Abstract:
A vehicle ( 1 ) provided with a frame having two frame sections ( 3,4 ) which are able to tilt with respect to one another. An opposite steer power transmitter ( 50 ) is connected to a steerable front wheel ( 13 ). The opposite steer power transmitter ( 50 ) is controlled as a function of the tilting moment on the frame sections ( 3,4 ). By this means, application of opposite steer is achieved, as a result of which the tilting section of the vehicle “drops into the bend” more rapidly and as a result of which increased maneuvrability is obtained. The opposite steer power transmitter ( 50 ) can be used on vehicles having an active tilting system with, for example, hydraulic tilting cylinders ( 9,9 ′), which are controlled by a sensor depending on the radius of the bend ( 24 ). The tilting moment generated by the tilting cylinders ( 9,9 ′) can optionally serve as a control signal for the opposite steer power transmitter ( 50 ).

Description:
BACKGROUND OF THE INVENTION 
     The invention relates to a vehicle provided with 
     at least three wheels, 
     a frame having a first frame section and a second frame section, the frame sections being tiltable with respect to one another about a tilt axis located in a longitudinal direction, 
     at least one front wheel which is connected to the first frame section and is rotatable about a front wheel steering shaft, 
     a steering wheel rotatably connected to the first frame section, 
     tilting means for tilting the first and the second frame sections relative to one another, and 
     a signal transmitter coupled to the tilting means in order, when driving round a bend having a predetermined bend radius, to generate a control signal for controlling the tilting means as a function of said bend radius. 
     A vehicle of this type is disclosed in WO 95/34459. This publication describes a self-balancing, preferably three-wheel vehicle having an active tilting mechanism, wherein the front frame section is tilted by means of hydraulic tilting cylinders when driving round a bend. In order to control the degree of tilt, the force or the moment on the front wheel is measured by a sensor, which in this case consists of a hydraulic rotary valve. In response to the signal from the sensor, the front frame section, which includes the driver&#39;s cab and the steering wheel, is tilted until the force or the moment on the front wheel becomes virtually zero. In this way a self-stabilising tilting mechanism is obtained which makes the vehicle stable when taking a bend at any speed. Since narrow vehicles as described in WO 95/34459 in general have too low a lateral stability and manoeuvrability to be able to participate in normal (car) traffic, a tilting system of this type increases the stability such that a narrow vehicle of this type can be a satisfactory means of transport. The tilting system described is fully automatic, as a result of which the driver is not expected to have any special skills for steering the vehicle. The known system provides a safe vehicle with a predictable response in all driving conditions to be anticipated. 
     When the driver turns into a sharp bend rapidly by exerting a moment on the steering wheel, a moment directed towards the inside of the bend will have to be exerted on the tiltable frame section in order to provide the desired tilting. The tiltable frame section will tend to tilt towards the outside, which tendency has to be counteracted by the tilting cylinders. Especially in the case of rapid manoeuvres in which the driver pulls hard on the steering wheel, the front wheel will be turned into the bend to a considerable extent and the moment on the tilting frame section which is directed towards the outside of the bend will be high, so that the tilting cylinders will have to exert a high moment in order to provide the desired tilting. As a result of this the maximum tilting speed is restricted. 
     SUMMARY OF THE INVENTION 
     It is therefore an aim of the present invention to provide a tiltable vehicle of improved manoeuvrability. In addition to an increase in the tilting speed, it is also an aim of the invention to provide a vehicle with which tilting means can be of relatively compact construction. 
     To this end the vehicle according to the present invention is characterised in that the tilting means comprise an opposite steer power transmitter which is connected to the front wheel and to the frame, which opposite steer power transmitter exerts a moment on the front wheel that is directed away from the center of curvature of the bend. By means of the power transmitter according to the present invention, an active front wheel servomechanism is created which compensates for the moment which is exerted by the driver on the front wheel and produces slight turning of the front wheel towards the outside. As a result, the front frame section of the tilting vehicle will “drop into the bend” more rapidly and the moments which have to be exerted by the tilting means on the front frame section in order to cause this to tilt are substantially reduced. If the power transmitter according to the invention is used in a tilting vehicle having an active tilting mechanism, that is to say where, when taking a bend, the front frame section is tilted with respect to the rear frame section by means of a separate drive device, the drive device to produce the tilting can be of relatively small construction as a result of the use of the opposite steer power transmitter. If the drive device comprises hydraulic cylinders, said cylinders can be small and/or operate at lower pressure. 
