Method for position regulation of an electric drive and for steering a motor vehicle by means of a steer-by-wire system

[0001] The invention concerns a method for positionally controlling an electric drive and for steering a motor vehicle with a steer-by-wire steering system as recited in collateral claim 1.

[0002] The positions of electric drives must be controlled in a wide variety of technical fields. Examples include not only the electric drives of machine tools and manufacturing robots, but also the valve actuators of steer-by-wire steering systems having hydraulic steering boxes.

[0003] All of these positional controls have in common the fact that the position of the electric drive must follow the prescribed setpoint with the least possible delay and without overshooting. It is also desirable that the position control be able to perform effectively with the lowest possible number of sensors, and further, that it exhibit robust control behavior that is insensitive to internal and external disturbances.

[0004] The position-control method of the invention is described hereinbelow with reference to the example of a valve actuator of a so-called steer-by-wire steering system having a hydraulic steering box. This does not imply any limitation of the protective scope of the invention to steer-by-wire steering systems. Method of steering a motor vehicle with a steer-by-wire system¹.

[0005] Steer-by-wire steering systems are distinguished by the absence of any continuous mechanical connection between the steering wheel and the steered wheels.

[0006] There are two sets of problems that must be overcome in the design of steer-by-wire systems. First, the driver's steering input must be transmitted from the steering wheel to the steered wheels, and second, the driver must receive feedback from the steered wheels to the steering wheel. The driver senses this feedback as a torque exerted on him by the steering wheel. This torque will be referred to hereinbelow as the steering feel.

[0007] Such a steer-by-wire system must be at least as good as a conventional servo steering system in terms of operational reliability and control behavior. Furthermore, it must be possible to integrate higher-order functions such as tracking control or vehicle dynamics control and crosswind compensation into the steer-by-wire system. Finally, a steer-by-wire system must be readily adaptable to different types of vehicles.

[0008] The object on which the invention is based is to provide methods for position control, particularly for steering a motor vehicle with a steer-by-wire system, that exhibit high control quality, operate safely and reliably, and permit the integration of higher-order functions.

[0009] This object is accomplished according to the invention by means of a method for positionally controlling an electric drive as recited in claim 1 and a method for steering a motor vehicle with a steer-by-wire system as recited in collateral claim 2, wherein

[0010] the steering-wheel angle is detected;

[0011] the steering-wheel angle is converted into a setpoint for the position of the steered wheels;

[0012] an actual value of the position of the steered wheels is detected;

[0013] a control difference between the setpoint and the actual value of the position of the steered wheels is generated;

[0014] the position of the steered wheels is adjusted according to the control difference;

[0015] a steering-wheel torque is adjusted according to the torque setpoints and/or the forces prevailing between the steered wheels and a steering regulator.

[0016] This method improves control quality and operational reliability, since the steering torque and the positions of the steered wheels are adjusted separately. This structure supports the conversion of the method into a modular software program. In addition, higher-order functions can be integrated readily and adaptation to different vehicles is simplified.

[0017] As a complement to the method of the invention, it is provided that a first controller outputs, as a manipulated variable derived from the control difference, a first setpoint of a valve actuator of a hydraulic steering system; that in parallel with the first controller, a compensator outputs, as a manipulated variable derived from the setpoint for the position of the steered wheels, a second setpoint of the valve actuator; that the first setpoint and the second setpoint are added to yield a setpoint of the valve actuator; and that the setpoint is the reference variable for a motor controller, so that nonlinearities of the hydraulic steering box are compensated for by the compensator and the subsequent control can thus be performed as a linear control.

[0018] As a complement to the method of the invention, it is provided that in the first controller, the control difference _{SW, setpinion} is amplified according to the rotation angle _pinion of the pinion in the region of the center position of the pinion, and that the product of the control difference _{SW, setpinion} and the amplification is integrated in an integrator to yield the first setpoint _{VA, set1}. This increases the control difference at the center position of the steering, as a result of which the manipulated variable of the steering controller is increased and the steering system therefore reacts sensitively to small changes in the driver's steering input when the steering wheel and the steered wheels are at or near center position.

[0019] In a further variant of the invention, it is provided that the motor controller is implemented as a cascade controller comprising a master controller and at least one slave controller, and that the control difference of the master controller is generated from the setpoint and the actual value of a rotation angle of the valve actuator.

