FR2764691A1 - Differential torque measuring device - Google Patents

Abstract

The device has a vertical axis which can be deformed along j and k directions by a force and a torque, respectively. The axis cooperates with two ferromagnetic U shaped pieces each carrying a coil. The U shaped pieces are separated by a horizontal ferromagnetic bar thus defining four magnetic circuits. The coils are connected in a bridge configuration and to a differential amplifier

Description

 The invention relates to a device allowing differential measurements of a force and a torque applied to a structure. It allows at a point of application the measurement of a force and a direction torque perpendicular to the direction of the force, that is to say two of the six quantities (3 forces, 3 couples) which characterize the actions at a point. The advantage of these quantities lies in the fact that they are characteristic of the mechanical stresses to which a structure is subjected.

 One of the applications of this invention is its use on an underwater vehicle powered by two thrusters connected together, to know the thrust of the thrusters and the torque between the two, this in order to make a thrust control of the vehicle. This type of control is simpler and more reliable than those practiced conventionally. Some of these vehicles are powered by two thrusters placed on each side. The invention makes it possible to measure simultaneously and at the same point the sum of the actions or thrust forces and the imbalance of the actions of each of the thrusters or torque.

 A certain number of devices already exist, which measure one of the two quantities (torque sensors, force or force sensors). There are also some which measure several quantities (2-axis, 3-axis force sensors).

 Patent FR 2 712 982 proposes a differential torque meter with permanent magnets. The device is formed of two magnetic circuits each excited by a magnet and having a central common branch. Toothed rings placed opposite move relative to each other under the effect of a couple. The reluctance of each circuit is modified, as is the operating point of the magnets. The teeth of the two toothed systems are adjusted so that the reluctances of each circuit vary in opposite directions for the same applied torque. The flux in the central branch is the algebraic sum of the fluxes in the two circuits. The induction measured in the central branch, using a Hall effect probe, is proportional to the applied torque and the processing electronics is simple.

 The device of French patent n ° 95.08620 uses coils instead of magnets and delivers a signal following a synchronous demodulation.

 These two devices allow the measurement of the torque on rotating shafts.

 The device of French patent no. 95.08960, meanwhile, allows the measurement of the torque applied to a fixed structure.

 There are many other devices allowing the measurement of a single torque, the differences of which are based on the technological choices made.

 Sensors using strain gauges are frequently used.

They measure the deformation of a structure under the effect of the applied torque. They are powered by sliding contacts, often replaced by a rotary transformer, as in the article by E. Zabler et al. entitled "a non contact strain-gage torque sensor for automotive servo-driven steering systems" in the journal "Sensors and Actuators" A, 41-42 (1994).

 Capacitive solutions also exist and are mentioned in an article by Reinoud F. Wolffenbuttel "non contact capacitive torque sensor for use on a rotating axel", IEEE Trans. Instr. Meas., Vol 39, N. 6. Dec. 1990. They use the relative displacement of the armatures of a capacitor as a function of the torsion of the shaft linked to the applied torque.

 Many torque measurement solutions have also emerged in recent years and use the magnetostriction properties of materials.

There have long been voluminous solutions for large trees, such as those proposed by O. Dahle in an article "the ring torductor -a torque-gauge, without slip rings, for industrial measurement and control", ASEA
Journal, 1960, Vol 33, No 3; they directly use the magnetostriction of the steel shaft which undergoes the torque. A magnetic circuit consists of a "core" carrying one or more coils, which is closed by the shaft. The reluctance of the circuit varies with changes in the magnetic permeability of the shaft. Later versions of this device consist of modifications to the shape of the external magnetic circuit (William J. Fleming, "magnetostrictive torque sensors
Comparison of branch, cross and solenoidal designs ", SAE technical papers series, 900264) or in its miniaturization (I. Sasada et al.," A new thin pickup coil for magnetic head type torque sensors ", IEEE Trans. Mag. Vol. 29, No. 6, Nov. 1993).

 The solutions presented by I. J. Garshelis in a number of articles (including "a torque transducer utilizing a circularly polarized ring", IEEE Trans.

Mag. Flight. 28, N "5, Sept. 1992 and" a torque transducer utilizing two oppositely polarized rings ", IEEE Trans. Mag., Vol. 30, No. 6, Nov. 1994) use a ring-shaped magnetostrictive material, polarized , and made integral with the tree which deforms in the form of a sleeve. The measurement of the field is made using Hall effect probes placed above the tree and is an image of the applied torque.

