Non-Contact Capacitive Sensing for Robotic Prostheses
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Solution Overview
Problem
Current sensing systems for robotic lower-limb prostheses, such as surface electromyography (sEMG) and capacitive sensing, face challenges including skin contact issues, sweat interference, skin damage, high costs due to complex sampling circuits, and limited applicability for transtibial amputees due to residual limb length constraints.
Innovation Solution
A non-contact capacitive sensing system using capacitance electrodes placed inside the prosthetic socket, forming coupling capacitors with the human body, which converts raw capacitance signals to digital signals through a capacitance-to-digital module and processes them using a digital filter module for noise removal and wireless communication, avoiding skin contact and optimizing electrode placement for effective locomotion mode recognition.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If sEMG electrodes are placed on the skin to record muscle signals, then motion information accuracy is improved, but skin damage and pressure sores occur due to tight adhesion
Solution Approach 1:
The patent introduces a capacitive coupling mechanism as an intermediary between the sensing electrode and the skin. Instead of direct contact, the electrode forms a capacitor with the skin through a non-contact coupling layer, eliminating the need for tight adhesion while still enabling signal detection. This intermediary approach resolves the contradiction by maintaining measurement precision through capacitive coupling while eliminating skin damage caused by mechanical adhesion.
2Reliability
If sEMG electrodes are tightly adhered to the skin to ensure good signal contact, then signal quality is improved, but pressure sores occur during long time use
Solution Approach 1:
The capacitive coupling mechanism serves as an intermediary that maintains signal quality without requiring continuous tight adhesion. The non-contact coupling allows the electrode to remain effectively connected to the skin throughout the day without causing pressure sores, thereby improving both reliability and duration of use.
3Measurement precision
If sEMG electrodes are used to record muscle signals, then motion information is obtained, but the system cost increases due to multi-stage amplifiers and filters
Solution Approach 1:
The patent replaces the complex mechanical/electrical signal conditioning system (multi-stage amplifiers and filters) with a capacitive sensing approach that directly detects muscle shape changes. This substitution eliminates the need for complex analog signal processing hardware, thereby reducing device complexity and cost while maintaining measurement precision.
4Adaptability or versatility
If capacitive sensing bands are placed on the skin to record muscle shape changes, then locomotion mode recognition is enabled, but sweat still impacts on performance
Solution Approach 1:
The non-contact capacitive coupling mechanism acts as an intermediary that isolates the sensing electrode from sweat on the skin. By forming a capacitor through a gap rather than direct contact, the system maintains locomotion mode recognition capability while being immune to sweat interference, as sweat cannot bridge the gap between the electrode and skin.
5Adaptability or versatility
If electrodes are placed inside the prosthetic socket for transtibial amputees, then sensing is enabled, but the system becomes limited by residual limb length
Solution Approach 1:
The patent positions the capacitive sensing electrode in a different spatial dimension relative to the residual limb - specifically, inside the prosthetic socket rather than on the residual limb surface. This dimensional relocation allows sensing to be performed independent of residual limb length, as the electrode can be placed within the socket structure regardless of how much residual limb remains.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
The system provides high repeatability and immunity to sweat, reduces skin pressure, and is cost-effective, enabling reliable locomotion mode recognition for both transtibial and transfemoral amputees by accurately converting capacitance changes into digital signals for efficient prosthesis control.
Implementation Method 1
the sensing front end is configured to form the coupling capacitors with human body
Data Source
AI summary
A non-contact capacitive sensing system for robotic lower-limb prosthesis, comprising a sensing front end, a signal sampling unit and a signal processing unit. The sensing front end is composed of capacitance electrodes inside the prosthetic socket, and the capacitance electrodes locate between the prosthetic socket and the stump sock. Each capacitance electrode forms a capacitor with the human body. The signal sampling unit is composed of the CTD module and the control module. The CTD module measures capacitance values by calculating the ratio of discharge-and-recharge cycles between the under-test capacitors and the reference capacitor. The signal processing unit comprises the filter module and the communication module. The capacitive sensing system is highly repeatable in signals, resistant to sweat, and reliably dressed on a human body. The system performs well regardless of residual limb length and residual muscle strength. It can be widely used in the field of robotic lower-limb prosthesis.