ES2358655A1 - Three-axis touchscreen sensor. (Machine-translation by Google Translate, not legally binding) - Google Patents

Abstract

The present invention relates to a three-axis touch sensor comprising an elastic body inside which a permanent magnet is located, said elastic body being placed on a rigid support, and three hall effect sensors located below said support rigid and placed so that they are inside the projection of the area of the magnet forming a triangle around the perpendicular axis of said magnet, so that when a force is applied on the elastic body a displacement of the magnet occurs causing a magnetic field that it is detected by the hall effect sensors, said magnetic field being indicative of the magnitude and direction of the displacement. (Machine-translation by Google Translate, not legally binding)

Description

Three axis touch sensor.

Field of the Invention

The present invention is framed within the field of robotics and, in particular, refers to sensors three-axis tactile devices capable of measuring deformations in objects caused by the forces acting on the foot of a robot humanoid

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Background

The robots, both the so-called industrial like humanoid robots, they need to know information about each other themselves and the environment around them. In this sense, the sensors proprioceptives are those that allow the measurement of the state internal robot, angular position of each of the degrees of freedom, battery charge, etc. On the other hand, the sensors exteroceptives are used for the perception and measurement of aspects external to the robot, such as temperature, pressure and Object location

A very important aspect is the measurement of forces acting on the sole of the humanoid robot's foot. For this, and within the area of exteroceptive sensors, they are gone developing various touch sensors that allow measuring deformations caused on objects as a result of mentioned forces. According to their physical behavior, these sensors Artificial tactiles are classified into piezoelectric sensors, capacitive sensors, resistive sensors, cell sensors load, strain gauges, magnetic effect sensors, accelerometers, biopotential sensors, analysis based sensors of color, etc.

However, the main limitations that present these sensors is that some only measure the forces perpendicular to the surface they are in and others, although they are capable of measuring forces on all three axes, they do not support the forces to which the sole of a robot's foot is subjected humanoid

Thus, several three-axis sensors have been developed to try to detect displacement or measure the force on all three axes. Kobayashi et al . [ IEICE Transactions on Electronics , 1991, J74-C-II, 5, 427-433] developed a three-axis touch sensor prototype using a two-dimensional array of silicone structures with fixed load cells. However, the manufacture of this type of sensors, besides being laborious, is excessively expensive. Ohka et al . [ Advances Information Storage Systems , 1999, 10, 313-325] proposed a method that measured a force vector using the variation of the index of refraction in a fingertip-shaped optical waveguide. The application of this touch sensor on the finger of a robot is quite simple but the sensor system additionally requires an imaging device. Shinoda et al . [ In Proc. of the 11th International Conference on Solid-State Sensors and Actuators , 2001, 2, 1430-1433] proposed a three-axis touch sensor element based on an ultrasonic cavity and tested the ability to detect the coefficient of friction by frequency measurement resonant of the cavity. However, this sensor allowed to measure the coefficient of friction only at the time of contact, and it was also difficult to reduce the size of the sensor element.

Ground displacement can be measured using piezoelectric devices. Thus, Son et al . [ In Proc. of the 1994 IEEE International Conference on Robotics and Automation , 1994, 471-476] developed a touch sensor with strips of piezoelectric film to detect incipient and contact displacement. However, the force vector was not able to measure it properly. Jockusch et al . [ In Proc. of the 1997 IEEE International Conference on Robotics and Automation , 1997, 3080-3086] prototyped a touch sensor using also a piezoresistive device and a piezoelectric device placed in layers to simultaneously measure the contact force and position and detect displacement However, this hybrid sensor was excessively complicated and its peripheral structure was very bulky.

Recently, Takenawa [ Proc. of the 2009 IEEE International Conference on Mechtronics , 2009] has developed a touch sensor for the measurement of forces in three directions based on the magnetic field. Said sensor is based on electromagnetic induction and its structure consists of an elastic body that contains a permanent magnet located on a rigid substrate and four encapsulated bovines. The tension is generated by each coil in response to the application of a force vector that deforms the elastic body and produces a displacement in the magnet. However, the main disadvantage of this sensor is that it depends on the variation of the magnetic field, so, in order to achieve an adequate signal or voltage when the field variation is small, it is necessary to increase the size of the coils or the number of spirals that constitute them, which greatly conditions the final size of the sensor.

