WO2010072391A1

WO2010072391A1 - Method and device of a position encoder

Info

Publication number: WO2010072391A1
Application number: PCT/EP2009/009181
Authority: WO
Inventors: Jan Jansen Doornenbal
Original assignee: Ab Skf
Priority date: 2008-12-23
Filing date: 2009-12-21
Publication date: 2010-07-01

Abstract

A device and a method of aligning a full or partial turn absolute rotary position encoder (310) with a predetermined desired rotary position of an axle (302). A trigger (335) is generated when the axle is in a predetermined triggering rotary position. The trigger initiates acquiring a position signal (315) from the absolute rotary position encoder (310). This acquired position signal is a base for determining a correction factor (355). The correction factor is then used to transform subsequent position signals (315) from the absolute rotary position encoder (310) into aligned position signals (365). The aligned position signals are equivalent to position signals the absolute rotary position encoder (310) would have generated had the absolute rotary position encoder been physically aligned with the predetermined desired rotational position on the motor axle (302).

Description

METHOD AND DEVICE OF A POSITION ENCODER

TECHNICAL FIELD

The invention concerns position encoders and is more particularly directed to alignment of position encoders on axles, especially on axles of electrical motors, such as brushless DC motors.

BACKGROUND

A brushed DC motor conventionally comprises a rotor and a stator. The stator comprises a plurality of magnets, such as permanent magnets. The rotor comprises armature coil windings. In a brushed DC motor, brushes make a mechanical contact with a commutator to make an electrical connection between the armature coil windings of the rotor and a DC electrical source. As the rotor rotates, the stationary brushes come into contact with different sections of the rotating commutator. The different sections of the commutator are coupled to the armature coil in such a way that a current is switched to always flow through the armature coil closest to the stationary stator magnets.

In a brushless DC motor the magnets and the armature coil windings have switched places. Thus the rotor suitably comprises a plurality of magnets, preferably permanent magnets, and the stator comprises the armature coils. This eliminates the problem of how to transfer current to a rotating armature. Brushless DC motors most commonly use an electronic controller instead of the brush commutator system. Brushless DC motors offer several advantages over brushed DC motors, including higher efficiency and reliability, reduced noise, longer service life due to elimination of brush erosion. Additionally ionizing sparks from the commutator are eliminated and there is an overall reduction of electromagnetic interference. Since the windings are part of the stator instead of the rotor, they are not subjected to centrifugal forces. Additionally since the windings are located around the perimeter, they can be cooled by conduction to the motor casing requiring no airflow inside the casing for cooling. This in turn means that the motor can be entirely enclosed and protected from dirt or other foreign matter.

The downside of brushless DC motors is that they require somewhat complicated electronic controllers. The controller additionally requires some means of determining the rotor's orientation/position. Some designs use hall effect sensors or a rotary encoder. The higher the power rating of the motor, the higher the accuracy of the rotor's orientation/position has to be to reduce losses and attain accurate control of the motor. Knowing the position/orientation of the rotor is important to attain optimal efficiency and be able to properly control torque, especially for low speeds and from standstill. There is still room for improvements in how to determine the rotor orientation/position, especially of a brushless DC motor.

SUMMARY

An object of the invention is to define a method and a device of aligning a rotary position of an absolute rotary position encoder on an axle with a desired/predetermined rotary position on the axle.

A further object of the invention is to define a device and a method that will avoid costly mechanical alignment of an absolute rotary position encoder.

The aforementioned objects are achieved according to the invention by a method and a device that will electronically align a rotary position of a full or partial turn absolute rotary position encoder, with a desired/predetermined rotary position on an axle by means of triggering at a predetermined triggering rotary position an acquisition of a position signal of the absolute rotary position encoder and utilizing the acquired position signal to transform subsequent position signals.

The aforementioned objects are further achieved according to the invention by a device and a method of electronically aligning a rotary position of a full or partial turn absolute rotary position encoder with a predetermined desired rotary position of an axle. According to the invention a trigger is generated when the axle is in a predetermined triggering rotary position. The trigger initiates acquiring a position signal from the absolute rotary position encoder. This acquired position signal is a base for determining a correction factor. The correction factor is then used to transform subsequent position signals from the absolute rotary position encoder into aligned position signals. The aligned position signals are equivalent to position signals the absolute rotary position encoder would have generated had the absolute rotary position encoder been physically aligned rotationally with the predetermined desired rotational position on the motor axle, such as physically aligning an alignment mark on the absolute rotary position encoder with an alignment mark on the axle.

