Triggering CSX Signal Disparity Voluntarily

[jcsb1994]

Hi all,

You might want to read this older post   https://community.cypress.com/t5/PSoC-4-MCU/Analysing-a-mutual-capacitance-sensor-s-signal-disparity...

I am trying to fully understand the behaviour of electric fields in mutual capacitive technologies  like CSX. I have run a series of tests with different overlay thicknesses under 0.5 mm (0 to 20mil with 2mil increments) to see how Capsense would fluctuate when a floating conductive object and when a grounded object would touch it.

I have used a copper plane the size of my CSX touchpad, glued to a plastic handle, and weighted to firmly press on the overlay.

Image 1: Hardware setup to test effects of copper plane on CSX touchpads with an overlay thickness lower than 0.5mm

I have recorded data for multiple overlay thicknesses and plotted (image below) the mean raw count reading of all the sensing elements of the touch pad, so I can compare the overall effect of overlay thickness on raw counts

Image 2:  Mean Raw counts for 0 to 20 mil overlay thicknesses

This graph shows how the capacitance increasing effect of the floating copper plane (reduced rawcounts at the middle of plot) is damped as the overlay thickens. Same goes with the capacitance decreasing effect of the plane once it is grounded. This makes sense as the copper plane is further away from the touchpad, its effect is lowered, whether it is grounded or not.

My question

What I do not understand however is that I  have not been able to trigger signal disparity with the copper plane, even when the tests were run without overlay (0 mil thickness on the plot). I would have expected the grounded copper plane (right section of the graph) to increase the capacitance, therefore lowering the rawcount compared to normal at 0 mil, but it actually shows the highest rawcount increase at 0 mil overlay thickness.

Basically, I would have expected a turnaround like the red line above at very low overlay thicknesses, but I have seen a line like the yellow one in my results.. How can this be explained?

I must mention that a finger does result in the capacitance increase, however (red line), when I press on the touchpad without overlay. I just do not understand why it is not the case with the grounded copper plane.

Thank you!!

 

[Hari · Accepted solution]

Hi @jcsb1994, 

A grounded copper plane will not allow the Tx field lines to reach the Rx, especially with a thin overlay. Electric field lines generated by the Tx electrodes are absorbed by the large ground plane, and this results in a very small current, IRx entering the Rx electrode - 

The field lines being taken up by the Copper plane causes the Cm between the Tx and the Rx to drop and therefore, the raw counts, which is inversely proportional, increases. 

On the other hand, a copper plane not connected to ground helps in transferring these field lines easily as we had discussed in the previous thread that is mentioned here as well. 

In short, anything that eases the field lines to reach the Rx electrode will increase the Cm and therefore, decrease the raw counts and anything that hinder electric field (having a large ground plane, increased distance between Tx and Rx etc) will decrease the Cm and increase the raw counts.

Best regards, 
Hari

[jcsb1994]

Hi @Hari ,

so sorry for the late reply. I have a lot of stuff to work on this semester.

I think I am familiar with the phenomena that hinder or ease flows of electric fields from Tx to Rx. Thank you for your post, however, it was very easy to read and understand.

1. I am mostly wondering why the finger starts easing the flow of Tx-Rx electric field lines at very small distances, and you made me realize that I will never be able to copy this behaviour with a grounded plane, but since the finger is grounded too, it should also capture all Tx lines, leaving none to reach Rx?? But it isn't the case, since the signal disparity makes the capacitance increase at small distances, strangely.

2. Anyway, maybe signal disparity is nothing to worry about with a sensor that it operating with a flexible grounded plane, since it doesnt seem to happen with a grounded copper plane, even at 0 distance.

[Hari]

Hi @jcsb1994 

The point to keep in mind is that a human finger is not a perfect conductor. We model the body as a capacitance to earth ground, CBodyDG, which is not the case with a conductive ground plane. So, with no overlay, the finger to sensor capacitance overtakes the body to ground capacitance, and hence, we observe a rise in the mutual capacitance.

Best regards, 
Hari

[jcsb1994]

Nice! would you believe it is safe to say that I cannot reach the CbodyDG<<Cfs condition with a board using a grounded layer like this one?

I will continue investigations, however from what I understand, and from my data so far, it seems like the conductive fabric, being a "perfect" conductor like the copper plane, it seems impossible to trigger the signal disparity even at the smallest distances from the capsense electrodes.

Thanks again for your help!

[Hari]

Yes, unless there is an external capacitor added to between the ground plane and device ground, signal disparity will not be triggered. Note that the ground plane must be connected to the device ground. 

Best regards,

Hari