Technical Field

This invention relates to a differential arrangement for electrically driven lift trucks, such as forklift trucks.

Background Art

In comparison to vehicles such as cars and lorries, lift trucks are designed to maximise manoeuvrability in very tight spaces, such as the narrow aisles of a warehouse. They therefore spend a relatively significantly high proportion of time being steered at extreme angles. At such angles the turning circles of inside and outside wheels have very different radii, and this will lead to slipping, skidding, loss of traction and undue tyre wear unless the inside and outside wheels are permitted or caused to be driven at significantly different speeds.

Differential steering for a hydraulically driven truck is generally not problematic as the hydraulic fluid in a circuit will follow a path of least resistance causing the inside wheel to resist being driven and allowing the motive hydraulic fluid to preferentially drive the outside wheel faster. For mechanically driven trucks and electrically driven trucks, some kind of differential may be required. These can be mechanically complicated and may involve significant engineering challenges or construction costs. A further problem associated with electronic differential steering systems is that they often rely on steering angle sensors which are mounted on the moving parts of the steering gear which renders them and the associated wiring susceptible to damage and may pose engineering problems in fitting these components within the steering gear's moving parts.

The present invention is directed at a differential steering arrangement specifically for an electrically driven lift truck to provide good steering with a simple set of components that can be built at a reasonable cost and easily fitted to a truck, and that provides an alternative solution to the problem of differentially steering an electrically driven lift truck.

Disclosure of the Invention

There is provided a lift truck comprising: a. a truck body having front and rear ends; b. at pair of driven, non-steerable wheels located towards one of said ends of the truck body; c. at least one steerable wheel located towards the other of said ends of the truck body; d. a pair of drive motors respectively associated with the pair of driven, non-steerable wheels; e. a steering mechanism which is responsive to a steering input to turn the at least one steerable wheel to an angle substantially tangential to a respective turning circle centred on a common turning circle centre located along an axis of rotation of at least one of the driven, non-steerable wheels; and f. a sensor adapted to provide a sensor output signal indicative of a linear distance between a first point which is fixed with respect to the vehicle body and a second point which moves with the at least one steerable wheel to be displaced towards or away from the first point as the steering angle of the at least one steerable wheels varies; g. a controller connected to said sensor to receive said sensor output signal, and connected to a throttle control to receive a throttle signal; wherein said controller is configured to determine from said sensor output signal the position of the common turning circle centre and to generate for each drive motor a respective independent control signal suitable to drive the driven non-steerable wheel associated with each drive motor at a rate proportional to the distance of that wheel from the common turning circle centre.

Preferably, said sensor is a linear sensor capable of varying in length by extending and retracting, which provides said sensor output signal in dependence on its length.

Preferably, the at least one steerable wheel is one of a pair of steerable wheels located towards the one of said ends of the truck.

Preferably, the pair of steerable wheels are connected by and steered with an Ackermann steering arrangement. Preferably, the first point which is fixed with respect to the vehicle body is a fixed-length beam member extending between the pair of steerable wheels forming part of the Ackermann steering arrangement.

Preferably, the second point which moves with the at least one steerable wheel is a point mounted on a radius arm of the Ackermann steering arrangement associated with said at least one steerable wheel, which radius arm is connected to a second radius arm associated with the other of the pair of steerable wheels, the radius arms being maintained at a fixed distance from one another and moved together to steer both wheels.

Preferably, the controller is configured to derive from the sensor output signal a lateral distance between the common centre of turning circles and a predetermined point on the geometry of the truck.

Preferably, the controller is configured to calculate the lateral distance of each driven wheel from the common centre of turning circles directly from the lateral distance derived according to claim 7 using stored constant values indicating the lateral distance of each driven wheel from the predetermined point on the geometry of the truck.

Preferably, said controller comprises a processor programmed with instructions which are effective to perform the determination of the position of the common turning circle centre and to generate said independent control signals.

Preferably, said controller comprises a plurality of cooperating processors which operate together to perform the determination of the position of the common turning circle centre and to generate said independent control signals.

Optionally, the drive motors each comprise a motor controller which is operable to receive and interpret a received control signal and to drive the motor at a speed determined by the control signal.