 The solutions of K. Harada and I. Sasada ("torque transducers with streesensitive amorphous ribbons of chevron-pattern", IEEE. Trans. Mag., Vol. MAG-20, N ". 5, Sept. 1984) also use the magnetostriction of amorphous materials glued in the form of thin ribbons on the deforming tree. The measurement is made via an exciter coil of the magnetic circuit and a receiver coil, the inductance of which varies according to the permeability of the material, therefore of the torque.

 There are also other devices allowing the measurement of a force or a force. The most conventional solutions are those with strain gauges, well known to those skilled in the art, and are marketed by many manufacturers (Trayvou, Kyowa, Interface, Strainsert, Tefal). The gauges are glued on test bodies of shape and properties adapted to the measurement of the force along the chosen axis.

 Technologies using the magnetostrictive properties of certain materials have also been used to create force measurement devices.

(K. Mohri, E. Sudoh, IEEE Trans. Mag., MAG-17, N "3, 1981).

 There are also devices which measure forces on several axes, such as the device described by D. Diddens et al. for the measurement of forces along three axes in "design of a ring-shaped three-axis micro forceltorque sensor", Sensors and actuators A 46-47 (1995). The technology used is that of strain gauges bonded to a drawn test body in order to deform in an exploitable way under the effect of a force. In fact, four complete bridges of gauges deliver signals which make it possible to calculate the applied forces, by means of the knowledge of the sensitivity matrix of the test body. All the measurement devices on several axes require the knowledge of the matrix of the stresses applied to the test body and the sometimes complicated drawings of parts in order to manage to decouple certain efforts and simplify the processing of the signals obtained.

Among the other technologies used, patent FR 2 680 874 presents a structure associating permanent magnets to create a field in an air gap whose value varies a lot for small variations of this air gap. Thus the force applied to the structure deforms it and modifies the air gap, therefore the value of
The induction measured using a Hall effect probe. This device measures the force along two axes, using several probes placed in the air gap for this purpose.

 Piezoelectric force sensors. use the properties of certain materials, such as quartz, for which a tension appears between two of their faces when they are subjected to a mechanical force. The exploitation of these properties leads to sensors, such as those from the Kistler company measuring forces along one or more axes. However, the very nature of the piezoelectricity property only allows the measurement of alternative or time varying forces, and not the measurement of the continuous component of a force.

 All the devices of the state of the art are devices which measure either the torque alone, or a single force, or several forces. None of the existing devices allows the simultaneous measurement of the two quantities torque and force.

 The object of the invention is to propose a device which is simple from the point of view of electronic design and processing, which measures both a force and a torque in a direction orthogonal to the force at a point of application, and this, even continuously.

 The invention therefore relates to a device for differential measurement of the torsional torque exerted on a shaft which is deformable in rotation along an axis k, and of the bending force applied in a direction j perpendicular to the axis k, characterized in what it includes two "U" shaped parts provided with coils on each arm of the "U", fixed by the bottom of the "U" to the axis, the arms of the "U" facing each other, integral with the axis, and defining with a bar which they frame and which is fixed in its middle to the axis, a magnetic circuit with two upper air gaps (e1, e4) and two lower air gaps (e2, e3), the two pieces in "U" shape and the bar being located in three different planes, offset parallel to each other so that under the action of a force exerted in direction j, the surfaces of the upper air gaps (e1, e4) vary in direction reverse of the surfaces of the lower air gaps (e2, e3), and, under the action of a couple exerted ant around the axis k, the surfaces of an upper air gap and of the lower air gap opposite with respect to the axis k vary in opposite direction from the surfaces of the other two air gaps.

 The device according to the invention has a simple construction and makes it possible to know the state of a mechanical device and the stresses applied to a part.

 The bar may have ends and in this case take the form of an "H".

 In a preferred embodiment of the device according to the invention, the surfaces of the air gaps are horizontal.

 In an alternative embodiment of the device according to the invention, the surfaces of the air gaps are inclined relative to the horizontal direction.

 In another embodiment of the device according to the invention, the surfaces of the air gaps are vertical.

 In another alternative embodiment, the surfaces of the air gaps are inclined relative to the vertical direction.

 Advantageously, for simple electronic processing, the coils are mounted on a magnetic bridge.

 The value of the bending force is thus proportional to the tension between a point located between the first two coils connected in series and a point located between the other two coils connected in series.

 The value of the torque is also proportional to the supply voltage minus the voltage between the point between the first two coils connected in series and the point between the other two coils connected in series.

