Tachogenerator for Electric Machines

The present invention relative generally to a tachogenerator and, more specifically, to a tachogenerator having improved signal output. Means for determining the rotary speed of rotary machines of any kind, in particular electric machines, sometimes in combination with the simultaneous determination of the angular position of the rotary part, are required for a plurality of applications and machines, for example in order to control the rotary speed of certain rotary drives, or in order to derive certain information from the momentary speed, for example for use as actual values for regulating or control purposes.

In the case of certain types of electric machines, which generally can be regarded as synchronous machines and may be described, from the functional point of view, as sort of brushless d.c. motors, it is important, in addition to gaining an actual-value signal indicating the rotary speed, to derive at least relative information on the respective rotor position so that one can determine from the position of the rotor which of the coils must be excited for generating corresponding magnetic fields for the operation of such an electric motor.

Quite generally, tachogenerators have been known in numerous configurations, depending on the demands placed on their accuracy for example as incremental pick-ups, using Hall generators, optoelectric means, or the like. Usually, and in most cases necessarily, such tachogenerators have associated with them a plurality of additional processing circuit components which serve, for example, for linearizing, amplifying or preparing the signals so obtained for further processing, for example for speed regulation. This is necessary because the useful level gained is very low in most of the cases and one cannot, therefore, do without such supplementary electronic means.

Now, it is the object of the present invention to provide a tachogenerator for use in connection with rotary drives of any nature, preferably improved by the addition of supplementary means for determining the rotary speed of the driving machine, which is of simple design and supplies immediately a perfectly linearized and, accordingly, highly precise output signal of high useful level.

In a different context it has been known, in connection with an inductor for rotary electric machines (AT-PS 219 701) to build up a stationary magnet system and a rotor supplying rotary energy in axial layers in which case flat windings and laminar conductors are arranged on the rotor. Due to the arrangement in axial layers, a plane air gap is obtained between the rotor and the stator parts. The stator part comprises a plurality of permanent magnets each of which is connected in parallel, via a common yoke, to a magnetic subsidiary circuit comprising an electric excitation winding. The parallel subsidiary circuits, therefore, contribute very specially towards generating the inductor flux in the machine when the excitation winding is excited, while a magnetic short-circuit is produced for the respective permanent magnet of the stator part when the excitation winding is interrupted. There does not exist any relation to the present invention.

The tachogenerator according to the invention solves the above problem and provides the advantage that a highly precise collector-less d.c. tachogenerator is created practically alone by the geometric structures and arrangement of the magnets belonging to or forming the rotor, and of the coils, and that the high useful output level of this tachogenerator is free from waviness, dissymmetries, interfering and stray influences and linearized up to the percentage range.

The invention, therefore, succeeds due to the special mechanical arrangement which also comprises the special shape of the coils used, in combination with the configuration of the magnetization lines of the rotor, in generating output signals which can be further processed, usually employed for control purposes, without the need to subject them to extensive electronic preparatory and correction measures.

The main reason for this lies in the fact that the EMF supplied by the individual coils exhibit a straight roof curve whose level is a direct measure of the rotary speed of the machine. The roof curve is perfectly linear, straight and free from waviness and other dissymmetries and disturbing influences, except that one may, as sometimes usual in connection with tachogenerators, select from the voltages supplied by the coil desirable sections for further processing, for example by fixing electronic thresholds and by suitable selection measures. In the case of the present invention, it can be the question only of selecting sections of the anyway linear and straight roof curves so that one only has to define the point in time at which the roof curve is to be clipped in the generally alternating output signal of the individual coil.

This is advantageously achieved by evaluating position signals which, according to one embodiment of the present invention, are gained by inductive means due to the fact that the rotor also supports a cam disc which, depending on the particular rotor position, provides full, partial or no screening of the magnetic flux emanating from separate transmitter heads relative to correspondingly positioned receiver heads. It is possible in this manner, by evaluating and comparing the output signals supplied by the receiver heads, which now have been modulated by the rotation of the rotor and the effect of the cam disc, to determine the respective rotor position of the driving electric machine, at least for certain relative positions at which, for example, predetermined coils should be excited, which coils may be distributed over the periphery in repeated arrangement, depending on the number of poles of the machine. This determination of the position is of course valid also for the tachogenerator so that it is also possible to select and cut out roof sections from the electric EMF signals of the tachogenerator coils by evaluating the positional signals.