Alternatively, the drive motors operate according to the characteristics of electrical power supplied by a power supply and modulated by a drive motor controller which receives said independent motor control signal and modulates the power supplied to the drive motor according to the characteristics of the motor control signal.

Brief Description of the Drawings

Fig. 1 is a front, right side perspective view from above of an electrically driven lift truck; Fig. 2 is a right side elevation of the truck;

Fig. 3 is a top plan view of the truck;

Fig. 4 is a bottom, right side perspective view from below of the truck;

Fig. 5 is a simplified top plan view of the wheels and associated mechanisms of the truck when steering straight ahead;

Fig. 6 is a simplified top plan view of the wheels and associated mechanisms of the truck when steering left;

Fig. 7 is a simplified top plan view of the wheels and associated mechanisms of the truck when steering right;

Fig. 8 is a perspective view of a detail of the front wheels and steering mechanism;

Fig. 9 is a perspective view of a linear sensor component;

Fig. 10 is a cross-sectional view of the linear sensor component when in the retracted state;

Fig. 11 is view partly in cross-section of the linear sensor component when in the extended state;

Fig. 12 is an enlarged and ghosted view of the steering arrangement as shown in Fig. 5;

Fig. 13 is a geometrical interpretation of the steering arrangement of Fig. 12;

Fig. 14 is an enlarged and ghosted view of the steering arrangement as shown in Fig. 6;

Fig. 15 is a geometrical interpretation of the steering arrangement of Fig. 14;

Fig. 16 is an enlarged and ghosted view of the steering arrangement as shown in Fig. 7;

Fig. 17 is a geometrical interpretation of the steering arrangement of Fig. 16; Fig. 18 is a ghosted view of the Fig. 7 view, with a geometrical overlay;

Fig. 19 is a geometrical diagram based on the overlay of Fig. 18; and

Fig. 20 is a further geometrical diagram based on the overlay of Fig. 18.

Detailed Description of Preferred Embodiments

Referring to Figs. 1-4, these show in different views a lift truck 10, which in this embodiment is a forklift truck of the type known as a side loader, with forks 12 carried on a mast 14 which is disposed in a recessed area behind a driver cab 16 containing the usual driver controls (including steering wheel and throttle controls etc.). While not in any way critical to the invention, the skilled person will understand that the forks are extendable away from the side of the truck using a pantograph extending mechanism carried on the mast, allowing a load to be picked up alongside the truck, raised above the horizontal level of the main truck body, and retracted to be carried over the body within the lateral footprint of the vehicle.

The invention is equally applicable to all other kinds of lift trucks that are electrically driven and which have (at least) one steerable wheel disposed towards either the front or rear, and at least two driven wheels at the end (front or rear) opposite the steerable wheel(s). Front and rear denote the directions of driving forward and reverse with neutral steering.

In the illustrated embodiment there are four wheels as best seen in Fig. 4, namely a left front wheel 18, a right front wheel 20, a left rear wheel 22 and a right rear wheel 24. The front wheels 18, 20 in this embodiment are steerable and undriven, while the rear wheels 20, 22 are unsteered (fixed angle) and driven by electric motors.

Referring to Fig. 5, the wheel arrangement is seen from above when at a neutral, straight-ahead steering angle. It can be seen that wheel 22 is driven by a motor 26, and wheel 24 is driven by a motor 28. The motors are independently controllable with signals from a controller (not shown) which has a processor programmed to perform the functions described further below, namely to take inputs from a throttle and a linear sensor associated with a steerable wheel, and to generate from those inputs appropriate control signals to drive the rear wheel motors 26, 28 at the correct speeds to follow a correct turning circle without slipping. Figs. 6 and 7 each show the same view as Fig. 5, but with the vehicle turning left (Fig. 6) or right (Fig. 7). It will be seen that in each of Fig. 6 and 7, a common centre of turning circles 102 is denoted, so that each wheel is tangential to a respective turning circle centred on point 102.

