deformation value and strain direction

The present invention relates to the method of measuring magnetic field strength, deformation value and strain direction, and sensor for measuring magnetic field strength, deformation value and direction. These solutions are designed to measure and control the position of moving or deformed elements.

The known methods and sensors for measuring magnetic field strength and deformation values are intended for measuring each of these values separately. They use the piezoelectric effect and the Hall effect.

From PL 205 355 is known a thin-film heterojunction magnetic field sensor based on the use of the Hall and Gauss phenomenon. This sensor at room temperature can be used as a measuring sensor, while at a temperature below room temperature as a signal sensor. In the sensor, the substrate is monocrystalline indium phosphide (InP) with a crystallographic orientation <001>, and the active layer is an undoped layer of gallium arsenide - indium (ln0.53Ga0.47 As) with a carrier concentration of 1 x 10 ²⁰ ÷ 1 x 10 ²¹ nr ³ . The active layer has the shape of an isosceles cross, in which the ratio of the length of the arm to its width is 0.5.

The object of the invention is to introduce an integrated method of measuring the magnetic field and deformation values, and an integrated sensor enabling the simultaneous measurement of these two quantities and the direction of deformation.

The essence of the method according to the invention is that

- with a single sensor, a simultaneous measurement of both the value of the magnetic field strength and the value and direction of deformation is conducted, wherein

- the received signal is the sum of the sensor response to the magnetic field and the deformation is separated by offset compensation,

- passing electrical current from the source along both sensor diagonals, one at a time, one way or the other, eight measurements of the signal value provided by the functional element are conducted, for each of the four possible directions of current flow, two values of the output signal corresponding to two possible configurations of voltmeter are measured,

- the values of measured voltages from eight configurations are averaged to obtain a signal component proportional to the magnetic field strength, which does not depend on the strain gauge signal, and thus the part of the signal corresponding to the magnetic field strength is extracted,

- in further signal processing, the magnetic component is subtracted from the total signal measured for both sensor diagonals to obtain a signal associated only with deformations. The essence of the device solution according to the invention is that the sensor contains one functional element for the simultaneous measurement of the magnetic field strength and the value and direction of deformation, the functional element being a thin conductive layer of material, preferably with a high Hall constant, mounted on the surface of a flexible dielectric, the conductive element has a layered dielectric substrate, the contour of the conductive layer has a symmetrical shape, with four electrical contacts located symmetrically.

In the case of using a two-layer substrate, two paths are located one below the other, so that the current flowing through them creates two loops with identical, as small as possible, cross-sectional areas, with each current loop having a different torsion.

In the case of using a three-layer substrate, two paths are located under the functional element and the cross-sectional areas of their loops are reduced to zero.

If a four-layer substrate is used, the paths are located one below the other and each path is separated from the next one by a thin layer of flexible dielectric.

The solution according to the invention, both of the method and the sensor, is presented below, with reference to the drawings in which the individual figures show:

FIG. 1 - simplified diagram of the sensor

FIG. 2 - side view of the sensor

FIG. 3 - examples of contours and shapes of the sensor functional element

FIG. 4 - (a) sensor wiring diagram as an equivalent system of resistors; (b, c) electrical subsystems constituting the system together (a).

FIG. 5 - (a) an example of Hall sensor characteristics; (b) enlargement of the characteristics for a range of ± 3 T fields, showing a linear relationship for small magnetic fields

FIG. 6 - exemplary Hall characteristics of a sensor bent along an axis lying at an angle of 45° to the X axis; the bending to one side results in a positive offset D (a), while the bending to the other one gives a negative offset (b)

FIG. 7 - measurement configurations on the example of a cross-shaped functional element. The arrow shows the direction of electric current flow. Markings at the arms of the cross indicate the polarity of the power source and voltmeter connections

FIG. 8 - change of Hall voltage for the functional element in the form of a bismuth layer 20 nm thick, measured at 1 mA direct current

FIG. 9 - dependence of 20 nm bismuth layer resistance for various deformations

FIG. 10 - exemplary implementation of conductive paths supplying the sensor and used to read the signal on a single-layer dielectric substrate. The functional element of the sensor is located on the right side at the crossroads of four paths

FIG. 11 - path configuration on a flexible dielectric substrate; a striped cross means the location of the functional element of the sensor, solid lines represent the paths on the upper side of the flexible plate; (a) a flexible two-layer substrate with conductive paths placed on both sides of the polymer film; (b) a three-layer substrate with paths placed in pairs one below the other; (c) a four-layer substrate with paths placed one below the other. FIG. 12 - exemplary implementation of a two-layer flexible sensor substrate with paths for power supply and signal reading together with metalized through holes and insulator layers on both sides of the substrate.

