GB1581231A

GB1581231A - Wheelchairs

Info

Publication number: GB1581231A
Application number: GB735/76A
Authority: GB
Original assignee: BIDDLE ENG CO Ltd
Current assignee: BIDDLE ENG CO Ltd
Priority date: 1977-04-12
Filing date: 1977-04-12
Publication date: 1980-12-10

Description

(54) IMPROVEMENTS IN OR REI,ATING TO WHEELCHAIRS (71) We, BIDDLE ENGINEERING CO. LIMITED, a British Company of, 103 Stourbridge Road, Halesowen, West Midlands, B63 3UB, do hereby declare the invention for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: The invention relates to motorised wheelchairs for use by invalids, such wheelchairs having at least one pair of electric motors, such motors driving respective wheels on opposite sides of the wheelchair.

The operation of the motors may be controlled either by the occupant of the wheelchair or by a person, hereinafter referred to as an attendant, walking adjacent to, e.g.

normally behind, the wheelchair.

Each wheelchair will normally be provided with a total of four wheels. However, an arrangement in which the wheelchair has three wheels is quite possible. In the case of a four wheeled wheelchair at least two of the wheels situated on opposite sides will be driven each by its own separate motor, and the other two wheels may be mounted to swivel about vertical, or approximately vertical, axes, such wheels being either idle or driven.

For operation of the wheelchair involving attendant control, it is desirable that steering the wheelchair shall be capable of being effected by the attendant without being overriden or significantly impeded by the drive applied to the wheels.

This consideration may also arise in any case where the occupant is able to effect steering by the swivelling wheels.

One object of the present invention is to provide an arrangement which more nearly, or more simply, meets this requirement than those hitherto available.

According to the present invention we provide a motorised wheelchair having at least one pair of electric motors driving respective wheels on opposite sides of the wheelchair, wherein the motor windings to be energised are connected to a power supply in a configuration such as to minimise counteraction of the speed differential of one wheel relatively to the other occurring during steering of the wheelchair, by reducing the increase in current that would flow through the motor driving the slower wheel and/or the decrease in current that would flow through the motor driving the faster wheel were both motors each to be energised directly with the voltage of the supply.

If the wheelchair is to be controlled solely by the occupant swivelling wheels provided on the wheelchair may be connected to a steering arm or some form of mechanical or hydraulic linkage through operation of which the occupant may cause one or both of the swivelling wheels to swivel about a substantially vertical axis.

Flrst control means may be provided whereby either the occupant or the attendant may control the voltage applied to the windings of the motors.

When the windings of each motor are connected in series with the other to a power supply it is found that only a very small force is required on one side of the wheelchair or over a steering mechanism to one of the swivelling wheels to cause the wheelchair to turn a corner. Such connection of the motor windings therefore is very suitable for attendant control operation. However the possible physical state of the occupant may not enable him to operate the steering means.

It is, therefore, a further object of the present invention to provide a wheelchair that overcomes or reduces this problem.

According to this aspect of the invention we provide a motorised wheelchair having at least one pair of electric motors driving respective wheels on opposite sides of the wheelchair, wherein the motor windings to be energised are connected to a power supply in a configuration such as to minimise coun teraction to the speed differential of one wheel relatively to the other occurring during steering of the wheelchair, by reducing the increase in current that would flow through the motor driving the slower wheel and/or the decrease in current that would flow through the motor driving the faster wheel were both motors to be energised to the voltage of the supply and wherein additional switching means are provided operative to electrically connect the windings of said motors in parallel with each other to the power supply, and second control means operative to control the voltage applied to each of the motors said second control means including a manually engageable part operable by the occupant.

Preferably said manually engage able part comprises a joystick control.

Preferably the second control means is operable to differentiate the voltage applied to each of the said motors.

A problem may arise when the windings of each motor are connected in series with each other to the power supply for the reasons which will now be discussed.

When the wheelchair begins to turn a corner the inside wheel is constrained to rotate at a slower speed than the outside wheel. Since the rotors of the motors are connected in series with each other the current flowing through the inside rotor will be the same as the current flowing through the outside rotor and cannot, therefore, increase to the extent that would occur were the motors to be connected in parallel to the supply because the effective voltage applied across both motors will be substantially con stant and the faster (outside) motor will develop an increased back emf.

