DEVICE AND METHOD FOR MEASURING A FORCE EXERCISED ON A BODY, AND CHARACTERIZING THE RIGIDITY OF A BODY USING SUCH A DEVICE

Abstract

The invention relates to a device (1) for measuring a force exerted on a body, the device (1) comprising a flexible rod (2) having a proximal end (2a) and a distal end (2b) intended to come into contact with the body and apply a force tending to deform it in the vicinity of a point of contact. The measuring device (1) also includes a camera (4), attached to the proximal end (2a) and oriented towards the distal end (2b) of the flexible rod (2), to provide at least one depth map of the body in the vicinity of the distal end (2b) of the flexible rod (2). The invention also relates to a method for measuring the force exerted on the body and to a method for characterizing the rigidity of a body using the measuring device (1). (Figure 1)

Description

Title of the invention: DEVICE AND METHOD FOR MEASURING A FORCE EXERCISED ON A BODY, AND CHARACTERIZING THE RIGIDITY OF A BODY USING A SUCH A DEVICE FIELD OF INVENTION

[0001] The present invention relates to a device and a method for measuring a force exerted on a body. This device and method find applications in the field of characterizing and modeling the rigidity of a body. TECHNOLOGICAL BACKGROUND OF THE INVENTION

[0002] There are a multitude of situations in which it is useful to have a characterization of the rigidity of a body.

[0003] In the medical field, it is common to examine a patient by palpation, that is, by direct physical contact. The distribution of rigidity in the part of the body being examined allows the practitioner to identify areas of the patient's body that may be abnormal, containing, for example, tumors.

[0004] The measurement of the rigidity of a part of a human body can find other applications, for example to design an object intended to be worn by a user (for example a prosthesis, an orthosis, or even a garment) which is not only adjusted to the morphology of the user, but which also takes into account the deformation of the user's body when the prosthesis, orthosis or garment is placed on the user's body and exerts pressure on that body.

[0005] Stiffness measurement is also useful for the complete modeling of a deformable body (a human body or a body of any other nature), the model then representing the deformation behavior of this body when it is subjected to external forces. The model can be useful for enabling the faithful replication of this object.

[0006] As recalled in the document "Capture and modeling of non-linear heterogeneous soft tissue" by B. Bickel et al., ACM Transactions on Graphics 28(3):89, July 2009, the work of characterizing and / or modeling a deformable body may involve implementing a system for measuring the deformation of the body when it is subjected to a force. More specifically, the measurement system proposed by this document includes a contact probe for applying a point force to the body. The probe includes a force sensor for measuring the force applied by the distal part of a rigid rod equipped with an application tip. The system also includes three imaging devices for recording the deformation of the surface of the The body is photographed from three different angles while the probe exerts force upon it. Activating a trigger by the contact probe operator allows for simultaneous measurement of the applied force and the three image captures. Markers are painted on the probe body and shaft to enable image processing to identify surface deformation and the probe's position / orientation.

[0007] A similar measurement system is described in US patent 9623608. This system comprises a robotic arm equipped with a force sensor, the arm being configured to apply a force to the body or object under test. The system also includes a plurality of imaging devices, arranged in a semicircle above the robotic arm, to track the deformation of the object during the measurement sequence. The object may carry means to assist in the processing of images acquired by the measurement system.

[0008] The document “The human touch: Measuring contact with real human soft tissues” by Pai et al., ACM Transactions on Graphics, 37, 1–12 (2018), proposes a portable contact probe comprising a head with three RGB cameras and a rigid rod, connected to the head, associated with a force and torque sensor. The distal end of the rod is designed to make contact with a patient's body to apply a force and deform it. A tracking system captures images via three cameras positioned in the room, allowing the probe to be located and tracked in a world-linked coordinate system. These images are digitally processed to establish an optical flow (i.e., the apparent movement of the body under the effect of the applied force), which allows for the analysis of the displacement of the patient's skin surface.

[0009] The document by Faragasso et al., “Real-Thne Vision-Based Stiffness Mapping” (2018), Sensors, 18, 1347, describes a device for measuring the deformation of a body or object. The device comprises a plurality of punches, each mounted on a spring and connected to a base. When the ends of the punches are applied against the object or body, the springs contract according to the applied force, their stiffness, and the stiffness of the object at the respective points of contact. The measuring device is equipped with a camera that captures an image of the springs to identify their contractions and thus determine the deformation of the body at each point of contact with the punches. Solving a system of nonlinear equations allows the stiffness of the object to be determined, assuming that this stiffness is the same under each punch.