FIG. 1 shows a simplified diagram of the sensor. The square, representing the functional element of the sensor, is a thin conductive layer of material, preferably with a high Hall constant, deposited on the surface of a flexible dielectric, such as for example a polymer film. The functional element should be made in the form of a thin layer, because the magnetic sensitivity of the sensor is inversely proportional to the thickness of the active element layer and at the same time provides high flexibility. At the vertices of the square, a current source and voltage meter are connected to the layer.

The side view of the sensor is shown in FIG. 2. The individual markings indicate: 1 - conductive layer, 2 - flexible dielectric substrate, 3 - contacts with which cables or PCB paths are connected, 4 - protective layer of insulator protecting the sensor surface.

The outline of the conductive layer need not be limited to a square and can be any symmetrical shape, such as, for example, a polygon, circle, circle with notches, rhombus, ellipse, star, or cross with electrical contacts connected as exemplified in FIG. 3 with black dots. Regardless of the shape of the functional element, it can always be described with an equivalent electrical diagram of the resistors shown in FIG. 4a. This system can also be considered as a combination of two subsystems shown in FIG. 4b and FIG. 4c.

The subsystem shown in FIG. 4b is responsible for measuring the Hall's component of the signal, depending on the value of the magnetic field strength. Neither a change in the resistance of the R1 - R4 resistors, nor a change in the resistance of the connecting cables have a significant effect on the value of the indication of such an element.

The second subsystem, shown in Figure 4c, is responsible for measuring the signal depending on the sensor deformation. This subsystem is a full strain gauge bridge. In this case, the magnetic field simultaneously changes the resistance of all resistors in the bridge, which means that the output signal remains unchanged. However, when the functional element is bent, the resistance of individual resistors in the bridge changes, and the signal values depend on the degree of deformation and the direction of bending. In the particular case of bending in the X or Y directions, the resistance in both parts of the bridge change in a proportional way to each other, which ensures that the output signal remains constant. Therefore, the functional element is not sensitive to bending in those directions. However, bending the sensor at any angle other than zero relative to the X and Y directions leads to an asymmetrical change in the value of individual resistors in the bridge. In particular, when bending at an angle of + 45°, the response from the bridge will have a positive sign, while bending at an angle of -45° will give negative signal values. This sensor characteristic allows for measuring both the deformation value and the bending direction.

The sensor was tested in its example implementation. The sensors are made of a bismuth layer with thicknesses in the range from 20 nm to 100 nm and shapes shown in Fig. 3 and Fig. 11. The functional element sizes were changed from 0.25 mm2 to 100 mm ² . The Hall characteristics of a prototype sensor made of a 20 nm thick bismuth layer, cross-shaped, with an active area size of 1 mm ² are shown in Fig. 5.

This characteristic is representative of all prototypes made and is non-linear in high fields (see Fig. 5a), while in the fields ± 3 T (see Fig. 5b) deviation from linearity does not exceed 2.6%. In cases where high measurement precision is not required, linear characteristics approximation can be used in fields below 3 tesla. Otherwise, additional calibration is required.

The sensitivity of measuring magnetic fields does not change under the influence of elastic deformations, which only cause an offset in the characteristics along the vertical axis. This offset is proportional to the deformation and depends on the bending direction, and its polarity allows to determine this direction.

An example of the characteristics of the sensor bent along the axis lying at an angle of 45° to the X axis is shown in Fig. 6. The simultaneous measurement of two values: the Hall signal and the offset, allows the simultaneous determination of the value of the magnetic field strength, the deformation value and the direction of sensor strain.

In practice, the simultaneous measurement of the values discussed above is difficult due to the fact that the measured signal is the sum of the sensor's response to the magnetic field and deformation. The offset compensation technique was used to separate those signals. To obtain a reliable measurement of the magnetic field strength and deformation values, eight measurements of the signal value provided by the functional element are made. The signal reading system, not shown in the figure, allows switching the direction of the direct current flowing through the functional element. The set of switches allows to connect the selected power source sensor and voltmeter with selected polarity together with the change of their polarity.

The integrated circuits form a matrix of analog switches controlled by a 3-bit digital code and conduct 8 possible configurations for connecting a voltmeter and a power source. In this way, electric current is passing along both diagonals of the sensor, one after the other, one way and the other. For each of the 4 possible directions of current flow, two values of the output signal corresponding to two possible configurations of the voltmeter are measured. All 8 measurement configurations showing the direction of current flow and polarization of the voltmeter are shown in Fig. 7.

By averaging the values of the measured voltages, a signal component proportional to the magnetic field strength is obtained. This value does not depend on the strain gauge signal, because for 4 configurations the voltage offset is positive and for the other 4 ones negative. Therefore, the arithmetic average of the voltages from 8 configurations leads to the separation of the part of the signal corresponding to the magnetic field strength.

