AU2011253801A1 - Mobile Base - Google Patents

Abstract

A mobile base (10) for use with a wheelchair. The mobile base (10) comprises two sides each provided with a front wheel (21), a rear wheel (22) and an intermediate drive wheel (23). The wheels are mounted onto a base portion (13), with a suspension means (33) connecting the front wheels (21) and the drive wheels (23) to the base portion (13). The suspension means (33) comprises of two rocker arms (41) each associated with a respective side of the base (13) portion and having a pivot axis. One front wheel (21) is connected to one rocker arm (41) to one side of the pivot axis for rotation about a first axis and one drive wheel (23) is connected to each rocker arm (41) to the other side of the respective pivot axis for rotation about a second axis. The second axis is higher than the first axis when the front wheel (21) and the drive wheel (23) are both resting on a flat horizontal surface

Description

P/00/011 28/5/91 Regulation 3.2 AUSTRALIA Patents Act 1990 ORIGINAL COMPLETE SPECIFICATION STANDARD PATENT Name of Applicant: Lendal Pty Ltd Actual Inventor Bill Clarke Address for service is: WRAYS Ground Floor, 56 Ord Street West Perth WA 6005 Attorney code: WR Invention Title: Mobile Base The following statement is a full description of this invention, including the best method of performing it known to me: 1 Mobile Base TECHNICAL FIELD [0001] This invention relates to a mobile base. [0002] The invention has been devised particular, although not solely, as a 5 mobile base for a powered wheel chair having six wheels comprising a pair of front wheels, a pair of rear wheels, and a pair of drive wheels deposed between the front and rear wheels. The mobile base thus provides a chassis for the wheel chair. [0003] The mobile base according to the invention may, however, be 10 applicable to any other mobile apparatus such as, for example, electric trolleys and other small vehicles, and industrial robots. BACKGROUND ART [0004] The following discussion of the background art is intended to facilitate an understanding of the present invention only. The discussion is not an 15 acknowledgement or admission that any of the material referred to is or was part of the common general knowledge as at the priority date of the application. [0005] It is known to provide electric wheel chairs with six wheels, being a pair of front wheels, a pair of rear wheels, and a pair of drive wheels positioned between the front and rear wheels. Typically, the front and rear wheels are castor 20 wheels. Such a wheel arrangement is utilised in an endeavour to provide stability when traversing uneven surfaces. It is particularly desirable to provide an arrangement which allows remaining wheels to maintain contact with the ground when one wheel, or one pair of wheels, encounters an obstruction such as a kerb or other bump, or a depression in its path. 25 [0006] One such electric wheel chair is disclosed in Australian patent 200221295 and features an arrangement in which the rear wheels and drive wheels on each side are interconnected through a cushioning swing arm which provides a suspension system.

[0007] The present invention seeks to provide a mobile base, which can be implemented in an electric wheelchair to offer a useful alternative to the arrangement disclosed in Australian patent 200221295 and which is suitable for implementation as an arrangement in which the front wheels and drive wheels on 5 each side are interconnected. SUMMARY OF INVENTION [0008] According to a first aspect of the present invention there is provided a mobile base comprising two sides each provided with a first wheel, a second wheel, and an intermediate drive wheel provided between the first and second 10 wheels, a base portion to which the wheels are mounted, and a suspension means connecting the first wheels and the drive wheels to the base portion, the suspension means comprising two rocker arms each associated with a respective side of the base portion and having a pivot axis, one first wheel being connected to each rocker arm to one side of the pivot axis for rotation about a first axis and 15 one drive wheel being connected to each rocker arm to the other side of the respective pivot axis for rotation about a second axis, wherein the second axis is higher than the first axis when said one first wheel and said one drive wheel are both resting on a flat horizontal surface. [0009] It has been found that the arrangement in which the second axis is 20 higher than the first axis when the one first wheel and the one drive wheel are both resting on a flat horizontal surface is particularly effective in allowing remaining wheels to maintain contact with the ground when one wheel, or one pair of wheels, encounters an obstruction such as a kerb or other bump, or a depression in its path. 25 [0010] Preferably, the pivot axis of each rocker arm is at a height intermediate the first and second axes when the first wheel and the drive wheel connected to the rocker arm are both resting on a flat horizontal surface. [0011] Preferably, the horizontal component of the distance between the pivot axis and the first axis of each rocker arm is greater than the horizontal component 30 of the distance between the pivot axis and the second axis when both the first wheel and the drive wheel are both resting on a flat horizontal surface.

