Electric motor assisted steering apparatus

The invention relates to an electric motor-supported steering system.

The term electric motor-supported steering devices includes active steering systems in which an electric motor has a driving action on a differential drive in order to superimpose a steering movement on the steering movement initiated by the driver depending on the driving situation. Such a steering device is known for instance from DE 39 08 217 A1. Furthermore, the aforesaid preamble also includes steering devices in which the task for the electric motor is merely to support the steering movement initiated by the driver by making available auxiliary steering force. In such steering systems, the electric motor is frequently drivingly connected to a rack of a steering gear.

The electric motors, embodied for instance as synchronous or asynchronous motors, are controlled using power electronics with a frequency converter. It converts the supply voltage from the vehicle onboard electric system to a plurality of clocked pulse-width modulated signals with frequencies in the 20 kHz range. The mean of the pulse-modulated signals is set such that the motor current desired depending on application is set via the inductivity of the electric motor. Depending on application, the motor current can be for instance sinusoidal or rectangular. The power electronics contains a plurality of power transistors. The switching processes performed thereby necessarily result in the transmission of interfering signals that affect electromagnetic compatibility (EMC). These are conducted for instance via the lines to the electric motor, the battery line for the steering device, and other electric connections on the steering device and there cause interfering emissions. In addition to the direct interfering emissions, there is overcoupling onto other lines in the motor vehicle and this causes further interfering emissions and/or galvanic interfering transmission to the onboard electric system.

Minor EMC interference results from long switching times, that is, from long control voltage rise and/or fall times and/or from long control current rise and/or fall times for building up and reducing the capacities of the power electronics or power transistors, but the power loss in the power electronics during long switching times is great, which can lead to the power electronics heating up sharply and thus in extreme cases to a functional failure of the power electronics and thus of the electric motor. When dimensioning the control of the electric motor, the switching times are set or established as slow as is permissible for a theoretical maximum ambient temperature or maximum power electronics temperature. That is, the design is based on the maximum permissible power loss and thus the maximum permissible heating at a theoretical maximum temperature.

The underlying object of the invention is to improve the EMC in a steering device.

The invention is based on the knowledge that, statistically, the theoretical maximum temperature underlying the design of the control of the electric motor is attained only rarely. The maximum temperature occurs only in vehicles used in hot climates, only for a very short period in a year, only during the hottest part of the day, only when the load on the vehicle's internal combustion engine is also at its maximum and the vehicle speed is slow, in particular when ascending mountains, and only when extreme steering maneuvers are performed at the same time. As a rule, the actual temperature of the power electronics or the ambient temperature of the power electronics is below this maximum temperature value so that over long operating periods even longer switching times could be attained, with resultant low EMC interference. Thus, most of the time high EMC interference is handled in a manner that is not noticed. In accordance with the invention, it is provided that the switching times for the power electronics are adapted to the actual temperature of the power electronics, or its ambient temperature, as measured by means of at least one sensor, in particular such that the switching times are maximized in order to cause the least EMC interference possible. In the framework of the invention, only the turn-on times or only the turn-off times or both together are varied.

Due to the invention, the EMC filter devices can be reduced and/or EMC emissions can be improved.

In accordance with the invention, the switching times for a power transistor in the power electronics can be changed by varying the control voltage rise speed and/or control current rise speed on the power transistors for building up or reducing the capacities of the power transistors. In particular a capacity must be built up between the gate terminal and the source terminal of a power transistor, preferably a MOSFET, so that the transistor conducts between source terminal and drain terminal. The drain or source terminal for the power transistor is connected to a coil of the electric motor. The time for building up or reducing this capacity is largely equal to the switching time of the power transistor.

In the simplest case, it is possible to select between at least two different switching times that can be provided for instance by selecting one of two activation lines in a power transistor that have different size ohmic resistances. If a pre-specified temperature is exceeded, the activation line with the smaller resistance with the resultant faster switching time is selected. If the temperature drops below the pre-specified level, the alternative activation line is used. In addition, it is conceivable that if the temperature drops below an even lower pre-specified power electronics temperature, the connecting lines will be used together for supplying the power transistor with control voltage and/or control current.

