Rotary transducer

A rotary variable differential transducer has first and second coil portions (50a and 50b) of a primary coil overwound by respective secondary coils (52 and 54). Pole pieces (42 and 44) are asymmetrically arranged about the axis of rotor (16) and are inductively associated with the first and second coil portions (50a and 50b), respectively. The rotor (16) has first and second formations which are respectively defined by front and rear arcuate portions (70b and 70c) associated with the pole pieces (42 and 44), respectively. The pole pieces (42 and 44) are mutually angularly displaced about the axis of the rotor (16). The first and second coil portions (50a and 50b) are mutually axially displaced relative to the rotor (16) and an intermediate common pole piece (32b) is disposed between them. An intermediate formation defined by an annular peripheral portion (70a) on the rotor (16) lies adjacent the common pole piece (32b), an output which is indicative of the angular position of the rotor (16) is taken from the second coils (52 and 54).

BACKGROUND OF THE INVENTION 
This invention relates to a rotary transducer and is more particularly, but 
not exclusively, concerned with a rotary variable transducer, especially a 
rotary variable differential transducer. 
It has been previously proposed to provide a rotary variable differential 
transducer having a body in which a primary and two secondary coils are 
wound on respective portions of a stator, such portions being 
equally-angularly spaced about the axis of rotation of a rotor mounted on 
a shaft in a plane perpendicular to the shaft. Each such stator portion 
extends over the same length of the rotor and carries an arcuate pole 
piece which is disposed close to the peripheral surface of the rotor. The 
rotor has an asymmetric pole piece which is arranged relative to the 
stator pole pieces associated with the primary coil and the secondary 
coils such that, upon angular movement of the shaft about its axis of 
rotation, the inductive coupling between the primary coil and one of the 
secondary coils increases whilst there is a corresponding decrease in the 
inductive coupling between the primary coil and the other of the secondary 
coils. The electrical output signals from the secondary coils can then be 
compared to give an indication of the angular position of the shaft 
relative to the body. Such a construction of transducer is relatively 
complicated to manufacture and can be difficult to assemble. Additionally, 
it does not have a particularly compact construction and the effective 
angular range is limited to .+-.60.degree. max approx. The linearity of 
signal output is dependent on the eddy current effects in the pole pieces. 
Use of solid magnetic pole pieces is desirable for constructional 
simplicity but eddy currents cannot be ignored in these. While eddy 
currents can be minimised by constructing the poles from thin laminations, 
their use makes assembly difficult and mechanical accuracy poor. 
GB-A-2231161 discloses a rotary position transducer having a fixed primary 
coil which is substantially coaxial with a rotor shaft, and two fixed 
secondary coils symmetrically disposed on opposite sides of the primary 
coil axis with their common axis at right angles to that of the primary 
coil. The rotor shaft carries an asymmetrical rotor which provides 
inductive coupling between the primary coil and each of the secondary 
coils. The asymmetry causes the combined output of the secondary coils to 
vary, possibly linearly, with the angular position of the rotor. With such 
a construction it is also relatively difficult to achieve consistent 
results in manufacture and signal output is low due to poor magnetic 
coupling through the relatively large air gap between the rotor and the 
stator pole pieces. The linear range is also limited to about 
.+-.45.degree. to .+-.60.degree.. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide an improved rotary 
transducer which can be capable of operating over a large angular range of 
up to .+-.90.degree. and which can be of compact construction, easy to 
manufacture and efficient in operation. 
According to the present invention, there is provided a rotary transducer 
comprising a first electrical coil, an electrical output, and an inductive 
coupling which inductively couples said electrical output with the first 
electrical coil and which includes a rotor rotatable about an axis 
relative to said first electrical coil so that, in use, the electrical 
output varies in dependence upon the angular position of said rotor about 
said axis, wherein the first electrical coil comprises first and second 
coil regions which are mutually displaced axially of the rotor, and 
wherein the rotor has first and second formations thereon which are 
mutually axially displaced and which are respectively inductively 
associated with the first and second coil regions of the first electrical 
coil. 
In the rotary transducer according to the present invention, the 
arrangement is such that, in use, as the rotor is rotated, the strengths 
of the magnetic fluxes in magnetic circuits associated with the first and 
second coil regions of the first electrical coil vary so as to enable a 
signal to be obtained at the output which is indicative of the rotary 
position of the rotor. 