In order to make efficient use of the power available, which usually comprises one or more batteries, the motors should be run at their most efficient speed which, in the case of the permanent magnet motors used, is when they are supplied with their maximum rated voltage. It will be appreciated that if one motor is virtually stalled, i.e. when the wheelchair turns a sharp corner, the voltage across the rotor of the outside wheel is increased.

It has been found in practice that if the wheelchair turns a sharp corner while maximum voltage is being supplied to the motors, the outside motor may overheat and in some cases it has been damaged or completely destroyed.

It is, therefore, a further object of the present invention to provide a control circuit that will overcome or at least reduce the above mentioned problem.

According to this aspect of the invention we provide a motorised wheelchair having at least one pair of electric motors driving respective wheels on opposite sides of the wheelchair wherein the motor windings to be energised are connected to a power supply in a configuration such as to minimise counteraction to the speed differential of one wheel relative to the other occurring during steering the wheelchair, by reducing the increase in current that would flow through the motor driving the slower wheel and/or the decrease in current that would flow through the motor driving the faster wheel were both motors to be energised at the voltage of the supply, and sensing means for sensing a change in the direction of the travel of the wheel chair and switching means responsive to the operation of the sensing means and so connected as to limit the power delivered from the supply.

The sensing means may comprise a part engage able by one of the wheels which swivel about a vertical axis when the wheel chair deviates from a straight ahead position. In particular the sensing means may comprise a switch carrying member and a switch operating member mounted about the swivelling axis. The swivelling wheels of one of said members being mounted for swivelling movement . with the wheel and the other being mounted on the wheelchair. The switch operating member may have projections or holes on or in the surface thereof respectively or other irregularities and the switch carrying member may be provided with a switch such that relative movement between these two members causes operation of the switch. In particular the switch operating member may be provided with magnets and the switch carrying member provided with a magnetically operated switch mounted such that on relative rotation between the two members the magnet or magnets passing in close proximity to the magnetically operated switch can cause operation thereof.

Alternatively the sensing means may comprise means for sensing electrical parameters depending upon a speed difference between wheels on opposite sides of the wheelchair.

Preferably said voltage reducing switching means only operates when the voltage control means is in a position such that at least half the maximum rated voltage of each motor is being supplied to the two motors in series.

The driving wheels of the wheelchair are preferably carried by an output shaft of each motor, said output shaft forming the sole support for each wheel in the manner described and claimed in British patent specification 1,198,905.

A wheelchair according to the present invention will now be described, by way of example only, with reference to the accompanying drawings wherein: FIGURE 1 is an illustration of a wheelchair having an attendant and occupant con trol means; FIGURE 2 is a circuit diagram of the attendant control means; FIGURE 3 is a circuit diagram of the firing circuit shown in block form in Figure 2; FIGURE 4 is a circuit diagram of one channel of the occupant control means; FIGURE 5 is a control circuit diagram in block form of the attendant and occupant control means; FIGURE 6 is an illustration of a four wheel drive wheelchair chassis having mechanical steering means.

Referring first ot Figure 1, the wheelchair illustrated comprises a frame which includes a pair of side members 10, 11, each side member 10, 11 including a back upright member 12 of tubular form, a front upright member 13 also of tubular form and a pair of tubular members 14 and 15 interconnecting the back and front members 12 and 13.

Three links 16, only one of which is shown, interconnect the side frame members 10 and 11. Each link such as 16 includes a pair of pivotally connected link elements which can be moved from a folded position (not shown) through a centre or aligned position to an over-centre position as shown in Figure 1 stretching and slightly relaxing the seat and back portions 22 and 21 and then serving to maintain the side frame members 10 and 11 spaced apart as shown in Figure 1.

Arm rests 19 with pads 20 are attached to respective side frame members 10 and 11.

The back panel 21 is formed of a sheet of flexible material secured between the back members 12 and side frame members to form a back rest. The seat panel 22 likewise is formed of a sheet of flexible material and is secured between the upper tubular members 14 to provide the seat portion of the chair.

An additional panel 23 also in the form of a sheet of flexible material is secured between the lower tubular members 15 to provide a parcel shelf.

