Human body model, capacitive sensor

K

[email protected]

Jan 1, 1970
0
Hi

I have this capacitive sensor I have made that works by applying a 5V
square wave onto a sensor plate. I then measure the current going to
that plate to estimate the capacitance from a human finger touch.

The funny thing is that when I touch it the first reading is higher
than the subsequent ones.

Is there a enhanced human body model beyond the simple one consisting
of a resistor and a capacitance to earth that can explain what I am
seeing?

Thanks

Klaus

J

Joerg

Jan 1, 1970
0
Hi

I have this capacitive sensor I have made that works by applying a 5V
square wave onto a sensor plate. I then measure the current going to
that plate to estimate the capacitance from a human finger touch.

The funny thing is that when I touch it the first reading is higher
than the subsequent ones.

Is there a enhanced human body model beyond the simple one consisting
of a resistor and a capacitance to earth that can explain what I am
seeing?

I don't know how you drive it but could it be that some charge bleeds
off or onto the plate?

One thing that does have such an effect is accumulated charge on the
body. Happens a lot on winter days and when wearing shoes where the
underside isn't leather but some type of plastics or rubber. The first
touch bleeds that off and when you don't move around too much between
that and the following touches the next ones won't register a surge.

J

Jim Thompson

Jan 1, 1970
0
Hi

I have this capacitive sensor I have made that works by applying a 5V
square wave onto a sensor plate. I then measure the current going to
that plate to estimate the capacitance from a human finger touch.

The funny thing is that when I touch it the first reading is higher
than the subsequent ones.

Is there a enhanced human body model beyond the simple one consisting
of a resistor and a capacitance to earth that can explain what I am
seeing?

Thanks

Klaus

Is the average value of the 5V square zero? Or is it 2.5V?

...Jim Thompson

P

PeterD

Jan 1, 1970
0
Hi

I have this capacitive sensor I have made that works by applying a 5V
square wave onto a sensor plate. I then measure the current going to
that plate to estimate the capacitance from a human finger touch.

The funny thing is that when I touch it the first reading is higher
than the subsequent ones.

K

[email protected]

Jan 1, 1970
0
Is the average value of the 5V square zero? Or is it 2.5V?

...Jim Thompson
--
| James E.Thompson, P.E. | mens |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| Phoenix, Arizona Voice480)460-2350 | |
| E-mail Address at Website Fax480)460-2142 | Brass Rat |
| http://www.analog-innovations.com | 1962 |

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The output is actually driven hard with a 2MHz square wave 10%
dutycycle for only 5us (ten pulses). Furtheremore the sensor is only
activated once every second. So normally the output is 0V hard driven
(push-pull output drive) and once every second some fast pulses are
present on the sensor plate. Output to the plate is isolated with some
Y-capacitors to provide protective impedance for the user.

Therefore it is likely the finger is discharged to ground when it
touches the plate (sensor normally at ground), so I would assume the
initial charge of the body would be insignificant which is why I have
this question of the human body model (maybe some sort of dielectric
absorbtion??). Of course charge may accumulate on the Y capacitors.

see simplified schematics at:

www.microdesign.dk/tmp/SimpleSensorOut.pdf

Thanks

Klaus

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