The front wheels 18, 20 are centred on the point 102 by means of the well-known Ackermann steering geometry, which uses a fixed axle beam having kingpins for each steered wheel, stub axles for each wheel extending from the kingpins, radius steering arms for steering the wheels left and right, and a fixed length tie rod or tracking rod (in this case provided by the piston rods extending from either end of a double-ended hydraulic steering cylinder) connecting the radius steering arms.

The skilled person will be well aware that in the basic Ackermann geometry a trapezoidal arrangement of axle beam, radius arms, and tracking rod gives a good approximation of steering where the steered wheels adopt an angle that allows them to follow a turning circle centred on a common point. The steering arrangement of this embodiment is a modified Ackermann geometry which uses curved radius arms to improve the tracking of the steered wheels. In this case the geometry is designed so that the common point 102 will always lie along a line defined by the axes of rotation of the rear wheels as seen in Figs. 6 and 7.

It can be seen from Figs 6 and 7 that increased steering angles draws the common point 102 closer to the truck along the line joining the rear axles, while decreasing the steering angle will move the common point 102 away from the truck until, with neutral steering the point is effectively at infinity.

The closer the common point 102 is to the truck, the greater the differential in turning speed required to avoid skidding or slipping of the rear wheels, since the rear wheels should each be driven at a speed that allows them to follow their own turning circles at a rate whereby they sweep the same angle of arc per unit time along the commonly centred turning circles.

Fig. 8 shows in greater detail the steering arrangement of the front wheels 18, 20. Each wheel is rotatable about a stub axle (not visible) which pivots about a respective kingpin 30, 32. The kingpins 30, 32 are disposed at either end of a chassis frame member 34 serving as a fixed axle beam in the Ackermann geometry. The wheel steering is effected by lateral movement of a pair of radius steering arms 36, 38 each pivotally mounted at a distal end 40, 42to the wheel forward of the kingpin, and at a proximal end 44 (only the proximal end of the right wheel's radius arm being visible) to an end of a respective piston rod 46, 48 extending from a double-ended hydraulic cylinder 50. Steering controls received from the steering wheel cause a piston within the cylinder 50 to be driven left or right, thereby extending one of the piston rods 46, 48 and simultaneously retracting the other. The radius arms 36, 38 are thereby steered in tandem and in turn the wheels are turned left and right at angles tangential to turning circles centred on a common point 102 (Figs. 6 and 7).

A linear sensor 52 is mounted on the steering mechanism, and extends between a proximal pivot mounting point 54 on the chassis and a distal pivot mounting point 56 on the distal end of the radius arm 38 of the right front wheel. The linear sensor 52 is telescopically extendable and due to the positioning of the distal pivot mounting point 56 relative to the kingpin 32, it will be appreciated that steering the vehicle to the right causes the linear sensor to extend and steering to the left causes the linear sensor to retract. The linear sensor can thus be used to measure the distance between its two endpoints 54, 56 and this in turn provides a measure of the steering angle of the right front wheel.

The linear sensor 52 is shown in Figs. 9-11. Fig. 9 is an external view, while Fig. 10 is a sectional elevation with the sensor fully retracted. Fig. 11 is an elevation with the sensor fully extended and shows the outer female component in cross section while the inner male component is shown without a cross-section.

The sensor 52 has a female component 58 with an outer sheath or tube 60 mounted in a socket 62, 64 at either end. The proximal socket 62 contains an electronic sensor package 66 connected to an external port 68 providing a power and signal connection to external components such as a power supply and signal processor. Extending centrally from the electronic sensor package within the bore of the sheath 60 is a cylindrical waveguide 70.

The sensor 52 also has a male component 72 which is received within and telescopically extendable from the female component 58 as seen in Fig. 11. The male component has an annular cylindrical shaft 74 which is received internally in tube 60 and which itself has an internal bore 76 receiving the cylindrical waveguide 70. The male component can slide with little resistance between the positions shown in Figs. 10 and 11.

Mounted on the end of the male component, on the internal surface of the shaft 74, and surrounding the waveguide 70 at that position, is an annular permanent magnet 78. It will be appreciated that as the male component 72 moves telescopically in and out of the sheath 60, the position of the magnet 78 will move relative to the waveguide, from a position where it is located within the proximal socket 62 as shown in Fig. 10, when the sensor is fully retracted, to a position where it is located within the distal socket 64 as shown in Fig. 11, when the sensor is fully extended. At intermediate positions, the magnet will be located somewhere along the length of the waveguide between these extremes.