Foot rests 24 are connected by clips 25 to support members 26 which are connected to the frame members 13 and side frame members 10 and 11. The front wheels of the wheelchair comprise a pair of castor wheels 29 privotally secured at the lower ends of the tubular front members 13 to swivel about vertical, or approximately vertical, axes. The rear wheels of the wheelchair comprise a pair of driving wheels 31 only one of which is visible. The wheels can have pneumatic tyres or solid rubber tyres. The driving wheels 31 are driven by respective electric motors 32LB, 32RB each of which may be of a permanent magnet type (i.e. has a stator including permanent magnets providing the motor field and a rotor having a commutator connected to rotor windings), and mounted to the wheel in a manner as described and claimed in British patent specificaton No.

1,198,905.

Each of the two back upright members 12 has at its upper end a handlebar 33 having a grip 34. The handlebar 33 on one side of the vehicle has attached thereto first control means comprising a control lever 35 to which is secured one end of the inner wire of a Bowden (R.T.M.) cable 36, the other end of which enters a switch box 37 secured to one of the side frame members of the wheelchair and is connected to the operating member of a variable resistor as hereinafter mentioned.

Secured to the arm rest 19 is a second control means 17 having a manually engageable joystick 18 projecting upwardly. The second control means 17 is connected by a multicore cable 27 to switch means in a switch box 37. Mounted on the switch box 37 are a pair of switches 28A, 28B, the former of which has an off position and a position to select attendant control of the wheelchair.

The switch 28B also has an off position and a position in which occupant control is selected. A light source 39 is provided which may be a filament bulb or a light emitting diode illuminated when either of the switches 28A or 28B are in a control selected position.

A further switch 38 may be provided either on the switch box 37 or alternatively on or adjacent either handlebar of the wheelchair, which switch is operative, when attendant control has been selected, to select drive for the wheelchair in either a forward or a reverse direction depending on the position of the operating member of the further switch 38.

Referring now in addition to Figure 2, the circuit for attendant control of the wheelchair will now be described in detail.

Power is supplied from batteries 30 (see Figure 1) which are connected in a circuit configuration to provide a 48 volt line L1, a 24 volt line L2 and an earth.

The rotor windings of the two motors 32LB and 32RB are connected in series with each other. The attendant control switch 28A controls either directly or indirectly contacts 28A1 to 28A4 (shown in Figure 2 in their off positions). The forward-reverse switch 38 is a two-pole two-way switch having contacts 38C1 and 38C2.

If switch 28A is switched to the on position contacts 28A1 to 28A4 change to their lower positions, the control chair will then be ready for attendant operation. The wheelchair will not move, however, since the left and righthand motor windings are effectively connected in a closed unenergised loop through resistor R1 and relay contact RLC1.

If the lever 35 is now operated by movement through a small distance, first of all microswitch M1 is closed to its upper contact thus energising relay coil RL1. Contact RLC1 will then change to its lower position thus opening the loop. Further movement of the lever 35 will cause the movable contact 46 of the variable resistor VR1 to move away from the earthed end thereof causing the voltage on the input line 44 to the firing circuit 40 to increase.

Operation of the firing circuit 40, shown in block form in Figure 2, will now be described in detail with reference to Figure 3.

24 volts are supplied to the terminal 60 with respect to the earth line. A zener diode 62 is connected across the circuit in order to stabilise the voltage at a value lower than the applied voltage. A uni-junction transistor 63 together with its ancillary components forms an oscillator which provides an output of sawtooth wave form. The level of the DC voltage on which the sawtooth wave form is superimposed can be varied by movement of the movable contact 46 of the variable resistor 47. A further preset variable resistor 48 may be provided in the circuit to avoid or reduce the possibility of the value of the variable resistor 47 being critical. The sawtooth wave form is applied to a field effect transistor 64, which characteristic of such device has a very high input impedance serving effectively to isolate the uni-junction oscillator from the following part of the circuit.

A thermistor 58 is connected in parallel with the resistor 59 to compensate for changes in temperature of the circuit due to changes in ambient temperature and heat generated by the control circuit itself.

The output from the field effect transistor 64 is fed through the preset variable resistor 65 to transistor 66 which only conducts when the potential applied to the base thereof is of a predetermined value. As the DC base component on which the sawtooth wave form is superimposed is raised transistor 66 will begin to conduct when the peak of the sawtooth reaches said predetermined value.

The output of the transistor 66 will therefore be of square wave form, the width of each pulse depending on the period of conduction of the transistor 66. Since the sawtooth wave form is of a constant size the period of conduction of the transistor 66 will depend on the magnitude of the DC base component on which the sawtooth wave form is superimposed. Transistors 67, 68 improve (square) the shape and amplify the pulses produced.