The features specified by the sub-claims permit advantageous improvements and further developments of the tachogenerator defined by the main claim. A particular advantage is seen in the fact that the tachogenerator is absolutely free from any iron, which means that it operates without any yoke structures. Accordingly, no iron back-shunt is required for the magnetization lines produced by the rotor, which takes the form of a special permanent magnet, so that no distortions, dissymmetries, or the like, which normally result from hysteretic effects, are encountered.

The individual coils operate in the manner of a multi-phase system, preferably a three-phase a.c. system, thus forming three output voltages set off by 120.degree., which preferably exhibit at least partially overlapping roof curves, so that by forming the before-described sections as described above, a continuous d.c. voltage can be obtained as output signal of the tachogenerator.

Another advantage of the present invention resides in the fact that a differential transmitter system can be provided by arranging two stationary coil carrier disks with correspondingly designed and positioned coils, in which case one of the coil carrier disks, with the coils mounted thereon, may be arranged axially on the one side of the rotor/permanent magnet and the other one o the other side thereof, in axial layers, relative to the driving shaft and/or the rotor of the tachogenerator. Such a differential transmitter arrangement suppresses axial displacements of the rotor which otherwise would result in variations of the measuring results; by connecting the coils on each side of the magnet rotor appropriately, it is possible to absorb any axial displacements, which may be due for example to an increase in length of the driving haft of the electric machine equipped with such a tachogenerator. In addition, the differential arrangement ensures that any dissymmetries, that may possibly result from the sense of rotation, are suppressed in the output voltages of the coils as the beginning and the end of each coil can be connected on both sides of the magnet rotor in such a manner that any influences resulting from the sense of rotation are balanced out.

Finally, a particularly advantageous improvement of the present invention is obtained when the individual tachogenerator coils, which overlap each other in the form of a circle, have a basic trapezoidal shape and the two oblique sides of the trapezoidal shapes of all coils arranged on the coil carrier disk form together the spokes of a wheel, while the longer side of the trapezoid provides particularly well suited correction means for influencing the shape of the measuring voltage supplied by each individual coil, and finally by all coils together, and this especially as regards the before-mentioned roof curve. By including these coil webs, which are formed by the base lines of the trapezoid and which normally extend either in the direction of, or in opposite direction to, the moving magnetic lines, into the active magnet area, at least partially, and forming them simultaneously in a special manner--whereby voltage components evaluated for correction purposes are induced also in the coil webs formed by those base lines and constituting the outer periphery of the wheel in the completed structure--it is possible altogether to influence the curve shape of the voltage supplied by each coil so that the desired roof curve shapes are obtained.

Certain embodiments of the invention will now be described with reference to the drawing in which:

FIG. 1 shows a side view of a first embodiment of the unit combining position-sensing means and a tachogenerator;

FIG. 2 shows a top view of the at least one coil carrier disk, with the arrangement of the coils and of the position-sensor heads;

FIG. 2a shows a detail of the layered arrangement of the coils and the mutually overlapping portions in the peripheral area of the "coil wheel";

FIG. 3 shows a diagram of a possible shape of the curve of the phase voltages supplied by the coils, as a function of time;

FIG. 4 shows a top view of the rotor which consists of a magnetic material and is provided on its periphery with an additional position-sensing cam disk, illustrating, in connection with the representation of FIG. 1, also the arrangement and interrelation of the individual poles; and

FIGS. 5a and 5b illustrate by way of diagrams the development of the supply voltage (FIG. 5a) supplied to inductive transmitter heads for position-sensing purposes, and the development of the signal received by separate inductive receiver heads, after modulation by the influence of the magnetic cam disk.