[0012] Preferably, the first and second wheels comprise castor wheels. [0013] Preferably, each first wheel comprises a front wheel and each second wheel comprises a rear wheel. [0014] Preferably, a biasing means is provided for biasing each rocker arm 5 into a first position and for yieldingly resisting pivotal movement of the rocker arm about the pivot axis away from the first position. [0015] Preferably, the first position of each rocker arm corresponds to the drive wheel resting on a flat horizontal surface and the front wheel being elevated in relation to the flat horizontal surface. With this arrangement, the rocker arm is 10 caused to rotate about the pivot axis from the first position to a second position when the drive wheel and the front wheel both rest on the flat horizontal surface, thereby loading the biasing means to cause it to urge the rocker arm to the first position. [0016] Preferably, the resistance means comprises at least one spring. 15 Preferably, the at least one spring comprises a tension spring. [0017] Preferably, the at least one spring has two opposed ends, with one end pivotally connected to the rocker arm and the other end pivotally connected to the base portion. [0018] Preferably, the second wheels are connected to the base portion 20 through a second suspension means. [0019] Preferably, the second suspension means comprises two independent suspension units each associated with one of the second wheels. [0020] Preferably, each suspension unit comprises a swing arm pivotally connected to the base portion and a resistance means connecting the swing and to the base portion, the respective second wheel being connected to the swing arm. [0021] Preferably, the resistance means for the swing arm comprises a spring and damper mechanism. 5 [0022] Preferably the swing arm has two opposed ends, with the second wheel being connected to one end, the resistance means being connected to the other end and the swing arm being pivotally connected to the base portion intermediate the two ends. [0023] Preferably, each drive wheel is driven by a drive means. 10 [0024] Preferably, the drive means comprises a drive motor and gearbox assembly mounted on the respective rocker arm. [0025] Preferably, the drive motor comprises an electric motor. [0026] Preferably, the drive motor and gearbox assembly extends forwardly from the second axis. 15 [0027] Preferably, the electric motors receive electrical power from at least one electric storage battery. [0028] Preferably, the base portion is adapted to carry at least two electric storage batteries for supplying electrical power to the drive motors. The batteries preferably supply power to a control system for the mobile base and any other 20 electrical devices, systems or components fitted to the mobile base. [0029] Typically, the batteries are of generally rectangular configuration in plan, having a longitudinal extent. [0030] For the purpose of accommodating such batteries in a compact arrangement, the base portion is preferably configured to receive the two 25 batteries. Preferably the base portion is configured to carry the two batteries in juxtaposition, one forwardly of the other. The forward battery is oriented such that its longitudinal extent is aligned with the fore-and-aft direction of the mobile base, and the rearward battery is oriented such that its longitudinal extent is normal to the fore-and-aft direction of the mobile base. [0031] With this arrangement, the base portion incorporates an exterior recess 5 adjacent each side of a section thereof accommodating the forward battery, with the recess contributing to a space for accommodating the respective forwardly extending drive motor and gearbox assembly. [0032] According to a second aspect of the present invention there is provided a mobile base comprising two sides each provided with a front wheel, a rear 10 wheel, and an intermediate drive wheel provided between the front and rear wheels, a base portion to which the wheels are mounted, and a suspension means connecting the front wheels and the drive wheels to the base portion, the suspension means comprising two rocker arms each associated with a respective side of the base portion and having a pivot axis, one front wheel being connected 15 to each rocker arm to one side of the pivot axis for rotation about a first axis and one drive wheel being connected to each rocker arm to the other side of the respective pivot axis for rotation about a second axis, and a biasing means for biasing each rocker arm into a first position and for yieldingly resisting pivotal movement of the rocker arm about the pivot axis away from the first position, the 20 first position corresponding to the drive wheel resting on a flat horizontal surface and the front wheel being elevated in relation to the flat horizontal surface, whereby the rocker arm is caused to rotate about the pivot axis from the first position to a second position when the drive wheel and the front wheel both rest on the flat horizontal surface, thereby causing the biasing means to act to urge the 25 rocker arm to the first position. [0033] According to a third aspect of the invention there is provided a wheelchair comprising a mobile base and a seat mounted on the mobile base, the mobile base being in accordance with the first or second aspect of the invention. [0034] According to a fourth aspect of the invention there is provided a 30 wheelchair comprising a mobile base and a seat mounted on the mobile base, the mobile base comprising two sides each provided with a front wheel, a rear wheel, and an intermediate drive wheel provided between the front and rear wheels, a base portion to which the wheels are mounted, and a suspension means connecting the front wheels and the drive wheels to the base portion, the suspension means comprising two rocker arms each associated with a respective side of the base portion and having a pivot axis, one front wheel being connected 5 to each rocker arm to one side of the pivot axis for rotation about a first axis and one drive wheel being connected to each rocker arm to the other side of the respective pivot axis for rotation about a second axis, wherein the second pivot axis is higher than the first pivot axis when the front wheel and the drive wheel are both resting on a flat horizontal surface. 10 [0035] According to a fifth aspect of the present invention there is provided a wheelchair comprising a mobile base and a seat mounted on the mobile base, the mobile base comprising two sides each provided with a front wheel, a rear wheel, and an intermediate drive wheel provided between the front and rear wheels, a base portion to which the wheels are mounted, and a suspension means 15 connecting the front wheels and the drive wheels to the base portion, the suspension means comprising two rocker arms each associated with a respective side of the base portion and having a pivot axis, one front wheel being connected to each rocker arm to one side of the pivot axis for rotation about a first axis and one drive wheel being connected to each rocker arm to the other side of the 20 respective pivot axis for rotation about a second axis, and a biasing means for biasing each rocker arm into a first position and for yieldingly resisting pivotal movement of the rocker arm about the pivot axis away from the first position, the first position corresponding to the drive wheel resting on a flat horizontal surface and the front wheel being elevated in relation to the flat horizontal surface, 25 whereby the rocker arm is caused to rotate about the pivot axis from the first position to a second position when the drive wheel and the front wheel both rest on the flat horizontal surface, thereby causing the biasing means to act to urge the rocker arm to the first position. [0036] Preferably, the wheelchair according to the third, fourth or fifth aspect 30 of the invention comprises an electric wheelchair.