The output of transistor 68 is then fed via resistor 69 to the power transistors 41 and 42 shown in Figure 2.

Varying the position of the variable contact of resistor VR1 will vary the speed of the motors since the average voltage and current of the pulsed input to transistors 41, 42 is a function of pulse width.

As already mentioned, to obtain maximum efficiency from the power available during normal running of the wheelchair, it is desirable that the motors should be run at, or near, their maximum power ratings. It will be appreciated that when the wheelchair is constrained by the attendant, or by the occupant, in this case steering the castor wheels, to turn a corner after straight movement in forward or reverse direction at full power, or almost full power, a speed difference will be imposed between the motors and one motor may be substantially stalled while the other motor may be speeded up. In practice operation under such conditions has been found to cause damage (overheating or in extreme cases burning out of the windings of one motor, namely that operating at the faster speed).

In order to overcome or reduce the possibility the control circuit shown in Figure 2 includes a power reduction switching circuit operation of which will now be described in detail.

As the lever 35 (attendant control) is operated from its "at rest" position, the power applied to the motors is increased.

When the lever 35 is operated to beyond three quarters of its full amount, microswitch M2 will operate connecting the line L3 to the line L4.

So long as the wheelchair is being driven in a straight line the control circuit will continue to operate normally. However, if the corner is turned the voltage across one motor (that having the greater speed) will rise while the voltage across the other motor will fall.

Thus if, for example, the voltage across the left-hand motor 32LB rises to 28 volts, the voltage on line L3 will decrease towards earth, the voltage level of line L4 will follow L3, and transistors TR1 and TR2 will start to conduct, and produce energisation of the coil of relay RL4, changing over relay contacts RL4C1 and RL4C2 to their left-hand positions.

The contact RL4C1 disconnects a high voltage (16 volts) zener diode Z1, from the circuit and connects a low voltage (6.2 volts) zener diode Z2 into the circuit providing "hold in" for relay RL4.

Contact RL4C2 connects a preset variable resistor PR1 effectively in parallel with the portion of VR1 below the slider to the earth line. This has the effect of reducing the voltage on the firing circuit input line 44. In practice it has been found that the preset value of resistance PR1 should be chosen such that the voltage on the line 44 is reduced by about a third of its former value.

The amount by which the voltage should be reduced will directly depend on the maximum power rating of the motors, and the maximum battery voltage.

As soon as the wheelchair reverts to movement in a straight line the speed differential between the motors will disappear, thus causing relay RL4 to de-energise restoring contacts RL4C1 and RL4C2 to their former positions and disconnecting the power reduction circuit PR1, RL4C2 from the firing circuit 40. The power applied to the motors will then again be dependent solely on the setting of VR1.

For left-hand turns the circuit associated with RL5 and which has the same configuration as that associated with RL4 will operate in place of the latter to bring the power reduction circuit into operation.

The capacitors C1 and C2 are present to provide a small time delay such that if the maximum allowable voltage across a single motor is exceeded only for a short time, the circuit will not come into operation.

The control of the wheelchair by the occupant using the second control means shown in Figure 4 will now be described in detail.

The circuit shown in Figure 4 is that applicable to one motor only, the remainder of the circuit being a duplication and associated with the other motor.

The joystick 18 as shown in Figure 1 is secured to the control box 17 and is biased to an "at rest" position from which it may be moved in a backward, forward or lateral direction. The joystick 18 is arranged either directly or indirectly to operate the sliders of two variable resistors, one of which is shown at 83 in Figure 4.

If the joystick is moved in a forward direction the slider of resistor 83 in both halves of the circuit will be moved by a like amount and the wheelchair will be driven in a forward direction. The speed of the wheelchair will be determined by the amount of movement of the joystick from its "at rest" position.

If the joystick is naoved in the reverse direction the wheelchair will move in reverse. If the joystick is moved in a lateral direction the variable contact of resistor 83 will move from its "at rest" position and power will be supplied to the associate motor driving it in a given direction, e.g. forward, while the slider of the corresponding resistor in the other half of the circuit will stay in its "at rest" position. Thus, one motor will operate while the other remains stationary and the wheelchair will turn to the right or left according to the direction of lateral movement of the joystick at a speed dependent upon the extent of the lateral excursion of the joystick.