Further signal processing consists in subtracting the magnetic component from the total signal measured for both sensor diagonals. In this way, a signal related only to deformations is obtained. Formulas used to obtain a signal depending on the strength of the magnetic field (V ) and a deformation (½ _¾ / _o/ ™):

Sign of signal value V _Deform depends on the direction of deformation. If the signal is positive during sensor deformation along one diagonal, then the elongation of the other diagonal will result in negative values. This feature allows measuring deformation in a given direction depending on how the sensor is installed. In addition, with knowledge of the mechanical parameters of the substrate, it is possible to obtain information about the value of stress acting on the sensor.

Moreover, the measuring system is equipped with a measuring amplifier and a current stabilization system, not shown in the figure. The measuring amplifier allows for adjusting the signal level to the voltmeter measuring range, and the current stabilizer provides a constant value of electric current for each of 8 measuring configurations.

For the prototype in question, the application of the above solutions allowed to obtain a nominal magnetic sensitivity of 5 W/T at room temperature, while the gauge factor was a = 12. The measured magnetic and deformation characteristics are presented in Fig. 8 and FIG. 9.

The structure of a flexible substrate with appropriate arrangement of conductive paths has a significant impact on the sensor's operation, in particular on its resistance to interference generated by alternating magnetic fields.

The simplest configuration of paths on a flexible dielectric substrate is to make them on one surface of the substrate in the arrangement shown in FIG. 10. The disadvantage of this solution is the generation of AC signals in each of the windings created by parallel conductors.

In the case of slow-changing or weak magnetic fields, such interference is very weak and does not significantly affect the accuracy of the measurement. In the case of fast-changing magnetic fields with large changes in intensity, the signal generated in the paths becomes comparable to the signal generated by the functional element, which leads to distortions in measurements and significantly affects their accuracy. A similar situation may also occur in the case of mechanical vibration of a sensor placed in a constant magnetic field.

In the proposed solution, this type of interference is significantly reduced by using one of the path configurations shown in FIG. 11.

The preferred path configuration, in the case of using a two-layer substrate, is shown in FIG. 11a. Circles in the drawing mark the metallized through holes, connecting together paths located on opposite sides of the dielectric substrate. In this solution, two paths were placed one below the other. The second pair of paths has been shaped in such a way that the current flowing through them forms two loops with identical cross-sections (Si = S2). The current flowing in each loop has a different rotation. The paths should be routed so that the Si and S2 fields are as small as possible. In this arrangement, the interference signals generated in each loop will have opposite signs, which will lead to their mutual blanking.

Even more resistant to interference is the structure shown in FIG. lib, where a three-layer dielectric substrate was used. It allows to lead both paths under the functional element. In this case, the cross-sectional areas Si and S2 shown in FIG. 11a are reduced to zero. Thanks to this solution, interference from an external magnetic field will be eliminated, but there may still be a short-term generation of interference when switching the current directions passed through the functional element.

To eliminate this source of interference also, a four-layer plate with paths one below the other should be used, as shown in FIG. 11c. The continuous lines were marked with the uppermost upper conductive layer and the dotted lines down the lower ones. Each of the paths is separated from the next one by a thin layer of flexible dielectric, and the metallized through holes, marked with circles on the drawing, are used to connect the respective layers located at different levels of the multilayer plate.

The paths on FIG. 12 are placed one above the other so that they do not form an additional winding, in which an additional signal could be induced. In this case, no other two paths can be conducted so that they do not cross with others and run one below the other. Therefore, they were arranged in two loops with the same surface area but the opposite direction of the current. In the variable magnetic field, the signal induced in each of those paths will have the opposite sign, as a result of which it will be compensated and will not make an additional contribution to the measured signal. When supplying a sensor with direct current, the last two paths are for current connection. In this case, the power supply stabilizer maintains a constant current value, additionally eliminating any interference generated in the sensor wiring.

FIG. 12 shows the substrate 5, the area 6 between the contacts on which the functional element is mounted, the lower path 7, the upper path 8, the contact pads 9 of the electrical connector.

As described above, according to the invention, one functional element is used to simultaneously measure the magnetic field strength and deformation values.

The problem of flexibility of the sensor, including its functional element, has been solved. The functional element according to the invention is made in the form of a thin layer, which can be repeatedly bent at small bending radii. The applied solution offers uninterrupted magnetic field measurement even with large deformations. The proposed path system for supplying and reading the sensor signal ensures the elimination of the influence of variable magnetic fields and interference.

One possible application of the integrated sensor is to mount it on a computer-controlled gripper. The sensor was placed on the curved surface of a rubber tube glued to the end of the gripper so that when compressed it was deformed along one diagonal. In this arrangement, the deformation of the tube with the sensor is proportional to the force with which the gripper squeezes objects. On the gripper's second arm there is a magnet whose field acts on the sensor. The measured magnetic field induction changes with the distance between the gripper arms, which allows to control their position. In this way, the program controlling the gripper can determine the sizes of objects touched, measure their elasticity and control the force with which they are compressed.