In view of the inconveniences they present the sensors proposed to date, the development of three-axis touch sensors that allow measuring the force in three directions properly and with one more size reduced for better implementation in the structure of a robot.

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Brief Description of the Invention

The authors of the present invention have noted that the replacement of four encapsulated coils, such as those used in the Takenawa sensor, for three effect sensors Hall allows not only to make an adequate measure of the force in three directions, but also greatly reduce the dimensions Touch sensor end. This is mainly due to the fact that Unlike the aforementioned coils, the voltage generated in the Hall type sensors depend on the value of the magnetic field and not their variation, so that these sensors or cells are not required sensors have a larger size when the variations of said Field are reduced. In addition, using the touch sensor of the invention is reduced to three the number of sensor cells and since these are located within the projection of the area of the magnet, with this provision is achieved occupy the minimum space and thereby obtain higher sensor density An additional advantage that it presents the sensor of the invention is to be a static sensor, that is, not you need a variation of the position of the magnet to measure a force, which allows for example the measurement of constant forces. Also, the measurement algorithm is based on a precalculated table that performs simple operations, which entails a lower cost computational

Thus, in a first aspect the present invention It is related to a three-axis touch sensor comprising:

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a elastic body comprising a first face and a second face, where both faces are substantially parallel;

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a permanent magnet located inside the elastic body, or outside it located adjacent to the first face of the body elastic, so that the magnet is arranged in parallel to the first and second sides of the elastic body;

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three Hall effect sensors located adjacent to the second elastic body face forming a plane parallel to said second elastic body face, and positioned so that they meet inside the projection of the magnet area forming a triangle around the axis of said magnet,

so that when a force is applied on the elastic body produces a displacement of the magnet causing a magnetic field that is detected by the Hall effect sensors, said magnetic field being indicative of the magnitude and direction of displacement.

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In a second aspect, the invention is directed to a robotic system comprising a touch sensor as has been previously defined.

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Brief description of the figures

Figure 1: Physical distribution of the elements which constitute the three-axis touch sensor.

Figure 2: Representation of the evolution of the error: (a) representation of the error in the measure; b) evolution of error in the measurement with respect to the distance to the plane of the magnet For a specific point.

Figure 3: Evolution of the magnetic field with the distance to the magnet axis for different distances to the plane of the magnet: (a) for all the space represented and (b) in the nearby area to the magnet

Figure 4: Block diagram of a sensor IC Hall.

Figure 5. Graphs of the tests performed for a displacement sensing cell in different axes: (a) sensor response to a z- shift; (b) sensor response to an x offset; (c) sensor response to a shift in y ; (d) sensor response to an xy offset.

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Detailed description of the invention

In a first aspect, the invention is directed to a three-axis touch sensor comprising an elastic body that it comprises a first face and a second face, where both faces are substantially parallel; a permanent magnet located inside of the elastic body, or outside it located adjacent to the first face of the elastic body, so that the magnet is is arranged parallel to the first and second faces of elastic body; three Hall effect sensors located so adjacent to the second face of the elastic body forming a plane parallel to said second face of the elastic body, and placed of way they are within the projection of the magnet area forming a triangle around the axis of said magnet.

Through the sensor of the invention it is possible measure the movement of a plane in contact with another initially parallel, thus obtaining an image of the differentials of contact forces in the three directions of the Cartesian space. This not only allows to register irregularities or contours on the which presses the sensor but also directions of the forces applied, possible material changes, landslides, etc.