A more finely graduated or continuous adaptation of the switching times to the temperature of the power electronics or to the environment of the power electronics can be provided in that a power transistor driver unit is provided for furnishing the control currents and control voltages for the power transistors of the power electronics (power output stage). For varying the switching times of the power electronics components it is now provided that the power transistor driver unit includes at least one variable, i.e. adjustable, current source for varying the turn-on time of a power transistor for the power electronics. If the variable current source is connected by means of a switch, a control current flows to the power transistor. A more rapid or slower switching process for the power transistor results depending on the current value selected. The control current rise speed and/or control voltage rise speed can be changed by means of the variable programmable current source. Preferably the power transistor driver unit contains a charging pump in order always to make available to the variable current source a constant input voltage, preferably 10 V, since the onboard electric system voltage is subjected to great fluctuations.

A logic unit is provided for controlling the variable current source as a function of the temperature measured by means of a sensor, and it is connected to the temperature sensor. The logic unit provides the control current rise speed or control voltage rise speed. The power transistor driver unit can be connected to the logic unit via a digital or analog interface. If a digital interface is used, serial data signals flow, whereby different current rise rates can be associated with the transmitted data signals. If it is an analog connection, the transmitted voltages or currents can be used directly as the control voltage for the adjustable current source.

In addition to or alternative to at least one adjustable current source for changing the rise times for the control voltage and/or control current for the turn-on process for a power transistor, at least one additional, separately adjustable current source can be provided in order to vary the turn-off time for the power transistor. It is also conceivable to provide a single common adjustable current source that can be used to commonly vary the turn-on and turn-off times of the transistor.

Additional advantages and useful further embodiments of the invention are explained in the claims, description of the figures, and drawings.

FIG. 1 is a schematic depiction of a steering device with electric motor and control device;

FIG. 2 is a schematic depiction of an electric motor for a steering device with control devices; and

FIG. 3 is a schematic depiction of an alternative control device.

Identical components and components with the same function are identified with the same reference numbers in the figures.

FIG. 1 is a schematic depiction of an electromechanical steering system 1 with steering means 2 embodied as a steering wheel. The steering means 2 is connected via a drag link 3 to a steering gear 4 that converts the rotary movement of the steering means 2 to a translatory movement that is transmitted via a linkage 5 to front vehicle wheels 6 for changing the vehicle wheel angle of lock. An electric servomotor 7 acts on the steering gear 4 for supporting the steering movement of the driver. During active steering, in addition or alternative to the electric motor 7 , another electric motor that acts on a superimposing gear in the steering column can be provided that can be activated in accordance with the invention.

For controlling the electric motor 7 , a frequency converter 9 with a plurality of power transistors (not shown in FIG. 1 , see FIGS. 2 and 3 ) is provided as power electronics 8 . The transistors contain their control voltage or their control current for building up and reducing the capacities of a power transistor driver unit 10 with a plurality of adjustable current sources (not shown in FIG. 1 , see FIG. 2 ). The power transistor driver unit 10 is connected with connecting lines 11 to the onboard electric system of the motor vehicle. Likewise, the frequency converter 9 is connected to the onboard electric system with connecting lines 37 for connecting the voltage or current to be modulated. The power transistor driver unit 10 is connected to a logic unit 13 that regulates or controls the current sources as a function of the temperature of the power electronics 8 measured by means of a sensor 12 or the ambient temperature of the power electronics 8 . The switching times of the semi-conductor components in the power electronics are influenced using the change in the control current rise speed and/or control voltage rise speed, depending on the temperature measured. The slower the rise time, the greater the power loss and thus the heat radiation and the lower the transmission of interfering signals. In accordance with the invention, the switching times can be slowed as much as is permissible at the current temperature, possibly less a safety temperature range, which results in a higher power loss but weaker interfering signals.

The temperature sensor 12 , together with the power transistor driver unit 10 and the logic unit 13 , can be arranged on a plate that together with the power electronics 8 is arranged on a metal block for removing heat. Likewise, it is possible to embody the temperature sensor 12 as an immediate component of the power transistor driver unit 10 . The measured temperature is then the ambient temperature of the power electronics 8 , which is largely equal to the actual temperature of the power electronics 8 or is at least proportional thereto.

FIG. 2 depicts an electric motor 7 that acts for instance directly drivingly on a steering gear for a steering device or on a superimposing gear (active steering).