     The opposite steer power transmitter according to the present invention can, however, also be used in vehicles without an active tilting mechanism, tilting being produced solely by the power transmitter. 
     The vehicle according to the present invention can have a tiltable frame which is as described in WO 95/34459 and which is provided with three or four wheels, or can comprise a tiltable frame as described in Netherlands Patent Application number 1 005 894. Furthermore, the vehicle according to the invention can include tiltable elements in the wheel suspension, wherein the first frame section is made up of the wheel axles and the second frame section is made up of the parts supported by the wheel axles, such as, for example, the driver&#39;s cab. In this case the wheel axles can remain in a constant position when the second frame section is tilted with respect to the road surface. 
     The invention is based on the insight that there are two ways of changing the degree of tilt of a vehicle:  1 . by exerting a tilting moment, as a result of which the vehicle is actively tilted towards the inside of the bend, and  2 . by a steering manoeuvre on the front wheels in the opposite direction to the desired tilt direction, as a result of which the vehicle will tend to lean towards the inside of the bend like a motorbike. 
     The use of the first method alone has the disadvantage that the tilting moment that can be exerted is restricted by the characteristics (width, mass of the tiltable frame section with respect to the fixed frame section) of the vehicle, as a result of which the achievable tilting speed (and thus the manoeuvrability) are limited. Moreover, a steering moment (towards the inside of the bend) exerted by the driver will cause the tiltable frame section to lean towards the outside, as a result of which part of the available tilting moment will already be used to prevent the tiltable frame section from dropping down further towards the outside. As a consequence the maximum tilting speed is reduced, possibly even to zero. 
     This can be improved by the opposite steer power transmitter according to the invention, which is connected to the front wheel/the front wheels and the frame section to which the front wheels are attached. Depending on the layout of the vehicle, said section does not have to be the tilting frame section. Said opposite steer power transmitter preferably exerts a steering moment on the front wheel(s) which is a function of (for example proportional to) and in the opposing direction to the tilting moment exerted between the tilting and the non-tilting frame section of the vehicle. 
     As a result of said steering moment on the front wheels, specifically directed towards the outside of the desired bend, the vehicle will tilt towards the inside more rapidly than in the absence of said power transmitter. In combination with an active tilting system, the maximum tilting speed of the vehicle will be appreciably increased. The steering moment that is exerted by the power transmitter is preferably proportional to the tilting moment and will decrease as the position comes closer to the desired tilt position. 
     Preferably, the vehicle according to the present invention has an active tilting mechanism in the form of a drive device which is connected to the first and the second frame section in order to exert a tilting moment between the first and the second frame section. With this arrangement the signal transmitter for controlling the opposite steer power transmitter can, in a preferred embodiment, be incorporated in said drive device, so that the tilting moment that is exerted on the tilting frame section is used as a control signal for the opposite steer power transmitter. The absence of a tilting moment on the frame sections means that the vehicle is in the correct tilt position when taking a bend and that the opposite steer power transmitter does not have to be activated. If there is a tilting moment on the front and rear frame sections, this means that the tilt position has to be modified when taking the bend and specifically has to be modified in the direction of the tilting moment. Said tilting of the frame sections can be effected both by the drive device and by the opposite steer power transmitter. When the opposite steer power transmitter is controlled by the tilting moment between the frame sections, the amplification factor is not critical: if the amplification factor is low, little opposite steer will be produced by the opposite steer power transmitter and the vehicle will tilt largely as a consequence of the active tilting produced by the drive device. If the amplification factor is very high, the front wheel will apply a large amount of opposite steer because of the opposite steer power transmitter, so that tilting of the vehicle will be produced mainly by application of said opposite steer and to a lesser degree by the action of the drive device. The front wheel will, however, never apply too great an opposite steer since in that case the vehicle would lean too far into the bend, with the result that the tilting moment on the frame sections would become negative and the operation of the opposite steer power transmitter, and consequently the application of opposite steer by the front wheel, would be terminated immediately. 
     Irrespective of the way in which the driver&#39;s input is translated into a tilt position and driving direction, it is possible to construct the signal transmitter for the opposite steer power transmitter as, for example, a “lateral acceleration sensor” which is mounted in the tilting frame section. Said sensor measures the error in the position of the tilting frame section. By using this signal to control the opposite steer power transmitter, the latter will actively steer the front wheel and reduce the error in the tilt position to zero. As a consequence the primary tilting system can be of simpler and more compact construction. Additional safety can also be built in by this means, for example by triggering a fitted emergency system or warning signal if a specific tilt position error angle is exceeded. In this case the signal from the acceleration sensor is a suitable input signal for the opposite steer power transmitter with an amplification factor to be chosen. 
