TouchXpress Moisture Immunity

by <a href="http://community.silabs.com/t5/Welcome-and-Announcements/Community-Ranking-System-and-Recognition-Program/m-p/140490#U140490"><font color="#000000"><font size="2">Hero Employee</font></font> </a> Stephen on ‎10-02-2017 02:09 PM

Question

How are TouchXpress devices, such as CPT007B and CPT112s, immune to moisture? 

Answer

TouchXpress devices can accommodate small droplets (water radius < sensor radius) as well as large droplets (water radius > sensor radius) of water. 

Internal behavior depends on how much water is on the sensor, in terms of sensor radius. 

Using Cap Sense Profiler and a CPT Evaluation Board is the easiest way to visualize this. 

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Scenario #1: Below is the behavior when water radius is 1.5x - 2x the sensor radius. See picture below:

 

cs_profiler_water.PNG

1. Start with no water on the board and touch a sensor. 

2a. When the Raw CS value (in green) > the Active Threshold (in black), the device says the sensor is touched. 
2b. When the Raw CS value (in green) < the Active Threshold (in black), the device says the sensor is untouched. 

3. When you add a lot water (water radius is 1.5x -2x the sensor radius), the Raw CS value increases a lot. See picture below:

 

CPT112S_water.JPG

This amount of water causes the Raw CS value > Active threshold, resulting in a false "sensor is touched". 

Notice the dynamic baseline did not change -- this is because the amount of water is large.

4. However, you can adjust the Active Threshold higher than the Raw CS value with water (ie to 14528 in this scenario), you can completely eliminate the false "sensor is touched". 

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Scenario #2: With much larger water radius (water radius much greater than sensor radius), we have a touch timeout feature, which resets (forces new baseline) the device when the Raw CS value > the Active Threshold (eg from water, or real touch) after x number of seconds. 

The idea here is that a user would not typically touch a sensor for multiple seconds. 

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Scenario #3: With small water radius, the baseline (red) will dynamically increase, and the Active Threshold will automatically increase relative to the baseline. 

Dynamic baseline auto-adjusts only when the Raw CS value increase is < 0.25x the Touch Delta (max - min Raw CS value on board with no water). 

When Raw CS value increase is < 0.25x the Touch Delta (Scenario #1), the baseline does not auto adjust. 

 

Demonstration:

https://www.youtube.com/watch?v=8PfBm9z59Ks&t=0m30s