Device for detecting rotor speed or position in a continuously excited electric motor

In an electric motor, a Hall sensor (3) is arranged in the vicinity of the outer circumference of the rotor (2) and of at least one of the permanent magnets (4-7) provided for operationally exciting the electric motor. At least one of the permanent magnets (4-7) is provided for generating the magnetic field inducing the Hall sensor (3). The rotor (2) has rotor teeth (2.1) and/or rotor slots (2.2) on the air-gap side and, in operation, produces magnetic field pulsations in proportion to the rotor speed.

FIELD OF AND BACKGROUND OF THE INVENTION 
The invention relates to a device for detecting the speed or position of a 
rotor in a continuously excited electric motor. 
A device for detecting rotor speed or rotor position in a continuously 
excited electric motor by means of a Hall sensor induced by magnetic field 
pulsation, which is proportional to the rotor speed is known from the 
French publication FR-A-2 693 054 and the corresponding German publication 
DE-A1-42 21 424. The Hall sensor in these cases is arranged on the outer 
circumference of the stator, which surrounds the rotor, in the vicinity of 
one of the stator-side permanent magnets, which are provided for 
operationally exciting the electric motor. 
The European reference EP-A-0 359 854 discloses a speed measuring device 
for a rotor-wound electric motor, in which an insulating end plate 
arranged at the end face of the rotor is constructed to rotate with the 
rotor and influence an assigned stator-side sensor either magnetically or 
optically and thereby produce an output which is proportional to the speed 
of the rotor. 
OBJECTS OF THE INVENTION 
It is an object of the invention to provide a device for detecting the 
rotor speed and/or rotor position in a continuously excited electric motor 
which, although mass-produced in an advantageously economic way, ensures 
sensitive, i.e. high-resolution, detection of rotor speed and/or position. 
SUMMARY OF THE INVENTION 
This and other objects are achieved by a device according to the invention. 
According to the invention, a device is provided for detecting the rotor 
speed or rotor position in a continuously excited electric motor by means 
of a Hall sensor. The Hall sensor is induced by pulsations in a magnetic 
field that are proportional to the speed of the rotor. The rotor of the 
electric motor, which has rotor teeth and/or rotor slots on the air-gap 
side, generates the pulsations as it rotates. At least one of the 
stator-side permanent magnets, provided for operationally exciting the 
electric motor, generates a magnetic field that further induces the Hall 
sensor and creates a magnetic bias. The Hall sensor is arranged on the 
stator side in the vicinity of the outer circumference of the rotor, as 
well as in the vicinity of at least one of the permanent magnets. The Hall 
sensor is designed specifically as a differential Hall sensor. 
In the device, the outlay for measurement can be substantially reduced by 
virtue of the fact that the permanent magnets, which are normally provided 
only to excite the electric motor operationally, are also used to create a 
static pre-magnetization for the Hall sensor. Additionally, the rotor 
teeth or rotor slots, which are conventionally provided for holding the 
rotor winding and have alternating permeance values thereby specified over 
the rotor circumference, are used for the pulsation, which is proportional 
to the rotor's speed, for the Hall sensor. Because of the high number of 
rotor teeth or rotor slots employed, in particular, in multipole motors, a 
high detection sensitivity of the rotor speed, or of the rotor position 
that can be derived therefrom, is nevertheless guaranteed by the 
invention. Again, because of the use of at least one differential Hall 
sensor it is possible to achieve reliable signal evaluation even in the 
case of small signal amplitudes or independently of fluctuations in the 
pre-magnetization induced by the permanent magnet. 
It is expedient to integrate two Hall probes with a slight spacial 
separation, e.g. of about 2.5 mm, onto a single chip. Where there is a 
change from rotor tooth to rotor slot, these probes cause a different 
magnetic field at the location of one or the other Hall probe. This is 
done in such a way that the signal of the two Hall probes can be separated 
from the magnetic bias, and can thus be evaluated independently of 
fluctuations in the magnetic bias, by a differential circuit. 