     Any system can be used to control the drive device for active tilting of the frame sections, for example the system as described in EP-A 0 592 377. With this system the drive device for tilting the front frame section relative to the rear frame section, which carries the drive, is obtained by angular rotation of the front wheel about the front wheel axle with respect to the frame. EP-A 0 020 835 describes a tilting vehicle wherein the drive device for tilting the frame sections is controlled by means of foot pedals or by a movement of the steering column, transversely to the axis of rotation thereof. Preferably, however, the drive device is controlled by a sensor which generates a signal that is dependent on the force or moment exerted on the front wheel. This type of control of the drive device is described in WO 95/34459. In a further preferred embodiment, the steering wheel is connected via a steering shaft to the first frame section, the steering shaft being rotatable with respect to the front wheel about its center line. The drive device is controlled by a turning angle sensor which measures an angle of rotation between the front wheel and the steering shaft. A construction of this type is described in International Patent Application number PCT/NL98/00534. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     One embodiment of a tilting vehicle according to the present invention will be described in more detail with reference to the appended drawing. 
     In the drawing: 
     FIG. 1 shows a diagrammatic perspective view of a vehicle according to the present invention when driving in a straight line, 
     FIG. 2 shows a perspective view of the vehicle according to FIG. 1 in the tilted position, and 
     FIG. 3 shows a hydraulic diagram for the vehicle according to FIGS.  1  and  2 . 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 shows a vehicle  1  having a frame  2 . The frame  2  comprises a front frame section  3  and a rear frame section  4 . The frame sections  3  and  4  are rotatably joined to one another at a point of rotation  5 . The rear frame section  4  is provided with a rear axle  6  with two rear wheels  7 , 7 ′. The drive device for active tilting of the front frame section  3  comprises tilting cylinders  9 , 9 ′, which are connected via a piston rod to a fixing plate  11  on the front frame section  3 . The cylinders  9 , 9 ′ are connected by their second end to the rear frame section  4 . The cylinders  9 , 9 ′ may be pneumatic or hydraulic cylinders. A gate valve  10 , which is driven by a cylinder  12 , is mounted parallel to tilt cylinder  9  between the rear frame section  4  and the front frame section  3 . The rear frame section  4  also supports drive means, such as an internal combustion engine or electric motor, for propulsion of the vehicle  1 . For the sake of clarity, this drive is not shown in the figures. 
     The front frame section  3  bears the front wheel  13 , which, via the front fork  14  and the front wheel steering shaft  15 , is rotatably supported in bearings in a front support  16 . A steering wheel  17  is attached via the steering shaft  18  to a second support  19  of the front frame section  3 . The steering shaft  18  can be turned via the steering wheel  17  independently of the front wheel  13  in the bearing of the second support  19 . A power transmitter, such as a torsion spring  61 , is connected to the steering shaft  18  in order to exert restoring force on the steering wheel  17  which increases with increasing angular rotation of the steering wheel  17 . 
     Transverse arms  20  and  21  are attached to the end of the front wheel steering shaft  15  and of the steering shaft  18  respectively, the free end of which transverse arms are connected to a respective part of the turning angle sensor  24 , which in the embodiment shown consists of a hydraulic cylinder. In FIG. 1 the angular positions of the front wheel  13  and of the steering wheel  17  are indicated by diagrammatic angular position indicators  22  and  23 , which are solely illustrative and will not be present in the final embodiment of a vehicle according to the invention. A power steering valve  25 , the respective ends of which are attached to the transverse arms  20  and  21 , is accommodated parallel to the turning angle sensor  24 . 
     Finally, the front fork  14  is attached via a transverse arm to a power steering cylinder  26 , the other end of which is attached to the front frame section  3 . 
     The front frame section  3  also supports a driver&#39;s seat and driver&#39;s cab, which for the sake of clarity have been omitted in the figure shown. 
     The rear frame section  4  also comprises an oil pump  28 , an accumulator  29  and a reservoir  30  for the hydraulic pressure medium. The tilting cylinders  9  and  9 ′ and the power steering cylinder  26  are driven by the oil pump  28 . Finally, a speed sensor  27  is connected to the rear axle  6  or front wheel  13  to provide speed-dependent activation and deactivation of the power steering cylinder  26 . 
     The components of the vehicle according to the invention which have been described above form part of the active tilting system that causes the front frame section  3  to tilt relative to the rear frame section  4  when taking a bend. The functioning thereof will be explained in detail below. 
