A flexible fingertip touch and slip sensor based on fiber grating and its detection method

Abstract

The present invention relates to a flexible fingertip touch and slip sensor based on fiber grating, which uses fiber grating as a sensor and is encapsulated in a silicone rubber fingertip, wherein the optical path portion includes the fiber grating with a reflection bandwidth, a Faraday rotator, a coupler, a sweeping light source module, a photodetector and a host computer; the fiber grating, the Faraday rotator, the sweeping light source module and the photodetector are respectively connected to four ports of the coupler to form a Michelson interference structure; the sweeping light source module performs wavelength scanning at a fixed repetition period, and the output light beam is split into two paths of light through a coupler and enters a detection arm and a reference arm respectively, the first path of light is reflected by the fiber grating and used as detection light, the second path of light is reflected by the Faraday rotator and used as reference light, and the reference light reflected by the reference arm and the detection arm and the detection light interfere to form a light that enters the photodetector. The present invention also provides a touch and slip detection method using the above sensor.

Description

Technical Field

[0001] The invention belongs to the technical field of flexible electronics, and in particular relates to a fingertip touch and slip sensor based on a fiber grating and a wavelength-sweeping coherence tomography structure. Background Art

[0002] With the advent of the era of intelligent robots, dexterous robotic arms are playing an increasingly wide role in production and life. They are no longer limited to operating on a production line with a fixed working condition, and their application areas have expanded to offices, homes, medical treatments and other places. Tactile perception is a basic function of organisms, and it is also one of the important means for robotic arms to directly interact with the external environment. It enables robotic arms to perceive the temperature, stress, strain, size, position, surface shape and other information of the contact target, which can help robots accurately detect and identify targets, thereby completing a variety of complex tasks. Slip perception is an important part of the robot's tactile sense, and the study of the robot's slip sense has attracted the attention of more and more researchers. Moreover, collecting and using these tactile sensing information can help robots more smoothly analyze and process the relationship with the surrounding environment, and optimize and improve the robot's perception performance.

[0003] The fingertip slip sensor is a sensing device that can detect whether a robot hand and an object are sliding. Detecting sliding by changes in vibration and static force is one of the main methods in the field of slip sensing. At present, the touch and slip sensor has used resistance, capacitance, piezoelectricity, optics and photoelasticity technology to sense the vibration, stress and deformation of components. However, most traditional sensor technologies can only accurately detect one of the physical quantities. To detect static and dynamic information at the same time, vibration and stress sensors must be installed at the same time. When the sensor is assembled on the robot hand, it will face problems such as miniaturization, multiple wires, heavy weight and complex installation. Optical fiber sensors have the advantages of small size, light weight, good stability, high sensitivity and easy reuse. They have good application prospects in the field of robot touch and slip sensing. There have been many research results on using fiber Bragg grating for robot touch and slip sensing. However, the existing solutions based on fiber Bragg grating only use the wavelength deviation of fiber Bragg grating as a detection quantity, which leads to the need to use array multiplexing method to complete the detection of slip, so it is difficult to use it in the field of fingertip touch and slip sensing. Summary of the invention

[0004] The present invention proposes a fingertip touch and slip sensor based on fiber grating and wavelength scanning coherence tomography, which can realize the simultaneous detection of dynamic and static dual parameters and provide a method for detecting touch and slip. The scheme provided by the present invention is as follows:

[0005] A flexible fingertip touch and slip sensor based on fiber grating, which uses fiber grating as a sensor and is encapsulated in a silicone rubber fingertip, and its optical path part includes the fiber grating 1 with a reflection bandwidth, a Faraday rotation mirror 2, a 2×2 coupler 3, a sweeping light source module 4, a photodetector 5 and a host computer 7;

[0006] The fiber Bragg grating 1, the Faraday rotating mirror 2, the sweeping light source module 4 and the photodetector 5 are respectively connected to the four ports of the 2×2 coupler 3 to form a Michelson interference structure, wherein the optical path where the fiber Bragg grating 1 is located is the detection arm, and the optical path where the Faraday rotating mirror 2 is located is the reference arm; the output of the photodetector 5 is sent to the host computer 7;

[0007] The scanning light source module 4 performs wavelength scanning at a fixed repetition period, and its output light beam is split into two light beams by a 2×2 coupler 3 and enters the detection arm and the reference arm respectively. The first light beam is reflected by the fiber grating 1 and used as the detection light, and the second light beam is reflected by the Faraday rotation mirror 2 and used as the reference light. The reference light reflected by the reference arm and the detection arm and the detection light interfere with each other to form the light transmitted to the photodetector 5; the host computer 7 is used to process the electrical signal collected by the photodetector 5.