Other characteristics and advantages of the present invention will appear on reading the following description of preferred embodiments, given by way of illustration and not limitation, and the appended figures in which:
FIG. 1 represents a first embodiment of the device according to the invention with horizontal gap surfaces;
FIG. 2 represents the electronic diagram of the device according to the invention;
FIGS. 3a and 3b represent two embodiments of the device according to the invention seen from the front, with airfoil surfaces inclined relative to the horizontal;
FIG. 4a represents another embodiment of the device according to the invention, seen from the front with vertical airgap surfaces; FIG. 4b represents the same embodiment seen in profile;
FIG. 4c represents another embodiment of the device according to the invention seen partly from the front, with air gap surfaces inclined relative to the vertical;
FIG. 5 represents particular embodiments of the device according to the invention.

 In FIG. 1, the device according to the invention consists of a part 1 which deforms under the combined action of a force in a direction j and a torque in a direction k. It is completed by two parts 2 and 4 in the shape of a "U" fixed by the bottom of the "U" to part 1, the branches or arms of the "U" facing each other.

 These two parts are fitted with coils. They are separated by a bar 3, also fixed in its middle to the part 1. The assembly thus constitutes a magnetic circuit defining four air gaps, two of which are higher and located above the bar 3: el, e4, and two are lower and located below the bar 3: e2, e3. We can also consider it as the assembly of four elementary magnetic circuits each corresponding to an air gap.

 Parts 1 and 3 constitute branches common to elementary circuits: part 1a is common to circuits 1 and 4, part 3a to circuits 1 and 2, part 1b to circuits 2 and 3, part 3b to circuits 3 and 4. Each circuit is excited by a Bi coil with i = 1, 2, 3 or 4 depending on the circuit.

 As can be seen in FIG. 1, the parts 2, 3, 4 are fixed to the part 1 in three different planes, offset parallel to each other so that under the action of a force in one direction j, the areas of the air gaps ei and e4 decrease, for example, while the areas of the air gaps e2 and e3 increase and vice versa for a force of opposite direction.

Similarly, under the action of a couple along k, the areas of air gaps el and e3 decrease, for example, while the areas of air gaps e2 and e4 increase and vice versa for a couple of opposite directions.

 In the embodiment shown in Figure i, the air gaps have horizontal planar surfaces.

 The variations in the area of the air gaps result in variations in the reluctance R (R = I /, uoS, I being the length of the air gap, S the section of the circuit which corresponds to the surface of the air gap, and sso the permeability magnetic vacuum) of each of the magnetic circuits and therefore by variations of the flux in the branches.

 The inductances of the windings also vary. However, the torsion angle of a shaft under the effect of a torque is proportional to this torque and the variation in area of the air gaps is proportional to the torsion angle. In addition, the bending of a tree under the effect of a force results in a linear displacement as a function of this force, and the variations in surface area of the air gaps are proportional to the displacement.

The variations of the reactances, Xi (i = i to 4), of the four coils are therefore the following:
X1 = X (i - aC + ss F)
X2 = X (i + aC - PF)
X3 = X (1 C -'3 F)
X4 = X (i + ctC + '3F)
with X = Lo and L = N2 / R, and where C is the applied torque, F the applied force, a and ss two constants, L the inductance at rest, o the pulsation of the coil supply, N sound number of turns, X the reactance value at rest.

 This expression means that the variations in reactance as a function of force and torque are linear.

 The coils are connected in full bridge, as shown in Figure 2. The coil B1 is connected at a point B in series with the coil B2 and the coil B3 is connected at a point A in series with the coil B4. These two associations are connected in parallel and the whole is supplied by an AC voltage E.

A first voltage, Vf, is taken between points A and B, and thus evaluated:
Vf = VA - VB
Classically Vf is equal to E (X1X4 - X2X3) / (X1 + X2) (X3 + X4) and therefore to '3EF
Using an operational amplifier assembly which adds the voltages taken at points A and B and subtracts this sum from the supply voltage, a voltage Vc = E - (VA + VB) is constructed, ie Vc = aE VS
These two alternating voltages are obtained very simply, and after demodulation they are continuous and proportional, one to the torque and the other to the force.

 All variations and operations are done in a linear fashion.

 This linearity at all levels is of great interest, since it allows very simple construction and use of information, and the continuous measurement of torque and force.

 The advantage of the device according to the invention lies in its simplicity: the parts are mechanically simple to produce. Piece 1, which serves as the test body, must be ferromagnetic. The mechanical properties of soft iron not being very good, this drawback can be remedied by making this part in a material with selected mechanical properties and by sleeving it with soft iron.

 Parts 2, 3 and 4 are ferromagnetic.

 The quantities delivered by the device are only sensitive to those that we want to measure, torque according to k and force according to j. For example, if the piece 3 is long enough in the direction of i, then the device is insensitive to a force applied along i. This decoupling of the quantities is essential for a simple processing of the measurements.

