A magnetoelastic torque transducer comprises a shaft, three stationary coils connected in series to magnetize the shaft and three coils to sense moment transmitted in the shaft, and a magnetic casing surrounding the coils. The shaft has three parallel annular zones provided with such anisotropy that the magnetic field in the shaft is deflected by an angle .alpha. relative to a generatrix to the shaft in the outer zones and is deflected by an angle .alpha. to a generatrix to the shaft in the middle zone, the axial extension of both outer zones being substantially half that of the intermediate zone. The angle .alpha. should be at least 45.degree. in order to obtain good sensitivity in the transducer. The excitation coils are arranged concentrically to the shaft, each being located outside an annular zone and the outer excitation coils being wound with substantially half as many turns as the central excitation coil. Concentric with each excitation coil is a measuring coil, the outer measuring coils being wound with substantially half as many turns as the central measuring coil and this being arranged to be inverse-connected to the outer measuring coils.

TECHNICAL FIELD 
The present invention relates to a transducer enabling contactless 
measurement of torsional moment on a rotating or stationary shaft. The 
transducer is of magnetoelastic type and designed as a symmetrically 
rotating body. The transducer is particularly useful in applications where 
the measuring zones on the shaft have a temperature gradient, i.e. when a 
thermal flux passes through the shaft. 
BACKGROUND ART 
Magnetoelastic torque transducers rotating symmetrically and similar in 
design to the invention are known through a number of patent 
specifications and articles, some of which will be commented upon below. 
The common measuring principle and common feature of these known solutions 
is that zones have been created in the magnetoelastic material with 
anistropy or magnetic orientation substantially coinciding with the 
compressive or tensile stresses appearing in a shaft, at .+-.45.degree. in 
relation to a generatrix to the shaft, when this is caused by the load to 
twist. In this way the reluctance in a zone whose direction of 
magnetization coincides with the tensile stress will be reduced and the 
reluctance in zones coinciding with the compressive stress will increase, 
thus causing a corresponding increase or decrease in the flow through the 
zones. This is applicable in the case of positive magnetostriction. 
By measuring the difference in reluctance between two zones in which the 
anisotropy in one zone is directed along the direction of pull and that in 
the other zone along the direction of pressure, a measurement of the 
torque is obtained which has little sensitivity to axial forces of 
flexural stresses. 
The difference in reluctance between the zones is usually measured by 
creating a time-dependent H-field via a primary coil concentric to the 
shaft, directed along the shaft and having equal amplitude in both zones. 
The difference in B-field between the zones is then measured using two 
identical secondary coils, one above each zone. The easiest way to do this 
is by inverseconnecting the secondary coils in such a way that the 
stresses induced in each coil are subtracted from each other. 
Phasesensitive rectification of the secondary signal obtained in this 
manner also enables torques of different direction to be distinguished. 
The difference between the torque transducers mentioned below, besides the 
choice of magnetostrictive material, lies in the different methods of 
creating the required anisotropy. 
According to Russian Pat. No. SU 667836 the anistropy is created by cutting 
slits in the surface of the shaft. 
Russian Pat. No. SU 838448 describes a transducer in which an attempt is 
made to increase sensitivity instead by producing the slits by 
roll-embossing this pattern in the surface. 
American U.S. Pat. No. 4,506,554 covers a transducer in which the 
anisotropy is achieved by using a sleeve with slit cut in the main stress 
directions. 
A very similar design is also described in IEEE Trans Magn Vol Mag-22 No. 5 
p. 403 (Sep. 1986) in an article entitled "Torque sensors using wire 
explosion magnetostrictive alloy layers" by J. Yamasaki, K. Mohri et al. 
Here a 100 micrometer thick "sleeve" has been produced, with 
through-"slits" on a stainless steel shaft by spraying the shaft with 
drops of a molten magnetoelastic alloy through a mask. The technique used 
consists of allowing a strong electrical discharge to pass through a 
conductor in the relevant material, whereupon the core is vaporized and 
the conductor explodes. 
An article by K. Harada, I. Sasada et al in IEEE Trans Magn Vol Mag 18, No. 
6, p. 1767 entitled "A new torque transducer using stress sensitive 
amorphous ribbons" describes a solution comprising glueing a 
stress-relieved foil of amorphous magnetostrictive material onto a shaft 
which has been prestressed with a certain torque. When the glue has dried, 
the prestressing will disappear and produce an anisotropy since the foil 
will now be prestressed. 