The aforementioned objects are still further achieved according to the invention by an electronic alignment device, which electronically aligns a full or partial turn absolute rotary position encoder at a mounted rotational position on an axle with a predetermined desired rotational position on the axle. This means that electronically it would seem as if the alignment mark on the rotary position encoder is physically aligned with the alignment mark on the axle, even though they are not. The absolute rotary position encoder generates position signals representing rotary positions of the axle. According to the invention the device comprises triggering means, determining means and transformation means. The alignment device transforms position signals generated by the absolute rotary position encoder into aligned position signals that are equivalent to position signals the absolute rotary position encoder would have generated had the absolute rotary position encoder been physically rotationally aligned with the predetermined desired rotational position on the axle. The triggering means are arranged to, when the axle is in a predetermined triggering rotational position, generate a triggering signal to receive a position signal generated by the absolute rotary position encoder as a triggered position signal from the absolute rotary position encoder. The triggering means also provides the triggered position signal to the determining means. The determining means are arranged to determine and store a correction factor based on at least the provided triggered position signal. The determining means will further provide the stored correction factor to the transformation means. The transformation means are arranged to transform position signals generated by the absolute rotary position encoder into aligned position signals by means of the provided correction factor.

In some embodiments the predetermined desired rotational position, such as the rotational position of an alignment mark on an axle, is equal to the predetermined triggering rotational position, such as the rotational position of the triggering means. In other embodiments the predetermined desired rotational position is different to the predetermined triggering rotational position, that is when an alignment mark is rotationally in a different position as a triggering means. In these cases the difference between the predetermined desired rotational position and the predetermined triggering rotational position can be expressed in a shift factor, and the determining means will then further base the determining of the correction factor also on the shift factor.

In some embodiments the triggering means suitably automatically generate the triggering signal by determining when the axle is in the predetermined triggering rotational position. This can then preferably be by means of a detector arranged to detect a marker or markings on the axle or in some embodiments when the axle is an axle of a brushless DC motor, it can advantageously be by means of receiving a signal from the motor. With other embodiments, the triggering means generates the triggering signal when an external signal is received. This can be a detecting device or a manual triggering when it is determined that the axle has the desired/predetermined orientation.

The different additional enhancements of the electronic alignment device according to the invention can be combined in any desired manner as long as no conflicting features are combined.

The aforementioned objects are also achieved according to the invention by a method of electronically aligning an absolute rotary position encoder at a mounted rotational position on an axle with a predetermined desired rotational position on the axle. The absolute rotary position encoder is coupled to the axle and generates position signals representing rotary positions of the axle. The method comprises a plurality of steps. In a first step it is determined when the axle is in a predetermined triggering rotational position. In a second step a triggering signal is generated when it is determined that the axle is in the predetermined triggering rotational position. In a third step a triggered position signal is received from the absolute rotary position encoder when the triggering signal is generated. In a fourth step a correction factor is determined based on at least the triggered position signal. The correction factor is at least representing a difference between the mounted rotational position and the predetermined triggering rotational position. In a fifth step the correction factor is stored. In a sixth step subsequent position signals generated by the absolute rotary position encoder are transformed into aligned position signals by means of the stored correction factor. The aligned position signals are equivalent to position signals that the absolute rotary position encoder would have generated had the absolute rotary position encoder been physically aligned with the predetermined desired rotational position on the motor axle. The predetermined desired rotational position can be equal or different to the predetermined triggering rotational position. If the predetermined desired rotational position is different to the predetermined triggering rotational position then suitably the difference between the predetermined desired rotational position and the predetermined triggering rotational position is expressed in a shift factor, and the step of determining a correction factor is further based on the shift factor.

By providing a method and a device for electronically aligning a rotary position encoder rotationally in relation to an axle the encoder is to encode, many advantages over traditional physical methods of physical alignment are obtained. Speed and accuracy of alignment are just two advantages, other advantages of this invention will become apparent from the detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described in more detail for explanatory, and in no sense limiting, purposes, with reference to the following figures, in which

Fig. 1 illustrates a typical application of the invention,

Fig. 2 illustrates a position encoder setup.

Fig. 3 illustrates a basic embodiment of the invention,

Figs. 4a, 4b illustrate different embodiments of trigger generation means according to the invention,

Fig. 5 illustrates a flowchart of a method according to the invention.

