Capacitive Touch Algorithm Tuning for Thick Overlays

Often the capacitive touch applications need thick overlay to be used over the capacitive touch pads to make the system mechanically robust. In addition to that, for aesthetic reasons, the overlay may be curved. This poses a serious issue of reducing the touch sensitivity. The curvature on the overlay distorts the sample reading from the buttons. Also it gives a variation in reading from button to button. To overcome these issues, the capacitive touch algorithm needs some tuning.      

When thick overlay of plastic or glass is used, the sensitivity goes down significantly. As you know, the human touch produces a basic capacitor, with finger and the touch pad being the electrodes and the overlay material is the dielectric material separating the plates. Because the thickness of the dielectric material increased(the overlay), the capacitance shift produced by touch reduces, thus reducing the sensitivity. Because of this, the signal to noise ratio reduces further down.

To overcome this, a slow moving average algorithm is used. First, the capacitive sensing module, such as Charge Time Measurement unit(CTMU) or Capacitive Sensing Module(CSM), is sampled 16 to 64 times(N Samples) every fixed interval. These N samples are accumulated and used as the current running sample. The averaging algorithm takes the current sample and takes a fraction of it and adds to the running average. The running average is not updated until 20 or more samples of slow moving average. By doing this the average changes very slow and the slight shift of the current sample allows to detect the touch.

Because of the low sensitivity, the difference between the current sample and the average data could be less than 1%. Each channel needs tuning on the number of samples for slow moving average to suit the touch key size and key characteristics.