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Topic: Capacitive Sensor Guarding (Read 610 times) previous topic - next topic

davivid

I was reading about ways to increase the accuracy of a Capacitive sensor and came across a technique known as Guarding. Using the CapSense Library, would I simply connect the Send pin to the guard conductor as well as the sensor conductor, and just connect the Receive pin to the sensor conductor? Thus in theory keeping the guard and sensor conductors at the same voltage? as suggested below:

from: http://www.sensorsmag.com/sensors/position-presence-proximity/capacitive-sensor-operation-part-1-the-basics-5844
Quote
Focusing the Electric Field
When a voltage is applied to a conductor, the electric field emanates from every surface. In a capacitive sensor, the sensing voltage is applied to the sensing area of the probe (Figures 3 and 4). For accurate measurements, the electric field from the sensing area needs to be contained within the space between the probe and the target. If the electric field is allowed to spread to other items--or other areas on the target--then a change in the position of the other item will be measured as a change in the position of the target. A technique called "guarding" is used to prevent this from happening. To create a guard, the back and sides of the sensing area are surrounded by another conductor that is kept at the same voltage as the sensing area itself (Figures 3 and 5). When the voltage is applied to the sensing area, a separate circuit applies the exact same voltage to the guard. Because there is no difference in voltage between the sensing area and the guard, there is no electric field between them. Any other conductors beside or behind the probe form an electric field with the guard instead of with the sensing area. Only the unguarded front of the sensing area is allowed to form an electric field with the target.



davivid

I have attached a diagram of how I was thinking this might work - however surely this just adds more sensor conductor to the mix. Any thoughts as how to do this correctly?


MarkT

Guard voltage is applied to the shield of cable to cancel any capacitance between sense electrode and environment except in the sensor.

For high sensitivity applications a dual-concentric coax cable is used - the outer shield is grounded, the inner shield is fed the guard voltage and
thus the inner conductor is both guarded and the guard voltage isn't allowed to affect the environmental fields.  Used for EEG/ECG etc from what I recall.

The CapSense library is nothing to do with all this, which is for analog voltage probes.
[ I won't respond to messages, use the forum please ]

davivid

Oh right - What would the technique / method of capacitve sensing be called that enables you to use a guard voltage in high sensitivity applications?

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