In "Torque transducers with stress-sensitive amorphous ribbons of 
chevron-pattern", published in IEEE Trans Magn Vol Mag-20, No. 5, p. 951, 
I.Sasada, A.Hiroike, and K.Harada describe how anisotropy can instead be 
created by glueing strips of amorphous magnetoelastic material onto a 
magnetic or non-magnetic transducer shaft in its principle stress 
directions. 
Offenlegungsschrift DE 3704049 A1 mentions a method of creating anisotropy 
by placing a conductor pattern on a shaft of magnetostrictive material. 
This conductor pattern has the same shape as the slits mentioned earlier. 
Torque transducers of this type generally offer good measuring qualities 
for the majority of applications. However, in a few special applications 
problems of measuring accuracy may occur. If a thermal flux passes through 
the shaft, i.e. if there is a temperature gradient in the shaft, this will 
affect the measurement. It is known from the literature that the 
permeability of magnetic material is extremely temperature-dependent. If, 
therefore, a temperature difference exists across the measuring zones of 
the shaft, the measured signal will not correctly indicate the torque, due 
to the varying permeability within the measured zone. Applications in 
which these problems may occur can be found in many areas. Temperature 
differences of several hundred degrees may occur, for instance, between a 
combustion engine and its gearbox or coupling. A thermal flux will 
therefore pass through the shaft connecting these parts and the measuring 
accuracy will thus be affected when measuring torque in the shaft. An 
electric motor may be located in a well heated machine room and, due to 
high load, may reach a maximum temperature permitted for the motor. The 
machine being driven may be located outdoors and connected to the motor by 
a shaft passing through a wall and there is every likelihood of the shaft 
acquiring a temperature gradient which may cause problems in the measuring 
accuracy. 
As far as we know, no information exists in the available literature, 
concerning the temperature-dependence of the permeability for the 
materials used in connection with magnetoelastic torque transducers of 
this type, i.e. both annealed and cold-rolled silicon steel. Some idea of 
expected magnitudes can, however, be deduced from a book published in 1951 
by D van Nostrand Company Inc, Princetown, N.J., "Ferromagnetism" by 
Bozorth. This includes measurements applicable to pure iron which has been 
stress-relieved to 800.degree. C. 
From this book, appendix 1, FIG. 3-8, it can be seen that both the initial 
permeability .mu..sub.0 and the maximum permeability .mu..sub.m increase 
as the temperature increases. It can be seen from the figures that 
.mu..sub.m increases by a factor of two at 200.degree. C., i.e. that 
EQU .mu.=.mu..sub.T=0 0.degree. C. (1+T/200) 
or, in other words, that .mu. increases by 0.5%/.degree.C. 
The magnetostrictive permeability alteration due to the tensile stress in a 
stress-relieved strip of the material used in the type of torque 
transducer under consideration has been measured and is at least 1%/MPa. 
For a moderately loaded transducer where the material is loaded to 20 MPa, 
therefore, maximal loading would give the same permeability alteration as 
a temperature alteration of 40.degree. C. This would thus give a neutral 
drift of 1/40=2.5% of max. signal per .degree.C. temperature difference 
between the measured zones. 
The above estimate is extremely rough and assumes, for instance, that the 
magnetostrictive permeability alteration for compressive stresses is 
substantially the same as that measured for tensile stresses. However, the 
estimated value should be representative for the maximum neutral drift 
caused by temperature gradients. This also shows that in applications 
where temperature gradients are likely in the shaft measuring zones, and 
where measuring must be rather accurate, measures must be taken to 
eliminate or greatly reduce the effect of the heat flux passing through 
the shaft. 
The invention to be described now shows a device which reduces the 
sensitivity to temperature gradients in magnetoelastic torque transducers 
in an extremely reliable manner.

DISCLOSURE OF THE INVENTION 
With the aid of FIGS. 1A-1D, we shall start by showing how a temperature 
gradient influences the zero signal, i.e. the output signal of the 
transducer when the shaft is not transmitting any torque. If the shaft is 
loaded, of course, the zero signal caused by a temperature gradient will 
be added to the relevant torque signal. 
In an unloaded shaft according to FIG. 1B is subjected to a temperature 
gradient according to FIG. 1A, due to the different permabilities 
prevailing in each measuring coil, the measuring coils will emit a .DELTA. 
zero signal in accordance with FIG. 1C, resulting in a .SIGMA.-zero signal 
in accordance with FIG. 1D. As is evident from FIG. 1B, the excitation 
coils of the transducer have been omitted. 