[0010] These prior art devices are particularly complex to implement. Some of them require identifying the position and orientation of the contact probe on the images provided by the cameras in order to reconstruct all the components of the force exerted on the body. The device described in the document Faragasso requires that the force be applied in the direction of spring contraction; this solution does not account for the existence of tangential components of the applied force. Furthermore, the force is applied over a relatively large area of ​​the body, making it difficult to pinpoint the point of application and limiting the effectiveness of the stiffness analysis.

[0011] One object of the present invention is to overcome, at least in part, the limitations of the prior art. In particular, it aims to provide a device for measuring a force exerted on a body that is simple to implement. This device can be used to characterize the rigidity of a body with satisfactory resolution. BRIEF DESCRIPTION OF THE INVENTION

[0012] With a view to achieving one of these goals, the object of the invention proposes a device for measuring a force exerted on a body, the device comprising: - a flexible rod having a proximal end and a distal end intended to come into contact with the body and to apply a force tending to deform it in the vicinity of a point of contact; - a camera attached to the proximal end and oriented towards the distal end of the flexible rod to provide at least one depth map of the body in the vicinity of the distal end of the flexible rod.

[0013] According to other advantageous and non-limiting features of the invention, taken alone or in any technically feasible combination:

[0014] - the measuring device includes a switch to actuate the shooting device; - the measuring device further includes a gripping handle attached to the shooting device and the flexible rod; - the switch is in the form of a trigger; - the measuring device includes a transmission unit for exchanging data with a remote computing device; - the measuring device includes a computing unit configured to process the depth map and establish the force exerted on the body; - the camera device also provides a digital image of the body in the vicinity of the distal end of the flexible rod; - the distal end has a tip with a shape or color chosen to be easily located on the map or image provided by the shooting device.

[0015] According to another aspect, the invention proposes a method for measuring a force exerted on a body, the method comprising: - a step of applying the measuring device as proposed above so that the distal end of the rod contacts the body at a point of contact and applies a force tending to deform the body in the vicinity of the point of contact; - an acquisition step, at a measurement time, of at least one depth map during the application step; - a depth map processing step to establish the force exerted on the body.

[0016] According to other advantageous and non-limiting features of this aspect of the invention, taken alone or in any technically feasible combination:

[0017] the processing step includes a substep of locating the distal end of the rod in the depth map to provide coordinates of this end in a coordinate system associated with the measuring device; The measurement process includes a preliminary step of calibrating the measuring device.

[0018] Finally, according to yet another aspect, the invention proposes a method for characterizing the rigidity of a body comprising acquiring, during the measurement method just presented, a plurality of depth maps in successive measurement times.

[0019] This characterization process may include a step of registering the plurality of depth maps which may optionally exploit visual registration markers arranged on the body. Brief description of the drawings

[0020] Other features and advantages of the invention will become apparent from the detailed description of the invention which follows with reference to the accompanying figures in which:

[0021] [fig. 1] Figure 1 represents a device for measuring the force exerted on a body according to the invention.

[0022]

[0023] [fig.2a] [fig.2b]

[0024] [fig.2c] Figures 2a, 2b and 2c schematically represent respectively the behavior of the measuring device before contact, in simple contact and in forced contact with a body under test;

[0025]

[0026] [fig.3a] [fig.3b]

[0027] [fig.3c] Figures 3a to 3c represent an example of the treatments implemented at during a process according to the invention;

[0028]

[0029] [fig.4a] [fig.4b]

[0030] [fig.4c] Figures 4a to 4c represent an example of the treatments implemented during a process for characterizing the rigidity of a body conforming to an aspect of the invention. DETAILED DESCRIPTION OF THE INVENTION

[0031] Device for measuring the force exerted on a body

[0032] In the example shown in Figure 1, a measuring device 1 according to the invention consists of a head 1b and a gripping handle 1a. The head 1b carries a flexible rod 2 having a proximal end to which the head 1a is connected. A distal end 2b of the flexible rod 2 is intended to come into contact with a body under test. More specifically, the measuring device 1 is manipulated by an operator to bring the distal end 2b close to the body, so as to make contact with it. A force is thus transmitted and applied to the body, tending to deform it in the vicinity of the point of contact.

