MPXM2102AS datasheet(522/670 Pages) MOTOROLA

522 / 670 page

AN1636

3–376

Motorola Sensor Device Data

www.motorola.com/semiconductors

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

MEAS +

* CZPO

Calculate Pressure

measure again

auto–zero command received

End

Freescale Semiconductor, Inc.

For More Information On This Product,

Go to: www.freescale.com

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