SYSTEM AND METHOD OF SIMULATING A STEERING RESISTANCE TORQUE ON A VEHICLE STEERING WHEEL

[0001] The present invention relates to a system and method of simulating a steering resistance torque on a vehicle steering wheel of a steer-by-wire system using a magnetorheological device and a motor.

[0002] Vehicle steer-by-wire systems are known and are ever so changing. Steer-by-wire systems eliminate the mechanical linkages between the steering wheel and the vehicle road wheels and allow for electronic steering of the vehicle by a driver thereof. Thus, in order to provide an appropriate feel to a steer-by-wire system, mechanisms are placed adjacent the steering wheel and steering shaft of the vehicle to generate resistive torque which resist a driver when the steering wheel is rotated for a vehicle turn during normal operation of the vehicle. This simulates for the driver an appropriate feel to the vehicle steering wheel as though the driver were driving a vehicle having a conventional mechanical steering wheel assembly.

[0003] Although current systems and methods of providing steering resistive torque are adequate, improvements can be made thereto. Typically, a motor is placed on the steering wheel shaft to generate torque applied on the steering wheel. This accomplishes a resistive torque on the steering wheel when the driver rotates the steering wheel during a turn. Torque generated by the motor has been identified as active force or torque on the steering wheel, since such torque is generated with a direction opposite the direction in which the driver rotates the steering wheel. The power required to generate such torque is relatively significant. However, manufacturers have been challenged in improving current systems and methods, specifically in areas of response time and power efficiency.

[0004] Thus, it is one aspect of the present invention to provide a system and method of simulating a steering resistance torque on a vehicle steering wheel of a steer-by-wire system.

[0005] It is another aspect of the present invention to provide a system and method of simulating a steering resistance torque for improved response time and power efficiency. In this aspect, the present invention provides an improved steer-by-wire system which responds in less time to actual steering wheel and vehicle parameters and operates with improved power efficiency. The present invention includes a steer-by-wire system which uses a motor and a magnetorheological (MR) device for variable damping on the steering wheel.

[0006] In this aspect of the present invention, implementation of both the motor and the MR device allows the system to generate an improved torque response, saving time and energy, depending on steering wheel and vehicle parameters. As mentioned above, torque generated by the motor has been identified as active force or torque on the steering wheel, since such torque can be generated in either rotational direction of the steering wheel. Generally, active torque is applied in a direction opposite the direction in which the driver rotates the steering wheel. However, torque generated by the MR device has been labeled as passive force or torque on the steering wheel, since such torque is generated with constant resistance that may react only in the opposite direction in which the steering wheel is rotated. Based on vehicle and steering wheel parameters, the system may determine whether active torque, passive torque, or a combination thereof will be applied on the steering wheel. The extent to which passive force is used instead of active force improves the power efficiency of the steering system because the magnetorheological device produces more torque than the motor, given the same amount of electrical energy.

[0007] Further aspects, features and advantages of the invention will become apparent from consideration of the following description and the appended claims when taken in connection with the accompanying drawings.

[0008] FIG. 1 is a schematic diagram of a steer-by-wire system of a vehicle steering wheel assembly having a magnetorheological device and a motor for simulating a steering resistance torque on a steering wheel;

[0009] FIG. 2 is a schematic diagram of a control logic used in the steer-by-wire system of FIG. 1 for simulating a steering resistance torque on the steering wheel;

[0010] FIG. 3 is one method of the control logic in FIG. 2 in accordance with the present invention; and

[0011] FIG. 4 is a graph depicting a magnetorheological performance curve for simulating hydraulic viscosity friction on a steering wheel.

[0012] FIG. 1 illustrates a steer-by-wire system 12 of a vehicle in which the steer-by-wire system 12 simulates a steering resistance torque on a steering wheel assembly including a steering wheel 20 and a steering shaft 22. Steer-by-wire system 12 includes a control logic to simulate a steering resistance torque on steering wheel 20 so that a driver of the vehicle experiences a steering feel during operation of the vehicle. As shown, sensor 30 is attached to steering wheel 20 to receive a driver input, such as actual angular velocity, on the steering wheel. Steering feel controller 32 is in electrical communication with sensor 30 and configured to receive an angular velocity signal therefrom to be processed for determining an amount of torque applied to the steering wheel.

[0013] Steer-by-wire system 12 further includes magnetorheological (MR) device 40 and motor 44. As shown, MR device 40 is attached to steering shaft 22 and is in electrical communication with steering feel controller 32. MR device 40 having an MR control is configured to receive an MR torque signal from steering feel controller 32, wherein the MR torque signal is indicative of torque to be applied by the MR device on the steering wheel 20 for simulating a resistance torque during normal operation of the vehicle. The MR torque is generated by the MR device and applied on the steering wheel.