It is the basic idea of the present invention that flat coils of trapezoidal shape, resembling the spokes of a wheel, are provided, preferably in overlapping arrangement, and that a driven, rotating magnet rotor is provided in axially layered arrangement, which means that it occupies a position in front of the flat coils, viewed in the axial direction, and that, in the driven state, its magnetization lines which are likewise mainly emitted in the axial direction intersect the coil windings, initially primarily those windings which form the spokes of the wheel, in the wheel-type overall design of the coils. This structure is obtained without any guiding means for the magnetization lines in the form of yokes, iron parts, or the like.

One embodiment of the invention then provides the important possibility of correcting the EMF emitted by the trapezoidal coils by means of the respective coil webs formed by the base lines of the trapezoid, which are likewise arranged within the active magnetic area, i.e. in the area where magnet lines emitted by the rotating rotor are still encountered. These top or base-line coil webs are subjected to special shaping measures after winding of the coils, which then already exhibit the basic trapezoidal shape, for the purpose of obtaining optimum correction results regarding the roof curve shapes of the emitted voltages.

Another embodiment of the present invention then provides additionally the possibility to determine simultaneously the rotary-angle position, if desired even very precisely, making use of inductive transmitter and receiver coils which are likewise arranged on the coil carrier discs and which are subjected to the influence of the magnetically active cam disc mounted on the rotor.

In the case of the embodiment illustrated in FIG. 1, the driven magnetic rotor of the tachogenerator 10 is designated by reference numeral 11 and coacts in axially layered arrangement with at least one stationary coil-carrier disk 12.

The rotor 11 comprises a disc 13 consisting of a magnetically permeable material and provided with a central bore for receiving the shaft 14, which may also be an output shaft end of an associated electric machine or of another rotary drive whose rotary speed and, if desired, rotary-angle position are to be determined.

The manner in which the shaft 14 carries the magnet rotor 11 need not be described here in detail, being not the subject of the invention. Similarly, it is not necessary to describe at this point the manner in which the at least one, according to a preferred embodiment the two coil carrier disks 12, 12' are supported. Preferably, they are located inside a suitable housing (not shown).

The material of the magnet rotor 11 may be a suitable magnetic sintered material. The magnetization of the magnet rotor 11 is such that--as will be understood best with reference to the representation of FIG. 1--north and south poles are arranged in alternating distribution about the circumference of each disk surface 13 so that star-shaped magnetization is obtained and certain disk segments form north and south poles in alternation succession, while on the other side of the annular disk 13 disk segments distributed similarly over the circumference form north and south poles, again in alternating succession but so that each pole positioned at a pre-determined angle is (necessarily) opposed by the corresponding counter-pole on the other outer surface, as can be seen best in the representation of FIG. 1. Such special magnetization forms can be produced by suitable iron yoke shapes. However, this need not be described here in detail because the nature, structure and distribution of the poles about the aperture or annular disk 13 forming the magnet rotor 11 are clearly illustrated in the representation of FIGS. 1 and 4.

For the measuring process performed by the tachogenerator according to the invention those magnet lines are of importance which leave the magnet rotor axially (and when coil carrier disks 12, 12' are arranged on both sides of the magnet rotor 11) on both sides of the magnet rotor and which then enter again the adjoining pole segments. An attempt at illustrating corresponding magnet lines has been made at 15 in FIG. 4. For inducing the desired voltages in the coils (this aspect will be described in greater detail further below) the invention, therefore, makes use only of those magnet lines which leave the annular surfaces of the magnet rotor 11 in the axial direction, while it makes no use of any magnet lines closing from both sides across the outer or inner marginal edges, although there magnet lines are (necessarily) also encountered because it is a fact that opposite poles must (of course) be arranged opposite each other on the same angle segment of the angular disk 13, as one pole alone can never exist.

It is understood that particular magnetization characteristics, i.e. for example the strength of the magnetization field in the individual poles of the magnet rotor, are adapted to the desired curve shape of the output voltage so that a doubled overall effect is obtained in connection with the special coil shape, which finally leads to the shapes of the output voltage curves (EMF induced in the coils) shown in FIG. 3.