Each variable resistor 83 is connected by way of its slider to a main control circuit 80, and also connected to two sensor circuits 81 and 82. The main control circuit 80 is connected by an output circuit 84 and switch 28B to one of the left-hand motors. The power supply 30 supplies power to the motors and to the control circuit.

The joystick is biased by spring means to its "at rest" position so that when the switch on the control box 37 is switched to select occupant control of the wheelchair the wheelchair will not move until the joystick is moved. Upon movement of the slider in one direction (forward) or the other direction (reverse) in consequence of movement of the joystick, the sensor circuit 81 or the sensor circuit 82 will respond respectively. If circuit 81 responds transistors 102 and 103 will switch on and relay RL2 will energise while if circuit 82 responds transistors 100 and 101 will switch on and relay RL1 will energise.

The general manner of operation of the circuit 80 is as already described for the circuit 40 in Figure 3. The components in firing circuit 80 are similarly referenced to the components in Figure 3 differing only in the prefixing of each reference numeral with the numeral 1.

The operation of the control circuit will now be described. If the joystick 18 is pushed in a forward, straight ahead, direction the slider on both of the variable resistors 83 (only one of which is shown) will be rotated by the same amount. The further the joystick 18 is moved in a forward direction from its "at rest" position the greater will be the power supplied to the motors and hence the faster the wheelchair will travel. If the joystick 18 is not pushed in a straight ahead direction but is pushed forward as well as to the left or to the right, one of the shafts of the variable resistors will be rotated more than the other thereby causing more power to be supplied to one motor than the other. This will cause the wheelchair to turn either to the left or to the right depending on which motor is supplied with the greater power.

If it is required to drive the vehicle in reverse either in a straight line or turning to the left or to the right while the joystick is moved backwards from its "at rest" position, such movement increasing resistance between the slider and the centre point of the variable resistor 83 resulting in an increase of voltage at the end of the variable resistor (83) connected to the base of transistor 100.

Transistor 100 will therefore be switched on which in turn switches on transistor 101 and energises the coiled relay RL1. Energisation of the coiled relay RL1 causes contact RLC1 to change over thus reversing the polarity of the supply to the motors. The control circuit will then function in a manner as aforedescribed.

Referring now to Figure 5, the complete control system for both attendant and occupant control of the wheelchair is shown in block form.

The attendant control comprises a lever 35 attached to handlebar 34 connected by a Bowden (R.T.M.) cable 36 to the attendant control circuit in control box 37 which in turn is connected to the batteries 30 and the motors 32LB and 32RB by switch 28A.

The occupant control comprises left and right-hand channel controls, both channels being illustrated but only the left-hand channel will be referred to. The joystick 18 is connected to the middle of the variable resistor 63 which in turn is connected to the control circuit 80 incorporating the null point sensor relay circuit 81 and relay reverse circuit 82. The main control circuit 80 is connected through output circuit 34 and switch 28B, the batteries 30 and motors 32LB and 32RB.

Referring now to Figure 6, a rolling chassis in the four-wheel drive wheelchair is illustrated. The chassis of the wheelchair is generally indicated at 110, the front of the chassis being provided with two sleeves 111,112.

Engaged within the sleeve 111 is a king pin 113 to which is secured a plate 114 on which is mounted a motor 115 having an output shaft 116 drivingly connected to the wheel 117.

A steering arm 118 is secured at one end to the upper end of the king pin 113 and is provided at its other end with a handgrip 119 and a control lever 120 connected to one end of the inner cable of a Bowden (R.T.M.) cable 121.

The metal plate 114 is provided with a rearwardly extending track rod 122 connected to one end of a connecting bar 123, the other end of which is connected to a track rod 124 secured to the metal plate 125 on the other side of the vehicle. Thus when the steering arm 118 is rotated by lateral movement of the handle 119, both wheels 117 and 126 will be moved through the same angle.

The wheelchair 110 is provided with two rear wheels 130 and 131, the wheel 131 is drivingly connected to and supported by the output shaft 132 of the motor 143. The motor 143 is supported by a plate 134 which is connected to a beam 135.

The wheel 130 and its driving motor 136 are connected to the beam 135 in a similar manner.

