Note: Descriptions are shown in the official language in which they were submitted.
CA 02287182 1999-10-21
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GR 97 P 1510 ~f_~ TRANSLATION
Description
Method for identifying the direction of rotation of a wheel by
means of Hall probes
The present invention relates to a method of identifying the
direction of rotation of a wheel u:~ing Hall probes which are
arranged in the circumferential direction of the wheel, via
evaluation signals phase-shifted rE:lative to one another.
In a large number of cases, it is desirable to detect not only
the position and speed of a rotation wheel, for example a
gearwheel, but also the direction of rotation of the wheel. In
general, a sensor which is capable of establishing the
position, speed and direction of rotation of a wheel is thus
sought.
For detecting the position and speed of a gearwheel, there is
already a differential dynamic Hall. sensor which measures the
difference field between two spatially offset Hall probes and
gives especially good results if the phase angle between the
two signals produced by the two Hall probes is 180. This is
because, in this case, one Hall probe lies over a tooth of the
gearwheel, while the other Hall probe lies over a gap between
two teeth of the gearwheel. With such a differential dynamic
Hall sensor, however, it is not possible to identify the
direction of rotation of the gearwheel.
This is because in order to identify the direction of rotation
of a gearwheel as well, yet another item of phase information
is necessary, and this can be made available by two Hall
sensors offset by 90m relative to cne another, as will be
explained below with reference to F'ig. 5 and Fig. 6.
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According to the prior art, for example, if two differential
dynamic Hall sensors 1, 2 with two Hall probes 3, 4 and 5, 6
each are arranged offset relative to one another by one fourth
tooth spacing in relation to the teeth 7 of a gearwheel 8, as
shown in Fig. 5, then output signals 9, 10 that are shifted
relative to one another by one fourth period are obtained from
the Hall sensors 1 and 2: as represented in Fig. 6, the
trailing edge of the output signal 9 of the Hall sensor 1 is
used to sample the output signal lc) of the Hall sensor 2. In
the case of signals 9, 10 running from left to right for one
direction of rotation of the gearwheel 8 in Fig. 6, the
trailing edge of the output signal 9 then always coincides
with a positive value of the output. signal 10 of the Hall
sensor 2, as is indicated by arrows 11.
If the direction of rotation of the gearwheel 8 is then
reversed, then the phase relation also changes: this can be
thought of in terms of "time" now running backward, so that
the output signals 9, 10 in Fig. 6 occur from right to left.
If the output signal 10 of the Hall. sensor 2 is then again
sampled with the trailing edge of the output signal 9 of the
Hall sensor 1, then a signal that is always negative is
obtained, since the trailing edge always coincides with a
negative value of the output signal 10, as is indicated by
arrows 12 in Fig. 6.
From the sign of the signal obtained by sampling the output
signal 10 with the output signal 9, it is thus possible to
assess the direction of rotation of the gearwheel 8. It can
also be seen that an arrangement of the Hall sensors 1, 2
offset by 90v~ is optimum since a maximum signal to noise ratio
is then obtained.
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DE 889 09 677 Ul discloses a device for identifying rotation
in which digital signal sequences are respectively derived
from at least three Hall probes by means of at least two
differential Hall ICs. For accurate rotational speed
identification, with a view to higher resolution, the
frequency of the output signal can be doubled relative to that
of only one single differential Ha:l1 IC. By phase comparison,
it is in principle also possible to identify the direction of
rotation.
DE 41 04 902 A1 discloses a method and an arrangement for
identifying a direction of motion, in particular a direction
of rotation. To that end, two sign~~ls, phase shifted by 900,
which are derived from two receivers arranged offset in the
direction of motion of a signal source, are formed by adding
and subtracting the output signals. From the sign of the 900
phase shift between the sum and difference signals, the
direction of rotation can be uniquely determined. This method
is, however, highly sensitive to DC: magnetic fields. There is
therefore an offset in the sum signal relative to the
difference signal, which is twice a.s great as the DC magnetic
field, with the result that reliable further processing of
these signals entails serious difficulties.
The object of the present invention is to provide a method for
directional identification of the direction of rotation of a
wheel by means of Hall probes, which allows reliable
identification of the direction of rotation without having to
demand exact coordination between the tooth spacing and the
Hall probe spacing. In particular, the method is intended to
be insensitive to DC magnetic fields.
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This object is achieved by a method having the features of
patent claim 1. Preferred refinements constitute the subject
matter of the dependent claims.
In order to implement the method, a first, a second and a
third Hall probe are arranged in such a way that the second
Hall probe is positioned between the first and third Hall
probes. Two evaluation signals, shafted by 90m, are obtained
from the output signals of the fir;;t to third Hall probes,
there preferably being a change in sign of the second
evaluation signal in relation to the first evaluation signal
when the direction of rotation changes.
The second Hall probe advantageous7_y lies exactly mid-way
between the first and third Hall probes, since the oscillation
amplitudes of the evaluation signa7.s are then maximum.
This method therefore requires only three Hall probes, which
can be fitted in one Hall sensor. Lrsing this sensor, the
direction of rotation, for example of a gearwheel, can be
determined reliably from a change in sign of the second
evaluation signal.
The method will be explained in more detail below with the aid
of the drawings, in which:
Fig. 1 shows a schematic representation of a sensor already
known according to the prior art;
Fig. 2 shows output signals of the Hall probes of this known
sensor;
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Figs. 3 and 4 show circuit diagrams of the evaluation
electronics for obtaining the output signals according to the
method described here
Fig. 5 shows an arrangement having two Hall sensors according
to the prior art, and
Fig. 6 shows output signals of the Hall probes of these known
Hall sensors.
Figs. 5 and 6 have already been explained at the start.