The oppositely arranged coil carrier disks 12, 12', which are separated by the magnet rotor 11, are insofar symmetrical and built up in the same manner so that it is absolutely sufficient to describe the structure of only one coil carrier disk with the associated coils, with reference to the representation of FIG. 2. A carrier consisting, for example, of a rectangular--as shown in FIG. 2--or a circular disk of a suitable, possibly magnetically and electrically inert (plastic) material is designated by reference numeral 16 and carries flat coil 18 which are adapted to the annular shape of the magnet rotor 11, but preferably inserted in a central circular opening 17, in the manner a spoked wheel may be inserted in a surrounding running tread.

It has been found in this connection that for achieving the best possible results each individual coil should present a substantially trapezoidal shape, which means that--as described in connection with the coil 18 in FIG. 2--it should comprise two oblique lateral parts 19a, 19b, which will be described hereafter as spoke-like webs, as well as a longer base web 20 and a shorter connecting web 21.

In order to build up the shape of a wheel from this coil shape, the longer base webs 20, which have been described above also as roof webs, are arranged towards the outside, i.e. towards the larger circumference of the circular structure.

The coils overlap each other and are assembled and connected, preferably by gluing, in this fan-like annular shape so as to provide the form of a complete wheel, with the partially overlapping inner connecting webs 21 forming the hub, the oblique lateral webs 19a, 19b forming the spokes of the wheel and the assembled and partially overlapping longer base webs 21 forming the hoop-like periphery. The entire annular structure of the flat coils is then mounted in the circular opening 17 of the coil carrier disk 16. In addition, the total arrangement of the coils in the form of the described wheel provides the following particularities. Due to the overlapping arrangement, each coil--in the representation of FIG. 2 the coil 18', overlaps the rear third of the preceding coil 18 by its forward spoke-like web 19a' so that the rear spoke-like web 19b of each preceding coil 18 is visible only through the inner, likewise trapezoidal coil opening, while the shorter connecting web (adjoining the hub) and the upper longer base web (forming the periphery of the wheel) rest fully on the corresponding connecting and base webs of the preceding coil.

In order not to obtain a stepped structure, but ensure that the wheel shape of the overall coil structure remains in one plane, each coil is positioned somewhat obliquely in the circular opening 17, as will be easily understood, because the forward part of each coil rests on the rear part of each preceding coil. The rear part of each coil is, therefore, somewhat depressed by the following coil--starting out from the top view of FIG. 2--while the leading part seems to be somewhat raised because it rests on the rear part of the preceding coil. Regarding the whole arrangement in a different way, one may also say that each coil seems to be bent off in the central portion of the connecting or base web, at the transition between its forward and rear parts. This central portion, therefore, serves as connection between a raised coil portion and a depressed coil portion so that each coil has certain portions arranged in two, of course closely adjacent, planes. This coil configuration can also be used for optimizing the final shape of the measured-value output voltage by empirical tests, and for subjecting it to final correcting influences. FIG. 2a illustrates this layered arrangement of the coils by way of the peripheral base webs 20 and their bent-off portions 20'.

The individual coils can be produced in the usual manner by winding them about a corresponding trapezoidal/angular arbor, although for the purposes of the present invention they have to be pressed into shape thereafter in order to bring them, and in particular the base web 20, which is of considerable importance for the straight linearized roof shape of the voltage curves according to FIG. 3, into the proper shape which actually results in the desired output voltage curves. A particularity of the invention actually consists in the fact that the base web 20 does not follow the circular shape of a wheel and/or of the opening 17 in the carrier disk 16, but exhibits a clearly straight, i.e. somewhat inwardly depressed shape so that the desired output voltage curve according to FIG. 3, i.e. mainly the desired roof shape, can be achieved by the nature of the fields emanating from the magnet rotor 11 during its rotary movement, in combination with the fact that the base webs 20 of each coil are (still) in the magnetically active area which is intersected by the field lines of the magnet rotor.