The functioning of the sensor developed consists of the measurement in each cell or each Hall sensor (unit of measurement) of the relative movement of the magnet, also called sneak, caused by the appearance of a force on the material elastic that sustains it. The variation of the magnetic field caused by the movement of the magnet is recorded by the three Hall sensors located in the fixed and rigid plane of the sensor, which produce An electrical signal

Figure 1 shows the structure of a sensor as described in the present invention with the arrangement of the different elements that constitute said sensor. In concrete, shows the arrangement of these elements when the magnet It is located adjacent to the first face of the body elastic.

The diameter of the magnet should be such that it covers the three Hall sensors, which are arranged in such a way that the detection areas are located on the vertices of a triangle surrounding the axis or center of the magnet.

Taking advantage of the linearity of the field with this arrangement you can calculate the position of the magnet with respect to the sensors by plane trilateration. The measure by three-dimensional trilateration would not be the most appropriate given that the magnetic field does not have a spherical symmetry but a symmetry cylindrical To perform the measurement, the field has been simulated Magnetic generated by the magnet and saved the data in a matrix. With the value of the three Hall sensors and looking at the matrix of simulated data above reduces the problem of three dimensions to two dimensions.

The measure therefore consists of an optimization algorithm. There are three Hall sensors in the same plane and in known positions. When taking a measurement, these sensors provide the magnetic field at each of the three points. Once the magnetic field is measured, two unknowns are solved: the distance from the Hall sensor plane to the magnet plane and the distance from each Hall sensor to the magnet axis. Considering the situation that the distance to the plane of the magnet is maximum, the search for data in the matrix is restricted to a single row, row m . In this row, each measured magnetic field value corresponds to a distance from the axis, that is, with a position in the row between 1 and n , or between 0 and ( n-1 ) according to the programming language used. There is thus a plane (the plane that is a distance m from the plane of the magnet), three points, which are the points where each of the Hall effect sensors are located and three distances, calculated by reading the matrix in row m . In this way, with two of the sensors and their two respective distances we establish the position of the axis of the magnet, and with the third sensor and third distance the error is measured. In the best case, the axis of the magnet will distract from the sensor three a distance R 3, then the error will be 0. In the rest of the cases, the procedure is to perform the previous steps but with a distance to the plane of the magnet of ( m -1) and calculate the error. It is repeated with a distance ( m -2) and thus up to a distance 1. In short, performing an abstraction would be to travel the matrix through each of its rows from m to 1, checking the error that returns when collating the data provided by the three sensors with the information found in each row of the matrix.

Checking each of the errors a very important characteristic is observed that is reflected in figure 2. If the distance to the plane of the magnet is represented on the x- axis and on the axis and the error module calculated above a relative minimum of the function between 1 and m . In the minimum, the error made is minimal and can be taken as a valid measure. In this way the distance to the plane of the magnet and the position of the axis of the magnet is completely defined.

Once the error in the measurement evolves as displacements occur in planes parallel to the plane of the magnet, the evolution of the magnetic field must be studied next when moving away from the axis of the magnet in each of the planes. Performing this analysis is the same as analyzing the evolution of the rows of the created matrix and, in a particular embodiment, a graph is created as shown in Figure 3. In this graph the distance to the axis of the magnet is related, which is represented on the x- axis, with the magnetic field module in the direction of the z- axis of the magnet that will be represented on the y- axis. This representation is made for different distances to the plane of the magnet with which a family of curves is obtained. Curve 1 is the one closest to the plane of the magnet and curve 30 the farthest.

It is observed in figure 3 that the magnetic field decreases as the sensor moves away from the axis of the magnet, not However, this is not true in all cases. In the figure 3 (a) is observed, and in Figure 3 (b) more clearly, that for distances close to the plane of the magnet the magnetic field grows as we approach the cylindrical edge of the magnet, decreases sharply near the edge (both on one side and on the other) and then grow back gently. This inconvenience can be overcome if the area of sensor movement to areas far enough from the plane of the magnet.

Consequently and in a preferred embodiment of the invention, the minimum distance between each of the sensors and the plane of the magnet is between 3 and 5 mm. Do not However, this distance depends on the size of the magnet used and the aspect ratio. A subject matter expert would know how to select the minimum distance in order to avoid the aforementioned inconvenient.

