
3–376
Motorola Sensor Device Data
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There are a lot of applications that will lend themselves
naturally to auto–zeroing. Typical applications are those that:
experience a zero–pressure condition at system start up,
are idle for a long time (zero pressure), take a pressure
measurement then go back to idle again.
For example, in a water level measurement in a washing
machine application, there is a zero pressure reference
condition when the water in the tub is fully pumped out.
Another application that is perfect for auto–zeroing is a
beverage fill level measurement; a zero reference condition
exists before the bottle is filled. HVAC air flow applications can
also use auto–zeroing; before system start up, an auto–zero
can be initiated. In other words, it can be used in applications
where a zero pressure condition can exist in order to
auto–zero the system.
An auto–zero command can be automated by the system
or can be commanded manually. Each system will have a
different algorithm to command an auto–zero signal. For
example, using the beverage fill level measurement as an
example, the system will auto zero the sensor before the bottle
is filled.
IMPLEMENTATION OF AUTO–ZERO WITH A
MICROCONTROLLER
Auto–zero can be implemented easily when the integrated
sensor is interfaced to a microcontroller. The auto–zero
algorithm is listed below:
1. Sample the sensor output when a known zero refer-
ence is applied to the sensor (atmospheric pressure is
a zero reference for gauge type measurement). Store
current zero pressure offset as CZPO.
2. Sample the sensor output at the current applied
pressure. Call this SP.
3. Subtract the stored offset correction, CZPO, from SP.
The pressure being measured is simply calculated as:
PMEAS = (SP
CZPO)/S.
Note that the equation is simply a straight line equation,
where S is the sensitivity of the sensor. The auto–zero
algorithm is shown graphically in Figure 3.
Figure 3. Flow–Chart of the Auto–Zero Algorithm
Start
Sample Current Zero Offset, CZPO
Sample Current Pressure, SP
P
MEAS
CP
CZPO
S
Calculate Pressure
measure again
auto–zero command received
End
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Freescale Semiconductor, Inc.
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