[0033] For safety reasons, and to avoid damaging the body at the point of contact, the distal end 2b may be fitted with a protective cap. This cap may have a contact surface, i.e., a surface intended to come into contact with the body, that is flat or rounded so as to ensure this protective function, without, however, being too large to preserve the point nature of the contact formed between the distal end 2b of the flexible rod 2 and the body under test.

[0034] By way of example, the protective tip may be a plastic sphere embedded on the distal end 2b of the flexible rod, and having a diameter on the order of a few millimeters, for example between 2 mm and 5 mm. As will be detailed later in this description, the shape of the tip and / or its color may also be chosen to be easily identified on a depth chart or image acquired during the use of the measuring device 1.

[0035] As already mentioned, the rod 2 allows a force applied to the measuring device 1 to be transmitted and this force applied to the body. This rod 2 is flexible, meaning that the reaction force applied to the distal end 2b of the rod 2 when it is in forced contact with the body tends to deform it, for example by bending. To a certain extent, the flexibility of the rod 2 makes the displacement of the distal end 2b of the rod independent of the displacement of the head 1b of the measuring device 1 to which it is attached. The material composing the flexible rod 2 and its shape are chosen so that, within the range of forces that are to be applied to the body, the rod 2 can deform elastically. When the body is a human body, this force can be between 0.1 N and 10 N.

[0036] It can therefore be foreseen that the rod 2 is made of a metal or a plastic. It may also have an elbow or be curved (as shown in the figure 1) to promote its elastic flexing when the measuring device 1 is applied to the body under test. The curved shape of the rod 2 may also have the advantage of placing the distal end 2b, when the rod 2 is not deformed, in the center (or near the center) of the field of view of a shooting device 4 of the measuring device 1, as will be detailed later in this description.

[0037] Continuing the description of Figure 1, the measuring device 1 also includes a camera 4, here a camera integrated into the head 1b of the device 1. The head 1b allows the relative positions of the camera 4 and the proximal end 2a of the rod 2 to be fixed so as to make them rigidly attached to each other. However, the head 1b is not an essential element of the measuring device 1, and other configurations of this device 1 could be considered in which the camera 4 is rigidly attached to the proximal end 2a of the rod 2 without requiring the presence of a head. For example, the proximal end 2a of the rod 2 could be directly attached to the camera 4 or to a housing of this device 4.

[0038] Regardless of how these two elements are joined together, the camera 4 is oriented towards the distal end 2b of the flexible rod 2. In other words, the orientation of the camera 4 is such that the distal end 2b of the flexible rod 2 is within its field of view, that is, within the area of ​​space perceived by the sensor or sensors of the camera 4. As mentioned previously, the device 1 can be configured so that, at rest, the distal end 2b is positioned at the center, or near the center, of this field of view.

[0039] When the measuring device 1 applies a force to the body under test (as will be shown in relation to the description of Figures 2), the orientation of the shooting device 4 towards the distal end 2b of the rod 2 also leads to a portion of the body located in the vicinity of the point of contact being brought into the field of view of the shooting device 4. Since this vicinity of the point of contact is likely to be deformed by the application of the force, the arrangement of the shooting device 4 also makes it possible to capture a deformation profile and an image of the deformation of the body under test.

[0040] According to the invention, the camera capture device 4 is provided with a sensor capable of supplying a depth map. The sensor performs the measurement in a plane, called the "image plane," which is equipped with a coordinate system (x,y) referred to as the "camera capture device coordinate system" in the remainder of this description. This coordinate system can be augmented with a depth direction z, perpendicular to the image plane, to form a complete coordinate system (x, y, z). Since the camera capture device 4 is fixed, without any degrees of freedom, to the head 1b of the measuring device 1, the camera capture device coordinate system also forms a reference point linked to the measuring device 1.

[0041] The depth map provided by the shooting device 4 can take the form of a point cloud whose coordinates are established in the full coordinate system (x, y, z). Alternatively, the map can be in the form of an image made up of elements of the sensor's image plane respectively associated with points of the imaged scene, each element of the image plane also being associated with distance information separating this plane of the shooting device 4 from the point in the scene in question.

[0042] Advantageously, but without this being an essential feature of the invention, the imaging device 4 can also provide a digital image of its field of view. To this end, the imaging device 4 may have optics, or any other means, for imaging the deformed surface of the body under test as well as the distal end 2b of the rod 2. The focal length of the optics can be chosen according to the desired extent of the field of view and therefore the extent of the imaged surface of the body under test. It is also advantageous to choose a depth of field sufficient to include the distal end 2b of the flexible rod 2 for the entire range of forces applied to the body, and therefore for the entire range of bending of the rod 2.