The three-phase system of the depicted electric motor 7 has three coils 14 , 15 , 16 offset in the circumferential direction that are supplied electrical energy by three phases U, V, W that are switched as a star circuit. The phases U, V, W are connected to power electronics embodied as a frequency converter 9 . The frequency converter 8 includes six power transistors 17 through 22 , so-called MOSFETs (metal oxide semiconductor/silicon field effect transistors), two power transistors 17 , 18 ; 19 , 20 ; 21 , 22 arranged in the drawing one above the other being allocated to each phase U, V, W. The power transistors in the power transistor pair are inversely clocked and supply the coils 14 , 15 , 16 with pulse-width-modulated voltage or current. A dead time is maintained between the switching of the power transistors of a power transistor pair in order to prevent simultaneous turn-on. The frequency converter 8 or [sic] is supplied with onboard electric system voltage that is switched or modulated by the power transistors 17 through 22 .

The power transistors are connected with their gate terminals G and source terminals S to a power electronics driver unit 10 . In this exemplary embodiment, the latter has two variable, displaceable current sources 23 , 24 for each power transistor 17 through 22 , whereby for reasons of clarity only current sources 23 , 24 for controlling power transistor 21 are illustrated. The current source 23 at the top in the drawing varies the control current rise speed or control voltage rise speed for the turn-on process of the transistor 21 , while the lower current source 24 varies the control current or control voltage rise speed for the turn-off process of the transistor 21 . The current sources 23 , 24 can be turned on/off via switches 25 , 26 .

For controlling the current sources 23 , 24 as a function of the temperature of the power electronics 8 , a logic unit is provided 13 that communicates with the power electronics driver unit 10 , in the depicted exemplary embodiment via digital interfaces 27 , 28 (e.g. SPI, I 2 C). Likewise, it is conceivable to use analog interfaces (pulse width modulation, voltage). The logic unit 13 is connected to the temperature sensor 12 that measures the temperature of the power electronics or its ambient temperature. The current sources 23 , 24 are controlled depending on the measured temperature, which changes the switching times for the power transistors 17 through 22 , in this case the power transistor 21 .

FIG. 3 depicts a simpler embodiment of the invention. Using this embodiment it is only possible to switch between three different switching times, but this by itself contributes to a clear improvement in the EMC. For reasons of clarity only the control of one power transistor 21 is depicted. The arrangement of the electric motor 7 and frequency converter 9 is the same as in the exemplary embodiment in accordance with FIG. 2 . The gate terminal G of the power transistor 21 is connected to two activation lines 29 , 30 that are switched in parallel and that have a common connection to a constant current source 31 with a nominal voltage of 10 V. Furthermore the source terminal S of the power transistor 21 is connected to the constant current source 31 via a control line 34 . Arranged in each activation line 29 , 30 is an ohmic resistance 32 , 33 , whereby the two resistances 32 , 33 are different in size. Each activation line 29 , 30 can be turned on and off using a switch 35 , 36 . For activating the switches 35 , 36 , they are connected to a logic unit 13 that itself is connected to a temperature sensor 12 for measuring the temperature of the power electronics 8 . As shown, the temperature sensor can for instance be embodied outside of the electronic components (to the right in the drawing) or additionally or alternatively for instance can be embodied as part of the power transistor driver unit 10 (center of the drawing). The switches 35 , 36 are actuated as a function of the temperature range of the measured temperature. Thus the activation lines can be used alternatively or cumulatively. Due to the resulting different size fall in the voltage based on the different resistances, there is an effect on the control current or control voltage of the power transistor 21 , so that there can be variation between three different switching times. It is conceivable to provide additional activation lines and thus finer graduation in the switching times.

In order to effect a turn-off process in the power transistor 21 , each activation line 29 , 30 is connected via a connecting line 38 , 39 , each of which has a switch 40 , 41 , to the control line 34 . By closing the switches 40 , 41 , the capacitor formed between gate terminal G and source terminal S is discharged, which causes the power transistor 21 to turn off the flow of current between gate terminal G and drain terminal D.

1 Steering device

2 Steering means

3 Drag link

4 Steering gear

5 Linkage

6 Vehicle wheels

7 Electric motor

8 Power electronics

9 Frequency converter

10 Power electronics driver unit

11 Connecting lines

12 Temperature sensor

13 Logic unit

14 Coil

15 Coil

16 Coil

17 Power transistor

18 Power transistor

19 Power transistor

20 Power transistor

21 Power transistor

22 Power transistor

23 Variable current source

24 Variable current source

25 Switch

26 Switch

27 Interface

28 Interface

29 Activation line

30 Activation line

31 Constant current source

32 Resistance

33 Resistance

34 Control line

35 Switch

36 Switch

37 Connecting line

38 Connecting line

39 Connecting line

40 Switch

41 Switch