Abstract:
A steering apparatus includes a rotatable steering shaft having an exterior, an input end for connecting a steering member and an output end for connecting to at least one steerable wheel of a vehicle. An electric motor is operatively connected to the steering shaft for rotating the steering shaft. The shaft is magnetized and serves as a torque sensor transducer. At least one magnetic field sensor is adjacent to the exterior of the steering shaft. The steering shaft and the at least one magnetic field sensor form a torque sensor for sensing torque applied to the steering shaft by the manually operable steering member and provide a signal to operate the electric motor to assist steering of the steering shaft by the manually operable steering member.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to power assist steering systems for vehicles and, in particular, to electric power assist steering systems employing torque sensors for vehicles provided with handlebars. 
         [0003]    2. Description of the Related Art 
         [0004]    Electric power assist steering systems are well-known and are used for such purposes as recreational vehicles and small utility vehicles. One type of electric power steering system includes an electric motor which is coupled to the steering shaft, typically including a worm mounted on the drive shaft of the motor which engages a worm gear mounted on the steering shaft. 
         [0005]    In some prior art examples the systems include a hollow steering shaft which has a narrower internal shaft (also known as the torsion bar) serving as a transducer for a torque sensor. Examples are found in U.S. Pat. Nos. 6,360,841; 7,182,169; 7,183,681; and 7,077,235. The steering shaft itself is typically made of several components. The result may be a system which lacks a sharp and direct steering feel due to play between the different components. 
         [0006]    These earlier steering devices with torsion bars are typically acceptable for multiple turn steering systems such as cars and trucks. A typical torsion bar in such a system has a ±4° to ±8° sensing range and a steering wheel range of 720° to 1440°. The torsion bar compliance is approximately 1% of the steering range. However, when a steering system of this type is employed in a vehicle with roughly 90° steering range, for example vehicles with handlebars or tiller steering, then the torsion bar compliance can be 9°-18° due to the higher torque applied to the steering shaft of such vehicles. 
         [0007]    Referring to stiffness, torsion bars employed in the past typically range from 120 Nm/rad. to 900 Nm/rad. The electric power steering shaft stiffness is dominated by the torsional region the steering shaft as referenced in SAE paper 2006-01-1320. Such systems use a relatively soft shaft (in torsional stiffness) because of the sensing technology employed. The more displacement, the higher the sensitivity that can be achieved with displacement sensors such as a potentiometer. 
         [0008]    Another example of using torsion bars with magnetoelastic sensing technology can be found in U.S. Pat. No. 6,360,841 with ±4 or ±8° of compliance. The stiffness is dominated by the torsional region. A low stiffness system makes it difficult however for handlebar vehicles to avoid oscillation during double lane change maneuvers if they were equipped with such systems. 
         [0009]    For instance, when a torsion bar of this type is employed in such a vehicle in a double lane change driving test, the steering system may experience significant overshoot and oscillation. Accordingly there is a need for an improved electric power assist steering system suitable for such purposes as recreational vehicles, particularly those employing handlebars for steering. 
       BRIEF SUMMARY OF THE INVENTION 
       [0010]    According to one aspect of the invention there is provided a steering apparatus which includes a rotatable steering shaft having an exterior, an input end for connecting a steering member and an output end for connecting with at least one steerable wheel of a vehicle. An electric motor is operatively connected to the steering shaft for rotating the steering shaft. The shaft has at least a portion thereof made of a magnetoelastic material and having a defined axially extending and circumferentially extending surface area which carries a magnetic field. At least one magnetic field sensor is adjacent to the exterior of the steering shaft. The steering shaft and the at least one magnetic field sensor form a torque sensor for sensing torque applied to the steering shaft by the steering member and provide a signal indicative of torque applied to the steering shaft by the steering member. 
