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Sensing Technologies

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Hi all! 

I have been trying to understand how grounding works with CSX reading section 7.5 of the Capsense design guide.  The section has good information, but I would understand better with a more visual approach. I have a few questions for the community:

 

1. Understanding Cfs (finger to electrodes) and CbodyDG (body to device ground)

JeSi_4326976_0-1610064858005.png

Figure 1: physical representation of figure 7-32 in capsense guide 

From my understanding, Cfs and CBodyDG should physically be represented this way. Cfs would be mainly influenced by distance (C = A/d) between the finger and the Rx/Tx electrodes. Too little distance boosts Cfs, creating the Cfs > CbodyDG condition. But it also mentions that water drops and directly touching the electrodes can also increase Cfs. Do these conditions also have to do with a small distance between the Cfs plate (finger or water drop) from the electrode plate? Or is there another important factor that weight is to increase Cfs drastically?

 

2. Understanding Cfm (the virtual capacitance between Cm and the finger)

JeSi_4326976_1-1610064857889.png

Figure 2: figure 7-32 in capsense guide

In this image, what does the node in the green circle represent? It seems as though 2 capacitors would be formed between a Tx and an Rx, but say if I would compare it with the finger drawing figure 1, there is only 1 capacitor as the Rx and Tx electrodes directly pair with each other. So how does Cfm work? Because we already have the pairing of Tx and Rx with the finger with Cfs, I am not sure of what it represents. In figure 1, would it be possible to draw the Cfm virtual capacitor to better visualize its effect on the system?

 

3. Understanding  current flow

If I am correct, the whole process of CSX is to measure AC current variations coming from the Tx electrode. A low IRx means a high impedance (capacitance) between Tx and Rx, and thus mean no finger is nearby. When a finger is nearby and pairs with Tx/Rx, do Cfm and CbodyDG actually create a closed circuit and we have electrons transferred from the user’s body to the PSoC? That is what Ibody represents?

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Hello @JeSi_4326976  ,

 

1.  But it also mentions that water drops and directly touching the electrodes can also increase Cfs. Do these conditions also have to do with a small distance between the Cfs plate (finger or water drop) from the electrode plate? Or is there another important factor that weight is to increase Cfs drastically?

Yes, in the case of directly touching the electrode, the effect is a direct cause of the small distance between the plates of the capacitor. Since C = eA/D, small D will increase the capacitance exponentially.

In the case of liquid drops, the result is due to the absence of Cbody as the liquid drop has no capacitance with respect to the earth. And since Cbody is absent, any added Cfs will directly impact the Cm and cause an increase in Cm.

 

2. Understanding Cfm (the virtual capacitance between Cm and the finger)

The electrical model of the CSX sensor has a split Cm to indicate that the Cfs, sensor to finger capacitance, isn't electrically connected after the Cm, which is the capacitance between the Tx and Rx electrode. Cfm is added to show the mutual capactiance change when the finger is pressed on the button. Cfm can be interpreted as dCm, which is the change in Cm when a finger touches the sensor. 

You can also see that without a finger, Cfm, Cfs and CbodyG will cease to exist and the capacitance between the Tx and Rx is effectively Cm.

Please let me know if you have further queries on this point.

 

3. Understanding  current flow

Let us consider 2 scenarios here - 

1 - where Cbody >> Cfs (good design) and 

2 - where Cbody << Cfs (signal disparity condition).

Case 1 will cause a drop in Irx whenever a finger touches the sensor since some current is taken from the normal Imt and is transferred to earth through our body (electrons travelling to PSoC from our body). 

Case 2 has an increase in Irx whenever we touch the sensor due to the additional current path for Irx through Cfs. This effectively means a higher Cm than the default condition. There is no effect of Cbody in this case.

 

Best regards, 
Hari

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