The tangential spacing between the end faces of two successive permanent 
magnets in the circumferential direction, which usually serves only for 
the insertion of spring clip parts used for bracing the permanent magnets 
against one another or against the motor housing, is expediently used to 
arrange the Hall sensor. As a result, it is easily possible, without the 
need for a separate and, in particular, additional installation space, to 
fix the Hall sensor advantageously both in the vicinity of the rotating 
rotor teeth or rotor slots and in the vicinity of the permanent magnets, 
which are used for the pre-magnetization of the sensor and are provided 
mainly for operationally exciting the electric motor.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIGS. 1 and 2 show a cup-shaped motor housing 1 of a continuously excited 
electric motor whose permanent magnets 4-7 are arranged on the inner 
circumference of the motor housing 1. The magnets 4-7 are shaped as 
partial shells, in conformance with the inner circumference of the motor 
housing, and serve to excite the motor operationally. Spring clips 8 or 
retaining clips 9 are used to position and secure the permanent magnets 
4-7. These clips 8,9 are inserted axially into tangential interspaces 
between the permanent magnets 4-7. More particularly, the clips abut 
respective end faces, situated tangentially opposite one another, of the 
circumference successively positioned magnets 4-7. The clips press these 
magnets both against one another and against the surface of the inner 
circumference of the motor housing 1. 
FIG. 2 further depicts a rotor, omitted in FIG. 1 for the sake of clarity, 
which is not yet provided with a rotor winding. The rotor 2 has rotor 
teeth 2.1 and rotor slots 2.2, which are distributed over the 
circumference alternating with one another and serve to hold the rotor 
winding. 
The Hall sensor 3 is arranged, for the purpose of determining rotor speed 
or position in accordance with the invention, in one of the tangential 
interspaces between the opposite end faces of one permanent magnet (e.g., 
4) and the next permanent magnet (e.g., 5) which follows on the 
circumference. As stationary components, the permanent magnets 4 and 5 
provide this Hall sensor 3 with a magnetic bias. However, since the Hall 
sensor 3 is arranged not only in the vicinity of the permanent magnets 4 
and 5 but also in the vicinity of the surface of the outer circumference 
of the rotor 2, the latter is advantageously also used to produce a 
different magnetic field, and thus an output signal at the Hall sensor 3, 
which is proportional to the speed of the rotor. It does this by 
alternating from tooth to slot, relative to the sensor 3, to produce a 
corresponding alternation in the permeance. 
The Hall sensor 3 is preferably designed as a differential Hall sensor with 
two Hall probes arranged with a mutual tangential spacing on a chip. 
Designing the Hall sensor 3 as a differential Hall sensor means that the 
alternation from rotor tooth 2.1 to rotor slot 2.2 produces a different 
magnetic field at the location of one probe of the differential Hall 
sensor in comparison to that at the other probe. The signals from the two 
probes can subsequently be separated from the magnetic bias by a 
differential circuit in such a way that fluctuations in the magnetic bias, 
which arise, for example, because of temperature effects, fail to 
influence the detection of the rotor speed. Structural and operational 
details regarding differential Hall sensors are known to those skilled in 
the art, e.g., from "Integrierte Differenz-Hall-IC zur Positions- und 
Drehzahlerkennung" (Integrated Differential Hall IC for Position- and 
Rotation Speed Recognition), Elektronik Informationen, which is 
incorporated into the present application by reference. 
Through the use of an appropriate conventional evaluation circuit (not 
shown), it is also possible to determine the rotor position, or the stroke 
path of a component moved by the electric motor in an actuator, such as, 
for example, a window in a window lifter drive, or a sliding roof in a 
sliding roof drive, of a motor vehicle. This is done on the basis of the 
number of pulses detected by the Hall probe 3, the frequency of which is 
proportional to the rotor speed, in conjunction with a high resolution of 
the respective rotor position, for example on the basis of an 
initialization position, fixed at the outset, and subsequent incremental 
pulse detection. To make this determination, it is expedient to provide a 
commutator motor fed from the DC system of a motor vehicle as the 
continuously excited collector motor. It is also expedient to use the 
device according to the invention for the purpose of stroke path 
monitoring with a quickly reacting anti-jamming protection system when the 
window or the sliding roof moves against an obstacle. Details regarding 
such window lifter drive systems and anti-jamming protection systems 
therefor are described, e.g., in U.S. Pat. No. 5,404,673 and U.S. Pat. No. 
5,436,539, the disclosures of which are hereby incorporated into the 
present application by reference. 
The above description of the preferred embodiments has been given by way of 
example. From the disclosure given, those skilled in the art will not only 
understand the present invention and its attendant advantages, but will 
also find apparent various changes and modifications to the structures 
disclosed. It is sought, therefore, to cover all such changes and 
modifications as fall within the spirit and scope of the invention, as 
defined by the appended claims, and equivalents thereof.