In Figs. 1 to 4, the same reference numbers as in Figs. 5 and
6 are used for corresponding components.
Fig. 1 shows a known Hall sensor 1~~, which has Hall probes 14,
15 and 16 that are arranged in the direction of rotation of
the gearwheel 8, Hall probe 15 being provided mid-way between
Hall probes 14 and 16. When the gearwheel 8 rotates, the Hall
probes 14 to 16 deliver output signals S1 to S3 (see Fig. 2),
which are approximately sinusoidal and will therefore be
treated as such below. Hall sensor 14 thus delivers the output
signal S1 which has a maximum value when the tooth 7 moves
past the Hall sensor 14, while a gap between the teeth 7 gives
a minimum value for the output signal S1. The same is true for
the output signal S2 of Hall probe 15, and for the output
signal S3 of Hall probe 16.
The output signals Sl to S3 can be digitized straightforwardly
with the aid of a comparator, so that the signals S1 to S3
take on a profile corresponding to the output signals 9 and 10
in Fig. 6. It should, however, be assumed below that the
signals are processed further in analog mode.
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In the method according to the invention, a first evaluation
signal A is obtained from subtraction of the output signal S3
from the output signal S1. Likewise, a second evaluation
signal B is obtained from addition of the output signal S3 to
the output signal S1 and subtraction of two times the output
signal S2 from this sum. In other words, the following
relationships are satisfied for this evaluation signals A and
B:
A = S1 - S3
B = S1 + S3 - :?~S2 (1)
The sinusoidal profile assumed abo~Te is then taken for the
signals S1 to S3, signal S2 being :shifted by phase p and
signal S3 being shifted by phase 2p relative to signal S1.
With t = time and w = angular velocity of the gearwheel 8, the
following is obtained:
S1 = sin(w~t)
S2 = sin(w~t + p)
S3 = sin(w~t + 2~p) (2)
From the system of equations (2), after a few rearrangements
with the aid of equations (1) the following relationships are
derived:
A = - 2 ~ sin (p) ~ c:os(w~t + p) (3)
B = 2 ~ (cos(p)-1) ~ sin (w~t+p) (4)
From equations (3) and (4) it can be seen that the two
evaluation signals A and B always have a phase shift of 90ra
relative to one another irrespective of the value of the phase
p. This means, irrespective of whether the Hall sensor 13
exactly matches the gearwheel 8, there is always a 90r~ "phase
system" in which at the zero crossing of the oscillation of
one evaluation signal the oscillation of the other evaluation
signal takes on its maximum. For example, for a rising zero
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crossing of the evaluation signal :~1 the value cos(w~t+p) - 0
is found, while the evaluation signal B then contains the
value sin(w~t+p) - 1.
It is nevertheless advantageous to have the best possible
coordination between the spacing o:E the Hall probes of the
Hall sensor 13 and the spacings of the teeth 7 of the
gearwheel 8, since the oscillation amplitudes of the
evaluation signals A and B then take on their maximum.
If, as explained above, it is then considered that reversing
the direction of rotation of the gearwheel 8 corresponds to
changing the time sign, then the following relationships are
obtained from equations (3) and (4):
A = - 2 ~ sin (p) ~ c:os (w~ t-p) (5)
B = - 2 ~ cos((p)-1) ~ sin (w~t-p) (6)
The signals resulting from this are: thus very similar to the
signals corresponding to equations (3) and (4), the only
difference being in the negative sign in signal B. This means,
however, that on sampling at the zero crossing of signal A,
the sign of signal B is inverted compared with before when the
direction of rotation is reversed, so that unique directional
identification can be established from the sign of evaluation
signal B in relation to evaluation signal A. In terms of
circuitry, this can for example be embodied with a D flip-
flop, in which, after digitizing, signal A is applied to the
clock input and signal B to the D input.
The method is, however, hot restricted to sampling at the zero
crossing of signal A. The sampling can also take place at
other values of signal A. Likewise, as an alternative to the
direction identification with the aid of the sign of signal B,
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the direction identification may take place by evaluating the
trend of the second evaluation signal B at the sample value.
In particular in the case of sinusoidal evaluation signals,
however, sampling at the zero crossing of signal A in
conjunction with the direction identification with the aid of
the sign of signal B represents a particularly preferred
method.
Further, one of the evaluation signals (A, B) can be employed
to produce switching edges, and the respective other
evaluation signal (A, B) can be sampled with these switching
edges, the direction of rotation being determined from the
relationship between the sample values of the edges.
The method thus allows for reliable identification of the
reversal of the direction of rotation of a gearwheel with
merely three Hall probes on a Hall sensor.
For producing the evaluation signals A and B, operational
amplifier circuits can in principle be readily used.
Transistor circuits are, however, also possible, as shown in
Figs. 3 and 4. In these Figs. 3 and. 4, Slp and Sln indicate
the output signals of Hall probe 14, S2p and S2n the output
signals of Hall probe 15, and S3p and S3n the output signals
of Hall probe 16.
In the circuit of Fig. 3, the output signal A is obtained
using the output terminals 17 and 18, while with the circuit
of Fig. 4 the evaluation signal B is obtained using the output
terminals 19 and 20.
In the circuits of Figs. 3 and 4, the Hall probes 14, 15, 16
are interconnected with the respective differential amplifiers
of the circuits in such a way that only difference fields are
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employed as drive, while a large superimposed magnetic bias
voltage causes only an in-phase shift which is effectively
suppressed without significant sidE: effects.
Circuits which are similar to the circuits shown in Figs. 3
and 4 have already been described. It is, of course, also
possible to employ other circuits f:or obtaining the evaluation
signals A and B, in respect of which the corresponding
operational amplifier circuits have already been mentioned
above.
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