Bidirectional DC motor

A bidirectional DC motor comprising a commutator (6) which can be connected to a constant voltage source by a first and a second brush. The second brush is disposed opposite one side of the first brush in a spaced disposition with respect to the periphery of the commutator, which spacing correlates with a desired motor characteristic to provide a relatively convenient arrangement for switching the direction of rotation of a DC motor. In addition to the second brush (8) provided for one direction of rotation, the commutator (6) also has a third brush (9) for the counter-direction of rotation and is disposed in a spaced disposition with respect to the periphery of the commutator (6), which spacing correlates with the desired motor characteristic for the counter-direction of rotation. The first brush (7) is connected to one pole (+) of the constant voltage source (10) and the two other brushes (8, 9) are connected to the other pole (-) of the constant voltage source (10) by a switch (S1, S2) respectively.

TECHNICAL FIELD 
The present invention relates to a bidirectional DC motor comprising a 
commutator which can be connected to a constant voltage source by brushes 
located in a spaced relationship around the periphery of a commutator to 
provide a desired motor characteristic. 
BACKGROUND ART 
DC motors are used in actuating drives and in particular are installed in 
motor vehicles for systems which raise windows adjust the sun roofs seat 
positions and operate windscreen wipers. The direction of rotation of 
these DC motors is switched by reversing the operating voltage by means of 
a suitable control device which is designed as a semi-conductor bridge 
circuit. A circuit of this type is comparatively expensive. 
SUMMARY OF THE INVENTION 
The object of the invention is to provide a relative convenient arrangement 
for the purpose of switching the direction of rotation of a DC motor. 
This object is achieved according to the invention by the fact that in 
addition to a second brush provided for one direction of rotation, the 
commutator also has a third brush which is provided for the 
counter-direction of rotation and which is disposed in a spaced 
disposition with respect to the periphery of the commutator. This spacing 
in turn correlates with the desired motor characteristic in the 
counter-direction of rotation, wherein the first brush is connected to one 
pole of the constant voltage source and the two other brushes are 
connected in each case by way of a switch to the other pole of the 
constant voltage source. 
Although it is known in connection with DC motors to allocate a third brush 
to the commutator, this is merely provided for the purpose of achieving a 
different motor rotational speed. 
The invention differs from the prior art in that a separate electrical 
circuit is provided in each case for the right-handed rotation and for the 
left-handed rotation. Thus, it is not necessary to reverse the voltage 
supply. In this manner, it is possible to allocate an evaluating circuit 
for the purpose of determining the direction of rotation and/or the 
rotational speed and also the adjustment path. This is based preferably on 
the fact that the periodic voltage oscillations on the commutator are 
detected. 
It is particularly favorable in the case of the invention that the two 
brushes which, for the purpose of achieving the two directions of rotation 
of the DC motor, are provided on one side and the other side of the 
(first) brush in such a manner that consequently different characteristics 
are allocated to the right-hand rotation and the left-hand rotation. 
Further advantageous embodiments of the subject matter in accordance with 
the invention are indicated in the subordinate claims and are further 
explained with reference to an embodiment illustrated in the drawing.

BEST MODE(S) FOR CARRYING OUT THE INVENTION 
As is evident from the drawing, a DC motor 1 consists of a stator 2 which 
is equipped with magnetic components 3. Disposed in the said stator is an 
armature 4 which is provided with electrical windings 5. The armature 
includes a commutator 6. Three (carbon)brushes 7, 8, 9 which consist of 
graphite are allocated to the commutator 6. 
A first brush 7 is connected to the positive pole (+) of the voltage source 
10, e.g. to the voltage supply network of a motor vehicle. The second 
brush 8 disposed opposite one side of the first brush 7 in a spaced 
disposition dependent upon the desired motor characteristic is connected 
by way of a switch S1 to the negative pole--(earth) of the voltage supply 
10 and fixes, together with the first brush 7, one direction of rotation 
of the DC motor. The third brush 9 disposed opposite the other side of the 
first brush 7--again in a spaced disposition dependent upon the desired 
motor characteristic--is likewise connected by way of a switch S2 to earth 
and together with the brush 7 fixes the counter-direction of rotation. 
The control can be achieved either by virtue of a single-pole relay (as 
shown in FIG. 1) or a semi-conductor component (as shown in FIGS. 2A and 
2B, wherein in conjunction with the latter it is especially favorable that 
the earth-side connection guarantees a relatively convenient structure. 
Special consideration can be given to a field-effect transistor as the 
semiconductor component. The switches can also be integrated into the 
motor so that a convenient connection method is produced. 
The second brush 8 and the third brush 9 are connected in each case by way 
of a resistance 12 to one line 11 which leads to an evaluating circuit 
provided for the purpose of detecting the direction of rotation and/or the 
rotational speed. This evaluating circuit evaluates the voltage 
oscillations occurring periodically on the commutator 6. The periodicity 
of this signal is produced from the rotational speed x number of poles. 
The resolution is correspondingly large. The position of the object being 
adjusted can be detected precisely by the integration which is dependent 
upon the direction of rotation. The rotational speed can also be 
calculated with a high resolution. 
While the best modes for carrying out the invention have been described in 
detail, those familiar with the art to which this invention relates will 
recognize various alternative designs and embodiments for practicing the 
invention as defined by the following claims.