Preferably, the rotary transducer includes first and second stator pole 
pieces inductively associated respectively with the first and second coil 
regions of the first electrical coil, each pole piece being asymmetrically 
arranged about the axis of the rotor, and at least one of the first pole 
piece and the first formation on the rotor being angularly displaced about 
the axis of the rotor relative to at least one of the second pole piece 
and the second formation on the rotor. 
The rotary transducer preferably further includes a common stator pole 
piece which is disposed between the first and second coil regions and 
which is inductively associated with both of said coil regions. The rotor 
preferably has an intermediate formation thereon which is disposed between 
the first and second formations and which lies adjacent the common stator 
pole piece. With such an arrangement, the intermediate formation on the 
rotor may have a continuous outer peripheral surface which lies adjacent a 
continuous internal peripheral surface of the common stator pole piece, or 
the intermediate formation on the rotor may have a discontinuous outer 
peripheral surface which lies adjacent a discontinuous inner peripheral 
surface of the common stator pole piece. 
The present invention is applicable to rotary transducers of the type 
wherein the first electrical coil is a primary coil and the electrical 
output is taken from one or more secondary coils which is/are variably 
inductively coupled to the first electrical coil via the rotor. The 
present invention is also applicable to rotary transducers of the type 
where the electrical output is obtained across one of said first and 
second coil regions, or by comparing the electrical outputs across the 
respective first and second coil regions. With such an arrangement, there 
are no secondary coils. 
In a preferred embodiment, two secondary coils are provided which are 
respectively inductively associated with the first and second coil regions 
of the first electrical coil. 
Whilst in the above description, the first electrical coil has been 
described as comprising first and second coil regions, it is to be 
appreciated that, in practice, such first and second coil regions may be 
comprised of individually wound coils connected together in series so as 
to define, in effect, two or more portions of the same coil. 
The two portions of said coil may be connected to produce the magnetic flux 
in the 2 halves of the rotor either to be in the same direction or to be 
in opposite directions. 
The benefits gained in this design are: 
1. Angular range up to .+-.90.degree.. 
2. Capable of achieving .+-.0.1% linearity over a range of .+-.80.degree.. 
3. Winding of the coils on to bobbins requires no special winding technique 
or special tools. 
4. Enables the use of low cost, flat laminated pole pieces to minimise eddy 
current distortion of the magnetic flux density in the air gaps. 
5. Easy to produce pole faces concentric with the shaft axis and thus 
achieve small air gaps with consequent high magnetic efficiency.

DETAILED DESCRIPTION OF THE DRAWINGS 
Referring now to FIGS. 1 to 3 of the drawings, the transducer comprises a 
generally cylindrical body 10 including front and rear end caps 12 and 14, 
and a shaft 16 mounted coaxially within the body 10. The shaft 16 is 
mounted in front and rear bearings 18 and 20 for rotation about its 
longitudinal axis relative to the body 10. The bearings 18 and 20 are 
respectively carried by front and rear bearing housings 22 and 24 forming 
part of the body 10. The shaft 16 projects externally of the body 10 
through end cap 12 and is sealed relative thereto by means of an O-ring 
seal 26. Spring clips 28 and a thrust washer 30 locate the shaft 16 
relative to the body 10 and load the bearings 18 and 20. Alternatively, 
the right hand spring clip 28 (as viewed in FIG. 1) may be replaced by a 
screw (not shown) which extends axially into the end of the shaft 16 and 
which has a head engaging the respective bearing 20. Such screw provides a 
convenient way of pre-loading the bearings 18 and 20. 
The body 10 further includes a main stator member 32 consisting of an outer 
cylindrical sleeve 32a with a central, inwardly extending annular flange 
32b. Opposite ends of the sleeve 32a are spun-over to engage with the end 
caps 12 and 14, respectively, in order to hold the assembly together. 
O-ring seals 34 and 36 serve respectively to seal the joints between the 
wall 32a and the end caps 12 and 14. The inner surface of the wall 32a is 
stepped on each side of the flange 32b so as to define annular abutment 
shoulders 38 and 40. Front and rear stator pole pieces 42 and 44 are 
trapped between the respective abutment shoulders 38 and 40 and the 
respective front and rear bearing housing's 22 and 24. 
Front and rear coil bobbins 46 and 48 are respectively held in position by 
the pole pieces 42 and 44 against opposite sides of the central annular 
flange 32b of the main stator member 32. 