The beam 135 is provided with a rod-like projection 136 which extends through a boss 137 secured to the space frame chassis of the wheelchair. Thus the beam 135 and hence wheels 130 and 131 may pivot relatively to the chassis of the wheelchair about the longitudinal axis of the rod-like projection 136.

It will be appreciated that the chassis of the wheelchair will be provided with a superstructure for carrying an occupant. There will also be provided on the wheelchair one or more batteries providing power for the motors and a control box for controlling the power supplied to the motors.

The circuit for controlling the motors of the wheelchair is substantially as illustrated in Figure 2. In this case, however, the motors 32LB and 32RB would be replaced by motors 133 and 136 respectively and in addition two further motors 115 and 127 are included in the circuit as shown in dotted outline. The control lever 120 controls the position of the slider of variable resistance VR1 illustrated in Figure 2.

Referring once again to Figure 1, an alternative to the electrical sensing means will now be described. A disc 8 is mounted on the front upright member of the wheelchair and another disc 9 is mounted on the wheel support for rotation about the swivelling axis of the wheel 29. The lower disc 9 is provided with one or more magnets (not shown) which on deviation of the wheel 29 from its straight-ahead position will oerate a magnetically operated switch mounted on the other disc 8.

Operation of the magnetically operated switch would operate in a similar manner to the electronic sensing means and would reduce the voltage supplied to the motors 32R and 32L. For example the magnetically operated switch could be operative to energise relay coil RL4 or RL5 or alternatively the magnetically operated switch itself could be used to connect the input line 45 of the firing circuit 40 to earth through preset variable resister PR1.

WHAT WE CLAIM IS: 1. A motorised wheelchair having at least one pair of electric motors driving respective wheels on opposite sides of the wheelchair, wherein the motor windings to be energised are connected to a power supply in a configuration such as to minimise counteraction of the speed differential of one wheel relatively to the other occurring during steering of the wheelchair, by reducing the increase in current that would flow through the motor driving the slower wheel a

Claims

**WARNING** start of CLMS field may overlap end of DESC **. right-hand channel controls, both channels being illustrated but only the left-hand channel will be referred to. The joystick 18 is connected to the middle of the variable resistor 63 which in turn is connected to the control circuit 80 incorporating the null point sensor relay circuit 81 and relay reverse circuit 82. The main control circuit 80 is connected through output circuit 34 and switch 28B, the batteries 30 and motors 32LB and 32RB. Referring now to Figure 6, a rolling chassis in the four-wheel drive wheelchair is illustrated. The chassis of the wheelchair is generally indicated at 110, the front of the chassis being provided with two sleeves 111,112. Engaged within the sleeve 111 is a king pin 113 to which is secured a plate 114 on which is mounted a motor 115 having an output shaft 116 drivingly connected to the wheel 117. A steering arm 118 is secured at one end to the upper end of the king pin 113 and is provided at its other end with a handgrip 119 and a control lever 120 connected to one end of the inner cable of a Bowden (R.T.M.) cable 121. The metal plate 114 is provided with a rearwardly extending track rod 122 connected to one end of a connecting bar 123, the other end of which is connected to a track rod 124 secured to the metal plate 125 on the other side of the vehicle. Thus when the steering arm 118 is rotated by lateral movement of the handle 119, both wheels 117 and 126 will be moved through the same angle. The wheelchair 110 is provided with two rear wheels 130 and 131, the wheel 131 is drivingly connected to and supported by the output shaft 132 of the motor 143. The motor 143 is supported by a plate 134 which is connected to a beam 135. The wheel 130 and its driving motor 136 are connected to the beam 135 in a similar manner. The beam 135 is provided with a rod-like projection 136 which extends through a boss 137 secured to the space frame chassis of the wheelchair. Thus the beam 135 and hence wheels 130 and 131 may pivot relatively to the chassis of the wheelchair about the longitudinal axis of the rod-like projection 136. It will be appreciated that the chassis of the wheelchair will be provided with a superstructure for carrying an occupant. There will also be provided on the wheelchair one or more batteries providing power for the motors and a control box for controlling the power supplied to the motors. The circuit for controlling the motors of the wheelchair is substantially as illustrated in Figure 2. In this case, however, the motors 32LB and 32RB would be replaced by motors 133 and 136 respectively and in addition two further motors 115 and 127 are included in the circuit as shown in dotted outline. The control lever 120 controls the position of the slider of variable resistance VR1 illustrated in Figure 2. Referring once again to Figure 1, an alternative to the electrical sensing means will now be described. A disc 8 is mounted on the front upright member of the wheelchair and another disc 9 is mounted on the wheel support for rotation about the swivelling axis of the wheel 29. The lower disc 9 is provided with one or more magnets (not shown) which on deviation of the wheel 29 from its straight-ahead position will oerate a magnetically operated switch mounted on the other disc 8. Operation of the magnetically operated switch would operate in a similar manner to the electronic sensing means and would reduce the voltage supplied to the motors 32R and 32L. For example the magnetically operated switch could be operative to energise relay coil RL4 or RL5 or alternatively the magnetically operated switch itself could be used to connect the input line 45 of the firing circuit 40 to earth through preset variable resister PR1. WHAT WE CLAIM IS:

