Energy-saving electric drive for small vehicles

An electric drive for small vehicles, especially wheelchairs, includes a d.c. motor, which is arranged as a wheelhub motor in the running wheel of the small vehicle. Efficiency and manufacturing costs are improved, while ensuring simple assembly, wherein the possibility of automatically locking the running wheels is provided, along with a short and lightweight design. The d.c. motor is an external rotor motor, whose external rotor forms the wheelhub or the wheel rim of the running wheel. The stator of the d.c. motor is stationarily connected to the small vehicle via a brake housing (3). A central spring pressure brake is provided which can be released electromagnetically and manually and brakes the external rotor of the d.c. motor in the case of a power failure.

FIELD OF THE INVENTION 
The present invention pertains to an electric drive for small vehicles, 
especially wheelchairs, comprising a d.c. motor, which is arranged as a 
wheelhub motor in the running wheel of the small vehicle. 
BACKGROUND OF THE INVENTION 
A drive of this class has been known from DE-PS 41 27 257 C2. This drive is 
intended for a small vehicle with a frame and at least two running wheels, 
especially for a wheelchair, and it comprises an electric motor, a gear 
mechanism and a clutch as the drive part, which is intended, e.g., for a 
large running wheel of the wheelchair. A planet gear is used as the gear 
mechanism. 
The components of the drive are arranged within a pot-like housing, which 
forms the wheelhub of the running wheel, wherein the wheelhub is mounted 
via a ball bearing on the toothed ring support of the planet gear. A 
locking clutch, which can be actuated by hand and which makes it possible 
to uncouple the wheelhub and the gear mechanism in the pushing operation, 
is provided between the planet gear and the wheelhub. The electric motor 
is designed as a multipole, electronically commuted, brushless d.c. motor 
with a permanent-magnet internal rotor, which is coupled with the planet 
gear arranged downstream via a corresponding drive shaft. This prior-art 
drive is characterized by a short overall length, so that it projects only 
slightly beyond the plane of the running wheel in question. A control 
device is also provided, which is arranged in the operating part of the 
wheelchair. The electric motors of the two running wheels of the 
wheelchair are energized by means of this control device via an electronic 
motor control unit, which is accommodated close to the respective motor in 
the corresponding running wheel hub. 
Other drives for small vehicles have also been known, in which gear 
mechanisms of many different types are arranged downstream of the electric 
motors. A worm gear, a spur gear, or a bevel gear, or combinations 
thereof, but even roller systems for increasing the torque are used, 
depending on the particular application. These reducing gears are 
necessary in the prior-art drives to reduce the speed of the electric 
motor, which is usually about 1,000-3,000 rpm, to the speed of the running 
wheels, which is up to about 50-100 rpm. 
It is common to all these drive systems that they must be composed of many 
individual parts, so that high material and manufacturing costs are 
incurred. The prior-art drives also have a high intrinsic weight. 
Furthermore, the gear mechanisms generate a high noise level during 
operation and reduce the efficiency of the entire drive due to their inner 
friction, which is always present. 
Clutches between the running wheel and the gear mechanism must additionally 
be provided for uncoupling for small vehicles which must temporarily be 
pushed by hand, e.g., wheelchairs, in order to achieve the lowest possible 
resistance to pushing. Other additional assembly and material costs are 
generated as a result. Such gear mechanisms also require lubricant, so 
that the wheelhub or the gear housing must additionally be specially 
sealed, which also leads to higher manufacturing costs. 
SUMMARY AND OBJECTS OF THE INVENTION 
Thus, the basic object of the present invention is to improve an electric 
drive of this class in terms of its efficiency and its manufacturing costs 
while maintaining the possibility of simple assembly, while a possibility 
of automatic locking of the running wheels shall be provided, along with a 
short and lightweight design. 
This object is attained by the d.c. motor being designed as an external 
rotor motor, whose external rotor forms the wheelhub or the wheel rim of 
the running wheel, and by the stator of the d.c. motor being stationarily 
connected via a brake housing to the small vehicle, and having a spring 
pressure brake, which can be released electromagnetically or magnetically 
and brakes the external rotor of the d.c. motor in the case of power 
failure. 