BRIEF DESCRIPTION OF THE DRAWINGS [0037] Further features of the present invention are more fully described in the following description of several non-limiting embodiments thereof. This description is included solely for the purposes of exemplifying the present 5 invention. It should not be understood as a restriction on the broad summary, disclosure or description of the invention as set out above. The description will be made with reference to the accompanying drawings in which: Figure 1 is a schematic side view of a mobile base forming part of an electric wheelchair according to a first embodiment, with the mobile base 10 shown supported on level ground; Figure 2 is a view similar to Figure 1, with the exception that a front wheel of the mobile base is shown riding over an obstacle; Figure 3 is a view similar to Figure 2, with the exception that the front wheel has passed over the obstacle and a trailing intermediate wheel is shown 15 riding over the obstacle; Figure 4 is a schematic plan view of the mobile base; Figure 5 is a schematic front view of the mobile base; Figure 6 is a schematic rear view of the mobile base; Figure 7 is a view similar to Figure 1, with the exception that the 20 intermediate drive wheel is shown removed so as to reveal a suspension means for the front and the intermediate drive wheels, as well as a drive arrangement for the intermediate drive wheel; Figure 8 is a further side view of the mobile base, viewed from the other side, with the intermediate wheel and the drive arrangement therefor 25 removed to reveal further parts of the suspension means; Figure 9 is a view similar to Figure 1, with the exception that certain dimensions are identified; and Figure 10 is a view similar to Figure 7, with the exception that certain other dimensions are identified.

