PSoC MCUs: Delivering Capacitive-Sensing & Inductive-Sensing for Today’s Embedded Designs
By: Michiyuki Yoneda, MCU Staff Product Marketing Engineer, Cypress
For the past several years, Human Machine Interfaces (HMIs) have increasingly become a key application for providing the point of contact between a user and a machine. Technologies such as resistive-sensing were first used to improve the interface between a machine and human but resulted in sub-optimal performance. Responsiveness, false touches, usability, and shorter operating life were issues faced with resistive-sensing.
Many of these HMIs have moved to capacitive-sensing and inductive-sensing techniques, which provide the robustness, intelligence, and ease-of-use that resistive-sensing could not. Cypress offers both types of these touch-sensing technologies, with CapSense® capacitive-sensing and MagSense™ inductive-sensing.
CapSense capacitive-sensing operates on the principle of monitoring the change in parasitic capacitance due to a finger touch. It provides two types of sensing modes: self-capacitance and mutual-capacitance. In self-capacitance mode, the net capacitance due to a finger touch and board capacitance (including PCB traces and PCB materials like FR4) is additive. Self-capacitance mode is useful in general touch application like buttons for touch-and-respond applications. In contrast, mutual-capacitance is well-suited for applications involving more complex sensing such as gestures, multi-touch, and sliders.
Mutual-capacitance sensing utilizes two different lines: Tx (transmitter) and Rx (receiver). The Tx sends a signal with respect to the system VDDD and GND. The Rx detects the amount of charge received on the Rx electrode.
One disadvantage of capacitive-sensing is that it cannot operate underwater. It also requires relatively strict design guidelines to be followed for error-free operation. Capacitive-sensing performance is also impacted by nearby LEDs and power lines on PCBs. Implementing auto-tuning with variation in trace capacitance, variation in capacitive sensing buttons, and different slider sizes and shapes also require different designs. Implementing capacitive-sensing with thicker glass material (display glass) and meeting capacitive sensor sensitivity with these types of materials are also implementation challenges in HMI applications. Cypress’ PSoC 6 and PSoC 4 MCUs provide CapSense blocks to enable this industry-leading technology to the masses.
Check these Development Kits out to get started with each available at RS:
PSoC 6 MCU:
PSoC 6 BLE Pioneer Kit (CY8CKIT-062-BLE) (136-7817)
PSoC 6 WiFi-BT Pioneer Kit (CY8CKIT-062-WIFI-BT) (175-4669)
PSoC 4 MCU:
PSoC 4100S Plus Prototyping Kit (CY8CKIT-149) (176-9834)
PSoC 4 BLE Pioneer Kit (CY8CKIT-042-BLE-A) (124-4184)
PSoC 4 L-Series Pioneer Kit (CY8CKIT-046) (124-4187)
Typical Capacitive Sensing Design
Inductive sensing enables next-generation HMIs that require high quality metal overlays in automotive, industrial, consumer and IoT applications. Inductive Sensing is based on the principle of electromagnetic coupling, between a coil and the target. When a metal target comes closer to the coil, its magnetic field is obstructed, and it passes through the metal target before coupling to its origin. This phenomenon causes some energy to get transferred to the metal target named as eddy current which causes a circular magnetic field. Eddy current induces a reverse magnetic field, in turn leading to a reduction in inductance.
Inductive Sensing Technique
To generate the resonant frequency, a capacitor is added in parallel to the coil to cause the LC tank circuit. As the inductance starts reducing the frequency shifts upward changing the amplitude throughout.
With the removal of a dielectric from the sensor, compared to capacitive-sensing, inductive-sensing is able to operate reliably in the presence of water. Thus, inductive-sensing brings touch sensing to a wide range of applications that involve liquids such as underwater equipment, flow meters, RPM detection, medical instruments, and many others. Inductive-sensing also supports biomedical applications. In general applications, inductive-sensing enables replacement of mechanical switches and proximity sensing of metal objects. For example, in automotive applications, inductive-sensing can be used to replace mechanical handles as well as detect car proximity. The PSoC 4700 MCU implements MagSense block to enable this exciting technology to the masses. Check out this development kit at RS:
PSoC 4700S Inductive Sensing Evaluation Kit (CY8CKIT-148) (181-3725)
Use Cases for Inductive-Sensing and Capacitive-Sensing
Capacitive-sensing is very useful in various applications. However, for certain use cases, inductive-sensing offers greater reliability, robustness and usability.
Consider the use case of a Bluetooth speaker that needs to be waterproof as its intended use is underwater. This use case requires that the product functions underwater and will be responsive to touch in this situation. Such operation needs to be simple, consistent, and reliable, even underwater.
With capacitive-sensing, this operation is partially possible using self and mutual capacitive-sensing. However, the device would only reject the water and cancel out any false touches. This is not fully waterproof but is liquid tolerant.
For this application, metal-over-touch using inductive-sensing would provide a consistent and reliable user performance. Alternatively, a mechanical button and/or dial could be used. However, a mechanical interface is costly compared to a coil printed on a PCB and connected to a few passive components. Additionally, a mechanical button can break or fail, providing a much shorter useable lifespan than an inductive button would.
The architecture of a waterproof Bluetooth speaker using inductive-sensing
All in all, both capacitive sensing and inductive sensing have a wide variety of use cases across many different markets. Cypress is an industry-leading in this space with PSoC architecture enabling anyone to integrate these advanced touch-sensing technologies in their next design.