By designing the d.c. motor as an external rotor motor, a reducing gear may 
be eliminated in the case of a corresponding number of poles, because the 
electric motor rotates at a correspondingly low maximum speed of about 
50-100 rpm. This speed can be adjusted to different running wheel 
diameters by varying the number of poles and also by a corresponding 
electronic drive, so that a running wheel speed adapted to the running 
wheel diameter and consequently a sufficiently high velocity of the small 
vehicle can always be achieved. The d.c. motor designed according to the 
present invention also has a substantially higher torque than a comparable 
gear motor, because the loss of efficiency of the gear mechanism as well 
as frictional losses of the motor seals are eliminated. 
The fact that the external rotor of the d.c. motor is designed at the same 
time as the wheelhub or the wheel rim of the running wheel leads to an 
extremely simple design of the drive, so that considerable savings are 
achieved in terms of material and assembly costs, and the drive will have 
a minimal intrinsic weight. 
The arrangement of the brake housing directly on the stator also makes 
possible a lightweight, inexpensive as well as short design of the drive. 
By the provision of the annular groove with a release magnet, which is 
designed as a ring magnet and pulls the pressing plate to release the 
spring pressure plate and disposing the release magnet in the brake 
housing opposite the stator and providing the central compression spring 
or plurality of springs uniformly distributed on a circular path so as to 
press the pressing plate together with the brake disc against the stator 
in the current-less state, the spring pressure brake is released on both 
the drive and the releasing magnet of the drive, and the small vehicle is 
ready to travel in the normal operating state, i.e., in the presence of a 
certain operating voltage. The spring pressure brake closes automatically 
in the case of malfunctions, e.g., power failure, so that the small 
vehicle is reliably braked, without the driver having to additionally 
respond. However, it is also advantageous during stopping that the spring 
pressure brake is automatically activated simply by switching off or 
removing the "ignition" key, so that no further activities of the operator 
are required during the parking of the small vehicle, either. 
In the case of a malfunction, i.e., when the spring pressure brake is 
activated, this brake can be released manually, so that the small vehicle 
can simply be pushed in the case of, e.g., a dead battery. A simple and 
compact design is achieved due to the central arrangement of the release 
bar and the wedge surfaces arranged at the collar of the release bar and 
at the pressing plate. To release the spring pressure brake, the release 
bar is rotated by about 90.degree. around its longitudinal axis, so that 
the wedge surfaces of the collar and of the pressing plate guided 
nonrotatably in the brake housing will move in relation to one another, as 
a result of which the pressing plate is moved in the axial direction 
against the spring force. Simple and reliable release of the spring 
pressure brake is thus guaranteed, and locking grooves, into which the 
wedge surfaces of the collar can snap after the rotation by 90.degree., so 
that an automatic rotation of the release bar is ruled out, may also be 
provided in the pressing plate. 
The brake housing preferably has, on a side opposite the external rotor of 
the d.c. motor, a control space for accommodating a power electronic unit. 
The control space can be closed by means of a housing cover. A release bar 
preferably passes centrally through the brake housing and the control 
space with the housing cover, projecting to the outside. 
The external rotor preferably has a pot-like design and comprises a round 
wheel disc with a circular, essentially cylindrical ring wall extending 
coaxially to the axis of rotation of the external rotor. On the 
cylindrical inner surface of the ring wall, a plurality of permanent 
magnets or electromagnets are preferably arranged as magnetic poles. The 
stator preferably has a central bearing hub in which the external rotor 
with its wheel hub shaft is mounted rotatably. The d.c. motor is 
preferably a multi-pole electronically commutable, brushless d.c. motor. 
The brake housing is preferably provided with a circular mounting flange 
via which the drive can be fastened to the frame of the small vehicle and 
a support disc is preferably provided with a radially outwardly directed 
supporting yoke, by which yoke the small vehicle is secured against 
tilting via the support wheel arranged at the outer end of the supporting 
yoke. The supporting disc is preferably provided on the mounting flange 
between the frame of the small vehicle and the brake housing. 
The various features of novelty which characterize the invention are 
pointed out with particularity in the claims annexed to and forming a part 
of this disclosure. For a better understanding of the invention, its 
operating advantages and specific objects attained by its uses, reference 
is made to the accompanying drawings and descriptive matter in which a 
preferred embodiment of the invention is illustrated.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
FIG. 1 shows an electric drive 1 for small vehicles, e.g., wheelchairs, 
comprising a brushless d.c. motor 2, a brake housing 3, a spring pressure 
brake 4 with magnetic brake release means 5, a manual release means 6, and 
a power electronic unit 7 arranged in the brake housing 3. 
The d.c. motor 2 has a pot-like external rotor 8 and an internal stator 9. 
