The present invention concerns a steering device for a vehicle, in particular an articulated frame vehicle driven by electric motors, in which an electric motor is arranged at each one of at least two wheels on opposite sides of the vehicle in order to drive the wheels .

In articulated frame vehicles, a steering force is applied to the joint that is present between the different parts of the vehicle in order to pivot these relative to each other. It is possible in very light vehicles to achieve the pivoting of the parts at the joint purely mechanically, but the heavier the vehicle is, the more difficult it becomes to transfer the steering force. In lighter vehicles, for example, direct coupling of a power transfer means from the steering column of the vehicle to the joint may be achieved, with the aid of, for example, a cable, a chain or similar, which transfers the pivoting force from the steering column to the pivot joint. For larger and heavier vehicle with a drive motor that can produce a hydraulic pressure with the aid of a servopump, there is generally no problem in finding solutions that make it possible to obtain sufficient servopower to facilitate the steering .

An articulated frame vehicle of this type with solely mechanical transfer of the steering force is revealed by, for example, EP 0772384 Bl. The problem with such steering is, as has been mentioned above, that it becomes heavy to turn the steering wheel, particularly for heavy vehicles.

A completely different steering principle for larger articu- lated frame vehicles, such as construction equipment, is known. It is obvious that these may have, as has been long known, hydraulic steering, in particular hydraulic pistons, between the two parts of the vehicle that accomplish the mutual pivoting of these parts. The driving of such construction equipment with the aid of the electric motors arranged in the wheels is also known through, for example, US 6,283,237 Bl, where a regulator unit can drive the wheels of the vehicle independently of each other by regulating the power supply to the driving wheels. The patent describes also how control signals from a steering wheel in the construction equipment can be supplied to the regulator unit of the vehicle, such that the regulator unit causes the wheels of the machine to rotate with different speeds, and in this way turn the machine. Such a solution requires complicated programming in order to achieve steering of the machine, and thus this is an expensive solution. Nor does it give any possibility of steering the machine unless all parts of the driving and regulatory system are working.

The aim of the present invention is to find a simple and practical steering device that can facilitate the steering of an articulated frame vehicle that is driven by an electric motor arrangedat the relevant wheel at one or several axles of the vehicle. This aim of the invention is achieved through the steering column of the vehicle being directly coupled through a power transfer means for the transfer of the turning force that is applied to the steering wheel to the pivot joint of the vehicle, whereby, according to the invention, there is arranged on the steering column also a force sensor that is arranged to sense the turning force that has been applied to the steering wheel and to produce an electrical output signal, which electrical output signal is arranged to be supplied to a regulator unit that controls the electric motor in the relevant wheel of the vehicle in such a manner that the wheel on one side of the vehicle is supplied with the signal as a small positive additive signal to the power supply for the driving of the electric motor in order to cause an increase in speed, and the wheel on the other side of the vehicle is supplied with the signal as a small negative subtractive signal to the power supply for the driving of the electric motor in order to cause a reduction in speed. According to one preferred embodiment of the invention, an inverter is arranged in order to -invert the output signal to the wheel on one side of the vehicle.

The invention will now be described in more detail in the form of an embodiment, illustrated with the aid of the attached drawings, where Figure 1 shows schematically in plan view a block diagram of an articulated frame vehicle, and Figure 2 shows schematically a wiring diagram for the functions for steering the vehicle and that are essential for the invention.

Thus, Figure 1 shows a block diagram of an articulated frame vehicle that may be, for example, a lawn mower. The vehicle has a forward part 1 and a rear part 2, which parts of the vehicle are united with each other through a pivot joint 3. The vehicle has four wheels, two forward wheels 4, 5 that are located at the forward part of the vehicle, and two rear wheels 6, 7 that are located at the rear part of the vehicle. A steering wheel 8 is arranged for steering the vehicle, and the steering column of the steering wheel is connected with the aid of a mechanical power transfer means 9 with a turning disc 10 that is fixed attached to the pivot joint 3, such that the mechanical power transfer means 9 can cause through the turning of the steering wheel 8 the turning disc 10 to be rotated around the pivot joint 3, such that the forward part 1 and the rear part 2 of the vehicle can be pivoted relative to each other. The vehicle is driven by electric motors, and comprises one electric motor 11, 12 in each one of the rear wheels 6, 7. Batteries or other sources of electrical power, not shown in the drawings, are arranged in the vehicle in order to provide the power required to drive the vehicle forwards, and for the supply of power to any units, such as a cutting unit, that may be arranged in the vehicle .