1. A motorised wheelchair having at least one pair of electric motors driving respective wheels on opposite sides of the wheelchair, wherein the motor windings to be energised are connected to a power supply in a configuration such as to minimise counteraction of the speed differential of one wheel relatively to the other occurring during steering of the wheelchair, by reducing the increase in current that would flow through the motor driving the slower wheel and/or the decrease in current that would flow through the motor driving the faster wheel were both motors each to be energised directly with the voltage of the supply.

2. A wheelchair according to claim 1 wherein the windings comprise rotor windings and are connected in series with each other.

3. A wheelchair as claimed in claim 2 wherein the motors have stators in which the field is set up by permanent magnets.

4. A wheelchair as claimed in any one of the preceding claims wherein a first control means are provided, said first control means being operative to control the voltage applied to the windings of the motors.

5. A wheelchair as claimed in claim 4 wherein said first control means is operative to vary the voltage applied to the windings of the motors in a variable manner.

6. A wheelchair as claimed in any one of the preceding claims wherein the wheelchair is provided with sensing means for sensing a change in direction of travel of the wheelchair and switching means responsive to the operation of the sensing means and so con

nected as to limit the power delivered from the supply.

7. A wheelchair as claimed in Claim 6 wherein the sensing means comprises means for sensing electrical parameters dependent upon speed difference between wheels on opposite sides of the wheelchair.

8. A wheelchair as claimed in claim 6 wherein the sensing means comprises a switch carrying member and a switch operating member, said switch operating member being operative to operate the switch on diviation by the wheelchair from a straightahead position.

9. A wheelchair as claimed in any one of claims 6, 7 or 8 wherein the switching means is operative only if the voltage across both motors is of a value equal to at least half the maximum rated voltage of each motor.

10. A wheelchair as claimed in any one of the preceding claims wherein additional switch means are provided, operative to electrically connect the windings of said motors in parallel with each other to the supply, and second control means operative to control the voltage applied to each of the motors, said second control means including a manually engageable part operable by the occupant.

11. A wheelchair as claimed in claim 10 wherein said second control means is adapted to control the power supplied to the motors in a variable manner.

12. A wheelchair as claimed in claim 10 or claim 11 wherein said control part engage able by the occupant comprises a joystick.

13. A wheelchair as claimed in any one of claims 1 to 9 wherein a mechanical or hydraulic steering mechanism operable by the occupant, is provided for steering the wheelchair.

14. A wheelchair as claimed in any one of the preceding claims wherein the wheelchair is provided with four wheels, each of said wheels being provided with an electric motor.

15. A wheelchair as claimed in any one of the preceding claims wherein the wheelchair includes a chassis, one pair of wheels being disposed respectively on opposite sides of the chassis but in transverse alignment with each other and being carried by mounting means enabling said wheels collectively to pivot relatively to the chassis about an axis extending both longitudinally and medially of the chassis.

16. A wheelchair as claimed in claim 15 wherein the mounting means comprises a beam on opposite ends of which the wheels are carried.

17. A wheelchair as claimed in claim 15 or claim 16 wherein said wheels are the rear wheels of the wheelchair.

18. A wheelchair as claimed in any one of claims 15 to 17 wherein each of said wheels is provided with an electric motor.

19. A motorised wheelchair substantially as hereinbefore described with reference to and as illustrated in Figure 1 of the accompanying drawings.

20. A wheelchair substantially as hereinbefore described with reference to and as illustrated in Figure 6 of the accompanying drawings.