The stator 9 is provided with a central bearing hub 10, in which the 
external rotor 8 is rotatably mounted via a wheelhub shaft 11 and two 
radial ball bearings 12. For its centered fastening, the external rotor 8 
has a wheel disk 13, which is screwed with a corresponding recess 14 onto 
a mounting flange 15 of the wheelhub shaft 11 almost without clearance on 
the stator side. On the brake housing side, the wheel disk 13 has a 
circular ring wall 16, which extends coaxially to the axis of rotation 17 
of the external rotor 8 and radially completely encloses the stator 9. 
The cylindrical inner surface 18 of the ring wall 16 is provided with a 
plurality of permanent magnets 19, which form the magnet poles of the 
external rotor. A correspondingly large number of poles is obtained by 
using a correspondingly large number of permanent magnets 19, as a result 
of which the necessary low rated speed is obtained along with a high 
torque of the d.c. motor 2. 
The outer jacket surface 20 of the ring wall 16 is designed as a wheel rim 
to accommodate a rubber tire (not shown in the drawing). If running wheels 
of a larger diameter are provided on a small vehicle, which may happen in 
the case of a wheelchair, the ring wall 16 may be provided in its axial 
end areas 21 and 22 with circular, radially outwardly projecting spoke 
webs 25, 26 (represented by broken lines), which are provided with cross 
holes 23, 24, so that a wheel rim of a larger diameter can be fastened to 
the external rotor via, e.g. spokes (not shown in the drawing), which are 
hung correspondingly in the cross holes 23, 24. 
The stator 9 has a circular cylindrical stator support 27, which extends 
coaxially to the wheelhub shaft 11 and on the jacket surface 28 of which 
stator laminations 29 with the corresponding stator windings 30, 31 
belonging to them are arranged. The inner distance between the mutually 
opposite permanent magnets 19 and the diameter of the stator laminations 
29 are adjusted to one another such that there is a small air gap 32 
between the permanent magnets 19 and the stator laminations 29. High 
performance capacity of the d.c. motor 2 is thus achieved at a low speed 
of rotation. 
The stator 9 is designed as a flat disk in its radial extent, between its 
bearing hub 10 and its stator support 27. The stator 9 forms one of the 
friction surfaces of the spring pressure brake 4 with its end face 33 
located toward the brake housing 3. A lightweight and compact design is 
achieved due to this embodiment. 
The brake housing 3 is screwed onto the stator 9 opposite the external 
rotor 8 in a centered manner. The brake housing 3 is designed as a stepped 
housing both in its outer contour and in its interior space, and on the 
stator side, it has an essentially round brake space 34, into which the 
wheelhub shaft 11 with a brake housing-side coupling pin 35 extends. The 
coupling pin 35 of the wheelhub shaft 11 has a groove toothing 36, which 
is used for the nonrotatable and axially displaceable mounting of a brake 
disk 37. Instead of the groove toothing 36, another nonrotatable and 
axially displaceable mounting of the brake disk 37 on the coupling pin 35, 
e.g., a simple tongue-and-groove connection, may be provided as well. The 
brake disk 37 is fitted into the brake space 34 with a radial clearance. A 
pressing plate 38, which presses the brake disk 37 against the stator 9 by 
means of a plurality of compression springs 39 (only two of which are 
visible in the drawing), is provided on the brake disk 37 on the side 
opposite the stator 9. The compression springs 39 are arranged uniformly 
distributed on a circular path, so that the pressing plate 38 and 
consequently the brake disk 37 are pressed uniformly against the end face 
33 of the stator 9. On its circumference, the pressing plate 38 has a 
plurality of guide lugs 40 which move guidedly on rails 41, with which the 
plate 38 is guided nonrotatably and axially displaceably in the brake 
housing 3 or in the brake space 34. Thus, the pressing plate 38 with its 
brake disk-side end face acts as the second friction surface of the spring 
pressure brake 4. 
A circular ring magnet 43 is provided on the rear side of the pressing 
plate 38 in the brake housing 3. The pressing plate 38 is pulled against 
the spring forces of the compression springs 39 when the ring magnet 43 is 
energized, so that the brake disk 37 can freely rotate together with the 
wheelhub shaft 11. It is also conceivable that a central coil spring or a 
plate spring assembly may be provided instead of a plurality of 
compression springs 39 to apply the necessary pressing force to the 
pressing plate 38. 