The front coil bobbin 46 contains a first coil portion 50a of a first 
electrical coil which is defined by a primary coil. The first coil portion 
50a is overwound by a first secondary coil 52. The rear coil bobbin 48 
contains a second coil portion 50b of the primary coil overwound by a 
second secondary coil 54. The axes of coil portions 50a and 50b and of the 
secondary coils 52 and 54 are coincident with the axis of rotation of the 
shaft 16. The main stator body 32 has its longitudinal axis coincident 
with that of the shaft 16 also. 
The front and rear stator pole pieces 42 and 44 are both identical but are 
asymmetrical about the axis of shaft 16. The shape of the rear stator pole 
piece 44 is illustrated in FIG. 2. The front stator pole piece 42 has a 
similar shape but is mounted in the housing 10 in an orientation which is 
180.degree. displaced relative to the rear stator pole piece 44. As can be 
seen from FIG. 2, the pole piece 44 has a solid region 56 which subtends 
an angle of 180.degree. at the axis, and a hollow region 58 of similar 
shape. The front stator pole piece 42 has corresponding solid and hollow 
regions 60 and 62, respectively (see FIG. 1). Arcuate abutment lugs 64 and 
66 on the respective bobbins 46 and 48 engage in the respective hollow 
regions 62 and 58 of the pole pieces 42 and 44 to ensure that these parts 
are assembled in the correct mutual dispositions. 
The rotary transducer further comprises a rotor 70 which is mounted on the 
shaft 16 and fixed for rotation therewith. The rotor 70 includes an 
intermediate formation defined by a centrally disposed annular peripheral 
portion 70a whose surface lies closely adjacent the inner periphery of the 
annular flange 32b of the main stator member 32. The rotor 70 further 
includes first and second formations which are respectively defined by 
front and rear arcuate portions 70b and 70c which are disposed at opposite 
axial ends of the rotor 70 in spaced relationship to the central annular 
peripheral portion 70a and which are disposed opposite the respective 
front and rear stator pole pieces 42 and 44. Each arcuate region 70b, 70c 
subtends an arc of 180.degree. about the longitudinal axis of shaft 16. 
The arcuate members 70b and 70c of the rotor 70 are mutually aligned in 
the longitudinal direction of extent of the rotor 70, as will be apparent 
from a consideration of FIGS. 1 and 3. For convenience, FIG. 2 only shows 
portion 70c of the rotor 70, such portion 70c being displaced by 
90.degree. as compared with its orientation as depicted in FIG. 1. 
The rotor 70 further includes an intermediate sleeve portion 70d by means 
of which the portions 70a and 70c are mounted on the shaft 16. Portions 
70a to 70d are of unitary constructions and, like the stator, are formed 
of a suitable ferromagnetic material. The coil bobbins 46 and 48 are 
formed of a suitable insulating material such as a synthetic polymer. 
In use, the first and second coil portions 50a and 50b are connected in 
series circuit with a suitable AC power supply. Output voltages are 
induced in the secondary coils 52 and 54, the magnitude of which being 
dependent upon the strength of the magnetic flux in the magnetic circuit 
which inductively couples each secondary coil 52, 54 with its respective 
first or second primary coil portion 50a, 50b. The magnetic circuit 
affecting secondary coil 52 is primarily that which exists in the 
surrounding circuit defined by portions 70a, 70d and 70b of the rotor 70, 
the front stator pole piece 42, the adjacent region of sleeve 32a, and the 
central flange 32b of the main stator member 32. In contrast, the magnetic 
circuit which mainly affects the secondary coil 54 is that which exists in 
portions 70a, 70d and 70c of the rotor 70, the rear stator pole piece 44, 
the adjacent portion of the wall 32a and the central flange 32b of the 
main stator member 32. The central flange 32b defines a central pole piece 
which is common to both of these magnetic circuits. The strength of the 
magnetic flux in each of such circuits depends to a very great extent upon 
the magnetic reluctance of any air gaps in such circuit. With the rotor 70 
in the position illustrated in FIG. 1, it will be seen that the arcuate 
portion 70b of the rotor 70 lies very closely adjacent the solid region 60 
of the front stator pole piece 42. Since the portion 70a is always very 
closely adjacent the central annular flange 32b of the rotor 70, it 
follows that there are only minimal air gaps in the magnetic circuit which 
primarily affects the secondary coil 52 and the magnetic flux will be 
high. As a result, the voltage which is induced in the secondary coil 52 
is relatively high. In contrast, as will be seen from FIG. 1, the arcuate 
portion 70c of the rotor 70 lies opposite the hollow region 66 in the rear 
stator pole piece 44. The result of this is that there is a relatively 
large air gap in the magnetic circuit which primarily affects the 
secondary coil 54. As a result of this, the magnetic flux coupling with 
secondary coil, 54, and the voltage which is induced in it is relatively 
lower than that in secondary coil 52. It will also be appreciated that, as 
the shaft 16 is rotated from the position illustrated in FIG. 1, there is 
a progressive decrease in the voltage induced in secondary coil 52 and a 
corresponding progressive increase in that which is induced in the 
secondary coil 54. In the position illustrated in FIG. 2, the air gaps in 
the magnetic circuits associated with the secondary coils 52 and 54 
respectively are the same and this corresponds to a "null" position of the 
transducer. It will therefore be appreciated that monitoring the 
difference in the signals from the secondary coils 52 and 54 enables 
accurate monitoring of the rotational position of the shaft 16 relative to 
the body 10. 