[0008] Furthermore, when the wavelength of the swept light source reaches the reflection wavelength of the fiber Bragg grating, the detection light and the reference light interfere with each other in the 2×2 coupler, and an interference signal of a scanning frame is formed after one scanning cycle; the upper computer receives the interference signal converted into an electrical signal through the photoelectric detector, and the signal demodulation scheme of tactile detection is:

[0009] The interference signal corresponding to the scanning frame when no contact occurs is selected as a reference signal;

[0010] De-noising the interference signal corresponding to the scanning frame;

[0011] The envelope of the interference signal corresponding to the scanning frame is extracted through Hilbert transform;

[0012] The offset between the envelope of the interference signal corresponding to the scanning frame and the envelope of the reference signal is calculated by cross-correlation to obtain the wavelength offset of the fiber grating of the interference signal corresponding to the scanning frame relative to the reference frame; whether contact occurs is determined based on whether the offset exceeds a preset threshold.

[0013] Furthermore, after the touch is detected, the slip detection is continued, and the signal demodulation scheme of the slip detection is:

[0014] Performing Hilbert transform on the interference signal corresponding to the scanning frame to extract its envelope;

[0015] Using the extracted signal envelope to perform de-envelopment and normalization operations on the interference signal;

[0016] According to the de-enveloped and normalized interference signal, the vibration signal of the scanning frame is demodulated;

[0017] The variance of the vibration signal of the scanning frame is calculated, and the variance is used as a measure of the vibration intensity detected in the current scanning frame; whether sliding occurs is determined based on whether the variance exceeds a preset threshold.

[0018] Compared with the prior art, the present invention has the following advantages:

[0019] 1) Compared with traditional touch and slip sensors based on resistance, capacitance, and piezoelectricity, the sensor proposed in the present invention has a simple and lightweight sensing structure, does not require complex wiring, and simultaneously realizes the detection of static stress and dynamic vibration on a fiber Bragg grating.

[0020] 2) Compared with traditional touch and slip sensors based on resistance, capacitance, and piezoelectricity, the sensor proposed in the present invention has good anti-electromagnetic interference capability, which can enable the robot equipped with the sensor to work in a more complex environment.

[0021] 3) Compared with the traditional fiber Bragg grating touch and slip sensor, the present invention uses light interference to detect vibration, thereby achieving high-frequency and more subtle vibration detection.

[0022] 4) Compared with the traditional fiber Bragg grating touch and slip sensor, since the present invention can use static force information and vibration information for comprehensive analysis at the same time, only one fiber Bragg grating is needed to complete the detection of touch and slip.

[0023] 5) Compared with the traditional touch and slip sensor based on the piezoelectric principle, the static information and dynamic information detected by the present invention are modulated into the time domain and frequency domain of the interference signal respectively, thus avoiding the aliasing of the dynamic vibration signal and the static force signal. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 It is a schematic diagram of the optical path of the touch and slip sensor proposed by the present invention.

[0025] Figure 2 It is a schematic diagram of the fiber grating packaging of the present invention.

[0026] Figure 3 The left and right figures are the scanning frame signals of the interference signal when it is modulated by stress and vibration, respectively.

[0027] Figure 4 The upper figure is the scanning frame interference signal after de-envelope; the lower figure is the vibration waveform demodulated by WSST instantaneous frequency.

[0028] Figure 5 Schematic diagram of sliding sensor test system

[0029] Figure 6Measurement waveforms of the sliding sensing process: static signal wavelength-time curve and vibration signal variance-time curve. DETAILED DESCRIPTION

[0030] How to use the touch and slip sensor proposed in the present invention to perform touch and slip detection is described below in conjunction with the accompanying drawings and specific embodiments.