[0014] Motor 44 is also attached to steering shaft 22 and is in electrical communication with steering feel controller 32. Motor 44 having a motor control is configured to receive a motor torque signal from steering feel controller 32, wherein the motor torque signal is indicative of torque to be applied by the motor on the steering wheel for simulating a steering resistance torque during normal operation of the vehicle. The motor torque is generated by the motor and applied on the steering wheel.

[0015] The steering wheel sensor is configured to measure rotational displacement of the steering wheel assembly. Any type of suitable sensor may be used to sense rotational displacement of the steering wheel assembly. This may include potentiometers, optical sensors, encoder sensors, resolver sensors, and any other suitable sensor.

[0016] The MR device is a torque producing device having magnetorheological low viscosity fluid which may be oil having ferrous particles suspended therein. As known, when current is applied across the magnetorheological fluid, a magnetic flux occurs and the ferrous particles align consistent with the magnetic flux. As a result, the relatively low viscosity fluid becomes a relatively high viscosity fluid, creating hydraulic friction on the steering wheel as the steering wheel is rotated. The MR device may be a magnetorheological damping device manufactured by Lord using Rheonetic magnetorheological fluid technology. Such MR device may have the specifications of a torque rating of 500 rpm, a breakaway torque of 10 Newton-meters (Nm), a braking response time of 10 milliseconds (ms), a current of 1 Amp, an electrical impedance of 1 ohm and a power consumption of 12 watts.

[0017] The motor used in this embodiment of the present invention may be any suitable motor capable of receiving the motor torque signal ranging between 0-5 amps and capable of producing motor torque ranging between 0.1-5.0 Newton-meters. In this embodiment, a motor amplifier may be used.

[0018] The motor amplifier is an electronic circuit which receives the motor torque signal from the controller and produces as output an electric signal whose voltage and current power the motor to produce the desired level of torque. The motor amplifier used may be any type of motor amplifier capable of receiving the motor torque signal ranging between about 0-5 amps and outputs torque ranging between 0.1-5.0 Newton-meters. In this embodiment, an MR amplifier (not shown) may be used to convert a digital signal to an electric signal to power the MR device. An MR amplifier may be used to receive the MR torque signal from the controller to convert the signal to an electrical signal whose voltage and current power the MR device to produce the desired level of torque on the steering wheel.

[0019] FIG. 2 depicts a control logic diagram 110 outlining signal flow of steer-by-wire system 12. In box 113, the MR damper control receives an actual angular velocity signal indicative of the actual angular velocity of the steering wheel to determine a coefficient K_MR for MR damping. In box 114, when determined, coefficient K_MR is multiplied by the actual angular velocity v to determine an MR torque represented by an MR torque signal u_MR. This may be represented as follows:

[0020] The MR device generates MR torque MR and applies the MR torque to the steering wheel. Then, a torque error signal e_MR is determined between the MR torque signal u_MR and a torque reference signal u_REF in box 116. The torque reference signal is a function of a steering wheel variable. The steering wheel variable may include current vehicle speed, angular velocity of the steering wheel, and steering wheel angle. In this embodiment, the torque error signal e_MR is a difference between the MR control signal and the torque reference signal. In determining the torque error signal e_MR, a summer in box 116 receives the MR torque signal and determines a difference between the MR torque signal u_MR and the torque reference signal u_REF. This may be represented as follows:

[0021] Then, the error signal is sent to the motor control.

[0022] In box 118, the MR difference signal e_MR is processed and multiplied by a coefficient K_d for the motor control to determine a motor torque signal u_M which is indicative of an amount of torque to be generated by the motor on the steering wheel. This may be represented by an equation as follows:

[0023] In box 123, the motor generates motor torque M according to the motor torque signal u_M. Then, a torque difference _diff is determined between the motor torque _M and a disturbance torque _R indicative of an amount of torque applied by a driver on the steering wheel. The steering wheel dynamics of the vehicle experiences the torque difference _diff resulting in an actual angular velocity of the steering wheel which may be again sensed by the sensor. This may be represented as follows:

[0024] FIG. 3 illustrates one method 210 of simulating a steering resistance torque on a steering wheel of steer-by-wire system 12 using an MR device and a motor. Method 210 is implemented by steer-by-wire system 12 in control logic diagram 110. Method 210 includes beginning the steering control by sensing an engine startup of the vehicle in box 212. This may be accomplished with various sensors known in the art for sensing an engine startup or merely sensing a key in the ignition keyhole. As shown in box 214, method 210 includes setting the coefficients K_MR, K_M and other reference values, e.g., reference torque value.

