electric motor 1 rotates opposite to the desired direction of rotation. The predetermined time period t is between 2 and 5 milliseconds. By setting the predetermined time period t in an interval between 2 and 5 milliseconds, the electric motor 1 is controlled for a sufficiently long period of time so that the rotor shaft 3 actually rotates opposite to the original desired direction of rotation. The predetermined time period t is also sufficiently short depending on the speed of the rotor shaft 3 so that the rotor shaft 3 rotates in the changed desired direction of rotation, in particular only for a certain part of a revolution.In step S5, the control unit 7 controls the inverter 8 depending on the angular position φ. The angular position φ corresponds to the actual angular position φ Ist . The motor winding 2 is energized according to the power requirement, so that the rotor shaft 3 of the electric motor 1 rotates at the target speed n Soll in the target direction of rotation.

Claims

1. Method for operating an electric motor (1), in particular a piston pump (9), wherein the electric motor (1) has a rotor shaft (3) and is actuated with a target rotational speed (ntarget) and a target rotation direction for the rotor shaft (3) depending on a power request, wherein an actual rotational speed (nactual) of the rotor shaft (3) is monitored, characterized in that the target rotation direction is changed for a specified time period (t) when an actual rotational speed (nactual) is equal to zero and a target rotational speed (ntarget) is not equal to zero and then the electric motor (1) is actuated again with the target rotational speed (ntarget) and the target rotation direction.

2. Method according to Claim 1, characterized in that a motor winding (2) for actuating the electric motor (1) is energized depending on an angular position (ϕ) of the rotor shaft (3), and in that an actual angular position (ϕactual) of the rotor shaft (3) is detected and provided with an offset (ϕOffset) for the specified time period (t) in order to determine the angular position (ϕ) therefrom, wherein the offset (ϕOffset) is selected in such a way that the rotation direction of the rotor shaft (3) is changed.

3. Method according to either of the preceding claims, characterized in that the specified time period (t) is between 2 and 5 milliseconds.

4. Control device (7) for an electric motor, characterized in that the control device (7) is specially prepared to carry out the method according to any of Claims 1 to 3.

5. Piston pump (9) comprising an electric motor (1), wherein the electric motor (1) has a rotor shaft (3), wherein the rotor shaft (3) has a cam (12) or an eccentric disc, wherein a piston (10) of the piston pump (9) bears against the cam (12) or the eccentric disc in such way that rotation of the rotor shaft (3) causes a longitudinal displacement of the piston (10) in the axial direction, characterized by a control device (7) according to Claim 4.