A torque transducer according to the invention is shown in FIG. 2B. The 
shaft here has three parallel annular zones provided with such anisotropy 
that the magnetic field in the shaft is deflected by an angle .alpha. 
relative to a generatrix to the shaft in the outer zones (3,5) and is 
deflected by an angle .alpha. to a generatrix to the shaft in the middle 
zone (4). The axial extension of each of the two outer zones is 
substantially half that of the intermediate zone. The angle .alpha. should 
be at least 45.degree. in order to obtain good sensitivity in the 
transducer. 
The excitation coils, not shown in FIG. 2B, are connected in series in the 
normal way. 
The measuring coils for the two outer zones are provided with an equal 
number of turns N and the coil for the central zone has a number of turns 
substantially corresponding to the sum of the two outer coils, i.e. 2N. As 
can be seen in FIG. 2B, the outer coils are series-connected and the 
measuring coil of the central zone is inverse-connected in relation to the 
two others. 
As revealed in FIGS. 2C and 2D, the resultant zero signal from a 
temperature gradient will be practically zero. A torque transducer of the 
type described can therefore by used for applications when a thermal 
current passes through the shaft. In the case of a stationary thermal 
flux, i.e. a linear temperature gradient, the influence of the thermal 
flux on the permeability, and thus on the measured signal, can thus be 
fully compensated and sensitivity to other more complicated gradients is 
greatly reduced. 
DESCRIPTION OF PREFERRED EMBODIMENTS 
FIG. 3 shows an embodiment of a magnetoelastic torque transducer according 
to the invention. The anisotropy is achieved here in the same way as in 
the transducer described in U.S. Pat. No. 4,506,554. 
A measuring sleeve 2 is secured about a shaft 1 in such a manner that it is 
unable to twist. The sleeve is provided with three annular zones 3, 4, 5, 
the axial length of zones 3 and 5 being equal and slits being arranged in 
the same direction in relation to a generatrix to the sleeve and inclined 
substantially 45.degree. to the generatrix. The axial length of measuring 
zone 4 is twice that of the other two and this zone is provided with slits 
having the same pitch but directed at 90.degree. to the slits in zones 3 
and 5. Each of the measuring zones 3, 4, 5 is surrounded by concentric 
bobbins 6, 7 and 8, stationary with the shaft, and by coils 9, 10 and 11, 
12; 13, 4, respectively. Coils 10, 12 and 14 are used for excitation with 
alternating current with the coils connected in series and with the coils 
9, 11 and 13 which are used to sense a torque applied on the shaft. Coil 
11 is inverse-connected to the two outer coils. The electrical connection 
can be seen in FIG. 4. An iron core 15 in the form of a rotary body with 
U-shaped generating surface surrounds the coils, the air-gap towards the 
shaft at the two annular outer parts 16, 17 of the iron core. 
The spaces 18 and 19 may be filled by circular discs of magnetic material, 
giving the excitation flux somewhat better distribution than if these 
spaces are filled with discs of insulating material. In this version the 
side walls of the bobbins may be made so wide that the bobbins can be 
fitted close together, or all coils can be wound onto one bobbin with 
turned openings corresponding to the three separate bobbins shown in the 
figure. In such case it is advisable for the excitation coil to be wound 
evenly over all the measuring zones. 
FIG. 4 shows that coils 10, 12 and 14 are supplied in series from an 
alternating current source 20, and the coils 9, 11 and 13 are connected so 
that coil 11 is inverse-connected to the other two and that the total 
signal is rectified in the controlled rectifier 21 and presented on an 
instrument 22. 
According to FIG. 4, the primary flow is directed in one and the same 
direction because the excitation coils are connected in seris. It is of 
course quite possible for the magnetic field to be directed in different 
directions in each half of the transducer. This method of connection can 
be seen in FIG. 5, i.e. with the excitation coils 10, 12a, 12b and 14 
wound and connected to obtain different magnetization directions. The 
central measuring coil must then of course be divided in order to obtain 
the desired inverse-connection. The two central coils are wound with half 
as many turns as the central coils described earlier. 
Since there will always be a certain leakage flux at the ends of such a 
construction, the flux at the central measuring zone will be slightly 
stronger than at the two outer zones if the central excitation coil has 
twice as many turns as the outer coils. To ensure minimum influence from 
the temperature gradient, therefore, the number of turns of either the 
central excitation coil or the central measuring coil may be made slight 
less than twice those of the two outer coils.