[0020] In particular, it is provided that the master controller is a position controller and that the manipulated variable of the master controller is a rotation speed setpoint of the valve actuator, that the control difference of a first slave controller implemented as a rotation-speed controller is generated from the rotation speed setpoint and the actual rotation speed of the valve actuator, that the manipulated variable of the rotation-speed controller is a torque setpoint of the valve actuator, and/or that a current setpoint of the valve actuator is generated from the torque setpoint via a torque/current characteristic, that the control difference of a second slave controller implemented as a torque controller is generated from the torque setpoint and the actual torque of the valve actuator, and/or that the control difference of a third slave controller implemented as a current controller is generated from the current setpoint and the actual current of the valve actuator, and that the current controller drives the valve actuator via a frequency converter.

[0021] The use of a cascade controller improves control quality in that the response behavior of the control is enhanced without any accompanying oversteering of the steered wheels.

[0022] Further complements to the method of the invention provide that a first disturbance variable M_dist1 is subtracted from the manipulated variable M_set of the master controller and that said first disturbance variable is calculated according to the following equation:

[0023] C_torsion bar being the torsion spring rate of the torsion-bar valve.

[0024] Incorporating a disturbance variable in this manner compensates for the oscillation caused by the torsion bar, and the control reacts even more quickly and accurately to changes in the driver's steering input.

[0025] The control quality can be further improved by subtracting a damping torque M_damp from the manipulated variable M,et of the master controller and calculating the damping torque M_damp according to the following equation:

[0026] where D represents a constant and a rotation speed.

[0027] In further complements to the method of the invention, the generation of the setpoint for the position of the steered wheels is performed in a speed-dependent manner and/or a first correction angle is superimposed on the steering-wheel angle by a tracking controller according to a steering-wheel course angle, and/or a second correction angle is superimposed on the steering-wheel angle by a vehicle-dynamics controller according to the road speed, the transverse acceleration and/or the yaw rate of the vehicle, so that the steering behavior and the driving stability of a vehicle equipped with a steer-by-wire system according to the invention are improved and surpass the driving behavior of a vehicle equipped with a conventional servo steering system. In addition, crosswind compensation, for example, can also be performed.

[0028] A further embodiment of the method according to the invention provides that the steering-wheel torque is generated according to the difference between the rotation angle of the valve actuator and the pinion angle, or that the steering-wheel torque is controlled according to the actual current of the valve actuator, thereby eliminating the need for a torque sensor.

[0029] The object cited at the beginning hereof is also accomplished according to the invention by means of a steer-by-wire system for a vehicle, comprising a steering wheel, a steering column, a rotation-angle sensor, a steering-wheel motor acting on the steering column, a steering actuator acting on the steered wheels via a steering box and a tie rod, and a control unit according to claim 15 sic, so that the advantages of the method according to the invention are also brought to bear in the steer-by-wire system according to the invention.

[0030] Further advantages and advantageous embodiments of the invention may be apprehended from the following drawing, the description of the drawing, and the claims.

[0031] In the drawing:

[0032] FIG. 1 is a schematic diagram of an exemplary embodiment of a steer-by-wire system,

[0033] FIG. 2 is a block diagram of an exemplary embodiment of a method according to the invention,

[0034] FIG. 3 is a block diagram of a first complement to the method of the invention, and

[0035] FIG. 4 is a block diagram of a second complement to the method of the invention.

[0036] FIG. 1 represents a steering wheel 1 with a steering column 3 of a steer-by-wire system according to the invention. Arranged on the steering column 3 are a first rotation-angle sensor 5, a second rotation-angle sensor 7 and an electric motor 9 for the steering wheel.