Figure 11 is a schematic side view of a mobile base forming part of an electric wheelchair according to a second embodiment, with the intermediate wheel and the drive arrangement therefore removed to reveal further parts of the suspension means; and 5 Figure 12 is an exploded view of Figure 11. DESCRIPTION OF EMBODIMENTS [0038] Referring to the drawings, there is shown a mobile base 10 for an electric wheelchair according to a first embodiment. In some drawings, the mobile base 10 is shown resting on a surface 11 (which would typically be a paved 10 surface or a floor surface). While not shown in the drawings, the mobile base 10 is adapted to support an upper structure which comprises a seat for supporting an occupant of the wheelchair. The upper structure would also typically incorporate a control system by means of which the wheelchair occupant could control operation of the wheelchair. 15 [0039] The mobile base 10 has a fore-and-aft axis 12 which is depicted in Figure 4. [0040] The mobile base 10 comprises a base portion 13 configured as a box structure 14 having a bottom 15, a top 16, two opposed sides 17, a front end 18 and a rear end 19. 20 [0041] Wheels 20 are provided on opposed sides of the base portion 13. The wheels 20 comprise a front wheel 21, a rear wheel 22, and an intermediate wheel 23 on each side of the base portion 13. The front and rear wheels 21, 22 are each in the form of castor wheels. The intermediate wheel 23 comprises a drive wheel adapted to be driven by a drive motor and gear box assembly 25, as will be 25 explained in more detail later. The drive motor and gear box assembly 25 comprises an electric motor 27 and a gearbox 28. The electric motor 27 is adapted to receive electrical power from a bank of electrical storage batteries 29.