[0031] The touch-slip sensor for simultaneous detection of dynamic and static dual parameters based on fiber grating and wavelength-scanned coherence tomography provided by the present invention is as follows.

[0032] 1) System optical path

[0033] The optical path of the system is as follows Figure 1 As shown, it includes a fiber Bragg grating 1 with a reflection bandwidth of 1 nm, a Faraday rotation mirror 2, a 2×2 coupler 3, a swept wave light source module 4, a photodetector 5, a data acquisition card 6, and a host computer 7.

[0034] The fiber Bragg grating 1, the Faraday rotating mirror 2, the sweeping light source module 4 and the photodetector 5 are respectively connected to the four ports of the 2×2 coupler 3 to form a Michelson interferometer structure, wherein the optical path where the fiber Bragg grating 1 is located is the detection arm, and the optical path where the Faraday rotating mirror 2 is located is the reference arm. The photodetector 5 is connected to the data acquisition card 6 through an SMA cable, and the data acquisition card 6 is connected to the host computer 7 through a PCIE interface.

[0035] 2) Fiber Bragg Grating Packaging

[0036] The packaging scheme adopted by the present invention is as follows Figure 2 As shown: the fiber Bragg grating is encapsulated in a silicone rubber fingertip as a sensor, and the silicone rubber fingertip is pasted on a silicone fingertip. The fingertip can be installed on the fingertip of an anthropomorphic robot of corresponding size as needed. Figure 2 The dotted line in the middle is the optical fiber encapsulated inside the fingertip, and the solid line is the optical fiber exposed outside.

[0037] Different from the traditional fiber Bragg grating sensing system, the system proposed in the present invention uses wavelength scanning coherence tomography to sample the interference spectrum of the sensing fiber Bragg grating, and is characterized by:

[0038] 1) The scanning light source module 4 provides detection light for the entire sensing system. The detection light is split into two beams through the 2×2 coupler 3 and enters the detection arm and reference arm of the interferometer respectively. The first beam is reflected by the fiber grating 1 and used as the detection light, and the second beam is reflected by the Faraday rotator 2 and used as the reference light. The reference light and the detection light reflected by the reference arm and the detection arm interfere in the 2×2 coupler to form interference light, and the interference light is transmitted from the 2×2 coupler to the photodetector 5. The photodetector 5 converts the interference light signal output by the 2×2 coupler into an analog electrical signal and transmits it to the data acquisition card 6 via an SMA wire. The data acquisition card 6 further converts the analog electrical signal into a digital signal through an analog / digital conversion and transmits it to the host computer 7 through the PCIE interface. The host computer 7 further processes the obtained signal.

[0039] 2) The wavelength of the detection light output by the sweeping light source module is linearly related to time, and the wavelength is scanned at a fixed repetition period. In a scanning cycle, when the wavelength of the sweeping light source reaches the reflection wavelength of the fiber grating, the detection light and the reference light interfere with each other in the 2×2 coupler, and an interference signal of a scanning frame is formed after a scanning cycle, and is transmitted into an electrical signal through the photodetector 5. Finally, the interference signal g(λ) collected by the data acquisition card 6 can be expressed as:

[0040]

[0041] Where R is the reflection spectrum of the fiber Bragg grating, which is reflected as the envelope of the interference signal, L is the optical path difference of the fiber Bragg grating, and n is the effective refractive index of the fiber. The envelope R(λ) of the interference signal g(λ) contains strain and temperature information. When the temperature strain or stress strain causes the central wavelength of the fiber Bragg grating to change, the envelope of the interference signal can be observed to shift in each scanning frame, such as Figure 3 As shown in the left picture.

[0042] 3) Vibration will cause phase modulation of the coherent signal during light wave transmission. The phase change can be decomposed into three terms:

[0043]

[0044] In the formula, β is the transmission constant of light, and α is the radius of the optical fiber. The first term is the phase change caused by the fiber length, which is the strain effect; the second term is the phase change caused by the refractive index, which is the photoelastic effect; the last term is the phase change caused by the change in the fiber radius, which is the Poisson effect. By performing periodic phase changes in the interference fringes, the output signal in (1) can be superimposed as:

[0045]

[0046] like Figure 3 As shown in the picture on the right.