Referring now to FIG. 4, there is illustrated an alternative embodiment 
which measures only 12.7 mm diameter.times.24 mm axial length and which 
also incorporates screening from the effects of external magnetic fields. 
Parts which are similar to the embodiment of FIGS. 1 to 3 are accorded the 
same reference numerals. In this embodiment, end cap 12 is formed of a 
ferromagnetic material and is extended axially so as to define an outer 
sleeve 12a in whose otherwise open end the ferromagnetic end cap 14 is 
sealingly secured. The sleeve 32a is disposed internally of the sleeve 12a 
with a small (&gt;0.1 mm) concentric gap (not shown) between the two achieved 
through spacers (not shown). This gap acts to separate sleeves 32a and 12a 
magnetically and thereby achieves magnetic shielding/screening of the 
inner magnetic circuits from external magnetic influences. In FIG. 4, the 
section chosen is such that an aperture 32c for connecting leads is 
illustrated as passing through flange 32b. A similar aperture passes 
through flange 32b in the embodiment of FIG. 1, but it cannot be seen 
because of the section chosen. In FIG. 4, coils corresponding to the coils 
50a, 50b, 52 and 54 of FIG. 1 are present but are not illustrated. Leads L 
are connected to such coils but are not fully illustrated in FIG. 4. 
FIG. 5 shows various designs of main stator member 32 and rotor 70 for 
different angular operating ranges with the same full scale output voltage 
variation. 
The rotary variable transducers as described above are simple to assemble 
and are of extremely compact construction. 
The design also has the advantage of mitigating the effect of eddy currents 
on the linearity of output signals from the secondary coils either by 
using pole pieces 42 and 44 made of solid ferrite material or of laminated 
metal construction, e.g. a stack of 30.times.0.05 mm thick silicon iron 
laminations. 
In an alternative embodiment to those described above, the secondary coils 
52 and 54 are wound on respective coil bobbins which are disposed 
laterally adjacent the respective coil bobbins 46 and 48 so as to lie 
respectively to the front and rear of the bobbins 46 and 48. 
In any of the above-described embodiments, the pole pieces 42 and 44 may be 
arranged in the same orientation i.e., with their solid regions 56 
mutually aligned and their hollow regions 58 mutually aligned. With such 
an arrangement, the front and rear arcuate portions 70b and 70c are 
mutually angularly displaced by 180.degree. instead of being mutually 
aligned in the longitudinal direction of extent of the rotor 70. 
The invention is considered to reside in any one or more of the following 
features: 
1. A shaft angle sensor in which 2 or more differential magnetic circuits 
are mutually displaced along the axis of the shaft. 
2. A shaft angle sensor in which the axis of the coil(s) or coil sections 
coincides with the axis of the shaft. 
3. A combination of 1 and 2 above but with axially separated primary coil 
portions only and output signal(s) taken from junction of such coil 
portions. 
4. A shaft angle sensor in which the primary and secondary coils (or coil 
sections) are wound one on top of the other or as a bifilar pair of coils 
on the same former (or formers). 
5. A sensor as in 4 above, in which the magnetic flux coupling any pair of 
portions of the primary and secondary coils is varied by the reluctance of 
the air gap(s) between a rotor and stator portion. 
6. A sensor as in 1, 2, 4 or 5 above in which the differential secondary 
coil portions are connected with voltages in series opposition or in 
series aiding.