In FIG. 3, the curves of three phase voltages, set off against each other by 120.degree., are shown. The base webs 20 of each coil provide notable shares of the EMF produced by each coil--substantially corrective components for the roof shape of the linked output voltages, which in FIG. 3 are represented as three-phase voltages.

It should be noted here expressly that the flat linear and--as has been explained repeatedly--desired straight roof shape of the output voltages, as shown by the diagram of FIG. 3, is not achieved by limiter action or by clipping by means of diodes or the like, but is directly the result of the geometric and structural conditions of the flat-coil arrangement, the shape of the coils and the shape of the axial magnetic field lines of the rotor 11. This also means that any change in the rotary speed will also cause the roof voltage level, i.e. the voltage level determined by the roof shape of the flat coils which, in the embodiment shown, constitute a three-phase a.c. system, to change proportionally, i.e. to rise or drop as indicated by arrow A. Whilst all hysteretic effects or other disturbing influences resulting therefrom are avoided due to the ironless construction (detection of the magnetic fields emanating from the magnet rotor 11 by the nature, shape, structure and number of the coils, directly and without any iron) the main EMF is obtained in the straight spoke-like webs 19a, 19b while counter-voltages may be induced also in the roof slopes of the flat coils, i.e. in the base webs 20 and, if desired, also in the connecting or hub webs 21, which then contribute towards the desired pulse shaping action: electric output pulse shaping by coil shaping.

The offset of the coils relative to each other, and the number of coils, is determined by the number of poles of the magnet, which is in principle freely selectable. If the magnet rotor consists of a six-pole excitation magnet, as in the illustrated embodiment, then the existing coils can be connected to three phase windings in the manner of an three-phase AC system so that--regarding again the illustrated embodiment which is, however, not to be regarded as restricting the invention--the following connections are obtained in the case of nine flat coils: The first, fourth and seventh for forming the first phase voltage, indicated in the diagram of FIG. 3 by I and represented by the full line; the second, the fifth and the eighth coil for forming the second phase voltage of the three-phase a.c. system (voltage curve II in FIG. 3, indicated in broken lines); and the third, sixth and ninth coil for forming the last phase voltage (indicated in the diagram of FIG. 3 by the dash-dotted line III).

Another contribution towards forming the output voltage is provided by the special nature of the magnetization of the individual poles of the magnet rotor 11. For example, a strong magnetization is sought in the central portion of the segment, while the magnetization is kept weaker in the marginal areas. Such different magnetization strength may be achieved--this is not shown in the drawing--by a special design of the magnetization means for the magnet rotor 11, for example by giving the yoke used for the magnetization process a corresponding, special shape and by determining the saturation areas in the magnetization yoke in such a manner that the desired axial direction of the magnet lines is obtained in the magnet rotor. These conditions are described at such length because, finally, empirical steps are required in connection with the determination of the particular output curve shape according to FIG. 3, if optimum results are to be achieved. Altogether, the roof curve, which is of interest for the present purposes, can be optimized by the following two parameters, namely

1. the geometry of the flat coils and

2. the configuration of the magnetization lines in the

magnet rotor.

The invention then makes use only of additional electronic means for cutting out certain roof curve sections from the trapezoidal voltages supplied by the coils, it being possible for these purposes to select the cut-out sections, for example, as indicated by arrow B in FIG. 3 which means that the roof sections can be clipped practically to provide a continuous line which is perfectly straight. In order to determine the loci, or better the moments t.sub.1 and t.sub.2, or t.sub.2 and t.sub.3, etc., for clipping the roof sections, one needs the position-detecting means which will be described further below.

Another advantageous embodiment of the present invention provides that the system is also capable of reacting to variations in the distance of the supporting means, which may possible occur, i.e. to any axial displacement of the magnet rotor 11 relative to the stationary coil-carrier plates 12, 12' in a manner to correct any such variations perfectly, although such variations in distance have a considerable influence on both shape and value of the output voltages if, as in the present case, only the axially emanating magnetization lines are evaluated.