DETAILED DESCRIPTION In order to clarify the method and device according to the invention, some examples of its use will now be described in connection with Figures 1 to 5.

Figure 1 illustrates a typical application of the invention, where a brushless DC motor 100 is driven/controlled by a motor controller 120 with the help of a position encoder, suitably a full or partial turn absolute position encoder 110. The position encoder 110 is coupled to the motor axle 102 of the motor 100. When the motor axle 102 turns, then the position encoder provides the motor controller 120 with a position signal 115 that represents the rotary position of the motor axle 102 and thus the positional relationship between a stator and a rotor of the motor. With the position signal 115, the motor controller 120 can convert the supplied power 125 into correct power signals 105 to feed to the motor. As mentioned previously it is very important for an electronic controller of a brushless DC motor to know the rotational relationship between rotor and stator to enable the motor to be driven as efficiently as possible.

Figure 2 illustrates a position encoder 210 setup. The position encoder 210 will detect the rotational 203 position of a motor axle 202 and transform/encode this physical position into an electrical position signal 215. Preferably in these applications, the position encoder is an absolute rotary position encoder 210 as this enables a motor controller to immediately know the relationship between rotor and stator even from power off, without having to rotate past a reference point. This is important to enable a smooth start up of the motor. The invention is from a practical point of view intended to be used in conjunction with full or partial turn absolute rotational position encoders. An absolute rotational position encoder will provide a relevant position signal when turned-on even though for example a motor axle has been physically rotated when the encoder was tumed-off. This in contrast with an incremental position encoder that will start from zero each time it is turned on until it has possibly rotated past a synchronization mark. Thus, an incremental position encoder will not detect at power-on that for example an axle has rotated during the encoder's turned-off period.

For an absolute rotary position encoder to accurately be able to give a representation of the rotational position of an axle, such as an axle of a brushless DC motor, it needs to be very accurately mounted directly on or coupled to the axle in question. Such a mounting is difficult and takes time. It is not sure that the attained results are those desired, due to for example a misalignment between axle and rotor.

According to the invention, a rotary position encoder, suitably an absolute rotary position encoder, coupled to for example an axle, is electronically aligned with the axle. An electronic alignment device according to the invention further has the possibility to further adjust a possible offset between the axle and for example a brushless DC motor's rotor.

Figure 3 illustrates a basic embodiment of the invention. Just as previously described, a position encoder 310 is coupled to measure a rotational position of a rotating 303 motor axle 302 and for this purpose generate a position signal 315. According to the invention, this generated position signal 315 is not directly transferred to, for example, a motor controller, but is transformed in a position signal adjuster/corrector 360 to in turn generate an adjusted/corrected position signal 365 that is equivalent to a position signal the absolute rotary position encoder 310 would have generated had the absolute rotary position encoder 310 been physically aligned with a predetermined desired rotational position on the motor axle 302. The predetermined desired rotational position on the motor axle defines a rotational starting/zero point for the absolute rotary position encoder. When coupled to a brushless DC motor, the motor axle 302 is directly linked with the motor's rotor and the rotational starting/zero point for the absolute rotary position encoder will define a rotational zero point between the rotor and a stator of the motor. The motor controller will then know the rotational relationship between the rotor and the stator by means of the absolute rotational position encoder. According to the invention the encoder is synchronized with the axle and thus the rotor to stator rotational position relationship.

In some situations it might be enough to just know the exact rotary position of an absolute position encoder on a motor axle to then make adjustments on received position signals at for example a motor controller. The adjustment would then be made based on the angular relationship between the position of the absolute position encoder and a rotor on the axle. Unfortunately it is near impossible to economically mount an absolute position encoder with such a high degree of accuracy that is necessary to for example efficiently control a brushless DC motor. The invention overcomes these difficulties by not having to physically mount an absolute position encoder at a very precise angular location. In fact, the invention could basically allow free angular placement of the encoder.

The output of the encoder 310, the position signal 315, is then according to the invention electronically adjusted to a desired angular location. This is according to the invention achieved by triggering means 330 that generates a trigger signal 335, position signal processing means 340 that generates a correction factor to be memorized 345, a correction factor signal memory 350, separately illustrated just for explanatory purposes, providing a memorized correction factor signal 355, and a position signal adjuster/corrector 360 that generates the adjusted/corrected position signal 365.