[0047] 4) The original signal collected by the present invention is in the form of a scanning frame sequence. When the sensor proposed by the present invention is used for touch and slip detection, the fiber grating wavelength offset and the dynamic vibration signal need to be demodulated from each scanning frame.

[0048] The signal demodulation scheme proposed by the present invention is:

[0049] Demodulating Fiber Bragg Grating Wavelength Deviation:

[0050] When force is applied to the sensor fingertip, the wavelength of the fiber Bragg grating will drift. The wavelength drift is manifested as the interference signal of the scanning frame shifting left and right on the time axis, such as Figure 3 As shown in the left picture.

[0051] In the first step, the interference signal corresponding to the scanning frame when no contact occurs is selected as the reference signal.

[0052] The second step is to perform Gaussian denoising on the interference signal corresponding to the current scanning frame.

[0053] The third step is to extract the envelope of the interference signal corresponding to the scanning frame through Hilbert transform.

[0054] The fourth step is to use cross-correlation to calculate the offset between the envelope of the interference signal corresponding to the scanning frame and the reference signal envelope to obtain the wavelength offset of the fiber grating of the interference signal corresponding to the scanning frame relative to the reference frame; and determine whether contact occurs based on whether the offset exceeds a preset threshold.

[0055] Demodulate vibration signal:

[0056] When the sensor detects vibration, it detects the vibration by demodulating the instantaneous frequency of the coherent signal of each frame.

[0057] The first step is to perform Hilbert transform on the interference signal corresponding to the current scanning frame to extract its envelope;

[0058] In the second step, the interference signal is de-enveloped and normalized using the extracted signal envelope.

[0059] The third step is to perform WSST transformation on the de-enveloped and normalized interference signal to demodulate the vibration signal of the current scanning frame, such as Figure 4 shown.

[0060] The fourth step is to calculate the variance of the vibration signal of the current scanning frame as a measure of the vibration intensity detected in the current scanning frame; and determine whether slippage occurs based on whether the variance exceeds a preset threshold.

[0061] The specific implementation platform includes: 1. a mechanical finger equipped with the sensor proposed by the present invention; 2. a press with a pressure gauge; 3. a sliding platform controlled by a stepper motor; 4. a wooden block with a certain roughness for testing the sliding sensation.

[0062] Before implementation, the wooden block is fixed on the sliding platform, and the wooden block and the sliding platform remain relatively still during the movement of the sliding platform.

[0063] Figure 2 This is a schematic diagram of the fingertip sensor. The sensing fiber Bragg grating is encapsulated in the fingertip of the sensor. During the implementation process, the mechanical finger equipped with the sensor is fixed so that the fingertip and the pressure gauge contact are always kept on the same vertical line, and the pressure gauge probe is against the back of the fingertip mechanical finger. Then the mechanical finger is placed close to the surface of the wood block so that the sensor fingertip is in parallel contact with the surface of the wood block.

[0064] Sensor measurement system such as Figure 5 shown.

[0065] The signals collected during the implementation process are demodulated using the demodulation algorithm proposed in the manual. The demodulated fiber Bragg grating wavelength offset-time curve and vibration signal variance-time curve are normalized and displayed simultaneously. Figure 6 middle.

[0066] The specific implementation process is as follows, divided into three stages:

[0067] Phase 1: The press applies pressure to the mechanical finger, and adjusts the pressure from 0 to 1N. The pressure gauge stops when the reading reaches 1N. The pressure is fixed at 1N, and the stepper motor of the sliding platform is driven to make the sliding platform move at a uniform speed of 2.5cm / s along the slide rail. In this phase, in addition to the normal force, there is also a tangential force. As the relative displacement between the wooden block and the sensor finger increases, the tangential force increases further. However, the tangential force has not increased enough to cause sliding at this stage, and there is no relative displacement between the wooden block and the sensor finger. Phase 1 corresponds to Figure 6 From t1 to t2, it can only be observed that the normalized value of the fiber Bragg grating wavelength drift increases from about 0.13 in the reference stage to 0.4, the corresponding strain is about 110με, the corresponding stress is about 980mN, and the normalized variance value of the vibration signal in this stage is always around 0. This means that there is only static force but no vibration in this stage.

