Datasheet
last update
November 30, 2010
reference
smtutin
page
6/27
5. Some theory about measuring sensor elements
5.1 About the three-signal technique and calibration
The three-signal technique is a technique to eliminate the effects of unknown offset and unknown gain in a
linear system. In order to apply this technique, in addition to the measurement of the sensor signal, two
reference signals are required to be measured in an identical way. Suppose a system has a linear transfer
function of:
offii
MkEM +=
We feed the system with three different input values:
E
1
=0, E
2
=E
re
and
E
3
=E
x
The three measured output signals (values) are
M
1
=M
off
M
2
=M
ref
=k*E
ref
+M
off
and M
3
=M
x
=k*E
x
+M
off
We then can calculate:
Eref
Ex
MoffMref
MoffMx
MM
MM
M =
−
−
=
−
−
=
12
13
When the system is linear, then in this ratio the influence of the unknown offset
M
off
and the unknown gain
k
of the
measurement system is eliminated and the calculated value of
M
represents the ratio between the values of the
(unknown) sensor element and the (known) reference element. This technique is called the three variable technique
or three signal technique. The values of offset and gain may vary over time, but since they play no role in the final
result, a UTI based measuring system is self calibrating.
The implementation of the three signal technique requires a microcontroller, which is used to digitize the
period-modulated UTI output signals and perform the data storage and calculations. Such a system combining
a sensing element (sensor), a signal-processing circuit, such as the UTI, and a microcontroller is called a
microcontroller-based smart sensor system. The auto-calibration properties of the UTI render such a system
insensitive to temperature effects.
5.2 The measurement of sensing elements
The output of the UTI is a period modulated signal. As an example,
Figure 2
shows two complete cycles of the
output signal from the UTI, each consisting of three phases.
V
DD
GND
1 cycle 1 cycle
T
x
T
off
T
ref
T
x
T
off
T
ref
Figure 2. The output signal of the UTI for a 3-phase mode