Psoc 4700

[Doménica Spielmann]

CapSensecapacitive-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.

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.

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:

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.

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.

Today whenever people talk about touch sensing, they are usually referring to capacitive sensing. Capacitive sensing has proven to be robust not only in a normal environmental use cases but, because of its water-resistant capabilities, also underwater. As with any technology, capacitive sensing comes with a new set of disadvantages. These disadvantages tend to more application-specific. And those have opened the door for the advent of inductive sensing technology.

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 (Figure 1). This phenomenon causes some energy to get transferred to the metal target named as eddy current that causes a circular magnetic field. That eddy current induces a reverse magnetic field, and that in turn leads to a reduction in inductance.

To cause the resonant frequency to occur, a capacitor is added in parallel to the coil to create the LC tank circuit. As the inductance starts reducing, the frequency shifts upward changing the amplitude throughout.

Consider another use case for proximity sensing using inductive sensing technology. A vehicle detection system needs to monitor when another vehicle approaches within 2 m and signal the driver on the dashboard or navigation panel. This functionality can be implemented using inductive sensing. A hardware board containing multiple coils at different locations using a flex cable, all around the dashboard, can be designed around the four corners and center of the headlight areas (Figure 3). Data from the inductive coils is collected by an inductive sensing controller such as the PSoC 4700S from Cypress Semiconductor. The controller would then analyze the data to determine the presence or absence of other cars in a 4-m vicinity around the vehicle.

From an engineering point of view, inductive sensing is rugged, environment-independent, and easy to design and develop. In addition, little tuning is required to achieve the desired closed loop for an application. Note: The controller need not be placed far away from the coils to improve SNR. Individual controllers can be used to optimize the design. The block diagram mentioned in Figure 4 is a principle representation.

In general, designing an inductive sensor is straightforward (Figure 4). A typical inductive sensor requires one or more inductive coils, as determined by the requirements of the application. To learn more about designing inductive sensing boards and controllers, make sure to check out the links on the Circuit Cellar article materials webpage. The sensor needs to be interfaced to the controller using suitable drivers or controllers to be understood by the microcontroller (MCU). This interface can be implemented using external components. However, to reduce system design and manufacturing complexity, PSoC integrates driver and converter circuitry to convert inductive sensor data into raw counts, which can then be processed using suitable algorithms.

Figure 4 shows the complete system block diagram of a typical PSoC-based Inductive Sensing board. A typical Inductive Sensing board using PSoC would require a programming and communication device, and here we use the PSoC 5LP device family. The sensor is interfaced with PSoC 4700S device, which communicates to external world using UART/I2C or any kind of feedback interface like LEDs. To program the inductive sensing controller, we need a suitable programmer either on board or using external programmers. You need to decide the maximum power to be provided. Here we have designed the system at 3.3 V, however it can range from 1.8 V to 5 V.

Designing an inductive sensing board using PSoC is straightforward compared to some other sophisticated systems. In an inductive sensing system, we need to take care of the design of tank circuit, which plays major role in tuning the circuitry for inductive sensing. Figure 5 shows the tank circuit involved in the functioning of the inductive sensing. Here L is the coil, Rs is the internal resistance. C is the tank capacitor, whose value is decided based on the frequency of resonance observed. Generally, the system is designed for higher frequency up to 1 MHz or 800 kHz for better performance, however lower frequency too can be chosen for it.

BUTTON OVERLAY DESIGN

The next important part in the system design of inductive sensing board is designing the metal assembly for the button. The overlay design has three major parameters that you need to decide:

Figure 6 shows the mechanical dimension of the overlay and adhesive layer for this kit. The thickness of overlay is an important parameter to decide the sensitivity of the coil response to MagSense. However, with a lower overlay thickness, the lifetime of the board reduces. A 0.5 mm thickness is typically an optimal choice from a button sensitivity and lifetime point of view. The metal target material determines the amount of deflection and response. We recommend using an aluminum overlay for inductive sensing application 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.

To tune the buttons and proximity sensor, it is necessary to measure the inductance and resistance of the coil by themselves and then with an aluminum overlay (for the buttons) and with the metal target at 2 cm (for the proximity sensor). Note here that, the proximity distance is directly proportional to the coil diameter. Once tuning has been completed, sensitivity can be adjusted by changing the resonant frequency by about 5% to 10% depending on your design. During this iteration, it is recommended to reduce the resonant frequency to detect the correct signal. In the CY8CKIT-148, there are a total of 4 inductive coils, out of which 3 are projected as buttons and 1 as proximity.

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