         [0011]    According to another aspect of the invention there is provided a vehicle having a frame, a plurality of wheels including at least one steerable wheel, a steering member for steering said at least one steerable wheel and an electric steering apparatus having a rotatable steering shaft. The shaft has an exterior, an input end for connecting the steering member and an output end for connecting with said at least one steerable wheel of the vehicle. An electric motor is operatively connected to the steering shaft for rotating the steering shaft. The shaft has at least a portion thereof made of a magnetoelastic material and having a defined axially extending and circumferentially extending surface area which carries a magnetic field, the magnetic field varying upon twisting of the steering shaft. There is at least one magnetic field sensor adjacent to the exterior of the steering shaft, the steering shaft and said at least one magnetic field sensor forming a torque sensor for sensing torque applied to the steering shaft by the manually operable steering member and providing a signal indicative of torque applied to the steering shaft by the steering member. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]    The present invention will be more readily understood from the following description of preferred embodiments thereof given, by way of example, with reference to the accompanying drawings, in which: 
           [0013]      FIG. 1  is a front elevational view of a three wheeled vehicle including a steering apparatus according to an embodiment of the invention; 
           [0014]      FIG. 2  is a side view, partly in section, of the steering apparatus according to an embodiment of the invention; 
           [0015]      FIG. 3  is an exploded, isometric view of the steering apparatus of  FIG. 2 ; 
           [0016]      FIG. 4  is a longitudinal sectional view of the steering shaft and torque sensor assembly thereof; 
           [0017]      FIG. 5  is an isometric view of the steering shaft and torque sensor assembly of  FIG. 4 ; and 
           [0018]      FIG. 6  is a perspective view of the magnetic field sensor assembly of the steering apparatus of  FIGS. 2 and 3 . 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0019]    Referring to the drawings and first to  FIG. 1 , this shows a three wheeled vehicle  10  which includes a pair of front, steerable wheels  12  and  14  and a rear wheel  16 . The vehicle includes a frame illustrated generally at  20  and a steering mechanism illustrated generally at  22 . There is a manually steerable member illustrated generally at  24  in  FIG. 1  which includes handlebars  26  for steering the front wheels  12  and  14 . It should be understood that this type of vehicle is illustrated by way of example only and the invention is also applicable to other types of vehicles with at least one steerable wheel. 
         [0020]    The handlebars are mounted on input end  30  of a steering shaft  32  illustrated in  FIG. 2 . Output end  34  of the steering shaft is operatively connected to the steering mechanism  22  in a manner well known in the art. Accordingly this arrangement is not described in more detail. The steering shaft forms part of steering apparatus  3   8  illustrated in  FIGS. 2 and 3 . This is an electric power assist type of steering apparatus. The general arrangement is well known in the art. 
         [0021]    The apparatus includes a housing  40 , illustrated in  FIG. 2 , including two halves  42  and  44  illustrated in  FIG. 3 . An electric motor  46  is mounted on the housing. A worm  48  is connected to the shaft of motor and meshes with a worm gear  52 , a sector gear in this example, which is mounted on splines  54  of steering shaft  32 . Thus rotation of the electric motor  46  causes rotation of the worm  48  which in turn rotates the steering shaft  32  and the sector gear  52 . A sector gear only is required in this embodiment because the handlebars  26  of the vehicle illustrated in  FIG. 1  can only be steered through an angle significantly less than ±180°, preferably less than ±90° and ±45° in this example. The sector gear in this embodiment is of metal, but could alternatively be made of plastic. In this embodiment the sector gear comprises approximately one quarter of a circle. The shaft has two spaced-apart bearing portions  31  and  33  only, illustrated in  FIG. 2 , supported by bearings  35  and  37 . This compares to prior art steering systems typically requiring three bearing portions. 
         [0022]    The steering apparatus includes a magnetic field sensor assembly  60  illustrated in  FIG. 5  which is mounted on the housing  40 . The sensor assembly  60  includes a sensor housing  62  which extends about the steering shaft  32 . As is known in the art, the assembly includes at least one magnetic field sensor. In this example there are eight magnetic field sensors, four of which, namely sensors  64 ,  70 ,  72  and  74  are illustrated in  FIG. 6 . These are arranged in sets of two sensors 90° spaced-apart about the shaft. Each set of two sensors, for example sensors  68  and  70 , and sensors  74  and  76  is mounted on a sensor mount  90  illustrated in  FIG. 6  which is connected to an annular member  92  having an opening  94  through which the steering shaft extends. It may be seen that the shaft is tapered from portion  99  towards the output end  34 , as seen in  FIG. 4 , thus allowing insertion of the shaft into the opening  94  of the sensor mount and corresponding opening  93  of the housing  40  illustrated in  FIG. 5 . 
         [0023]    Four of these magnetic field sensors  68 ,  70 ,  74  and  76  are illustrated in the sectional view of  FIG. 4 . These magnetic field sensors are located and oriented relative to steering shaft  32 , acting as a torque sensor transducer, so as to sense the magnitude and polarity of the field arising in the space about the shaft as a result of the reorientation of the polarized magnetization due to torsional stress from the quiescent circumferential direction to a more or less steep helical direction. The magnetic field sensor assembly  60  provides a signal output reflecting the magnitude and direction of torque applied to the shaft  32 . In one example the magnetic field sensors are integrated circuit Hall effect sensors. 
