Electricity: motive power systems – Limitation of motor load – current – torque or force
Reexamination Certificate
2000-07-12
2002-06-04
Ro, Bentsu (Department: 2837)
Electricity: motive power systems
Limitation of motor load, current, torque or force
C318S254100, C318S721000, C318S431000
Reexamination Certificate
active
06400109
ABSTRACT:
The present invention concerns an electronically commutated motor and a method for operation of this motor according to the preamble of independent patent claims
1
and
9
.
For the sake of simplicity and for better understanding, a three-phase motor excited by a permanent magnet is primarily discussed in the description, whose phases are arranged in known fashion. The present invention, however, is not restricted to a variant with three phases, but is fully applicable to variants with more than three phases and/or more than one permanent magnetic pole pair.
The drive signals for the individual phase are generated by corresponding position sensors. Hall elements or Hall ICs are preferably used according to the prior art as position sensors. The mutual rotation angle spacing or spacing of the individual Hall sensors relative to each other stated in radians is dependent on the number of magnetic pole pairs, the exact commutation times only being guaranteed when the spacing tolerance of the position sensors equals zero. Under practical conditions this requirement cannot be achieved, so that current supply of the phase windings driven by the corresponding Hall sensors occurs either too early or too late.
Such incorrect commutations result in an increase in torque ripple of the motor, which, on the one hand, leads to intensified vibrations and, on the other hand, hampers exact position or torque control.
Positioning accuracy for the Hall sensors of ±0.1 mm can only be accomplished at extremely high cost and satisfies the requirements less, the more the sensor system of the motor is miniaturized.
This problem is much more strongly pronounced in particular in motors with a larger number of pole pairs, since in this case, the mechanical angular spacing corresponding to a “electrical” rotational angle, under which the Hall sensors must be arranged, becomes increasingly smaller.
In particular, when the outside dimensions of these motors become increasingly smaller in adjustment to corresponding incorporation conditions so that the radius of the circle on which the Hall sensors are arranged also diminishes, the resulting percentage angular error resulting from this is increased at the same positioning accuracy. The smaller the motor and the larger the number of magnetic pole pairs, the more critical the position tolerances of the Hall sensors are as angle errors with reference to the commutation times of the corresponding phase windings.
The underlying task of the invention is therefore to generate commutation signals produced by Hall sensors more precisely and less subject to tolerance so that commutation is no longer dependent on the corresponding dimension-related scatter with reference to arrangement of these Hall sensors on the circuit board.
The solution to this task occurs with the technical teachings of the independent patent claims
1
and
9
.
It is important according to the invention that only the output signals of a single position sensor are evaluated as signal source after startup of the motor. Startup occurs, as previously, with allowance for the tolerance-burdened output signals of at least two position sensors operating with the same timing frequency.
All additional commutation times are derived from the timing frequency of this one signal source, in which the timing frequency of this one signal source is a whole number multiple (corresponding to the number of phases) of the second frequency with which the other position sensors necessary for motor startup conforming to direction of rotation are driven. An additional Hall sensor can be provided as signal source with higher timing frequency, which is driven via an additional magnetic track with a correspondingly higher number of pole pairs. The higher timing frequency, however, can also be derived from one of the position sensors operating with lower timing frequency whose low timing frequency is prepared and multiplied accordingly by a method of signal technology appropriate for this.
The Hall sensors are arranged on a fixed circuit board, whereas the control magnetic disk rigidly connected to the rotor rotates at spacing Z
1
around the common Z axis. In the case of an additional Hall sensor H
4
, the Hall sensors H
1
, H
2
and H
3
are arranged on an arc around Z with radius R
1
by 30° mechanical (corresponding to 60° electrical), in which R
1
corresponds to the average radius of the inner magnetic track
1
(two pole pairs). The additional Hall sensor H
4
, for example, is arranged diametrically to H
2
on an arc with radius R
2
, in which R
2
corresponds to roughly the average radius of the outer magnetic track
2
(3×2=6 magnetic pole pairs).
For example, if the Hall sensor is switched from a north pole at logic “1” and accordingly from a south pole at logic “0”, a state diagram still to be explained in the later drawings is obtained during rotation of the control magnetic disk. Since one can establish immediately after the first state change by means of a truth table whether the motor is rotating in the desired direction, the Hall sensor H
4
timed with the higher frequency can be switched to shortly after startup of the motor, but at the earliest after the first state change.
After switching the commutation times are exclusively stipulated by the output signal of Hall sensor H
4
. This state can be maintained until the motor is stopped again. However, it is important that before restartup, i.e., at the latest during shutdown, the state signals of the other Hall sensors must be reactivated so that during the next startup of the motor the actual rotor position can be clearly verified. To be able to guarantee this, the signals of the other Hall sensors H
1
-H
3
must also be evaluated in this operating state.
It is therefore proposed according to the invention in a first approach to the solution, starting from the prior art, to provide a fourth Hall sensor H
4
and a second control magnetic track in order to be able to derive the exact commutation times from the higher timing frequency of the additional Hall sensor.
It is recognized from the resulting truth table that in a three-phase motor excited by a permanent magnet only three Hall sensors are required in principle for distinct position and directional rotation recognition. In addition to Hall sensor H
4
with the higher timing frequency and an additional second control magnetic track, only two additional Hall sensors, for example, H
2
and H
3
are therefore required instead of the initially proposed three Hall sensors H
1
, H
2
and H
3
. This solution is more cost effective, since overall, according to the prior art, only three Hall sensors are again used. In this case the motor is started with H
2
, H
3
and H
4
and after starting, at the earliest after the first phase change, only H
4
is evaluated.
From this point, the signals of the other Hall sensors are therefore no longer considered so that the commutation times are stipulated free of tolerance merely by the phase change of Hall sensor H
4
. Driving of the power transistors that switch the coil currents occurs, for example, via an upline &mgr; processor.
It is therefore proposed according to the invention in the second approach to the solution to use the minimum number of position sensors from the prior art, but in which one of these position sensors is arranged on an arc with a different, for example, larger radius and is driven via a second magnetic track with a correspondingly higher number of pole pairs. This position sensor then delivers the higher timing frequency required for determination of the exact commutation times.
In a third approach to the solution according o the invention, it is proposed to derive the higher timing frequency required for determination of the exact commutation times by an appropriate and known method of signal technology for the timing frequency of a position sensor H
1
or H
2
or H
3
driven with the usual low timing frequency, for example, by frequency multiplication.
The invention is now characterized by the fact that the state signals of
Precision Motors Deutsche Minebea GmbH
Ro Bentsu
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