[0042] The base portion 13 is configured to define a storage zone 31 for accommodating the batteries 29. [0043] The front wheels 21 and the intermediate wheels 23 are connected to the base portion 13 by a first suspension means 33. The rear wheels 22 are connected to the base portion 13 by a second suspension means 35. [0044] Each front wheel 21 is mounted for rotation on a respective first axle 24 for rotation about a first axis. [0045] Each intermediate drive wheel 23 is mounted for rotation on a drive output shaft 26 of the drive motor and gear box assembly 25 for rotation about a second axis (corresponding to the central longitudinal axis of the drive output shaft 26). [0046] The first suspension means 33 comprises two rocker arms 41 on opposed sides of the base portion 13. Each rocker arm 41 is pivotally mounted to the respective side 17 of the base portion 13 by pivot pin 43 for pivotal movement about a generally horizontal pivot axis transverse to the fore-and-aft axis 12 of the mobile base 10. [0047] The rocker arm 41 has forward end section 45 and a rearward end section 47 on opposed sides of the pivot pin 43, as best seen in Figure 8 of the drawings. The front wheel 21 is connected to the forward end section 45. More particularly, the forward end section 45 incorporates a castor housing 49 at its free ends which receives the pivot shaft 51 of the front castor wheel 21. [0048] The rearward end section 47 of the rocker arm 41 is configured as a mounting plate 53 onto which the drive motor and gearbox assembly 25 is mounted. [0049] A biasing means 61 is provided for biasing each rocker arm 41 into a first position and for yieldingly resisting pivotal movement of the rocker arm 41 about the pivot pin 43 away from the first position. [0050] More particularly, the biasing means 61 is configured to bias the rocker arm 41 into the first position in which it is oriented such that the drive wheel 23 rests the flat horizontal surface 11 and the front wheel 21 is elevated in relation to the flat horizontal surface 11, as depicted in Figure 8. Further, the arrangement is such that the rocker arm 41 is caused to rotate about the pivot pin 43 from the first position to a second position when the drive wheel 23 and the front wheel 21 both rest on the flat horizontal surface 11, as shown in Figures 1 and 7. Rotation of the rocker arm 41 from the first position to the second position loads the biasing means 61 to cause it to urge the rocker arm towards the first position. [0051] In this embodiment, the biasing means 61 comprises a tension spring 63 configured as a coil spring. The spring 63 comprises an elongated body 65 having opposed ends, with one end 67 being pivotally connected to the base portion 13 and the other end 69 being pivotally connected to the rocker arm 41. In the arrangement shown, the end 69 is pivotally connected to lug 71 projecting from the mounting plate 53. [0052] The biasing means 61, according to the first embodiment, serves to provide a pre-load on the rocker arm 41. The pre-load on the rocker arm 41 in turn applies a preload on the respective intermediate drive wheel 23 in normal operating conditions as the wheelchair travels over a flat horizontal surface. The pre-load arises from tensioning of the spring 63 as the rocker arm pivots from the first position to the second position. The pre-load enhances traction between the drive wheel 23 and the surface 11. [0053] Further, the pre-load on each rocker arm 41 may aid in the ability of the wheelchair to pass over obstacles, including in particular climbing kerbs. This is because the bias of the spring 63 is conducive to elevation of the respective front wheel 21, thereby assisting its passage over an obstacle such as a kerb. [0054] Still further, the pre-load on each rocker arm 41 may provide cushioning as the front wheels 21 drop from a kerb or ride over an obstacle. [0055] Still further, the pre-load on each rocker arm 41 provides an anti-pitch mechanism which reduces the likelihood of the rear wheels 22 lifting from the surface 11 (typically, the ground) when the wheelchair comes to a sudden stop or when travelling down a steep incline. [0056] The rear wheels 22 are connected to the base portion 13 through the second suspension mean 35. The second suspension means 35 comprises two independent suspension units 81, each associated with one of the rear wheels 22. Each suspension unit 81 comprises a swing arm 83 pivotally connected to the base portion 13 by pivot pin 85. The swing arm 83 has a trailing end which incorporates a castor housing 87 in which the pivot shaft 89 of the respective rear wheel 22 is received. [0057] A resistance means 91 is provided for yieldingly resisting swinging movement of the swing arm 83. The resistance means 91 comprises a spring and damper mechanism 93. The spring and damper mechanism 93 comprises an operating assembly 95 incorporating a coil spring and a hydraulic shock absorber. The operating assembly 95 has two opposed ends, one end 98 being pivotally connected to the base portion 13 and the other end 99 being pivotally connected to an extension portion 101 on the swing arm 83, as best seen in Figure 8 of the drawings. With this arrangement, the resistance means 91 biases the swing arm 83 into a position in which the respective rear wheel 22 contacts the surface 11 and can yieldingly pivot out of that position to provide cushioning and to also ride over an obstacle encountered. [0058] Referring now in particular to Figures 9 and 10, the second axis (corresponding to the central longitudinal axis of the output shaft 26) is higher than the first axis (corresponding to the central longitudinal axis of the axle 24) when the front wheel 21 and the intermediate drive wheel 23 are both resting on a flat horizontal surface 11. This is best seen in Figure 9 in which the height of the second axis X 2 above the surface 11 is greater than the height of the first axis X1 above the surface 11. [0059] Further, the pivot axis of each rocker arm 41 is at a height X 3 intermediate the first and second axes when both the front wheel 21 and the intermediate drive wheel 23 connected to the rocker arm 41 are both resting on a flat horizontal surface 11, as depicted in Figure 10. [0060] Still further, the horizontal component Y of the distance between the pivot axis of the rocker arm 41 and the first axis (corresponding to the central longitudinal axis of the axle 24) is greater than the horizontal component Z of the distance between the pivot axis of the rocker arm 41and the second axis (corresponding to the central longitudinal axis of the output shaft 26) when both the front wheel 21and the intermediate drive wheel 23 are resting on a flat horizontal surface 11, as depicted in Figure 10.