This partial problem is solved by arranging two coil-carrier plates 12, 12', carrying coils in the described manner, on both sides of the magnet rotor 11 so that a differential arrangement is provided which generally ensures that if the one coil arrangement receives a smaller amount of magnet lines, due to any variation in the axial distance, this amount is additionally applied to the other coil arrangement so that any variations in the distance, i.e. any axial displacement of the magnet rotor 11, is balanced out if the coils mounted on the coil-carrier plates 12 and 12' are connected in a suitable manner. This also simplifies considerably the (initial) adjustment of the system. Another advantage of such a differential arrangement, which is derived from the known induction laws, is seen in the fact that any distortions resulting in the output voltages of the coils due to the sense of rotation, are also balanced out since--as will be easily understood--one coil arrangement can be connected in such a manner that--to say it in simple words--it perceives the inductive effects of the magnetic rotor 11 as being directed in the one sense when the other coil arrangement perceives the same effects as being directed in the other sense, and vice versa. This can be achieved, for example, by directing the overlapping portions of the coils in clockwise or counter-clockwise direction. If the rotor then reverses its sense of rotation, the only thing that happens is a reversal of the conditions and corrective influences existing at the coils.

According to another embodiment of the invention, the means for determining the rotor position mentioned before are not only related to the position of the magnet rotor 11, for clipping the output voltage roof curves, but relate also to the rotor of the electric machine driving the tachogenerator which may for example have the general design of a synchronous machine and the function of a brushless d.c. motor so that, accordingly, it is an important feature that specific coils can be excited at specific moments in time by electric circuits, rather than--as heretofore usual--by means of a mechanical commutator.

For determining the position in connection with a combined solution comprising the tachogenerator, a cam disk 22 consisting of a magnetically permeable material (iron disk) is fixed on the rotor, preferably in a central position, while inductive transmitter heads 23 and inductive receiver heads 24 are provided in axially opposite positions, and properly aligned relative to each other, conveniently directly on carrier plates 12, 12' arranged on both sides so that when the cam disk 22 rotates the said heads face each other in fully or partially screened, or in fully exposed relationship, depending on the shape and rotary position of the cam disk 22. To say it in other words, the iron disk forming the cam disk 22 interrupts or releases the free path between the transmitter and the receiver heads 23, 24 in the manner of an aperture diaphragm, the transmitter and the receiver heads being designed as ferrite heads operating on an inductive basis, similar to known proximity switches. Accordingly, an alternating magnetic field is generated in each of the transmitter heads, which during rotation of the cam disk 22 is masked to a greater or lesser degree so that it is no longer received at all, or is received only in very weak form, by the receiver head.

This means properly speaking that the receiver head or heads receive an image of the movement of the magnet rotor, the magnetically permeable material (iron) of the cam disk 12 providing a greater or lesser screening effect.

As regards the type, number, structure and distribution of the transmitter and the receiver heads, as well as the shape of the cam disk 22, these may generally be selected according to the particular requirements. The illustrated embodiment uses three transmitter heads and, accordingly, three receiver heads 23, 24 which--as will be seen from the representations of FIG. 2 and FIG. 4, in connection with the cam disk 22--are distributed over the circumference of the magnet rotor in such a manner that in the rest position, for example, one pair of transmitter and receiver heads is not masked at all, another one, for example the one following in the sense of rotation, is fully masked, and a third one, again viewed in the sense of rotation, is masked in part, due to the special shape of the cam disk 22. This condition is repeated cyclically when the shape of the cam disk 22 is designed, for example, in the manner represented in FIG. 4, i.e. when its basic shape resembles that of an isosceles, for example with rounded edges and sides which are a little bulged.

It is easily seen that, when the inductive transmitter and receiver heads are distributed as shown in FIG. 4, the first pair of transmitter/receiver heads, viewed in the sense of rotation indicated by arrow C, is masked by the moving edge 22a initially only in part, then fully, while at the same time the next following pair is initially masked in part, then released in full, and the last pair is fully masked.