[0068] Stage 2: The sliding platform continues to move. In this stage, the tangential force increases to the critical point of static friction-sliding friction. Then the wooden block and the sensor fingertip undergo relative displacement and sliding friction occurs. The vibration caused by sliding friction is transmitted to the fiber Bragg grating and coupled to the interference signal of the scanning frame in the form of instantaneous frequency change. Stage 2 corresponds to Figure 6From t2 to t3 in the figure, it is observed that the normalized variance of the vibration signal fluctuates from the lowest 0.1 to the highest 0.6, and the normalized wavelength drift of the fiber Bragg grating also fluctuates significantly from the lowest 0.25 to the highest 0.75 compared with the previous stage. The corresponding strain range is about 53με to 189με, and the corresponding stress is 223mN to 1750mN. At this stage, it can be found that the stress value exceeds the fixed 1N applied, because the strain of the fiber Bragg grating is the result of the combined action of the normal force and the tangential force (friction force).

[0069] Stage 3: The sliding platform stops moving and gradually removes the pressure on the mechanical finger. The pressure gauge reading gradually changes from 1N to 0. Stage 3 corresponds to Figure 6 From t3 to t4 in the figure, it can be seen that the normalized variance of the vibration signal becomes 0, and then the normalized wavelength offset of the fiber Bragg grating gradually returns to the reference value of about 0.1.

Claims

1. A flexible fingertip touch and slip sensor based on fiber grating, which uses fiber grating as a sensor and is encapsulated in a fingertip made of a flexible material, and its optical path includes the fiber grating with a wide reflection bandwidth, a Faraday rotator mirror, a coupler, a swept light source mode, a photodetector and a host computer; The fiber Bragg grating, Faraday rotator, sweeping light source module and photodetector are connected to the four ports of the coupler respectively to form a Michelson interference structure, where: The optical path where the fiber Bragg grating is located is the detection arm, and the optical path where the Faraday rotator is located is the reference arm; the output of the photodetector is sent to the host computer; The swept light source module performs wavelength scanning at a fixed repetition period. Its output light beam is split into two light paths by a coupler and enters the detection arm and the reference arm respectively. The first light path is reflected by the fiber grating and used as the detection light, and the second light path is reflected by the Faraday rotation mirror and used as the reference light. The reference light reflected by the reference arm and the detection arm interferes with the detection light to form a light that is transmitted to the photodetector; the host computer is used to process the electrical signals collected by the photodetector.

2. The flexible fingertip touch and slip sensor according to claim 1, characterized in that: The coupler is a 2×2 coupler.

3. The flexible fingertip touch and slip sensor according to claim 1, characterized in that: The flexible material is silicone rubber.

4. A method for detecting touch and slip sensation using the flexible fingertip touch and slip sensation sensor according to any one of claims 1 to 3, characterized in that: When the wavelength of the sweeping light source reaches the reflection wavelength of the fiber Bragg grating, the detection light and the reference light interfere with each other in the coupler, and an interference signal of a scanning frame is formed after one scanning cycle. The upper computer receives the interference signal converted into an electrical signal through the photoelectric detector. The signal demodulation method of tactile detection is as follows: An interference signal corresponding to a scanning frame when no contact occurs is selected as a reference signal; Performing denoising on the interference signal corresponding to the scanning frame; The envelope of the interference signal corresponding to the scanning frame is extracted through Hilbert transform; The offset between the envelope of the interference signal corresponding to the scanning frame and the envelope of the reference signal is calculated by cross-correlation to obtain the wavelength offset of the fiber grating of the interference signal corresponding to the scanning frame relative to the reference frame; whether contact occurs is determined based on whether the offset exceeds a preset threshold.

5. The method for detecting tactile sensation according to claim 4, characterized in that: After the touch is detected, the slip detection is continued. The signal demodulation method of the slip detection is: Perform Hilbert transform on the interference signal corresponding to the scanning frame to extract its envelope; Using the extracted signal envelope, the interference signal is de-enveloped and normalized; According to the de-enveloped and normalized interference signal, the vibration signal of the scanning frame is demodulated; Calculate the variance of the vibration signal of the scanning frame, and use the variance as a measure of the vibration intensity detected in the current scanning frame; Whether sliding occurs is determined based on whether the variance exceeds a preset threshold.

Images