[0043] For the sake of clarity, it is recalled that a digital image is defined by image elements, designated "pixels," which carry at least one numerical value of light intensity, grayscale levels, or color. The pixels are generally arranged in a matrix, in rows and columns. Such a digital image can, however, be recorded in the image capture device 4 and transmitted in another form, for example, a compressed form, but in all cases the image can be manipulated to represent an image composed of pixels arranged in a matrix.

[0044] When the image capture device 4 combines both distance information and color or light intensity information, this device may, for example, consist of an RGB-D camera, which is described in documents US20070216894 and US7433024. In addition to the traditional image sensor, which allows, for example, the determination of a radiation intensity value in the red, green, and blue range for each pixel, an RGB-D camera also projects infrared radiation, the reflection of which is captured by an infrared image sensor. The measured radiation is processed to determine the distance between the sensor and any objects in the field of view, according, for example, to the technology described in the aforementioned documents. The infrared sensor and the image sensor are calibrated against each other, so that it is possible to associate depth information with the intensity information for each pixel.

[0045] It should be noted that the data from the camera 4 conforming to this description are provided in any format, which is not limited to that of a digital image whose pixels are augmented with depth information. This data may also be presented, for example, as already mentioned, in a point cloud format. In such a format, each point of the cloud is represented by its coordinates in the full coordinate system (x, y, z), possibly augmented with color information.

[0046] Regardless of the format in which the data established by the shooting device 4 are conditioned, these will be designated by the term "images" for the data relating to the intensity of the visible radiation perceived on the image plane of the image sensor, and by "depth map" for the data relating to the distance separating the objects of the scene from the shooting device 4. And these data are located in the same coordinate system associated with the shooting device 4, and therefore with the measuring device 1.

[0047] The image capture device 4 can take a succession of images and depth maps, for example every 20 ms, to create an animated sequence. When the measurement device 1 is used to actually analyze the deformation of a body, the image capture device 4 provides data that can be represented as an animated sequence of the deformation of that body, and includes both images and successive depth maps of the deformation.

[0048] To initiate the acquisition of an image and / or a map, or a sequence of such images and maps, the measuring device 1 can be equipped with a switch 5 that can be operated by the operator. The operator can, for example, manually activate the switch and initiate the image acquisition(s) while moving the measuring device 1 towards the body under test and applying a force to the body that tends to deform it. At the end of this step, an animated sequence of images and a depth map illustrating the deformation of the body is available. These images naturally include the distal end 2b of the flexible rod 2, which is positioned at all times within the field of view of the image acquisition device 4.

[0049] For practical reasons, it may be provided that the switch 5 is in the form of a trigger located on the grip handle la of the device 1, as shown in Figure 1. This allows the operator to use the measuring device 1 with one hand and to operate the switch 5 while manipulating the device 1 to apply it to the body under test.

[0050] The data generated by the imaging device 4, digital images and depth maps, or any other data generated from this information, can be transmitted to a remote computing device for processing. For this purpose, The measuring device 1 may include a transmission unit 6 connected to the image capture device 4. This transmission unit 6 may be a simple connector for plugging in a cable to connect the measuring device 1 to the computing device. Alternatively, it may be a more complex electronic circuit for conditioning the data before transmitting it, for example, wirelessly, to the computing device. The transmission unit may also allow the image capture device 4, and more generally the measuring device 1, to be configured and / or operated from the remote computing device.

[0051] The measuring device 1 may also include a computing unit, for example a microcontroller, a graphics chip, an FPGA, specifically configured to perform processing on the data provided by the shooting device 4, before this processed data is transmitted to the remote computing devices.

[0052] These treatments can correspond to a simple formatting of the images and depth maps provided by the shooting device 4, for example to provide them in the form of a cloud of colored points as mentioned previously, or to reduce their resolution, or to extract richer information such as the force exerted by the flexible rod 2 on the body under test which is the subject of the measurement process described below.

[0053] Figures 2a, 2b and 2c schematically represent the behavior of the flexible rod 2 before the distal end 2b contacts a body under test 3, at the moment of contact of this distal end 2b with the body 3 without applying any forces on this body, and when the device 1 is brought close enough to the body 3 for a force to be applied to it.