The triggering means 330 will generate a trigger signal 335 either by manual intervention, by for example a switch that is activated when the axle is visually in a correct position, semi-automatically by using for example a locking mechanism to put the axle in a predetermined rotary position and then activate a switch, or completely automatic where measuring means are detecting the rotary position of the axle and generating the trigger at an appropriate moment when the axle is in the predetermined rotary position. The trigger signal 335 will allow the position signal processing means 340 to acquire a triggered position signal 315 from the position encoder 310. This position signal 315 will then make the position processing means 340 aware of what the encoder 310 believes the rotational position of the axle 302 to be. The position processing means 340 has been pre-programmed or is programmable with what the axle's 302 rotational position actually is when a trigger signal 335 is received. That is what the rotational relationship is between a triggering rotational position on the axle and a predetermined rotational position. The position processing means 340 will then determine what correction factor signal 345, 355 is needed to adjust/transform position signals from the encoder.

To facilitate further processing, it is also possible according to the invention to electronically align a rotational position of an axle with a rotor of a motor coupled to the axle. It can be that the rotor is not aligned properly with the axle and thus needs a correction. This would then possibly further need an additional shift factor 342 to be taken into account when determining the correction factor signal. An additional shift factor 342 could also be taken into account when the trigger signal 335 is generated at a different rotational position of an axle than a desired/predetermined rotational position. This angular difference can then be taken into account when determining the correction factor signal.

It is important to observe that the triggering means 330 and the position processing means 340 are only needed during the alignment. Once the alignment has been performed, only the correction factor signal memory 350 and the position signal adjuster/corrector 360 are needed.

Figures 4a and 4b illustrate different embodiments of the trigger means generation according to the invention. Figure 4a illustrates a simple switch 470 that generates the trigger signal 435. The switch could be actuated manually by a person, when that person gets an indication that the axle is in the correct position. The indication may for example be visual or a physical rotation stop of the axle. The switch 470 could also be automatically actuated when the axle attains a predetermined rotational position, such as a rotation stop with a switch that is hit by an arm mounted on the axle.

Figure 4b illustrates an alternative embodiment of the trigger, where the trigger comprises a detector 480 and a marker 482. The marker 482 is mounted or present on an axle 402 of a motor 400 to which an absolute rotational position encoder 410 is mounted. The marker 482 may take many different forms, it may for example be passive or active. The detector 480 may detect the marker in many different manners, such as optically or magnetically for example. The trigger in this example will give a trigger signal 436 when the detector 480 determines that the marker and thus also the axle 402, is in a specific rotary position.

Another version of generating a triggering signal is to have the possibility to variably control/change the electronic alignment. This could be done manually or automatically. It is then observed, either manually or automatically, how the motor runs with these different alignments. Automatically it could be done by measuring and optimizing for example torque. When an optimal alignment is determined to have been obtained, then this is triggered and stored.

Figure 5 illustrates a basic flowchart of a method according to the invention. In a first step 590, a physical rotational position of the motor axle is determined. In a second step 592 a trigger is generated to read a rotary position encoder signal value when the rotational position of the motor axle is determined to be in a triggered rotational position. In a third step 594 the read position sensor signal or a shifted read position sensor signal is stored/saved as a correction factor. The correction factor comprises a difference between a mounted rotational position of the rotary position encoder and the triggered rotational position of the motor axle. And finally in a fourth step 596 subsequent position sensor signals are adjusted/corrected with the correction factor.

The invention is based on the basic inventive idea of electronically aligning a rotary position encoder by reading the encoder value when for example an axle is in a known rotational position. This will eliminate having to physically align the encoder.

The invention is not restricted to the above-described embodiments, but may be varied within the scope of the following claims.

FIGURE 1 - illustrates a typical application of the invention, 100 a brushless DC motor,

102 a motor axle,

105 motor power connections,

110 a position encoder, such as an absolute position encoder,

115 a position signal output from position encoder, 120 a motor controller,

125 a power input to the motor controller.

FIGURE 2 - illustrates a position encoder setup, 202 a motor axle, 203 a motor axle direction of rotation,

210 a position encoder,

215 a position signal from the position encoder.

FIGURE 3 - illustrates a basic embodiment of the invention, 302 a motor axle,

303 a motor axle direction of rotation,

310 a position encoder,

315 a position signal from position encoder,

330 triggering means, 335 a trigger signal, in some embodiments also shift factor signal,

340 position signal processing means, reads the position signal when the triggering signal is received, in some embodiments also with additional shift factor, and transforms this into a correction factor,

342 optional shift factor, 345 correction factor signal to be memorized,

350 correction factor signal memory,

355 memorized correction factor signal, 360 position signal adjuster/corrector,

365 adjusted/corrected position signal.

