Coplanar Capacitive Keypad and Antenna Layout for EMI Control
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Solution Overview
Problem
Capacitive touch keypads in access control devices are susceptible to noise interference from nearby smart card reader antennas, leading to false key presses and increased size, cost, and complexity when separate printed circuit board assemblies are used to separate the components.
Innovation Solution
Integrating the capacitive touch keypad matrix and antenna on a single printed circuit board assembly, with parallel connections between key locations and the controller to reduce loop size and susceptibility to electromagnetic interference, allowing a substantially coplanar arrangement.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Volume of moving object
If the antenna and capacitive touch keypad matrix are placed in close proximity to one another, then the overall device size is reduced, but noise interference and electromagnetic coupling increase causing false key presses
Solution Approach 1:
The capacitive touch keypad matrix is divided into multiple isolated key contacts, each surrounded by guard traces that segment the electromagnetic field. This segmentation prevents the antenna's electromagnetic radiation from coupling across the entire matrix, isolating interference to individual key locations where it can be more easily managed.
Solution Approach 2:
Guard traces are introduced as intermediary conductive elements between the antenna and the capacitive key contacts. These guard traces act as electromagnetic shields, intercepting and redirecting electromagnetic fields before they can couple onto the key contacts, thereby mediating the interaction between the antenna and keypad matrix.
2Object-affected harmful factors
If separate printed circuit board assemblies are used to house the antenna and capacitive touch keypad matrix, then noise interference is reduced, but device size, cost, and complexity increase
Solution Approach 1:
The antenna and capacitive touch keypad matrix are merged onto a single printed circuit board assembly. By integrating both components on the same PCB, the design eliminates the need for separate PCBAs, reducing overall device complexity, size, and cost while managing electromagnetic interference through careful layout and guard trace implementation.
Solution Approach 2:
Different regions of the printed circuit board are assigned different functional qualities: the antenna region is optimized for electromagnetic radiation, while the keypad matrix region is optimized for capacitive sensing. Guard traces with specific electrical characteristics are placed locally around key contacts to provide targeted electromagnetic shielding only where needed, rather than requiring complete physical separation.
3Device complexity
If the antenna and capacitive touch keypad matrix are integrated on a single printed circuit board assembly, then device size and complexity are reduced, but electromagnetic coupling and false key presses may occur
Solution Approach 1:
Guard traces are pre-positioned around each capacitive key contact before the device operates. This preliminary arrangement of conductive shields ensures that when the antenna transmits electromagnetic signals, the key contacts are already protected from coupling, preventing false key presses before they can occur.
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
Reduces noise interference and coupling with electromagnetic fields, minimizing size and cost while maintaining effective operation of both components.
Implementation Method 1
capacitive touch keypad matrix
Implementation Method 2
antenna radiating in close proximity to the keypad matrix
Implementation Method 3
parallel connections between key locations and the controller to reduce loop size and susceptibility to electromagnetic interference
Data Source
AI summary
An exemplary an access control device generally includes a keypad assembly, an antenna, and a controller. The keypad assembly includes a capacitive touch keypad matrix, and the antenna is substantially coplanar with the capacitive touch keypad matrix. The controller is connected with each of the keypad assembly and the antenna such that the controller is operable to receive access codes via each of the keypad assembly and the antenna.