The electronic sensor package 66, waveguide 70 and permanent magnet 78 are available in a combined package from MTS Systems Corporation, Cary N.C., USA under the trademark MH Series. A combined sensor and housing, similar to that shown in Fig. 9 is available as the MH- Safety Canopen sensor.

The electronic sensor package 66 measures the position of the magnet along the waveguide (and therefore the amount of linear extension of the sensor) by emitting an interrogation RF pulse which travels along the waveguide towards the distal end. This creates a momentary magnetic field which travels along the waveguide at known speed and extends outside the waveguide. The permanent magnet 78 also creates a magnetic field surrounding the waveguide at its position. When the pulse reaches the position of the magnet, the momentary interaction of the magnetic fields releases a torsional strain pulse that travels back as an ultrasound pulse along the waveguide to be detected at the electronic sensor package 66 and converted to an electrical signal. As the speed of the RF interrogation pulse and the ultrasonic return pulse are known, the delay between interrogation pulse and return pulse detection provides a highly accurate measure of the distance travelled, and therefore the position of the magnet along the waveguide. An output signal from port 68 can be calibrated to determine the length of the sensor unit between the mounting points at either end 80, 82.

Thus as the right front wheel is steered, the sensor unit provides a continuous output signal which can be calibrated to a length value indicating the lateral displacement of the pivot 56 relative to the pivot 54 (Fig. 8).

We now describe how the value from the linear sensor is used to drive the rear motors at differential speeds corresponding to the turning circles dictated by the steering angle.

As noted earlier, the front axle uses an Ackermann steering setup, which increases the steer angle of the inner wheel when cornering, to give a true turning point for both front wheels which is positioned along the extended axis of the rear axle. (While there is no physical axle connecting the two rear wheels here, there is a notional axle defined by the common axis passing through the axes of rotation of the two rear wheels.)

Referring to Fig. 12, a triangle is overlaid on a ghosted view of the front steering arrangement when in a straight-ahead steering angle. Fig. 13 shows the same triangle, enlarged. In this geometry, the linear sensor forms one side (a) of a triangle, running from the chassis pivot (CP) i.e. the point on the chassis on which the proximal end of the linear sensor is pivotally mounted, to the radius arm pivot (RP) on the distal end of the radius arm closest to the wheel. A second side (b) of the triangle is defined between the radius arm pivot point (RP) and the steering kingpin from which the wheel's stub axle extends. The third side (c) of the triangle runs from the chassis pivot point (CP) to the kingpin (KP).

The steered angle of the wheel can be measured as the angle a, which is the included angle at the kingpin (KP) between sides b and c. Note that the vector direction of the triangle side b, which changes as the wheel is steered angle, need not be co-linear with the front-to-rear axis of the truck when the truck is steering straight ahead. In other words, when shown on the page in Fig. 14, that side need not be vertical on the page, as the connection between the kingpin and the radius pivot point may be at an offset angle to the wheel track. However, that offset angle is fixed and known, and once the angle a can be calculated, the steering direction of the wheel is thereby known, i.e. at a given neutral angle a the wheel will be steering straight ahead or at zero degrees. Any deviation of the steering angle above or below that neutral angle is the steered angle of the wheel i.e. if the calculated value of a is the neutral angle minus 10 degrees, the wheel is being steered 10 degrees to the left.

Figs. 14 and 15 show corresponding ghosted views of the steering arrangement, and of the enlargement of the overlaid triangle defined between the same points, when the steering is to the left. Figs. 16 and 17 show corresponding ghosted views of the steering arrangement, and of the enlargement of the overlaid triangle defined between the same points, when the steering is to the right.

It can be seen from a comparison between Figs. 13, 15 and 17 that in the three steering positions, the lengths b and c are fixed, while the length a of the linear sensor varies as it is extended and retracted. The steering angle a likewise varies as the wheel is steered (as of course do the other two internal included angles of the triangle). Once the angle a is known, the steered angle of the right front wheel is known and simple geometry will define where, along the axis connecting the axes of rotation of the rear wheels, the common centre of the turning circles for all wheels should be located.