[0054] In Figures 2a and 2b, no force is applied to the body 3, and it therefore presents its "natural" shape, without deformation. This is also the case for the flexible rod 2. The distal end 2b is thus separated from the measuring device 1 by a rest distance d0. This distance can be, for example, that separating the distal end 2b from the proximal end 2a of the flexible rod 2, or from any other fixed point of the measuring device 1.

[0055] In Figure 2c, the measuring device 1 is brought close enough to the body 3 that the rod 2 applies a force to this body 3 at the point of contact. The body 3 is deformed at the point of contact and in its vicinity. Since the rod 2 is flexible, it is also deformed in bending, so that the distance dl separating its distal end 2b from its proximal end 2a is reduced, compared to the rest distance dO shown in Figures 2a and 2b.

[0056] It is noted that, knowing the distance separating the distal end 2b from the measuring device 1, the force applied by the rod 2 (and therefore by the measuring device 1) on this body 3.

[0057] In the illustration of Figures 2a to 2c, the measuring device 1 is brought close to the body 3 to apply a force, via the rod 2, which has no tangential component. More generally, the measuring device 1 can be brought close to the body 3 in various directions in order to impart to the body 3 a force having a tangential component. In this case, the distal end 2b of the flexible rod 2 moves not only along a depth z direction in the full frame attached to the device, but also along directions perpendicular to this depth z direction. In other words, the coordinates of the distal end 2b of the rod 2 are likely to vary along the three components (x, y, z) in a frame attached to the measuring device 1 when the latter is moved to apply a force to the body using the flexible rod 2.

[0058] In this general approach, it is noted that it is possible to determine all the components of the force applied to the body 3 by the measuring device 1 from the position of the distal end 2b of the rod 2 in the frame linked to the measuring device 1.

[0059] In certain simple cases, the relationship linking the position of the distal end 2b to this force can be formulated explicitly, for example from the dimension of the rod 2 and / or stiffness coefficients. But more generally, this relationship can be established by a preliminary calibration step during which the measuring device 1 is applied to a force sensor pre-calibrated according to 3 components (called a "tri-axis" sensor).

[0060] For example, the sensor can be mounted on a rigid platform that can be moved in a controlled manner in three orthogonal directions. The measuring device 1 is fixed opposite the platform and directly above the force sensor so that the distal end 2b of the flexible rod 2 contacts this sensor. A series of measurements is then taken by moving the rigid platform in the three possible directions to collect:

[0061] -the components fx, fy, fz of the force applied by the flexible rod 2 on the sensor, and such that these components are provided by the sensor;

[0062] - the coordinates (x, y, z) of the distal part 2b of the rod 2 in the frame linked to the shooting device 4. These coordinates can be taken manually from the depth map provided by this device 4 or by digital processing of this map, as will be explained in another part of this description.

[0063] In this way, pairs of measurements {(x, y, z); (fx, fy, fz)} can be collected, relating the position (x, y, z) of the distal end 2b of the rod 2 to the components fx, fy, fz of the force applied by this rod 2. These pairs of measurements can be used to determine, by interpolation, the components of the force applied by the device. measurement 1 on the body for any position of the distal end 2b in a reference frame linked to the measuring device 1 (such as that associated with the shooting device 4).

[0064] In other words, in a measuring device 1 according to the invention, the position of the distal end 2b of the rod 2 vis-à-vis a reference point of the measuring device 1 can be uniquely related to the components of the force applied by this device 1 on a body 3.

[0065] This feature of the measuring device 1 is exploited in order to provide a method for measuring the force exerted on a body 3 which is particularly simple to implement, and which is the subject of the following section of this description.

[0066] Method for measuring the force exerted on a body

[0067] The measurement method comprises a first step of applying a measuring device 1, as described above, to a test body 3. As already mentioned, during this step the measuring device 1 is brought close to the body 3 to force the distal end 2b of the flexible rod 2 into contact with this body 3. A force is thus exerted on the body 3, this force tending to deform the body 3 at the point of contact and in its vicinity. The flexible rod 2 is also deformed. This step can be carried out by an operator or, alternatively, by an automated system, such as a robotic arm whose end is equipped with the measuring device 1.

[0068] Once the forced contact of the measuring device 1 on the body 3 has been established, the device 1 can be moved away from the body 3 and the process can be interrupted, or repeated at an identical or distinct point of contact.

[0069] The measurement method also includes an acquisition step during which at least one depth map is acquired, in a measurement instant, using the shooting device 4. As already mentioned, it will preferentially be carried out to acquire a plurality of maps in successive measurement instants, so as to collect an animated sequence of the deformation of the body under test 3 caused by the forces exerted by the measuring device 1 over time.