         [0024]    As stated the steering shaft acts as a transducer for the torque sensor. It includes one or more axially distinct, magnetically contiguous, oppositely polarized circumferential bands or regions. The shaft in this example is formed of a ferromagnetic, magnetostrictive material having the desired crystalline structure. The shaft has at least a portion  99  thereof made of a magnetoelastic material and having a defined axially extending and circumferentially extending surface area which carries a magnetic field, the magnetic field varying upon twisting of the shaft. The steering shaft in this embodiment is a solid one-piece member of case hardened steel,  9310  steel in this example although other materials could be used. 
         [0025]    Accordingly it may be seen that all of the torque applied to the handlebars  26  is transferred to the steerable wheels  12  and  14  by the steering shaft  32 . Collarless circularly magnetized torque transducers are known as disclosed in U.S. Pat. No. 6,047,605, the disclosure of which is incorporated herein by reference. Alternatively, as is known in the prior art, the shaft could have a magnetized ring extending about the shaft adjacent to the magnetic field sensors. In this case the shaft could be of a non-magnetic materials such as stainless steel. 
         [0026]    Three important criteria of a steering system of this type are, in descending order of importance, torque range, stiffness, and control bandwidth. 
         [0027]    First, considering torque range, the torque control scheme should be set so as to distinguish foreseeable operating torque from the driver during driving conditions. Typically, in a steering system without power assist, torque user torque can range from 25 Nm to 50 Nm during most of the driving. With maximum payload and maximum frictional surfaces such as hot pavement, the user torque can reach 80 Nm. Accordingly the sensing range in this example is set at ±90 Nm. 
         [0028]    The material and size of the steering shaft is selected so that the torque range is 20-30% of the yield strength of the torque sensor region  99  for the torque sensing range. The diameter of the sensing region of the shaft in this example is approximately 21 mm. As stated the shaft in this example is of AISI 9310 steel which is carburized to a case hardened range. This references to U.S. Pat. No. 6,553,847 which is incorporated herein by reference. However other materials may be used to fit the sensor requirement. 
         [0029]    It is also important to balance the torque sensitivity and torque range that can be measured. In the present system the torque sensitivity is 1% F.S., ±0.9 Nm. for a ±90 Nm range. 
         [0030]    The stiffness of the electric power steering system is defined as the rotational stiffness from the input shaft to the output shaft. The stiffness in the area  99  of the sensors is 18,100 Nm./rad., 19,900 Nm./rad. adjacent the input and output splines and 31,425 Nm./rad. for the rest of the steering shaft in this example. The total stiffness of the electric power steering system in this example is 6200 Nm./rad. For example, if there is  90  Nm of torque experienced in the shaft, there will be 0.8° compliance. It should be noted that these figures are for a particular example and can vary significantly in different embodiments of the invention. The stiffness may be between 2900 Nm./rad. and 6200 Nm./rad, and preferably between 4400 Nm./rad. and 6200 Nm./rad. 
         [0031]    In another example, stiffness in the center sensing region is 9500 Nm./rad. The total steering shaft stiffness is 4400 Nm./rad. Both this design and the one discussed above have yielded satisfactory steering performance. With a minimum stiffness of 2900 Nm./rad such that the diameter of the sensing region of the shaft is approximately equal to 12 mm with approximately 1° of compliance in typical driving, the steering performance has been found to be appropriate. 
         [0032]    The stiffness of a steering shaft according to the present invention is significantly greater than conventional prior art electric steering systems. By increasing stiffness of the shaft 3.5-7 times, the natural frequency of the system increases, typically in square root relationship. Therefore the natural frequency is increased by 1.9-2.6 times. The increased natural frequency increases the control bandwidth. For a change of steering angle, such as one during a double lane change, overshoot and oscillation is minimized. 
         [0033]    Improvement on the stiffness also improves steering accuracy. The compliance of the steering shaft is minimal compared with the steering angle. The vehicle dynamic or nimbleness is maintained. 
         [0034]    Also, with the stiffness of the shaft being as high as discussed above, and with a maximum torque sensing range being 20-30% of the yield strength of the shaft, the steering assembly can be much simplified to remove the rotational stop features required in, for example, U.S. Pat. No. 6,360,841 during manual steering in the case of malfunction of the power assistance mechanism. 
         [0035]    A steering system according to the invention is also useful for jet boats where the typical steering range is less than 360° (less than ±180°). 
         [0036]    As will be apparent to those skilled in the art, various modifications may be made within the scope of the appended claims.