A control space Which is radially limited by a circular ring wall 45, is 
provided on the brake housing 3 on the rear side. The control space 44 can 
be closed by means of a housing cover 46. An electronic board 47, on which 
the power electronic unit 7 for driving both the ring magnet 43 and the 
stator windings 30, 31 is provided, is arranged in the control space 44. 
This power electronic unit 7 can be connected via detachable adapters to a 
corresponding electronic control unit of the small vehicle, which is 
arranged on the small vehicle. Trouble-free operation of the power 
electronic unit tis ensured due to the arrangement of the power electronic 
unit 7 in a control space 44 enclosed by metal on all sides, because the 
power electronic unit 7 is shielded on all sides from the outside-against 
electromagnetic or electric interference fields. 
The manual release means 6 comprises a central release bar 48, which is 
arranged coaxially to the wheelhub shaft 11 and extends behind the 
pressing plate 38 with a circular, radially outwardly directed collar 
arranged at its inner bar end. The release bar 48 passes through the 
pressing plate 38, the brake housing 3 and the housing cover 46 from the 
inside to the outside, and it projects from the housing cover 46. At its 
outer end 51, the release bar 48 is provided with an actuating lever 52, 
which is arranged extending at right angles to the release bar 48. In the 
area of the inner partition 53 between the brake space 34 and the control 
space 44 of the brake housing 3, on which partition the power electronic 
unit 7 or its board 46 is mounted on the outside, the release bar 48 has a 
crossbolt 54 passing through it, with which the release bar 48 is axially 
supported on the outer surface 55 of the partition 53 located within the 
control space 44. 
To manually release the spring pressure brake 4, the release bar 48 is 
rotated by about 90.degree. around its longitudinal axis by means of the 
actuating lever 52. Elevations 59 forming wedge surfaces 57, 58, which 
move the pressing plate axially against the spring force of the 
compression springs 39 during the rotary movement in cooperation with 
corresponding wedge surfaces 60, 61 of depressions 62 arranged in the 
pressing plate 38 on the collar side, are provided on the radially 
projecting collar 50 on the actuating lever side or pressing plate side. 
For locking in the released position, locking grooves of a small axial 
depth, into which the elevations 59 of the collar 50 can snap, may be 
provided in the pressing plate 38. A plurality of elevations 59 and 
depressions 62 may be provided on the circumference of the web flange 50 
and of the pressing plate 34, respectively, in order to reduce the 
frictional forces during the rotation of the release bar 48, and the 
release distance of the pressing plate 38 equals at most a few mm. During 
release, the release bar 48 is supported, with its crossbolt 54 axially 
stationary, on the outer surface 55 of the partition 53. 
For mounting on a small vehicle, the brake housing 3 has a circular 
mounting flange 63, which is provided with corresponding tapholes 64 
uniformly distributed on the circumference. 
FIG. 2 shows a side view I of the electric drive 1 according to FIG. 1 with 
a support disk 65, which is arranged on the mounting flange 63 and can be 
fastened to the tapholes 64 of the mounting flange 63 by means of screws 
(not shown in the drawing). The support disk 65 is provided with a 
supporting yoke 66 and is provided with a support wheel 68 at its radially 
outer end 67. The diameter of the support disk 65 is smaller than the 
diameter of the outer jacket surface 20 of the ring wall 16 of the 
external rotor 8, so that this jacket surface can roll directly on the 
ground, without the support disk 65 touching the ground, if the ring wall 
16 is used directly as the wheel of the small vehicle or wheelchair. 
The thickness of the support disk 65 and the axial distance between the 
mounting flange 63 and the actuating lever 52 at the brake housing 3 are 
selected to be such that a sufficient space is left between the support 
disk 65 and the actuating lever 52 of the release bar to fasten the 
electric drive 1 together with the support disk 65 to the frame of a 
wheelchair, without the risk of jamming of the hand of a person during the 
actuation of the actuating lever 52. 
The embodiment according to the present invention provides a lightweight, 
compact, high-performance drive which can be manufactured at a low cost 
for small vehicles, which drive has optimal efficiency. A small vehicle 
can be retrofitted with this electric drive in a simple manner, and its 
power electronic unit can be adapted, due to a corresponding design, to a 
possibly existing analog, digital or computer-controlled electronic 
control unit. 
While a specific embodiment of the invention has been shown and described 
in detail to illustrate the application of the principles of the 
invention, it will be understood that the invention may be embodied 
otherwise without departing from such principles.