FIGURES 4a and 4b - illustrate different embodiments of trigger means generation according to the invention,

400 brushless DC motor,

402 motor axle,

405 motor power connections,

410 position encoder, such as an absolute position encoder, 435 trigger signal, in some embodiments also shift factor signal,

436 trigger signal, in some embodiments also shift factor signal,

437 trigger signal, in some embodiments also shift factor signal, 470 switch,

480 detector of a marker, can be optical, magnetic, capacitive etc. to generate trigger signal, and possibly with a shift signal,

482 marker, to be detected by the detector and give rotational position of axle in relation to the detector,

FIGURE 5 - illustrates a flowchart of a method according to the invention, 590 first step of determining the physical rotation of the motor axle,

592 second step of generating a trigger to read rotary position encoder signal value, 594 third step of saving the read rotary position encoder signal or a shifted read rotary position encoder signal as a correction factor, 596 a fourth step of adjusting/correcting subsequent rotary position encoder signal values with the correction factor,

Claims

1. An electronic alignment device to electronically align an absolute rotary position encoder at a mounted rotational position on an axle with a predetermined desired rotational position on the axle, the absolute rotary position encoder generating position signals representing rotary positions of the axle, the alignment device comprising triggering means, determining means and transformation means, characterized in that, the alignment device transforms position signals generated by the absolute rotary position encoder into aligned position signals that are equivalent to position signals the absolute rotary position encoder would have generated had the absolute rotary position encoder been physically aligned with the predetermined desired rotational position on the axle, and in that: the triggering means are arranged to, when the axle is in a predetermined triggering rotational position, generate a triggering signal to receive a position signal generated by the absolute rotary position encoder as a triggered position signal from the absolute rotary position encoder, and to provide the triggered position signal to the determining means; the determining means are arranged to determine and store a correction factor based on at least the provided triggered position signal, and to further provide the stored correction factor to the transformation means; the transformation means are arranged to transform position signals generated by the absolute rotary position encoder into aligned position signals by means of the provided correction factor.

2. The electronic alignment device according to claim 1 , characterized in that the predetermined desired rotational position is equal to the predetermined triggering rotational position.

3. The electronic alignment device according to claim 1 , characterized in that the predetermined desired rotational position is different to the predetermined triggering rotational position.

4. The electronic alignment device according to claim 3, characterized in that the difference between the predetermined desired rotational position and the predetermined triggering rotational position is expressed in a shift factor, and in that the determining means further bases the determining of the correction factor on the shift factor.

5. The electronic alignment device according to any one of claims 1 to 4, characterized in that the triggering means automatically generate the triggering signal by determining when the axle is in the predetermined triggering rotational position.

6. The electronic alignment device according to claim 5, characterized in that the triggering means determines that the axle is in the predetermined triggering rotational position by means of a detector arranged to detect a marker or markings on the axle.

7. The electronic alignment device according to any one of claims 1 to 4, characterized in that the triggering means generates the triggering signal when an external signal is received.

8. A method of electronically aligning an absolute rotary position encoder at a mounted rotational position on an axle with a predetermined desired rotational position on the axle, the absolute rotary position encoder being coupled to the axle and generating position signals representing rotary positions of the axle, characterized in that, the method comprises the steps of: generating a triggering signal when the axle is in a predetermined triggering rotational position, - receiving a triggered position signal from the absolute rotary position encoder when the triggering signal is generated; determining a correction factor based on at least the triggered position signal, the correction factor at least representing a difference between the mounted rotational position and the predetermined triggering rotational position, storing the correction factor, transforming subsequent position signals generated by the absolute rotary position encoder into aligned position signals by means of the stored correction factor, the aligned position signals being equivalent to position signals the absolute rotary position encoder would have generated had the absolute rotary position encoder been physically aligned with the predetermined desired rotational position on the motor axle.

9. The method of electronic alignment according to claim 8, characterized in that the predetermined desired rotational position is equal to the predetermined triggering rotational position.

10. The method of electronic alignment according to claim 8, characterized in that the predetermined desired rotational position is different to the predetermined triggering rotational position.

11. The method of electronic alignment according to claim 10, characterized in that the difference between the predetermined desired rotational position and the predetermined triggering rotational position is expressed in a shift factor signal, and in that the step of determining a correction factor is further based on the shift factor signal.