Introduction to Capacitive Sensing Part3
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Overview of a few of the important characteristics to consider when selecting a Capacitive Sensing solution and how they affect the overall design
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Introduction to Capacitive Sensing Part3
- 8. Material Overlays and Thickness
Hinweis der Redaktion
- Welcome to the introduction to Capacitive Sensing Solutions. In the module, we will go over a few of the important characteristics to consider when selecting a solution and how they affect your overall design.
- When considering the right capacitive sensing solution for your product, it’s important to think about the needs of your design. Where will this product be used? What kind of environment? Is battery life the most important factor in the design or is it durability? CapSense is a programmable and flexible capacitive sensing solution that can be fine-tuned to fit your design needs. We will now go over how CapSense can meet your particular specifications as well as some of the options you have when using CapSense.
- Both the CSA and CSD methods regularly update the baseline from scan to scan. With CSA, the raw counts from each scan is processed through an LPF that creates trend table over time. So as temperature changes on a particular device, as this graph shows, the baseline is adjusted as well. The trend that is tracked by the baseline automatically compensates for the effects of temperature and humidity. In addition, by using a combination of shield electrodes and a guard sensor, and enabling the CSD method for capacitive sensing, you can make your design essentially waterproof. This solution provides reliable touch detection and elimination of false detection caused by water droplets and streams of water. The shield electrode reduces the influence of water droplets at the physical level and the guard sensor resets the decision logic operation at the logic level.
- Depending on the type of product you are designing, power consumption may or may not be crucial. The benefit of the CapSense solution is that because it is programmable, you can choose the right design for your power needs. For example, if you had a battery-powered, handheld device, power consumption would be extremely important. One method to manage the overall average power consumption and thus battery-life is to set up 3 different operating zones. One zone would be the fast-response zone which means each sensor is scanned every 200 us. The solution would enter this zone when the buttons & sliders were under constant activity. In periods of little to no activity, the solution could enter a slow response zone which reduces the number of scans to every 100 ms or so. Finally, if there has been no activity for a long time, the solution could enter a deep sleep mode, thereby saving power. One example of a real CapSense system using the power-saving, slow-response mode with a three-button scan every 100 milliseconds consumes less than 50µA of average current.
- In today’s electronic world noise is a fact of life. Both conductive noise, like from power lines, and radiated noise, from mobile handsets or fluorescent lights, are always present and have to be accounted for. To be effective, the goal is to increase the signal-to-noise ratio…thereby eliminating false touches. There are two simple techniques to filter out noise within the CapSense firmware. This can be done with a Finite Impulse Response, or FIR, filter or with an Infinite Impulse Response, or IIR, filter. Both are low pass filters as the frequency of finger presses is very low in comparison to noise. There are tradeoffs for using each type of filter as shown here in this table so the final decision should depend on your product needs. For all configurations of CapSense, a minimum Signal-to-Noise ratio (SNR) of 5:1 needs to be achieved for all sensors. This system level requirement is specified so that the CapSense User Module can discriminate between ON and OFF states for each sensor in the presence of noise. This minimum level of performance will provide built-in immunity to common levels of external sources of noise, such as RF transmissions, AC line noise, and switching power supply noise.
- In products like mobile handsets, RF interference is a major concern. RF can affect the accuracy of a capacitive sensing solution because of the change in the electric field surrounding the sensor. This is especially true when the RF chips in a mobile handset are in close proximity to the capacitive sensors. The RF signal is AC but the effect on CapSense counts is DC due to the diodes on the CapSense input. A positive shift in counts can cause false button presses. A negative shift can prevent real button presses from being sensed. There are a few solutions to this however: The CapSense firmware is already able to handle low levels of RF through its Finger and Noise Threshold settings. Normal operation can continue without any additional firmware or hardware. For higher levels of RF, though, there are two options. If the RF chip is in the same system as CapSense, it is easy to “turn off” CapSense when RF is active. Another option is to dampen the resonance by adding resistors in series with CapSense.
- The type of overlay material you select and the thickness of that overlay will have a large effect on your design’s signal-to-noise ratio, durability, ESD resistance, as well as accuracy. Again, there are many tradeoffs when considering the type of material and thickness which will have to be based on your products needs. Here are a few things to consider…. As the thickness of the overlay material increases, both signal and noise decreases. However, the thicker the overlay material, the more resistant it is to ESD. The electrostatic voltage on the human body can reach 15 KV. The overlay in the CapSense system will protect the PSoC from permanent damage when the thickness guidelines of the table are followed. A layer of Kapton tape works well in applications needing extra ESD protection. Of course, the thicker the glass, the harder it is to break and be vandalized as well. These are all factors to consider for your design. The advantage of using CapSense is that because it is programmable, it can be adjusted to your design.
- To summarize, CapSense gives you both configurability and flexibility. You can create the solution that fits your needs…even when your needs change last minute.
- This concludes the Capacitive Sensing Solutions training. Capacitive-sensing based systems are more durable and more user-friendly than traditional mechanical buttons and sliders Because the human body is a conductor, the presence of a finger on a capacitive sensor changes the overall capacitance detected PSoC is a device that has an array of hardware blocks that can be configured to perform a variety of functions With it’s programmability and easy-to-use software design tools, Cypress’s CapSense solutions are both flexible and configurable
- CapSense Successive Approximation (CSA) is able to detect the presence or absence of a conductive element by counting how long it takes CMod to go from VStart to VRef CapSense Sigma-Delta (CSD) is able to detect the presence or absence of a conductive element by looking for a change in duty cycle caused by the time it takes to charge CMod There are many factors to consider in a capacitive-sensing solution. CapSense can be configured to meet your needs.
- Thank you for taking the time to view this presentation on PSoC . If you would like to learn more or go on to purchase some of these devices, you can either click on the link embedded in this presentation, or simple call our sales hotline. For more technical information you can either visit the Cypress site – link shown – or if you would prefer to speak to someone live, please call our hotline number, or even use our ‘live chat’ online facility.