Capacitive sensors replace mechanical switches to make products more portable

Capacitive Sensors The success of scroll wheels in products such as the iPod is prompting other MP3 manufacturers to consider using capacitive sensors to improve the user interface of their products and make their products look simpler and more beautiful. In fact, the application of capacitive touch sensors is not limited to MP3 players, but can be used in any product that currently uses traditional mechanical switches, especially small portable products such as menu control buttons on new mobile phones. The style of these advanced menu control switches can be easily changed by using highly reliable and cost-effective capacitive touch sensors. The capacitive touch sensor interface usually consists of a capacitive sensor, a capacitance-to-digital converter (CDC) and a host processor. The sensor is manufactured using traces or flexible circuits on standard two-layer or four-layer PCBs, so no external components and materials are required. Sensitivity: Accurate and flexible A reliable sensor must be unaffected by changes in the external environment and be able to maintain accurate sensitivity levels under all operating conditions. Due to changes in temperature or humidity, the properties of the PCB material will change, so the output level of the printed circuit capacitive sensor will drift. For example, this may occur when a user moves from a car with air conditioning on to a hot and humid environment. To avoid intermittent contact errors, the CDC must include real-time drift compensation. As environmental conditions change (e.g., temperature or humidity rises), the environmental parameters of the sensor drift. During periods when the user is not in contact with the sensor, the environmental parameters are measured by the CDC. To compensate, the high-end and low-end threshold levels are dynamically adjusted to determine a valid sensor contact. The PCB can also be plagued by parasitic capacitance, which can be as large as 20pF, which can shift the threshold. When the capacitance is at the threshold, the capacitive touch sensor is considered pressed, so the threshold shift changes its sensitivity. To compensate for parasitic capacitance, a DAC can be programmed to offset the input to the CDC. This parasitic capacitance is consistent from PCB to PCB, so it can be simply adjusted when the PCB is manufactured, eliminating the need for external RC tuning components and minimizing the cost of materials, assembly, and test. Adjusting the offset of each sensor individually allows the designer to fully utilize the resolution of the converter. In addition, electromagnetic noise emitted by the host processor board can cause unpredictable sensor behavior if it couples into the capacitive sensor and sensor traces. This can result in degraded performance, but there are simple ways to help minimize the effects of electromagnetic interference (EMI) in the sensor: First, the CDC should be mounted on the sensor board. This minimizes the sensor trace length, which reduces the chance of EMI being coupled into the trace. In addition, using a four-layer sensor board with a solid ground plane can provide additional EMI shielding for the sensor. If these two methods are not effective in isolating EMI noise from the EMI, a grounded metal shield can also be placed above the sensor board cavity. The capacitive sensor electric field can also couple into the conductive metal shell or metal coating of the product, resulting in unpredictable sensor behavior. This imposes a mechanical constraint that the edge of the capacitive sensor must be kept a certain distance from the edge of the metal surface. In addition, the sensitivity of the capacitive sensor is also related to the thickness of the plastic directly above the position sensor. If the plastic is too thick, the flow field lines cannot effectively pass through the plastic, making the sensor performance unreliable. Typically, the distance between the housing and the sensor should be greater than 1.0mm, and the plastic thickness should be less than 4.0mm to keep the sensitivity within the appropriate range. It is also important to keep the user proactive about the sensitivity. Traditional mechanical switches have a familiar sensitivity and tactile feedback, and for capacitive sensors, these parameters must also be considered and optimized. Different sensors may require unique sensitivities, depending on the switch function or the physical location of the switch in the product. Also, one set of sensitivity settings may not be suitable for all users, so the user should be allowed to set different sensitivity levels - ideally if they can be selected through a sensitivity control menu. For example, the AD7124 supports these sensitivity requirements by allowing a separate 16-bit sensitivity control register to be programmed for each sensor. These registers can also be embedded in a host firmware and provided in a menu display, allowing the user to select different sensitivity levels to meet their specific needs. Other advantages: energy saving and simplified assembly If every sensor input is sampled during the period when the user is not in contact with the sensor, battery power is wasted. To maximize battery efficiency, the CDC should be able to detect when the user stops contacting the sensor. And automatically switch to low power mode. When the sensor is contacted again, it automatically re-enters normal operating mode. To save more power, a complete shutdown mode should also be included. In portable products, turning off the sensor switch is usually done by setting a mechanical switch or selecting a blocking mode from a control menu. Another benefit of using capacitive sensors instead of traditional mechanical switches is that the manufacturing and assembly process is much simpler. Traditional mechanical switches require each switch to be manually inserted into a dedicated hole on the plastic housing, while a single capacitive sensor board containing all these switches can be placed under the plastic housing in one step. The sensor board mounting hole with a positioning notch and some glue are sufficient to complete the installation and position calibration of the sensor board.