[0037] The steered wheels 11 of the motor vehicle, which are not shown in FIG. 1, are connected to one another via a tie rod 13 and are steered by a steering actuator 15. Steering actuator 15 is based on a hydraulic steering box that is known per se. The steering box of steering actuator 15 is implemented as a rack-and-pinion steering box comprising a rack 17 and a pinion 19. In conventional steering systems, the pinion 19 is driven by the steering column 3. In the steer-by-wire system of the invention, pinion 19 is driven by an electric valve actuator 21. Disposed between valve actuator 21 and pinion 19 is a torsion-bar valve 23. Torsion-bar valve 23 controls the servo assistance of steering actuator 15 by releasing, to a greater or lesser extent, a hydraulic connection between a feed pump 25 and a double-acting working cylinder 27, which is shown in merely indicative fashion in FIG. 1. Working cylinder 27 acts on tie rod 13. Feed pump 25, torsion-bar valve 23 and working cylinder 27 are interconnected via connecting lines 29. Provided on the rotor of valve actuator 21 is a third rotation-angle sensor 31, which detects the rotation angle of valve actuator 21. Provided on pinion 19 is a fourth rotation-angle sensor 33, which detects the rotation angle of pinion 19. Rotation-angle sensor 33 can be supplanted by a position sensor 40.

[0038] Arranged in torsion-bar valve 23 is a torsion bar (not visible in FIG. 1), which twists in dependence on the torque transmitted from valve actuator 21 to pinion 19. On the one hand, the twist of the torsion bar is utilized in torsion-bar valve 23 to drive working cylinder 27, and on the other hand, an angular difference between the third and fourth rotation-angle sensors can be used to determine the amount of torque applied by valve actuator 21. This eliminates the need for a torque sensor on steering actuator 15, as well. Any transmission gearing that may be present between valve actuator 21 and pinion 19 must also be taken into account.

[0039] Valve actuator 21 is driven via a valve-actuator frequency converter 35 and a steering actuator 37. The reference variable of steering actuator 37 is a steering setpoint _{SW, set}, which is generated according to the rotation angle _SW of the steering wheel 1 as measured by first rotation-angle sensor 5 and/or second rotation-angle sensor 7 and, for example, the road speed of the vehicle. The control system of steering actuator 15 and the steered wheels 11 is laid out in detail in FIG. 2.

[0040] FIG. 2 is a block diagram of a control system according to the invention for the steering actuator. The steering control system consists of a steering controller 41 and a motor controller 43.

[0041] Steering controller 41 in turn consists of a controller 45 and a compensator 47. Controller 45 controls the rotation angle _pinion of pinion 19. The reference variable of controller 45 is the setpoint wheel angle _{SW, set} imposed by a setpoint generating system (not shown in FIG. 2). Provided in parallel with controller 45 is a compensator 47 that serves to offset nonlinear effects of the steering actuator 15, especially of the hydraulic steering. From the output variables of controller 45 and compensator 47, a setpoint *_{VA, set} is generated. This setpoint *_{VA, set} is the reference variable of motor controller 43. Motor controller 43 is implemented as a cascade controller and comprises, in the exemplary embodiment shown, a master controller 49 implemented as a position controller, a first slave controller 51 implemented as a rotation-speed controller, optionally a second slave controller 53 implemented as a torque controller, and a third slave controller 55 implemented as a current controller.

[0042] Master controller 49 has the task of adjusting the angle _VA measured by third rotation-angle sensor 31 at valve actuator 21 in such a way that _VA follows the setpoint _{VA, set} without overshooting. The output variable of master controller 49 is a rotation speed setpoint n_set that serves as the reference variable for first slave controller 51. First slave controller 51 evaluates the difference between the setpoint rotation speed n_set and the actual rotation speed n of valve actuator 21, which can be deduced, for example, from the change in rotation angle _VA over time. The output variable of first slave controller 51 is a torque setpoint M_set. In the case of torque control, a current setpoint I_set is generated from the difference between the torque setpoint M_set and an actual torque M_actual determined at valve actuator 21, as noted above. Optionally, the second slave controller 53 can also be omitted and the current setpoint I_set obtained by means of a torque/current characteristic.

[0043] A third slave controller 55 then adjusts the current supply to valve actuator 21 by comparing the current setpoint I_set to an actual current I_actual measured at valve actuator 21 and delivering a drive signal to a frequency converter 57².

[0044] Implementing motor controller 43 as a cascade controller with a master control loop governed by a master controller 49 and multiple slave control loops improves the quality of the control of rotation angle _pinion. When a disturbance occurs, the change that begins earliest in time, for example a change in the rotation speed n, the torque M or the current I, is sufficient to trigger a control process through the slave controller, an approach that assists the control system as a whole. This makes it possible for the rotation angle _pinion of pinion 19 to follow the steering-wheel-angle setpoint _{SW, set} rapidly, but without overshooting.