[0070] The acquisition step can be initiated and interrupted by the switch 5 or by the trigger of the measuring device 1 operated by the operator during the application step. Alternatively, the acquisition can be initiated by the remote computing device to which the measuring device 1 can be connected, particularly when the latter is part of an automated measuring system.

[0071] Regardless of how the acquisition step is triggered, it shall be temporally included in or overlapping with the application step of the measuring device 1, so that at least one depth map of the body can be acquired while the flexible rod is in forced contact with it. body.

[0072] When the shooting device 4 allows it, this acquisition step also acquires digital images of the body 3 simultaneously with the acquisition of depth maps.

[0073] The data acquired during this step can be stored in a buffer memory of the shooting device 4 or the measuring device 1. They can be processed by the computing unit and / or transmitted to the remote computing device for processing via the transmission unit 6 to which the shooting device 4 is directly or indirectly connected.

[0074] Whether this data is processed locally or by the remote computing device, a measurement method according to the invention includes a step of processing this data (at least one depth map and, optionally, at least one digital image) aimed at establishing the force exerted by the device 1 on the body 3, at the time of measurement.

[0075] To this end, the processing step includes a first substep aimed at locating, using a digital processing technique, the distal end 2b of the flexible rod 2 in the depth map or, when available, in the digital image. This substep can be facilitated by the presence of a protective tip with a specific shape, allowing its outline to be clearly distinguished in the depth map. This locating substep can include conventional digital processing in the field of computer vision and, for example, be implemented by a neural network. Upon completion of this substep, it provides the coordinates, in the (x, y, z) coordinate system associated with the imaging device 4, of the distal end 2b of the rod 2.

[0076] When this processing is performed on the digital image, the tip can be chosen to also have a specific color, thus facilitating its identification in the image. The tip's location can be very simply expressed as the column and row positions of a pixel in the image. The pixel positions and the depth map are then used to determine the coordinates, in the (x, y, z) coordinate system associated with the imaging device 4, of the distal end 2b of the rod 2.

[0077] If the depth map has a lower resolution than the image, this substep may include or correspond to the search for the point on the depth map closest to the coordinates identified in the previous substep, in order to extract consistent information from the depth map.

[0078] In all cases, and regardless of the processing implemented during this substep, the output of this substep provides the coordinates, in the (x, y, z) coordinate system associated with the imaging device 4, of the distal end 2b of the rod 2 at the time of measurement. When several successive measurements are performed, a list is available. of these coordinates.

[0079] In a second substep of the processing step, the position of the distal end 2b of the rod 2 at the time of measurement is converted into a force. This conversion is performed using the calibration function or table of the flexible rod 2, which associates the position of the distal end 2b with an applied force. As previously mentioned, this function or table may have been developed in a calibration step prior to the steps comprising the measurement process itself.

[0080] Figures 3a to 3c illustrate an example of implementing the processes just described. In this example, the image capture device 4 is an RGB-D camera, and the color information provided by the digital images is used to identify the tip of the flexible rod 2. Figure 3a shows the measuring device 1 in forced contact with a body 3 at the distal end 2b of the flexible rod 2.

[0081] To simplify the representation, the body under test 3 has here only one dimension, and an image of this body is therefore composed of pixels xl to xn arranged in a line along an X axis which forms the frame of the shooting device 4. Each pixel xi is, in this example, associated with a measurement of red color intensity, provided by an image sensor of the shooting device 4, and with a depth measurement z, provided for example by a device with an infrared image sensor.

[0082] In the example shown, the body 3 is considered to have an exposed surface that is essentially blue in color, which contrasts well with the red tip of the flexible rod 2 and makes it easier to locate this tip in the image.

[0083] The illustration in Figure 3c represents the depth map, here the distance zi associated with each pixel xi, and the illustration in Figure 3b the red IR intensity image, that is to say the red color intensity IRi associated with each pixel xi.

[0084] The first substep of the identification process is facilitated by choosing a color that is very distinct from that of the distal end 2b of the rod 2 compared to that of the body 3. Thus, in the image composed of pixels associated with a red color intensity, it is easy to identify pixel xO, whose red intensity is clearly marked in relation to the other pixels, which correspond to the body under test 3, here blue in color. In practice, algorithms from the field of computer vision can be implemented to locate the distal end 2b of the rod 2 in the image and to choose a pixel representative of the position of this end in the image.