 The device according to the invention measures the force and the torque differently. This is linked to the use of a full bridge and to the fact that the inductors vary in opposite directions, two by two, for each of the types of stress, torque or force. The advantage of a differential structure lies in its linearizing effect of electrical characteristics and in its first-order insensitivity to drifts linked to temperature variations, as well as in an increase in the sensitivity of the device compared to a simple structure of the same characteristics.

 The initial adjustment of the relative positions of the different parts is done so that the four reluctances are equal for the force and torque values chosen as rest values, these values being able to be zero or not.

 The air gaps can have other shapes, adapted to obtain better sensor performance, in particular in sensitivity. The bar can in particular have an "H" shape as shown in FIG. 5a.

 FIG. 4a also shows a form of "H", in which the ends of the bar form with the branches of the "U" air gaps with vertical surfaces. The profile of Figure 4b shows the two U-shaped parts and the bar in offset parallel planes.

 By way of example, the air gaps can have surfaces in an inclined plane, as in FIGS. 3a, 3b, 4c and 5b.

 The device according to the invention finds its application in the optimization of the dimensions to be given to mechanical parts subjected to forces.

Claims (10)

 1.A device for differential measurement of the torsional torque exerted on a shaft which is deformable in rotation along an axis k, and of the bending force applied in a direction j perpendicular to the axis k, characterized in that it comprises two "U" shaped parts (2,4), provided with windings on each arm of the "U", fixed by the bottom of the "U" to the axis, the arms of the "U" facing each other, integral with the axis, and defining with a bar (3) which they frame and which is fixed in its middle to the axis, a magnetic circuit with two upper air gaps (e1, e4) and two lower air gaps (e2, e3), the two U-shaped parts (2,4) and the bar (3) being located in three different planes, offset parallel to each other so that under the action of a force exerted in direction j, the surfaces upper air gaps (e1, e4) vary in opposite direction to the surfaces of the lower air gaps (e2, e3), and, under the action of a couple acting around from the k axis, the surfaces of an upper air gap and of the lower air gap opposite with respect to the k axis vary in opposite direction from the surfaces of the other two air gaps.  2. A device for differential measurement of the torsional torque acting on a shaft which is deformable in rotation along an axis k, and for measuring the bending force applied in a direction j perpendicular to the axis according to claim 1, characterized in that that the bar has an "H" shape  3.Device for differential measurement of the torsional torque exerted on a shaft which is deformable in rotation along an axis k, and for measuring the bending force applied in a direction j perpendicular to the axis according to one of claims 1 or 2, characterized in that the surfaces of the air gaps are horizontal.  4. Device for differential measurement of the torsional torque exerted on a deformable shaft in rotation along an axis k, and for measuring the bending force applied in a direction j perpendicular to the axis k according to one of claims 1 or 2, characterized in that the surfaces of the air gaps are inclined relative to the horizontal direction.  5. Device for differential measurement of the torsional torque exerted on a shaft which is deformable in rotation along an axis k, and for measuring the bending force applied in a direction j perpendicular to the axis k according to one of claims 1 or 2, characterized in that the surfaces of the air gaps are vertical.  6.Device for differential measurement of the torsional torque acting on a shaft which is deformable in rotation along an axis k, and for measuring the bending force applied in a direction j perpendicular to the axis k according to one of claims 1 or 2, characterized in that the surfaces of the air gaps are inclined relative to the vertical direction.  7. Device for differential measurement of the torsional torque exerted on a deformable shaft in rotation along an axis k, and for measuring the bending force applied in a direction j perpendicular to the axis k according to one of claims 1 to 6, characterized in that the coils are mounted on a magnetic bridge.  8. A device for differential measurement of the torsional torque acting on a shaft which is deformable in rotation along an axis k, and for measuring the bending force applied in a direction j perpendicular to the axis k according to claim 7, characterized in that the value of the bending force is proportional to the tension between a point located between the first two coils connected in series and a point located between the two other coils connected in series.  9. A device for differential measurement of the torsional torque exerted on a shaft which is deformable in rotation along an axis k, and for measuring the bending force applied in a direction j perpendicular to the axis k according to claim 7 or 8, characterized in that the value of the torque is proportional to the supply voltage minus the voltage between the point between the first two coils connected in series and the point between the two other coils connected in series.  10. A device for differential measurement of the torsional torque exerted on a shaft which is deformable in rotation along an axis k, and for measuring the bending force applied in a direction j perpendicular to the axis k according to claim 7 or 8, in which the variations in the area of the air gaps result in variations in reluctance, characterized in that the four reluctances are equal for the force and torque values chosen as rest values, these values possibly being zero or not.