The centre of the turning circle for the left front wheel is not needed, because the Ackermann geometry will, to an acceptable degree of precision, mechanically centre the turning circles of both undriven front wheels to about the same point (and in any event as the wheels are undriven they are free to adjust their rotation rates).

The centres of the respective turning circles for the two driven rear wheels are important however, as if these wheels can be driven at rates that cause them each to track their own turning circles, centred on a common point with the centre of the turning circle of the front right wheel, without slippage, then a good differential drive has been achieved.

Therefore, calculation of the angle a allows the drive speeds of the two rear wheel motors to be adjusted above and below a nominal speed given by a throttle input, so that each wheel has an adjusted speed that causes it to follow a turning circle (assuming a non-zero steering input angle above a de minimis threshold) at exactly the right speed to avoid slippage and to drive the truck along the desired steering angle at the desired velocity according to the operator's throttle input.

The output of the linear sensor is calibrated to a length value, which is the length a in the triangle. The lengths b and c are fixed and known and may be stored in memory available to a processor which performs a steering angle calculation to derive the steering angle a.

The calculation of the steering angle a is obtained from the cosine rule, which provides that in the triangle shown in Figs. 13, 15 and 17 the angle a is given by: er = cos ^-1 [(Z> ² + c ² — a ² )/2bc]

As indicated before, b and c are constant values, and are permanently stored in a storage available to the processor and loaded into working memory when program instructions for calculating the angle a are executed. The program instructions take as an input a value from the linear sensor, which is calibrated by the processor to a length value a, and using the values a, b and c, a simple trigonometric function (such as is available in most programming language mathematical libraries or which can be specifically encoded or provided in a look-up table) will give the value of the angle a. Once a is known, the processor can either pass this as a raw value, or as an adjusted value offset to give the true directional angle of the right front wheel as discussed above (e.g. 0 degrees for straight ahead, or a positive or negative angle for right or left steering), to a drive motor control function. The drive motor control function may form part of a common program with the steering angle calculation, or may be a separate set of program instructions running on the same or a different processor.

Typically, there will be a single processor with an overall control a program which receives the inputs from the linear sensor and from a throttle sensor (which indicates the desired truck velocity from an operator's throttle input_. The control program will typically have a module or function to derive the steered angle of the right front wheel from the linear sensor input, and a module or function to convert the throttle input to a pair of output motor control signals which are respectively directed to the left and right rear wheel motors.

In order to convert a right front wheel steered angle to a motor speed for each wheel, again a consideration of the geometry will show how this may be done using only the derived right front wheel angle and a throttle input.

Firstly, if the truck is detected to be steering straight ahead, and optionally within a small angle below a threshold where differential steering is not required, the motors may be driven at a common speed determined by the throttle input.

Referring to Fig. 18, the geometry of the truck when steering left is shown again, as in Fig. 6, but with an overlay of a centre line 100 denoting the central front-rear axis of the truck's geometry. The centre line 100 is used as a datum from which the horizontal offsets of the wheels can be readily measured. The common centre of the turning circles for the four wheels is indicated at 102, and for each wheel a respective point 104 (left front), 106 (right front), 108 (left rear) and 110 (right rear) is indicated to derive a fixed geometry between the four wheels, the centre line and the common centre of the turning circles. It may be noted that the points 108, 110 used for the rear wheels are the central ground contact points of the wheels, which follow the turning circle circumference, while the points 104, 106 used for the front wheels are the kingpin positions, which are fixed with respect to the chassis (unlike the front wheel ground contact positions which move relative to the chassis due to the Ackermann steering).

Fig. 19 reproduces the geometry from Fig. 18. The steered angle of the right front wheel (which has been calculated in the preceding section) is indicated as a. This is the true steering angle of the right front wheel and may have been calculated from a using a known offset. The complementary angle (where = 90° - a) is indicated, and is one internal angle of a right- angled triangle which has perpendicular sides whose lengths are indicated as H and V.

