Comparing capacitive and inductive sensing for embedded applications
Consider another use case employing proximity sensing. A vehicle detection system needs to monitor when another vehicle approaches within two meters and signal the driver on the dashboard or navigation panel. This functionality can be implement using inductive sensing. A hardware board containing a coil around the dash board can be designed around the four corners and center of the headlight areas (see Figure 2). Data from the inductive coils is collected by an inductive sensing controller such as the PSoC 4700S from Cypress. The controller would then analyze the data to determine the presence or absence of other cars in a four-meter vicinity around the vehicle.
Figure 2: Using inductive sensing to determine vehicle proximity in an automotive application. (Source: Cypress Semiconductor)
Capacitive sensing could also be used for vehicle proximity sensing. However, capacitive sensing is more sensitive to noise due to changes in the environment like rain and snow. Inductive sensing is rugged, environment-independent, and easy to design and develop from an engineering point of view. In addition, little tuning is required to achieve the desired closed loop for a particular application.
Implementing Inductive Sensing
In general, designing an inductive sensor is fairly straightforward (see Figure 3). A typical inductive sensor requires one or more inductive coils, as determined by the requirements of the application. The sensor needs to be interfaced to the controller using suitable drivers or controllers to be understood by the microcontroller. This interface can be implemented using external components. However, to reduce system design and manufacturing complexity, some inductive controllers integrate driver and converter circuitry to convert inductive sensor data into raw counts which can then be processed using suitable algorithms.
Figure 3: Inductive Sensor Block Diagram (Source: Cypress Semiconductor)
Figure 4 shows the design flow for a typical inductive sensing application. First, assess how sensitive the system needs to be. Sensitivity determines the coil size and its number of turns are decided. The application also impacts the shape of the coil. For example, a slider interface requires a series of squares or an elongated rectangle. The next step is to calculate the tank capacitor and the inductance based on the number of turns, spacing, width, and diameter.
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Figure 4. Design flow for a typical inductive sensing application. (Source: Cypress Semiconductor)
In comparison, capacitive sensing requires measurement of the theoretical capacitance with the required dielectric constant. Then, during the layout, the designer must follow take strict layout guidelines like ground shielding, making sure sensor traces have equal length for a constant Cp, and so on. For more details, Capacitive Sensing Design Guide.
Once these parameters are decided, the next step is to begin the mechanical design, specifically the overlay, also known as the metal target. An overlay comprises two materials whose specifications need to be decided: the metal target and the adhesive. The metal target material determines the amount of deflection and response. We recommend using an aluminum overlay for inductive sensing application over here because of its better deflection and response. For button applications, a higher Newton force on the overlay causes deflection throughout the overlay, leading to undesirable false triggering throughout the coils. For this use case, the user should only be able to press the buttons just enough to generate feedback. Pressing the overlay harder can even deform the overlay.
Once all these things are intact, the board is designed and fabricated. The advantage of using a design environment, such as PSoC Creator IDE, is that it can help developers accelerate design of a user-friendly inductive sensing tuner graphical user interface.
Both capacitive and inductive sensing enable developers to build intuitive, touch-based user interfaces to make their products more intuitive and easier to use. Because of its versatility, capacitive sensing has become the technology of choice in a great many applications. However, for applications where water tolerance is required, inductive sensing provides a robust and cost-effective alternative.
Ronak Desai is a system engineering manager at Cypress. He is responsible for the development of development kits and hardware design. He has 18+ years of experience in the embedded industry, including the design and development of embedded products and set-top box applications. He has worked extensively using embedded hardware and software platforms and has experience with microcontrollers, embedded applications, set-top boxes, RF, and CATV.