[0085] Knowing the pixel xO (or more generally the coordinates in the frame associated with the camera 4) corresponding to the distal end 2b, it is easy to determine from the depth map the measured distance dO associated with this pixel, from the depth map illustrated in Figure 3c. Then the pixel and distance information is converted to determine the coordinates of the end distal 2b of the rod 2, which is converted to obtain the force applied to the body 3 at the time of measurement, that is, at the time when the image and depth map were acquired. If a plurality of images and depth maps have been acquired, the substeps can be repeated on each instance of the plurality, and the corresponding forces determined.

[0086] It should be noted that the information provided by the depth map, here, for each pixel of the digital image, the distance separating the exposed surface of the body 3 from the camera 4, is the combination of two phenomena: the deformation of the body 3 which tends to move its surface away from the measuring device 1 and the deformation of the rod 2 which tends to move the measuring device 1 closer to the body 3. However, the estimation of the applied force depends only on this latter distance (or more precisely on the position (x, y, z) of the distal end 2b of the rod 2 with respect to the measuring device 1), so that the method can estimate this force unambiguously.

[0087] Method for characterizing the rigidity of a body

[0088] To be complete however, it should be noted that the information provided by a single measurement (i.e. a depth map and, possibly, a digital image) is not sufficient to characterize by itself the rigidity of the body under test 3 at the point of contact.

[0089] As a preliminary point, it should be noted that the rigidity of a body can be characterized in many ways. For example, it can be represented by the deformation distance of the body at the point of contact, depending on the applied force, for example, of increasing intensity or different directions. The term "deformation distance" refers to the distance D between the point of contact when no force is applied and this point of contact when a specific force is applied by the measuring device 1, as shown in Figure 2a.

[0090] Alternatively, this deformation can be characterized and represented by a deformation profile (for example established from the points (xi, zi) of the depth map, to take up the example of figures 3a to 3c) of the body 3 in the vicinity of the point of contact, rather than by a simple deformation distance D at the point of contact.

[0091] Regardless of how the rigidity of the body 3 is characterized, it relies on the availability of a plurality of depth maps, allowing the deformation of the body 3 to be evaluated for at least two different applied forces. Advantageously, this consists of a first map corresponding to the natural shape of the body 3 (in the absence of any applied force) and at least a second map corresponding to the shape of the body 3 when the measuring device 1 applies at least one determined force, according to its 3 components fx, fy, fz.

[0092] By way of example, the map of is shown in dotted lines in Figure 4a The depth Cl, calculated by the measuring device 1 in Figure 2a, is shown when the distal end 2b of the flexible rod 2 is in contact with the body 3, without exerting any force on this body (zero applied force). The depth map thus defines the undeformed profile of this body 3 within the field of view of the camera 4. On this depth map, at the point of contact between the rod 2 and the body 3, is the distance dO, corresponding to the distance separating the distal end 2b of the rod 2 from the head 1b of the measuring device 1, when this rod 2 is not flexed.

[0093] In Figure 4a, the depth map C2 generated by the measuring device is shown in solid lines when the distal end 2b of the flexible rod 2 exerts a force on the body 3, as depicted in Figure 2a. Since the rod 2 is flexed, the distance dl between the end 2b of the rod 2 and the body 3 is reduced, being less than the distance d0. Furthermore, because the measuring device 1 has generally approached the body under test 3 to apply a deformation force, the depth map C2 generally exhibits smaller values ​​than the depth map Cl of the undeformed body. Applying the measurement method described above allows the force applied to the body to be identified according to its three components (fx, fy, fz).

[0094] Figure 2a shows that the body 3 is deformed in the vicinity of the contact point, but not in peripheral areas Z of this neighborhood. However, a shift is introduced between the depth maps shown in Figure 4a.

[0095] Also, during a characterization process according to the invention, the deformation profiles of the body 3 established using the first and second depth maps (or these first and second maps directly) are registered with each other. More generally, all the depth maps C1, C2 acquired during the process are registered with each other. In this way, the profiles produced by the imaging device 4 carried by the head 1b of the measuring device 1 are registered with each other.

[0096] This registration step may consist of numerically matching the undeformed areas of the depth maps Cl, C2, for example, at least one peripheral area Z in Figure 4a. An undeformed area is sufficiently far from the point of contact so that its offset from one map to the other is not due to the deformation of the body 3, but only to the displacement of the measuring device 1. Figure 4b shows the two depth maps Cl, C2 of Figure 4a, after application of this registration step. Figure 4c shows the depth maps Cl, C2 placed in a coordinate system (x', z') attached to the body under test 3, to present the undeformed and deformed profiles of the body 3.

