How Capacitive
Sensors work
and
How to use them effectively.
Compliments
of Lion
Precision.
Copyright © 2007 Lion Precision. www.lionprecision.com
View the Complete Tutorial (www.capacitive-sensing.com)
A Capacitive
Measurement System
Capacitive sensor dimensional measurement requires three basic
components:
a probe that uses changes in capacitance to sense changes in
distance to the target,
driver electronics to convert
these changes in capacitance into voltage changes,
a device to indicate and/or record
the resulting voltage change.
Each of these components is a critical part in providing reliable,
accurate measurements. The probe geometry, sensing area size,
and mechanical construction affect range, accuracy, and stability.
A probe requires a driver to provide the changing electric field
that is used to sense the capacitance. The performance of the
driver electronics is a primary factor in determining the resolution
of the system; they must be carefully designed for a high-preformance
applications. The voltage measuring device is the final link
in the system. Oscilloscopes, voltmeters and data acquisition
systems must be properly selected for the application.
What is
Capacitance?
Capacitance describes how the space between two conductors affects
an electric field between them. If two metal plates are placed
with a gap between them and a voltage is applied to one of the
plates, an electric field will exist between the plates. This
electric field is the result of the difference between electric
charges that are stored on the surfaces of the plates. Capacitance
refers to the “capacity” of the two plates to hold
this charge. A large capacitance has the capacity to hold more
charge than a small capacitance. The amount of existing charge
determines how much current must be used to change the voltage
on the plate. It’s like trying to change the water level
by one inch in a barrel compared to a coffee cup. It takes a
lot of water to move the level one inch in the barrel, but in
a coffee cup it takes very little water. The difference is their
capacity.
When using
a capacitive sensor, the sensing surface of the probe is the
electrified plate and what you’re measuring (the target)
is the other plate (we’ll talk about measuring non-conductive
targets later). The driver electronics continually change the
voltage on the sensing surface. This is called the excitation
voltage. The amount of current required to change the voltage
is measured by the circuit and indicates the amount of capacitance
between the probe and the target. Or, conversely, a fixed amount
of current is pumped into and out of the probe and the resulting
voltage change is measured.
How Capacitance
Relates to Distance
The capacitance between two plates is determined by three things:
Size of the plates: capacitance increases as the plate size increases
Gap Size: capacitance decreases
as the gap increases
Material between the plates (the
dielectric):
Dielectric material will cause the capacitance to increase or
decrease depending on the material
In ordinary
capacitive sensing, the size of the sensor, the size of the target,
and the dielectric material (air) remain constant. The only variable
is the gap size. Based on this assumption, driver electronics
assume that all changes in capacitance are a result of a change
in gap size.
The electronics are calibrated to output specific voltage changes
for corresponding changes in capacitance. These voltages are
scaled to represent specific changes in gap size. The amount
of voltage change for a given amount of gap change is called
the sensitivity. A common sensitivity setting is 1.0V/100µm.
That means that for every 100µm change in the gap, the
output voltage changes exactly 1.0V. With this calibration, a
+2V change in the output means that the target has moved 200µm
closer to the probe.
Focusing
the Electric Field
When a voltage is applied to a conductor, an electric field is
emitted from every surface. For accurate gaging, the electric
field from a capacitive sensor needs to be contained within the
space between the probe’s sensing area 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.
To prevent this from happening, a technique called guarding is
used. To create a guarded probe, 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. When the excitation
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 to cause current flow. Any conductors
beside or behind the probe form an electric field with the guard
instead of the sensing area. Only the unguarded front of the
sensing area is allowed to form an electric field to the target.
Effects
of Target Size
The target size is a primary consideration when selecting a probe
for a specific application. When the sensor’s electric field
is focused by guarding, it creates a field that is a projection
of the sensor size and shape. The minimum target diameter for
standard calibration is 30% of the diameter of the sensing area.
The further the probe is from the target, the larger the minimum
target size.
Range of
Measurement
The range in which a capacitive sensor is useful is a function
of the area of the sensing surface. The greater the area, the
larger the range. The driver electronics are designed for a certain
amount of capacitance at the sensor. Therefore, a smaller sensor
must be considerably closer to the target to achieve the desired
amount of capacitance. The electronics are adjustable during
calibration, but there is a limit to the range of adjustment.
In general, the maximum gap at which a probe is useful is approximately
40% of the sensing surface diameter. Standard calibrations usually
keep the gap considerably less than that.
Much more
information available
Please view that complete tutorial at www.capacitive-sensing.com for detailed information
on these important topics:
Multiple Channel Sensing
Effects of Target Material
Measuring Non-Conductors
Maximizing Accuracy: Target Size
Maximizing Accuracy: Target Shape
Maximizing Accuracy: Surface
Finish
Maximizing Accuracy: Parallelism
Maximizing Accuracy: Environment
And find the precise definitions for these important terms:
Sensitivity
Sensitivity Error
Offset Error
Linearity Error
Error Band
Bandwidth
Resolution
Resolution Calculation
Article published with the agreement of Lion Precision.
For more details of their products, click here...
© Lion Precision 2007 All Rights Reserved