[0061] It has been found that such an arrangement is particularly effective in allowing remaining wheels 20 to maintain contact with the surface 11 when one wheel, or one pair of wheels, encounters an obstruction such as a kerb or other bump, or a depression in its path. This is illustrated in Figures 2 and 3. In Figure 2, it can be seen that the intermediate drive wheel 23 and the rear wheel 22 remain in contact with the ground as front wheel 21 rides over obstacle 11a. Similarly, in Figure 3, it can be seen that the front wheel 21 and the rear wheel 22 remain in contact with the ground as the intermediate drive wheel 23 rides over obstacle 11 b. [00621 The base portion 13 defines the storage zone 31 for accommodating the batteries 29. The batteries 29 are of conventional shape, being generally rectangular in plan and thereby having a longitudinal extent. [0063] In the arrangement shown, the base portion 13 is configured to carry the two batteries 29a, 29b in juxtaposition position, one forwardly of the other, as best seen in Figure 4. The forward battery 29a is oriented such that its longitudinal extent is aligned with the fore-and-aft axis 12 of the mobile base 10, and the rearward battery 29b is oriented such that its longitudinal extent is normal to the fore-and-aft axis 12 of the mobile base. [0064] The base portion 13 is configured to accommodate this arrangement of the batteries 29. Such an arrangement is particularly advantageous, as it provides a compact construction as will become apparent later. [0065] More particularly, the base portion 13 comprises a leading section 111 configured to accommodate the forward battery 29a and a trailing section 113 configured to accommodate the reward battery 29b. Because of the orientation of the forward battery 29a, the width of the leading section 111 can be narrower than that of the trailing section 113. Accordingly, the side 17 of the base portion 13 can be configured to incorporate a stepped section 115, as best seen in Figure 4. With this arrangement, the side 17 comprises a front portion 121, a rear portion 123, and a lateral portion 125 extending between the front and rear portions 121, 123 and providing the stepped section 115. [0066] The stepped section 115 thus provides a recess 127 which provides additional space for accommodating componentry of the mobile base 10. In particular, the recess 127 on each side of the base portion 13 provides additional space for accommodating the forward end section 45 of the rocker arm 41. [0067] This space is utilised in the present embodiment to accommodate the respective drive motor and gear box assembly 25 located on the corresponding side of the base portion 13. In particular, the motor and gearbox assembly 25 is oriented such that it extends forwardly of the pivot pin 43 and utilises the available additional space. This facilitates a reduction in the track width between the two front wheels 21 and the two intermediate drive wheels 23, thereby facilitating a compact arrangement. [0068] Referring now to Figures 11 and 12, there is shown a mobile base 10 for a wheelchair according to a second embodiment. The second embodiment is similar in some respects to the first embodiment and corresponding reference numerals are used to identify corresponding parts. [0069] The biasing means 61 for the second embodiment comprises tension spring 63 and a further spring 129, both configured as coil springs. As in the case of the first embodiment, the spring 63 comprises the elongated body 65 having opposed ends, with the end 67 being pivotally connected to the base portion 13 and the other end 69 being pivotally connected to the outwardly projecting lug 71. The further spring 129 is similar to the spring 63, comprising an elongated body 131 having opposed ends, with an end 133 being pivotally connected to the base portion 13 and another end being pivotally connected to the inwardly projecting lug 135. [0070] Referring to Figure 12, the mobile base 10 comprises a rotational limiting means 137 for the rocker arms 41. The means 137 comprises of a static assembly 139 and a rotatable assembly 141. The static assembly 139 comprises of a plate 143, containing one or more arcuate slots 145, mounted at the inner end of the pivot pin 43, which is connected to the base portion 13 as shown in Figure 12. The rotatable assembly 141 is secured to the rocker arms 41 and, a clamping plate 147 and a mounting plate 149, the mounting plate having at least one projecting pin 151 as shown in Figure 12. The individual components of the rotatable assembly 141 are secured together via a fastening means which can comprise of bolts. [0071] The rotatable assembly 141 interacts with the static assembly 139 by virtue of the projecting pins 151 locating in corresponding arcuate slots 145 in the static assembly 139. The rotatable assembly 141 is capable of rotating about the pivot pin 43, however the range of rotation is limited by the travel distance of each projecting pin 151 in its respective arcuate slot 145. [0072] The rotational limiting means 131 can be incorporated into the first embodiment for limiting the range of rotation of the rocker arms 43. [0073] From the foregoing, it is evident that the present embodiments each provide an electric wheelchair which is of relatively compact construction and which can traverse obstacles such as kerbs while maintaining a relatively stable condition as well as effective traction. [0074] It should be appreciated that the scope of invention is not limited to the scope of the embodiment described. In particular, while the embodiment has been described in relation to a mobile base for a wheelchair, it should be understood that the mobile base according to the invention may find application in various other types of mobile apparatus such as, for example, electric trolleys and other small vehicles, and industrial robots. [0075] Throughout this specification, unless the context requires otherwise, the word "comprise" or variations such as "comprises" or "comprising", will be understood to imply the inclusion of a stated integer or group of integers but not the exclusion of any other integer or group of integers.