[0045] The aforementioned setpoint generating system 39 is laid out in greater detail in FIG. 3³. Taking as a point of departure a driver steering input materialized in the form of a steering-wheel angle _SW, the steering-wheel angle setpoint _{SW, set} is modified according to the speed v of the vehicle. This permits speed-dependent conversion of the rotary motion imposed on the steering wheel 1 into rotary motion of the pinion 19, which acts on the rack 17.

[0046] Optionally, the steering-wheel angle setpoint _{SW, set} can also be influenced by further higher-order functions. For example, provided in FIG. 3 is a tracking controller 59, which is connected to the setpoint generating system via an interface A. The tracking controller 59 superimposes a first correction angle _{SW, var1} on the driver's steering input _SW in accordance with a course angle _C. This influences the steering-wheel angle setpoint _{SW, set} in such a way that the vehicle follows a given course, or at least the driver receives feedback via the steering wheel when he strays from a given course.

[0047] A further higher-order function is implemented in a vehicle-dynamics controller 61. A second correction angle _{SW, var2} is superimposed on the driver's steering input _SW as a function of the speed v, transverse acceleration a_y and yaw rate of the vehicle. The steering-wheel angle setpoint _{SW, set} is generated from the driver's steering input _SW, the speed-dependent conversion ratio and optionally the first correction steering angle _{SW, var1} and the second correction angle _{SW, var2}. This steering-wheel angle _{SW, set} is the input variable of controller 45 from FIG. 2. Tracking controller 59 and the vehicle-dynamics controller 61 can also, of course, be switched off. Additional functions, such as, for example, a crosswind compensation system (not shown), can also be integrated into the setpoint generating system in like manner.

[0048] The advantages of the steer-by-wire system of the invention are, among other things, that the overall structure of the motor control system and the steering control system is easy to understand, and that higher-order and collateral functions can be integrated simply and independently of one another. With this structure, it is possible to generate situation-dependent setpoints _VA for the valve actuator 21, to design the controllers so that they are robust and fault-tolerant, and to apply the steering control system with a particular focus on making it driver-adaptive and user-friendly.

[0049] The steering-wheel control system can be implemented as a closed-loop or an open-loop system. As noted in FIG. 1, the manual torque setpoint M_{M, set} is determined by means of the angular difference _VApinion. Alternatively, the manual torque setpoint M_{M, set} can be determined from the motor current I_actual of valve actuator 21. In a further alternative, the manual torque setpoint M_{M, set} can be generated via a family of characteristic curves or a performance graph (containing vehicle-dependent or driving-situation-dependent parameters). To prevent any erroneous determination of the manual torque setpoint M_{M, set}, both of the aforesaid methods of determining the manual torque setpoint can be used in parallel and a cross-check performed. The manual torque setpoint is delivered to a steering-wheel controller 63, which drives the steering-wheel motor 9 via a frequency converter 65 in such a way that the manual torque M_M is transmitted to the steering wheel 1. In the design of the steering-wheel controller 63, special emphasis is placed on the torque ripple of the steering-wheel motor 9 to ensure that the steering feel is comfortable for the driver.

[0050] FIG. 4 is a block diagram of a portion of the first controller 45 of FIG. 2. To enable the control system to respond with particular sensitivity to changes in the steering-wheel angle setpoint SW, set, which is a measure of the driver's steering input, the rotation angle of the pinion _pinion is branched off before the control difference _{SW, setpinion} and is amplified according to the rotation angle _pinion. As indicated by the characteristic curve 67 shown in FIG. 4, the amplification factor is high at the center position of the steering system and the steering device and falls away symmetrically from this center position. This signal modified by the characteristic curve is multiplied by the control difference _{SW, set pinion}. The resulting signal is then routed to an amplifier K_i and subsequently to an integrator 69. The output variable of the integrator 69 is the first angle setpoint of the valve actuator, _{VA, set1}. This measure improves the response behavior of the steering system according to the invention during straight travel and causes the steering system to respond with greater sensitivity to even the smallest changes in the driver's steering input.