In this triangle, H is the horizontal (as represented on the page) or lateral distance between right front kingpin point 106 and the common centre of the turning circles 102. V is the vertical (as represented on the page) or front-to-rear distance between the right front kingpin point 106 and the common centre of the turning circles 102 (which is aligned with the axles of the rear wheels).

V is a fixed and known distance which is stored in memory, is obtained from the calculation of the raw steering angle a or the actual wheel steering angle a. Therefore the horizontal offset of the common centre of the turning circles which each wheel is required to steer at this steering angle is calculated as H = V * tan( ?). From this and the known geometry of the truck it may be convenient to calculate and store the horizontal offset between the truck's centre line 100 and the common centre of turning circles 102, indicated in Fig. 19 as He (for H central).

Referring to Fig. 20, the horizontal offsets of the rear wheel points 108 and 110, denoted as HL and HR respectively, can be derived by simple addition or subtraction from He, since the offset values of each wheel from the centre line may be stored in memory for this purpose.

Fig. 20 shows an arc from each of the turning circles followed by the left rear wheel point 108, the right rear wheel point 110, and the intersection of the centre line 100 and the line joining the rear wheels. Each arc is denoted with a length proportional to the required speed. So if for example the throttle input is interpreted as requiring a truck speed of (say) 3 m/s, the left wheel will travel slower and the right wheel travel faster, with the centre line of the truck travelling at the required speed. This requires driving the left and right rear wheels at different speeds indicated by the lengths of the respective arcs.

It will be noted that HL, He and HR are radii of three turning circles centred on point 102. For a desired velocity vel, which the centre line should achieve based on the throttle input, the left wheel should be driven at a rotation rate which will cause the wheel to travel over the ground at a speed of vel * (HL/HC) while the right wheel will be driven at a rotation rate causing it to travel over the ground at a speed of vel * (H _R /Hc). The module calculating the output signals to the left and rear drive motors therefore outputs a signal which adjusts the desired velocity according to the throttle input proportionally to these factors.

The motor control signal may be received and interpreted directly by the motors, where those motors have appropriate electronic circuitry built in. Alternatively, the control signals may be sent to a power control unit or units which receive the control signals and which generate electrical power whose characteristics directly drive the motor at the required speed.

It will be appreciated that the same calculations will apply when turning right, and that for small steering angles the system designer may choose to drive the wheels at the same velocity.

It will also be appreciated that while the embodiment described has steerable front wheels and unsteered, driven rear wheels, the front wheels might be driven and unsteered in other embodiments and the rear wheels steered and undriven.

It will also be appreciated that a three-wheel vehicle, having one undriven steered front (or rear) wheel and two driven rear (or front) wheels will benefit from the invention in almost identical manner, with only the geometry of the triangles needing to be adjusted to account for the position of the wheel at which the linear distance measurement is taken.

Furthermore, while the described embodiment shows a linear sensor used to measure the lateral displacement of a component of the right front wheel steering gear, and calculate the steering angle of this wheel, there is no reason why this must be the case. The sensor could instead measure the lateral displacement of a component associated with the left front wheel and calculate the steering angle of that wheel (or of either rear wheel if they were the steered wheels). A sensor could also be used to measure the lateral displacement of the right front wheel and then calculate (from the Ackermann geometry) the resultant steering angle of the left front wheel, though this would usually be a less direct approach.

It is also to be noted that the calculation of an angle associated with a steered wheel such as the angles a, a or is only an intermediate value that assists in determining the lateral offset of the common centre of turning circles. It is also possible to achieve the same benefits with a direct calculation of the required motor drive speeds from the linear sensor input, with the steering angle being made implicit within the calculation.

While the illustrated embodiment has two driven wheels at one end and two undriven steered wheels at the other end, the system can be adapted to a truck which has electric drive on three or four wheels. The geometrical calculations given above can be extended to determine the distances between the front wheel kingpins and the common turning circle, and thereby to determine an appropriate drive speed for one or both front wheels also. This can be further improved in precision by adjusting the calculations to take into account the offset between the centre of the ground contacting surface (for any given steering angle) and the kingpin position, noting that this is steering-angle dependent.