     The vehicle as shown in FIGS. 1 and 2 also comprises tilting means in the form of an opposite steer power transmitter  50  which on one side is connected to the front fork  14  and on the other side is connected to the front frame section  3 . By means of the opposite steer power transmitter  50 , which is controlled by the moment that is exerted by the tilting cylinders  9 ,  9 ′, as will be explained in more detail below, when taking a bend the front wheel  13  is rotated away from the center of curvature of the bend, so that tilting of the front frame section  3  towards the center of curvature of the bend is obtained as a result. The mode of operation of the active tilting mechanism by means of the tilting cylinders  9 , 9 ′ is as follows: in the straight position shown in FIG. 1 the angle α between the steering shaft  18  and the front wheel steering shaft  15  is 0°, so that the turning angle indicators  22  and  23  are parallel to one another. The turning angle sensor  24  and the cylinder  12  of the gate valve  10  are hydraulically coupled to one another. Movement of sensor  24  causes movement of follower cylinder  12 , as a result of which gate valve  10  moves. As a result gate valve  10  opens, a pressure difference is produced on the tilting cylinders  9 , 9 ′ and the tilting section starts to move. As a result, gate valve  10  moves back to the mid position and when this is reached the pressure difference between tilting cylinders  9 , 9 ′ disappears and the tilting movement stops. Each position of the turning angle sensor  24  leads to a specific position of cylinder  12  and thus of the tilting frame. The slight turning of the front wheel will hardly be noticed by the driver and will be corrected subconsciously by a small steering correction. 
     When the steering wheel  17  is turned from the straight driving position until the turning angle indicator  23  makes an angle with respect to the turning angle indicator  22 , the turning angle indicator  22  initially remains in the straight ahead position as a result of the inertia and other dynamic characteristics of the front wheel  13 . Tilting cylinders  9 , 9 ′ are activated by the turning angle sensor  24 , so that a pre-determined tilting of the front frame section  3  relative to the rear frame section  4  takes place. Depending on the degree of tilt of the front frame section  3 , the front wheel  13  will turn in a little, depending on the vehicle&#39;s speed, until an angle a is obtained between the front wheel  13  and the steering wheel  17 , as indicated in FIG.  2 . The tilt angle will finally be β°. In FIG. 2 α is determined by the diagrammatic turning angle indicators  22  and  23 . The angular rotation of the steering wheel  17  is thus translated partly into the tilt angle β and partly into a front wheel angle, both of which assume the ideal relationship during any driving speed. 
     The value α measured by the turning angle sensor  24  is equal to the steering angle of the steering shaft  18  minus the angular rotation of the front wheel steering shaft  15 . The angle α is used to control the tilt angle β of the vehicle in accordance with the equation β=f(α). The tilt angle β is determined entirely by the steering measurement α. Both the turn of the steering wheel  17  with respect to the front wheel  13  (the range in degrees of the steering measurement α) and the translation of the steering measurement α into the tilt angle β can be chosen such that optimum driving feel is obtained. For example, it is possible to choose β=cxα where c is constant. 
     FIG. 3 shows, diagrammatically, the hydraulic system of the vehicle  1  according to the present invention. In FIG. 3 the front frame section  3  and the rear frame section  4  are indicated by rectangles shown in broken lines. Furthermore, in FIG. 3 the same reference numerals are used as in FIGS. 1 and 2 for the same components. As can be seen from FIG. 3, the oil pump  28  is driven by the engine  31  of the vehicle  1 . In this case, the engine  31  can be an electric motor or an internal combustion engine. However, it is also possible to drive the oil pump  28  by a separate motor which is fitted in the front frame section  3 . 