Thus, the magnetic field module is decreasing as the point moves away from the axis of the magnet and that way the data in the rows of the array are ordered from greater to Minor. The fact of having all these values ordered implies the advantage of being able to use strategies that allow saving a lot processing time, that is, much more algorithms rapid.

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The fact that the sensors are always within the projection of the area of the magnet is because outside that area, the magnetic field begins to vary in shape abrupt and it happens that for two distances to the plane of the different magnet the same magnetic field value is obtained. This situation is not desirable because the measurement algorithm is based, as we have seen, on field checks at different distances to the magnet plane and if there are points that coincide it could happen that the solutions are multiple.

In a particular embodiment of the invention, Hall sensors are arranged in such a way that the areas of detection are located on the vertices of a triangle equilateral whose circumcenter coincides with the axis or center of the magnet.

By distributing the sensors in equilateral triangle shape achieves considerable savings in the computational expense given that the data generated can be minors getting the same sensitivity. To illustrate this advantage will be taken the example that the distance to the center of the magnet varies between 1 mm and 3 mm for sensor 1, sensor 2 varies between 2 mm and 3 mm and that of sensor 3 between 3 mm and 5 mm. To do this, it you must save a table that includes values between 1 and 5 mm, if We take 256 samples, the sensitivity will be 5 mm / 256 which means 0.02 mm / sample or, otherwise, to achieve a sensitivity of 0.05 mm / sample, the number of samples to be saved will be 100 samples (5 mm / 0.05 mm). If the magnet is placed in the circumcenter of the equilateral triangle formed by the three Hall sensors, the distances will vary for example between 2 mm and 4 mm, so for to obtain a sensitivity of 0.05 mm, only 40 are needed samples instead of the previous 100.

On the other hand, and as mentioned above, as the distance to the magnet axis decreases, the variation of the magnetic field in the z axis decreases with respect to this displacement, which causes the sensor sensitivity with respect to these displacements in The indicated area is smaller. Therefore, in a preferred embodiment of the invention, the minimum distance between each of the sensors and the axis of the magnet is at least 7% of the radius of the magnet.

In a particular embodiment of the invention, Hall effect sensors with PWM digital output can be used, sensors with analog output or programmable sensors with output analogue A diagram with the blocks is shown in Figure 4 functional components of a Hall effect sensor. These sensors they also comprise temperature compensation elements, of compensation of mechanical stress and signal amplification, since the Hall sensor is very dependent on temperature and mechanical stresses

In a preferred embodiment of the invention, the Hall sensor is a Hall sensor with analog output, more preferably it is an Allegro Al322 sensor. These sensors provide an output voltage proportional to the applied magnetic field. They have a 0 Gauss field output voltage of 50% of the Vcc supply voltage and a sensitivity of between 2.5 and 5 mV / G. Each device integrates a Hall element with temperature compensation, an amplifier with low output impedance and an offset cancellation system with a high frequency clock that allows higher sampling frequencies that result in greater accuracy and faster processing capacity Of the signal. This technique produces a device with a very stable output voltage and immune to mechanical stress.

As regards the composition of the magnet, and In a particular embodiment, this may be a ferrite magnet, a Neodymium magnet or samarium magnet.

Ferrites are characterized by presenting a great remanence after magnetization, provide a flow Moderate magnetic depending on volume. They are composed of ferrite, cobalt and barium alloys. On the other hand, the magnets of Neodymium are composed of rare earths, neodymium alloy, iron and boron and coated by a standard nickel, silver or bath Zinc to prevent corrosion. Samarium magnets are a sintered alloy of iron, samarium and cobalt and others secondary elements

The essential conditions that the magnet are not to saturate the Hall effect sensor at any time and that your weight is as low as possible. In this sense, the magnet that presents a better balance of the mentioned properties is the ferrite magnet for what is proposed as a preferred embodiment of the invention.

