Torque transducer

A torque transducer for measuring the torque in the outgoing crankshaft of automobile engines comprises two two-pole magnetic cores, arranged perpendicular to each other, with coils around the poles. The coils of one core are adapted to be supplied with alternating current for generation of an alternating field in the surface of the shaft. The second core with its coils is adapted to sense the changes in the alternating field which are caused by the torque of the engine. The poles of the primary core are located opposite to diametrically opposed points on the crankshaft.

BACKGROUND OF THE INVENTION 
The present invention relates to a torque transducer, preferably intended 
to be used when measuring the torque in the outgoing crankshaft of 
automobile engines. 
In the automobile industry there is a need of a torque transducer which 
shall be capable of being placed between the engine housing and the 
flywheel and which more or less surrounds the crankshaft. Since the space 
in the longitudinal direction is very limited here, there is no room for 
an annular torque transducer, for example of the type which is shown and 
described in U.S. Pat. No. 3,011,340. In addition, the power requirement 
and the cost of such a multi-polar transducer are far too high. A 
cruciform transducer according to U.S. Pat. No. 2,912,642 would be more 
favourable considering the power requirement and the cost, but using a 
normal, concentrated mode of construction such a transducer will surround 
only a small part of the circumference of the shaft and the shaft must 
therefore rotate practically a full turn before the internal stress 
configuration prevailing in the shaft surface has been scanned over the 
whole circumference, so that a measurement value independent of the 
internal stresses can be formed. Since the automobile industry desires a 
response time of at the most 50 ms, the lowest engine speed for correct 
measurement -- if the necessary filter time constant could be made 
negligible by using a high supply frequency -- would be 60 .multidot. 
1000/50 = 1200 revolutions per minute, which is not acceptable. It must be 
possible to determine the torque at considerably lower engine speeds. 
SUMMARY OF THE INVENTION 
With a transducer according to the present invention, two diametrically 
opposed scanning points are obtained on the circumference of the engine 
shaft. Thus, the lowest speed for correct measurement is reduced to 600 
revolutions per minute, which is below the range of revolutions in 
question and is therefore acceptable. According to the invention, the 
shaft circumference is scanned twice per revolution by constructing the 
primary core so as to encompass half the shaft and with two salient poles 
provided with windings, which poles are located diametrically opposite to 
each other. On the other hand, only one secondary core is required, which 
is suitably attached to the primary core symmetrically between the primary 
poles by way of non-magnetic spacers to separate the fluxes in the two 
cores. The secondary core can be made of solid, magnetic material since 
the secondary flux is so small that the surface layer determined by the 
penetration depth of the flux is quite sufficient. For reasons of space in 
the axial extension of the transducer the distance between the secondary 
poles must be made only about one-sixth of the optimum distance, which is 
normally equal to the distance between the primary poles along the shaft 
surface. The reduction in sensitivity thus obtained is, however, only to 
about one-third, which results in quite satisfactory level of the 
measurement signal. The fact that it is possible to obtain two complete 
scannings of the torsional stress in the circumference of the shaft for 
each revolution using only one secondary core may seem like a 
contradiction. The explanation thereof will be given later on in 
connection with the detailed description of the transducer.

DETAILED DESCRIPTION OF THE INVENTION 
The transducer shown in FIGS. 1 and 2 is mounted on a base plate 1. The 
primary core 2 of the transducer is attached to the plate by means of 
screws 3. The core has two primary poles 4, each supporting a winding 5 
for generation of a magnetic flux in the surface of the engine shaft. The 
position of the engine shaft, when the transducer is mounted, is marked by 
the circle 6 in FIG. 1. The primary core supports the secondary core 7, 
which is fastened right between the primary poles by means of spacers 8 of 
nonmagnetic material and rivets 9. The secondary core supports the two 
secondary windings 10. The two cores with windings are covered by a casing 
11 which is attached to the base plate 1. Both the casing 11 and the plate 
1 have a slot 12 open downwardly which, at the top, terminates in an 
arc-shaped edge 13, the radius of which is somewhat larger than the radius 
of the engine shaft. The slot makes it possible to place the transducer in 
position around the engine shaft. The turned cavity 14 of the base plate 1 
is intended to fit over a corresponding flange on the engine housing 
around the outgoing crankshaft. This makes it possible to ensure a well 
centered mounting of the pole surfaces of the transducer in relation to 
the shaft. However, this presupposes that the outside of the flange is 
concentric with the shaft. From FIG. 2 it is also clear that the pole 
distance of the secondary core is chosen so that the distance between the 
outer edges of the two secondary coils 10 is substantially equal to the 
extension of the coils of the primary core in the longitudinal direction 
of the engine shaft. 
FIG. 3 shows the field configuration for a stress-free condition in the 
evolved shaft surface with projections P.sub.1 and P.sub.2 of the primary 
poles 4 and projections S.sub.1 and S.sub.2 of the secondary poles 7. FIG. 
4 shows the field configuration of a torque-loaded shaft, which is 
indicated by the principal stresses .+-..sigma.. It should be noted in 
this connection that the filed configurations show the H field and the 
magnetic equipotential lines perpendicular to the H lines. The projections 
of the secondary poles are drawn with a considerably smaller distance than 
the distance between the projections of the primary poles, since the 
secondary core in the object of the invention must be made short because 
of the fact that the available distance in the axial direction of the 
transducer is limited. This means that the transducer has less sensitivity 
than in the normal embodiment when the same pole distance is used or both 
the primary and the secondary circuits. In the stress-free state according 
to FIG. 3, the field configuration becomes completely symmetrical and the 
two secondary poles S.sub.1 and S.sub.2 are positioned right opposite to 
the same equipotential line, which results in the secondary flux and the 
secondary voltage becoming zero. 
When a torque is applied, such as shown in FIG. 4, the field configuration 
is distorted and this distortion is almost entirely caused by the changes 
in the potential falls in the vicinity of the four points a, b, c, d where 
the predominant part of the potential fall in the shaft surface is 
concentrated and where the field strength at the same time is 
substantially parallel to either of the principal stresses .+-..sigma.. 
The distortion in the centre of the field, where the magnetic potential 
difference between the secondary poles is sensed, is thus a resulting 
phenomenon which is caused by the shear stresses adjacent to the primary 
poles. Therefore, with two diametrically located primary poles the whole 
shaft circumference can be scanned during half a revolution and the 
average value of the internal stresses of the shaft be formed during the 
corresponding time. 
In both FIGS. 3 and 4 dashed rings S.sub.o mark the positions for the 
projections of the secondary poles in the field configuration in the 
normal case, when the same pole distance is used for both primary and 
secondary cores. FIG. 3 shows that in the unloaded state the projections 
of the secondary poles lie on the equipotential line .chi. = 0. In the 
loaded state according to FIG. 4, this line is distorted so that points 
S.sub.o are located at a certain distance from the line, which distance is 
determining for the secondary signal. FIG. 4 also shows that although the 
secondary poles are moved considerably closer to one another, they are, 
however, located at a distance from line .chi. = 0, which should be 
sufficient for generation of a secondary voltage which is sufficient for 
measuring the applied torque.