[0025] In box 216, the method includes sensing an actual angular velocity of the steering wheel and reading the reference values. For example, the angular velocity sensor, which may be attached to the steering wheel, may receive a driver input during operation of the vehicle. As the driver rotates the steering wheel, the sensor senses the angular velocity of the turn and sends an actual angular velocity signal indicative of the actual angular velocity of the steering wheel. The actual angular velocity signal may be sent to the steering feel controller for further processing. The sensor may also read the torque reference and determine a torque reference signal to be used as described below.

[0026] Method 210 further includes receiving the actual velocity signal indicative of the actual angular velocity of the steering wheel inputted by the driver. In box 218, method 210 includes comparing the actual angular velocity with a threshold angular velocity of the steering wheel. The threshold angular velocity may range between 5-10 per second.

[0027] It is to be noted that a software program may be configured in the steering feel controller to perform the steps of method 210. However, steps of method 210 may be performed in separate units within the steer-by-wire system.

[0028] The method 210 further includes calculating an MR torque signal indicative of MR torque to be applied by the MR device on the steering wheel. The MR torque signal is based on the comparison in box 218 of the actual angular velocity with the threshold angular velocity of the steering wheel. In box 220, the step of calculating the MR torque signal is accomplished by determining the MR torque based on a friction coefficient K_MR and the actual angular velocity, if the actual angular velocity is less than the threshold angular velocity. When the actual angular velocity is determined to be less than the threshold angular velocity, the friction coefficient K_MR is multiplied by the actual angular velocity to compute the MR torque signal indicative of the MR torque to be applied by the MR device on the steering wheel. This may be represented as follows:

[0029] The friction coefficient K_MR is indicative of a slope of a magnetorheological performance curve (discussed in greater detail below) compensated to simulate hydraulic viscosity friction on the steering wheel. In this embodiment, the performance curve is based on the actual angular velocity of the steering wheel and the vehicle speed.

[0030] In box 222, the step of calculating the MR torque signal further includes determining the MR torque based on a maximum torque signal d, if the actual angular velocity is not less than the threshold angular velocity. When the actual angular velocity is determined to be not less than the threshold angular velocity, the maximum torque signal d is used to produce the MR torque signal indicative of the MR torque to be applied by the MR device on the steering wheel. This may be represented as follows:

[0031] The MR device then applies the MR torque on the steering wheel.

[0032] It is to be noted that when the actual angular velocity is determined to be zero, no MR torque is generated by the MR device.

[0033] FIG. 4 depicts a curve K_MR representing a magnetorheological performance curve compensated to simulate hydraulic viscosity friction on a steering wheel. In this embodiment, the curve is based on force (torque) and velocity (angular velocity of the steering wheel). Coefficient K_MR represents the slope of the curve and maximum torque d represents substantially horizontal portions of the curve. The steering feel controller then sends to the MR device MR control signal u_MR indicative of the MR torque to be generated.

[0034] In box 224, a motor torque signal um is calculated based on the error signal e_MR as mentioned above. As described above, the error signal e_MR is determined by calculating the difference between the MR signal u_MR and a torque reference signal u_REF as mentioned above. The motor torque signal u_M is indicative of motor torque by the motor applied on the steering wheel. In this embodiment, the motor torque signal u_M is calculated by multiplying the error signal e_MR with a coefficient Kd of the motor to produce the motor torque signal u_M. This may be represented as follows:

[0035] The motor torque signal is converted to a corresponding amount of motor torque to be generated by the motor. Torque is then applied on the steering wheel by the MR device and the motor in box 226.

[0036] The MR torque ranges between about 0-5 Newton-meters. The MR torque signal ranges between about 0-1 amps. The motor torque ranges between about 0-5 Newton-meters. The motor torque signal ranges between about 0-5 amps.

[0037] In use, the present invention provides an improved steer-by-wire system by implementing a motor and an MR device. The use of both a motor and an MR device for generating resistive torque on the steering wheel allows both active torque, passive torque, and combinations thereof to be applied on the steering wheel. This provides an improved power efficiency, resulting in a more competitive steer-by-wire system.

[0038] While the present invention has been described in terms of preferred embodiments, it will be understood, of course, that the invention is not limited thereto since modifications may be made to those skilled in the art, particularly in light of the foregoing teachings.