Claims (30)

1. A mobile base comprising two sides each provided with a first wheel, a second wheel, and an intermediate drive wheel provided between the first and second wheels, a base portion to which the wheels are mounted, and a suspension means connecting the first wheels and the drive wheels to the base portion, the suspension means comprising two rocker arms each associated with a respective side of the base portion and having a pivot axis, one first wheel being connected to each rocker arm to one side of the pivot axis for rotation about a first axis and one drive wheel being connected to each rocker arm to the other side of the respective pivot axis for rotation about a second axis, wherein the second axis is higher than the first axis when said one first wheel and said one drive wheel are both resting on a flat horizontal surface.

2. A mobile base according to claim 1, wherein the second axis is higher than the first axis when the one first wheel and the one drive wheel are both resting on a flat horizontal surface.

3. A mobile base according to claim 1 or 2, wherein the pivot axis of each rocker arm is at a height intermediate to the first and second axes when the first wheel and the drive wheel connected to the rocker arm and both resting on a flat horizontal surface.

4. A mobile base according to claim 1, 2 or 3, wherein the horizontal component of the distance between the pivot axis and the first axis of each rocker arm is greater than the horizontal component of the distance between the pivot axis and the second axis when both the first wheel and the drive wheel are both resting on a flat horizontal surface.

5. A mobile base according to any of the preceding claims, wherein each first wheel comprises a front wheel and each second wheel comprises a rear wheel.

6. A mobile base according to any of the preceding claims, wherein a biasing means is provided for biasing each rocker arm into a first position and for yieldingly resisting pivotal movement of the rocker arm about the pivot axis away from the first position.

7. A mobile base according to claim 6, wherein the first position of each rocker arm corresponds to the drive wheel resting on a flat horizontal surface and the front wheel being elevated in relation to the flat horizontal surface. With this arrangement, the rocker arm is caused to rotate about the pivot axis from the first position to a second position when the drive wheel and the front wheel both rest on the flat horizontal surface, thereby loading the biasing means to cause it to urge the rocker arm to the first position.

8. A mobile base according to claim 6 or 7, wherein the resistance means comprises at least one spring.

9. A mobile base according to claim 8, wherein the at least one spring has two opposed ends, with one end pivotally connected to the rocker arm and the other end pivotally connected to the base portion.

10. A mobile base according to any of the preceding claims, wherein the second wheels are connected to the base portion through a second suspension means.

11. A mobile base according to claim 10, wherein the second suspension means comprises two independent suspension units each associate with one of the second wheels.

12. A mobile base according to claim 11, wherein each suspension unit comprises a swing arm pivotally connected to the base portion and a resistance means connecting the swing arm to the base portion, the respective second wheel being connected to the swing arm.

13. A mobile base according to claim 12, wherein the resistance means for the swing arm comprises a spring and damper mechanism.

14. A mobile base according to claim 12 or 13, wherein the swing arm has two opposed ends, with the second wheel being connected to one end, the resistance means being connected to the other end and the swing arm being pivotally connected to the base portion intermediate the two ends.

15. A mobile base according any of the preceding claims, wherein each drive wheel is driven by a drive means, comprising a drive motor and gearbox assembly mounted on the respective rocker arm.

16. A mobile base according to claim 15, wherein the drive motor comprises an electric motor.

17. A mobile base according to claim 15 or 16, wherein the drive motor and gearbox assembly extends forwardly from the second axis.

18. A mobile base according to claim 16 or 17, wherein the electric motors receive electrical power from at least one electric storage battery.

