SaM - Capacitive Proximity Sensors

Remeber:

A conductive proximity sensor, without a target and with a conductive target:

• No conductive target found, the measured capacitance will be a small value equal to $C_{\infty }$
• Conductive target found, then $C=\epsilon \frac{A}{d}$
• So what happens is this:
  

What happens, instead, if I have a large, conductive target, which is also not grounded? ⇒ The parasitic capacitance formed between the ground and target can be ignored. More specifically if: $C_S\ll C_{GT}$(And this is true if the area of the sensor is much smaller that that of the target) Then the parasitic capacitance $C_{GT}$ can be ignored:

• $C_{MEAS}$: measured capacitance.
• $C_S$ : sensor’s capacitance.
• $C_{GT}$ : parasitic capacitance between ground (${}_G$) and target (${}_T$).
• ==If the sensor is small (if its area $A$ is small) relative to the large target==: Then we have that $C_S\ll C_{GT}$ and $C_{MEAS}\simeq C_S$.
  ⇒ So it's like always having a grounded conductive target, which is good.

We have seen an example of a capacitance sensor, with real world values, remember this: ==A circular capacitive proximity sensor with radius $r=10\text{ mm}$, measuring an object $1\text{ mm}$ away will assume a capacitance of about $2.7\text{ pF}$==.

Remember, and this is very important, impedance of a capacitance in module is this one: $|Z|=\frac{1}{\omega C_0}$

• ==So small capacitance means high impedance==.
• If proximity capacitive sensor have a reasonable size, they will be a very high impedance sensor.
• ==This can be mitigated by operating this sensor at high frequency, but we know that the higher the frequency the most critical is the design of the electronics==.
• ==So we have to cope between these two requirements to keep this impedance small enough to be measured and to operate at a not too high frequency, in order for the design and for the parasitic effect which comes together with the high frequency electronic design==.

Actually proximity sensors can be used also for non-conductive target, under the assumption that the electrical permittivity of the medium of the target is different from the one of the vacuum:
When we have the target close to our measuring plate, the electrical field will find a medium with a different permittivity
⇒ ==Like for conductive targets, the capacitance measured by the sensor changes (it increases), if we get closer to the target, even if it is non-conductive==.
For this reason also a non-conductive target can be sensed.

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Memory Card

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Index

• Capacitive Proximity Sensors
• Large Non-Grounded Conductive Target for Capacitive Proximity Sensors
• ~Ex. Simple Sensor
• High Impedance Sensor

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Capacitive Proximity Sensors

• In the middle there is the sensing plate, which is polarized (biased)
• ==In a proximity sensor, what is needed is to sense the proximity of a conductive target==.
  So we imagine to have a target, which is moving, and can arrive close to the sensing plate.
• This represents two conditions:
  1. No conductive target found, the measured capacitance will be a small value equal to $C_{\infty }$
  2. Conductive target found, then $C=\epsilon \frac{A}{d}$

How can I sense the presence of this target?

This is a commercial proximity sensor:

So what happens is this:

• $d$ : the distance of the target.
• $C$ : measured capacitance, which goes to the infinite value, which is the one related to this first condition (where no conductive target are present).

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Large Non-Grounded Conductive Target for Capacitive Proximity Sensors

What happens, instead, if I have a large, conductive target, which is also not grounded? ⇒ We need to account for a non-ignorable Parasitic Capacitance:

• $C_{MEAS}$: Capacitance Measured
• $C_S$ : Sensor Capacitance.
• $C_{GT}$ : Parasitic Capacitance between ground (${}_G$) and target (${}_T$).
• ==If the sensor is small (if its area $A$ is small) relative to the large target==: Then we have that $C_S\ll C_{GT}$ and $C_{MEAS}\simeq C_S$.
  ⇒ So it's like always having a grounded conductive target, which is good.

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~Ex.: Simple Sensor

Remeber: ==A circular capacitive proximity sensor with radius $r=10\text{ mm}$, measuring an object $1\text{ mm}$ away will assume a capacitance of about $2.7\text{ pF}$==.

Here we can see a numerical example of a simple capacitive sensor (no parasitic capacitances considered):

• $A$ : area of the sensor (radius of 1 cm)
• ${\epsilon }_0$ : absolute permittivity (we consider to have air as the isolating material, usual scenario).
• $C(d)$ : Sensor capapitance
• $C_0=C(d=1mm)$ : Capacitance for the sensor distance 1 mm from the target.

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High Impedance Sensor

Remember, and this is very, very important, impedance of a capacitance in module is this one:

• ==So small capacitance means high impedance==.
• So proximity capacitive sensor, if they have a reasonable size, they will be a very high impedance sensor.
• ==This can be mitigated by operating this sensor at high frequency, but we know that the higher the frequency the most critical is the design of the electronics==.

==So we have to cope between these two requirements to keep this impedance small enough to be measured and to operate at a not too high frequency, in order for the design and for the parasitic effect which comes together with the high frequency electronic design==.

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Non-Conductive Target for Capacitive Proximity Sensors

• Actually proximity sensors can be used also for non-conductive target, under the assumption that the electrical permittivity of the medium of the target is different from the one of the vacuum.
• When we have the target close to our measuring plate, the electrical field will find a medium with a different permittivity
  ⇒ ==Like for conductive targets, the capacitance measured by the sensor changes (it increases), if we get closer to the target, even if it is non-conductive==.
  For this reason also a non-conductive target can be sensed.