The output signal of each receiver head, therefore, consists of an output voltage modulated by an envelope corresponding to the representation of FIG. 5b, when the associated transmitter head is supplied with an alternating signal of constant amplitude and a frequency of, for example, 20 KHz. The modulation of the signal received provides an image of the rotary movement, at least of the relative rotary movement. One then knows, for example, which coils of a driving primary machine have to be excited and/or the output voltage roof curves of which coils of the tachogenerator have to be clipped according to FIG. 3 for the purpose of arriving at a common d.c. voltage proportional to the rotary speed. By "relative position" it is understood in this connection that certain machines have repeating geometrical structures, depending on their number of poles. In these cases it is not important to know the absolute angular position within the given 360.degree., but it will absolutely suffice to have information regarding the position of repeating, i.e. insofar similar, poles relative to specific coils.

The systems involved being usually three-phase a.c. systems, one therefore arrives at the triangular design of the cam disk 22 also on this basis. It goes, however, without saying that in the case of other multi-phase systems and quite generally in the case of other driving systems, for example if one wishes to know the absolute position of the rotor, other distribution patterns of the inductive transmitter and receiver heads are of course also possible.

The modulated output voltages of the receiver heads 24 according to FIG. 5b may be supplied, for further electronic processing, for example to threshold circuits which respond to certain positions of the envelope shape and initiate corresponding switching or control operations, or the like. It is also possible to provide phase-comparator circuits which then detect and evaluate the phase position of the envelopes relative to each other.

Another preferred additional effect of the cam disc 22 consists in the fact that the stray fields emanating from the magnet rotor 11 due to its own magnetization might under certain circumstances influence the transmitter/receiver heads of the position-sensing system, and that the aperture disc or cam disc 22 acts in this respect in the manner represented at 25 in FIG. 1; Due to its magnetically permeable structure, the cam disc will short-circuit these fields so that these axially extending magnetic lines are prevented from influencing the position-sensing system and, besides, also from influencing the flat coils of the tachogenerator in an undesirable way.

There is still another aspect that deserves attention. Apart from the fact that the special arrangement of the inductive transmitter and receiver heads also permits to determine the sense of rotation, namely by detecting and comparing the modulated output voltages supplied by the transmitter heads, and their relative phase positions, the shape of the envelope obtained also enables a so-called auxiliary commutation signal to be derived for the driving machine. This is due to the fact that for achieving optimum efficiency, the electric machines cannot or should not be commutated with a purely sinuscidal curve, i.e. controlled with correspondingly sinusoidal currents, because the magnets and fields involved are distorted. For, if one proceeds from a corresponding, geometric distortion or variation in shape of the cam disk 22 which finally effects the modulation of the envelope and is, therefore insofar changed it is also possible, without the initially purely sinusoidal modulation, to produce other shapes, too, which then are the image of the desired or necessary commutation characteristic for the driving electric motor. Such an auxiliary commutation characteristic derived from the changed envelope signal may then also serve as regulating signal, i.e. as reference value for the current regulator of the driving electric machine. The regulator for the current supply of the driving motor then makes use of the envelope signal changed in this manner at least as a correction value for the currents to be supplied to the machine. Due to the use of inductive transmitters and receivers for determining the position, in conjunction with a cam disk, the invention therefore provides possibilities which go far beyond the mere derivation of a position-indicating signal.

Conveniently, the number of poles of the tachogenerator should be adapted to, i.e. conform with, the number of poles of the driving electric machine. Accordingly, a six-pole driving machine would require a tachogenerator of the type described, while an eight-pole or twelve-pole electric machine should conveniently be designed as eight-pole or twelve-pole tachogenerator, already for enabling the position sensing operation to be carried out correctly. If a three-phase system is obtained in the case of the driving electric machine, then a minimum number of three inductive transmitter/receiver heads will be required for position sensing, while in the case of an eight-pole machine four transmitter/receiver heads will be required, etc.

All features mentioned or shown in the above description, the following claims and the drawing may be essential to the invention either alone or in any combination thereof.