19. A mobile base according to claim 18, wherein the base portion is adapted to carry at least two electric storage batteries for supplying electrical power to the drive motors.

20. A mobile base according to claim 19, wherein the batteries additionally supply power to a control system for the mobile base and any other electrical devices, systems or components fitted to the mobile base.

21. A mobile base according to claim 19 or 20, wherein the batteries are of generally rectangular configuration in plan, having a longitudinal extent.

22. A mobile base according to claim 21, wherein the base portion is configured to carry the two batteries in juxtaposition, one forwardly of the other, the forward battery is orientated such that its longitudinal extent is aligned with the fore-and-aft direction of the mobile base, and the rearward battery is orientated such that its longitudinal extent is normal to the fore-and-aft direction of the mobile base.

23. A mobile base according to claim 22, wherein the base portion incorporates an exterior recess adjacent each side of a section thereof accommodating the forward battery, with the recess contributing to a space for accommodating the respective forwardly extending drive motor and gearbox assembly.

24. A mobile base comprising two sides each provided with a front wheel, a rear wheel, and an intermediate drive wheel provided between the front and rear wheels, a base portion to which the wheels are mounted, and a suspension means connecting the front wheels and the drive wheels to the base portion, the suspension means comprising two rocker arms each associated with a respective side of the base portion and having a pivot axis, one front wheel being connected to each rocker arm to one side of the pivot axis for rotation about a first axis and one drive wheel being connected to each rocker arm to the other side of the respective pivot axis for rotation about a second axis, and a biasing means for biasing each rocker arm into a first position and for yieldingly resisting pivotal movement of the rocker arm about the pivot axis away from the first position, the first position corresponding to the drive wheel resting on a flat horizontal surface and the front wheel being elevated in relation to the flat horizontal surface, whereby the rocker arm is caused to rotate about the pivot axis from the first position to a second position when the drive wheel and the front wheel both rest on the flat horizontal surface, thereby causing the biasing means to act to urge the rocker arm to the first position.

25. A wheelchair comprising a mobile base according to any one of the preceding claims and a seat mounted on the mobile base.

26. A wheelchair comprising a mobile base and a seat mounted on the mobile base, the mobile base comprising two sides each provided with a front wheel, a rear wheel, and an intermediate drive wheel provided between the front and rear wheels, a base portion to which the wheels are mounted, and a suspension means connecting the front wheels and the drive wheels to the base portion, the suspension means comprising two rocker arms each associated with a respective side of the base portion and having a pivot axis, one front wheel being connected to each rocker arm to one side of the pivot axis for rotation about a first axis and one drive wheel being connected to each rocker arm to the other side of the respective pivot axis for rotation about a second axis, wherein the second axis is higher than the first axis when the front wheel and the drive wheel are both resting on a flat horizontal surface.

27. A wheelchair comprising a mobile base and a seat mounted on the mobile base, the mobile base comprising two sides each provided with a front wheel, a rear wheel, and an intermediate drive wheel provided between the front and rear wheels, a base portion to which the wheels are mounted, and a suspension means connecting the front wheels and the drive wheels to the base portion, the suspension means comprising two rocker arms each associated with a respective side of the base portion and having a pivot axis, one front wheel being connected to each rocker arm to one side of the pivot axis for rotation about a first axis and one drive wheel being connected to each rocker arm to the other side of the respective pivot axis for rotation about a second axis, and a biasing means for biasing each rocker arm into a first position and for yieldingly resisting pivotal movement of the rocker arm about the pivot axis away from the first position, the first position corresponding to the drive wheel resting on a flat horizontal surface and the front wheel being elevated in relation to the flat horizontal surface, whereby the rocker arm is caused to rotate about the pivot axis from the first position to a second position when the drive wheel and the front wheel both rest on the flat horizontal surface, thereby causing the biasing means to act to urge the rocker arm to the first position.

28. A wheelchair according to claim 27, 28 or 29 wherein the wheelchair comprises an electric wheelchair.

29. A mobile base substantially as herein described with reference to the accompanying drawings.

30. A wheelchair substantially as herein described with reference to the accompanying drawings.