     The accumulator  29  is located on the pressure side of the oil pump  28 . Via the  4 / 3  gate valve  10 , the tilting cylinders  9 ,  9 ′ can be connected by their respective lines  32 ,  33  to the high pressure line  34  which is connected to the accumulator  29  or to the return line  35  which opens into the reservoir  30 . The gate valve  10  is operated by the cylinder  12 , which is coupled via lines  36  and  37  to the turning angle sensor  24 . The cylinder  24  is connected on one side to the transverse arm  20  of the front wheel steering shaft  15 , whilst the piston rod of the cylinder  24  is connected to the transverse arm  21  of the steering shaft  18 . The piston of the cylinder  24  is moved depending on the relative angular rotation between the steering shaft  18  and the front wheel steering shaft  15  (α). This turn is followed by the cylinder  12 . In the upright position shown, where the angle α is 0°, both tilting cylinders  9 ,  9 ′ are connected to the high pressure line  34 , so that the front frame section is upright. On steering movement of the steering wheel anti-clockwise (seen from the driver&#39;s position), the piston will move to the left in the cylinder  24 . As a result the piston in the cylinder  12  will be pushed towards the valve  10  and the right tilt cylinder  9 ′ will be connected to the high pressure line  34 . The left tilt cylinder  9  will be connected to the return line  35 . The valve  10  and the cylinder  12  are connected on one side to the rear frame section  4 , which is indicated diagrammatically by the broken line  38 , and on the other side are connected to the front frame section  3 , shown diagrammatically by the broken line  39 . As a result, on tilting to the left, the cylinder  12  will be moved away from the valve  10 , so that the valve  10  assumes the mid position again and the movement of the piston rods of the tilting cylinder  9 ,  9 ′ which piston rods are coupled to one another ,is stopped. 
     In FIG. 3 the opposite steer power transmitter  50  according to the invention is formed by the double-acting cylinder  50 , the piston rod of which is connected to the arm of the front fork  14  of the front wheel  13 . The housing of the cylinder  50  is attached to the front frame section  3 . A line  51  is connected between the tilt cylinder  9  and the valve  10  to the line  33  of tilt cylinder  9 . A line  52  is connected between the tilt cylinder  9 ′ and the valve  10  to the line  32  of cylinder  9 ′. The signal transmitter for the opposite steer power transmitter  50  is in this case thus formed by the tilting cylinders  9 ,  9 ′. 
     If the driver turns the steering wheel to the left, the valve  10  will be moved such that tilt cylinder  9 ′ is connected to the high pressure line  34  and tilt cylinder  9  is connected to the pressure return line  35 . As a result the line  52  of cylinder  50  will likewise be connected to the high pressure line  34  and the line  51  will be connected to the pressure return line  35 . As a result the piston in the cylinder  50  will move to the left and the front wheel  13  will be turned to the right. As a result, the front frame section  3  will tend to “drop into the bend” to the left, seen from the driver&#39;s position, and the moments to be exerted by the tilting cylinders  9 ,  9 ′ will be restricted. 
     FIG. 3 also shows the speed-dependent power steering by means of the power steering cylinder  26 , which is connected via the  4 / 3  power steering valve  25  to a switching valve  40 . The switching valve  40  is operated by a speed sensor  27 , for example in the form of a gear pump. In the situation shown the speed of vehicle  1  is insufficient to cause the pump  27  to move the valve  40  against the spring pressure. At the low speed the valve  40  is switched such that the line  41  of the valve  25  is connected to high pressure line  34 . The line  42  is always connected to the return line  35 . On turning the steering wheel a pressure difference will be built up over the piston of the cylinder  26  by opening of the valve  25 , as a result of which turning of the front wheel  13  is obtained. When the vehicle speed increases the pressure in the switching line  43  will increase sufficiently to switch over valve  40 , so that both lines  41  and  42  are connected to the return line  35 . Smooth deactivation of the power steering can be obtained by not instantly switching line  41  from high pressure  30  to the return line but allowing the pressure to decrease gradually (for example by means of a pressure control valve which is controlled by the speed sensor). The power steering cylinder  26  is deactivated in this way. By engaging the power steering at low speeds, the front wheel  13  will follow the steering wheel movements when the steering wheel  17  is turned, so that the angle a remains virtually equal to 0°. As a result, tilting of the vehicle is prevented. A certain free turn of α, for example+1°, remains possible. 
     Although in the embodiments described above the sensor  24  is of hydraulic construction, the invention is not restricted to this and optical, electrical or mechanical sensors can also be used for controlling the tilting cylinders. In principle, the hydraulics of the tilting cylinders can also be replaced by any other system, such as, for example, an electrical system. Furthermore the present invention is not restricted to vehicles having a front tilting section having one wheel and a rear, stationary section having two wheels, but it is possible for the front frame section to have two wheels and not to be of tilting construction, whilst the rear frame section is able to tilt and has one wheel, or it is possible to use a four-wheeled frame as is described in Netherlands Patent Application number 1 005 894. It can also be employed in a three-wheeled or multiple wheeled vehicle where one frame section is made up of the wheel suspension. 
     Finally, the opposite steer power transmitter  50  can also be used in vehicles which are tiltable without an active drive in the form of tilting means  9 , 9 ′ and it can also be used in tilting vehicles with an electronic construction, such as, for example, of the Citroën Activa or Mercedes Life Jet type.