In a further aspect, the present invention It is aimed at a robotic system comprising a touch sensor three axes as previously defined.

In a particular embodiment, the sensor of the invention can be applied to the foot of a humanoid robot. The foot make a contact with the ground every time you step on what, placing the sensor in the sole of the foot, the elastic material absorbs contact and in this way you can know which irregular it is the ground, what portion of the foot area makes the contact, complementary information to locate the breakeven point due to the better information of the distribution of forces, if the contact is slipping by the absence of tangential forces expected, etc.

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Execution Example

A functional test of a position / force sensor as described in the present invention is shown below. Figure 5 shows the results obtained for this sensor. Before each of the originated disturbances, the traces for the x coordinate in which the center of the magnet is located, the y coordinate and the z coordinate are shown.

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The sensor has been deformed in each of the three directions of the Cartesian space. All these measures they have been carried out manually, the measuring elements being a King's foot, folio, pen and orthogonal view of the human eye.

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Single offset in z

At rest, the z coordinate has a value of 18.7 mm. A force so that the z coordinate decreases to 15 mm is exerted, this is a \ Delta z = -3.7 mm. The results are shown in Figure 5a. It is observed that it is measuring a minimum of 11.75 mm, while the x coordinate varies between 0 and -1.07 mm and the coordinate varies between 5.20 and 3.60 mm.

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Unique offset in x

In this test a displacement is made on the x axis trying to keep the distance y and the distance z constant. The displacement in x, \ Delta x = + 3 mm shown in Figure 5b.

The desired x - axis shift is observed, as x varies between -0.53 and 3.47 mm, ie, \? X = 4.1 mm, but displacements are also observed in the y and z axes.

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Single offset in y

A displacement is made on the y- axis of 3 mm keeping the x and z coordinates constant, although due to the physical constraints of the material, when performing the displacement in and the elastic material requires a small displacement in z . The results are shown in Figure 5c.

A desired variation in the y axis is observed, since it varies between 4.00 and 9.21 mm, that is, Δ y = 5.21 mm, although variations in x and z are also observed.

Joint offset in xy

This measure has been carried out approximately to have a qualitative graph, rather than quantitative. The results are shown in figure 5d where a Correct response to this type of deformation.

Claims (10)

1. A three-axis touch sensor that understands:

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a elastic body comprising a first face and a second face, where both faces are substantially parallel;

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a permanent magnet located inside the elastic body, or outside it located adjacent to the first face of the body elastic, so that the magnet is arranged in parallel to the first and second sides of the elastic body;

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three Hall effect sensors located adjacent to the second elastic body face forming a plane parallel to said second elastic body face, and positioned so that they meet inside the projection of the magnet area forming a triangle around the axis of said magnet,

so that when a force is applied on the elastic body produces a displacement of the magnet causing a magnetic field that is detected by the Hall effect sensors, said magnetic field being indicative of the magnitude and direction of displacement.

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2. Touch sensor according to claim 1, in the that the three Hall effect sensors are arranged forming a equilateral triangle whose circumcenter coincides with the axis of the magnet. 3. Touch sensor according to claims 1 or 2, where the minimum distance between each of the sensors and the axis of the magnet is at least 7% of the radius of the magnet. 4. Touch sensor according to any of the claims 1 to 3, wherein the maximum distance between each of the sensors and the axis of the magnet match the radius of the magnet. 5. Touch sensor according to any of the claims 1 to 4, wherein the minimum distance between each of the sensors and the magnet plane are between 3 and 5 mm 6. Touch sensor according to any of the claims 1 to 4, wherein the Hall effect sensors are non-programmable sensors with analog output. 7. Touch sensor according to any of the claims 1 to 5, wherein the magnet is a ferrite magnet. 8. A robotic system comprising a sensor three-axis touch as defined in any of the claims 1 to 7. 9. Robotic system according to claim 8 which It is a humanoid robot. 10. Robotic system according to claim 9, where the three-axis touch sensor is located at the foot of the robot humanoid