There may be arranged, for example, a main switch, such as a pedal 13, in the vehicle for the driving of the vehicle, see Figure 2, which pedal must be held depressed in order for the vehicle to receive any power at all for its propulsion or for the units arranged at it. A regulator 14, an "accelerator pedal", is arranged at the vehicle for the regulation of the speed of the vehicle, with which accelerator pedal the power supply to the electric motors 11, 12 is regulated. The power supply to the motors is regulated by the accelerator pedal 14 such that both motors operate at the same speed by this means. The regulation of the power to the two motors takes place in a separate regulator unit 15, 16 for each motor 11, 12.

The steering of the vehicle will acquire considerable inertia with the design described above, due to the weight of the vehicle. The force that must be applied to the steering wheel 8 in order to turn with the aid of the mechanical power transfer means 9 the turning disc 10 and thus the pivot joint 3 of the vehicle is quite large. In order to solve the problem of the steering having high inertia, therefore, there is arranged on the steering column of the vehicle a force sensor 17 that will sense the torque that is applied to the steering column with the aid of the steering wheel 8, and that will sense the direction in which the torque is acting on the steering column. The force sensor 17 emits an electrical signal, positive or negative, the magnitude of which depends on the torque applied. This electrical signal is led through an amplifier 18 onwards to each one of the regulator units 15, 16 for the electric motors, where the signal to one of the regulator units 15 is led though an inverter 19 that inverts the polarity of the signal, such that the two regulator units 15, 16 receive signals from the force sensor that have the same magnitude but dif- ferent polarities. These signals from the force sensor 17 are applied to the regulator units 15, 16 and they are added to the speed-regulation signal that is supplied from the accelerator pedal 14, where thus one side in this way is supplied with a small additive signal and the other side by a subtrac- tive signal of the same magnitude. If the output signal from the force sensor 17 is negative, this signal will be added as a negative signal to the first side, i.e. it will have an effect of a negative addition, i.e. it will be a signal that reduces the speed. The negative signal is inverted on the opposite side, which thus gives an increase in speed.

If the steering wheel 8 is then turned, a positive electrical signal will be supplied to one wheel in order to drive it in one direction, and a negative signal will be supplied to the other wheel in order to drive it in the opposite direction. If no speed-regulation signal is supplied from the accelerator pedal, the vehicle will attempt to pivot around the pivot joint 3 in the direction in which the steering wheel is turned, through one of the motors being driven forwards and the other being driven backwards. If the accelerator pedal 14 is at the same time depressed, the signal from the steering wheel 8 and the force sensor 17 will be supplied as a small positive additive signal to the wheel on one side and as a negative, speed-reducing signal, to the wheel on the other side, which means that the two wheels are driven at different speeds and that the vehicle in this way turns. The output signal from the force sensor is thus supplied with different polarities to the opposite sides of the vehicle, and it is added to or subtracted from the driving signal from the accelerator pedal 14.

A steering device according to the invention may be arranged also on an articulated frame vehicle that is driven by more than two wheels, for example when all the wheels of the vehicle are driven, for example four or more wheels, and where also in such a case the output signal from a force sensor on the steering column is supplied to the regulator units for the drive motors, but with a negative signal supplied on one side of the vehicle and a positive signal supplied to the other side of the vehicle during turning.

Thus, there is obtained in a very simple way with the steering device according to the invention power-assisted steering for an articulated frame vehicle, such that the force that must be applied to the steering wheel in order to achieve the required steering is considerably reduced from the force that is required with fully mechanical manually steering. The steering according to the invention provides a servo effect that reduces the requirement for manual force, and it does so with a minimum of additional components. Thus, it provides a cost-effective and secure solution to the need for steering force. The steering device ensures also that the degree of turn of the steering wheel that is required in order to achieve the steering intended can be kept at a limited level, and this means that the steering properties are improved from those of a vehicle in which it has been necessary to have a high gearing ratio for the transfer from the steering column, in order to reduce the force required.

The steering device according to the invention is primarily intended for light and relatively cheap electrically powered vehicles, such as, for example, lawn mowers and other small vehicles. It is, however, possible to use the steering principle for all types of vehicle that have electric motors for the individual wheels to drive the vehicle.

It is also obvious that it is not necessary for the vehicle to have a conventional steering wheel: there can instead be arranged a control located on a steering rod, with the equivalent function as a steering wheel and steering column, or other equivalent steering means .