[0097] To facilitate this depth chart alignment step, the body 3 can be equipped with visual markers, designated as alignment markers, which can be identified on the digital images provided by the imaging device 4. These markers can be particularly useful for detecting a deformation having a rotational component of the body 3, which would not be detectable on the depth charts.

[0098] Once the depth maps have been registered to each other, it is possible to characterize the stiffness of the body 3, for example by identifying the deformation distance D or the deformation profile (which appears on the depth maps in Figure 4a and on the profile maps in Figure 4c), this distance D or profile being associated with the determined force f applied to the body. As a further example of characterizing the stiffness of the body 3, the displacement vector of the point of contact of the rod 2 on the body 3 can be provided. This vector is derived by taking the difference between the coordinates of the distal end 2b of the rod 2 in the two registered depth maps. The vector can be associated with the applied force determined during the measurement procedure to fully characterize the body 3.

[0099] Of course the invention is not limited to the described embodiment and alternative embodiments can be made without departing from the scope of the invention as defined by the claims.

[0100] In particular, the device and the measurement and characterization methods just described can be used to characterize the rigidity of a body of any kind. This may include, but is not limited to, a human body. It may also include any object whose rigidity one wishes to characterize.

[0101] The device and the method can find many practical applications, for example to develop a digital model of the characterized object, this model making it possible to represent or simulate the deformation of an object or body when it is subjected to external forces.

Claims

Claims

1. Device (1) for measuring a force exerted on a body (3), the device (1) comprising: - a flexible rod (2) having a proximal end (2a) and a distal end (2b) intended to come into contact with the body (3) and to apply a force tending to deform it in the vicinity of a point of contact; - a shooting device (4) integral with the proximal end (2a) and oriented towards the distal end (2b) of the flexible rod (2) to provide at least one depth map of the body (3) in the vicinity of the distal end (2b) of the flexible rod (2).

2. Measuring device (1) according to the preceding claim comprising a switch (5) for actuating the shooting device (4).

3. Measuring device (1) according to one of the preceding claims further comprising a gripping handle (1a) integral with the shooting device (4) and the flexible rod (2).

4. Measuring device (1) according to claims 2 and 3 wherein the switch (5) is in the form of a trigger.

5. Measuring device (1) according to one of the preceding claims comprising a transmission unit (6) for exchanging data with a remote computing device.

6. Measuring device (1) according to one of claims 1 to 5 comprising a computing unit configured to process the depth map and establish the force exerted on the body (3).

7. Measuring device (1) according to one of the preceding claims wherein the imaging device (4) also provides a digital image of the body (3) in the vicinity of the distal end (2b) of the flexible rod (2).

8. Measuring device (1) according to the preceding claim in which the distal end (2b) comprises a tip having a color to easily identify the image provided by the imaging device (4).

9. Measuring device (1) according to one of the preceding claims in which the distal end (2b) comprises a tip having a shape chosen to be easily located on the map provided by the imaging device (4).

10. A method of measuring a force exerted on a body (3), the method comprising: - a step of applying the measuring device (1) according to one of the preceding claims so that the distal end (2b) of the rod (2) contacts the body (3) at a point of contact and applies a force tending to deform the body (3) in the vicinity of the point of contact; - a step of acquiring, at a measurement instant, at least one depth map during the application step; - a step of processing the depth map to establish the force exerted on the body (3).

11. Method for measuring a force exerted on a body (3) according to the preceding claim in which the processing step comprises a sub-step of locating the distal end (2b) of the rod (2) in the depth map to provide coordinates of this end (2b) in a reference frame associated with the measuring device (1).

12. Method for measuring a force exerted on a body (3) according to one of claims 10 to 11 comprising a prior step of calibrating the measuring device (1).

13. Method for characterizing the rigidity of a body (3) comprising the acquisition, during a measurement method according to one of claims 10 to 12, of a plurality of depth maps at successive measurement times.

14. Method for characterizing the rigidity of a body (3) according to the preceding claim, comprising a step of resetting the plurality of depth maps.

15. Method for characterizing the rigidity of a body (3) according to the preceding claim in which the body (3) comprises registration marks making it possible to register the plurality of deformation maps.

Images