MPXM2102AS datasheet(536/670 Pages) MOTOROLA

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AN1668

Washing Appliance Sensor Selection

Prepared by Ador Reodique

Sensor and Systems Applications Engineer

INTRODUCTION

North American washing machines currently in production

use mechanical sensors for water level measurement func-

tion. These sensors are either purely mechanical pressure

switch with discrete trip points or electromechanical pressure

sensor with an on–board electronics for a frequency output.

High efficiency machines require high performance sen-

sors (accuracy, linearity, repeatability) even at lower pres-

sure ranges. Benchmarks indicate that these performance

goals is difficult to achieve using current mechanical pres-

sure sensors1.

In Europe, where energy conservation is mandated, washing

machine manufacturers have started to look at electronic solu-

tions where accuracy, reliability, repeatability and additional

functionality is to be implemented. North American and Asia

Pacific manufacturers are also looking for better solutions.

From surveys of customer requirements, a typical vertical–

axis machine calls for a sensor with 600 mmH2O (24 “ H2O ~

6 kPa) sensor with a 5 % FS accuracy spec. Certain appli-

ances call for a lower pressure range especially in Europe

where horizontal axis machines are common.

SENSOR SOLUTIONS

For the typical 600 mmH2O, 5 % FS spec, an off the shelf

solution available today is the MPX10/MPX12, MXP2010 and

the MPXV4006G sensor. The MPX10 (or the MPX12) is 10

kPa (40 “ H2O) full–scale pressure range device. It is uncom-

pensated for temperature and untrimmed offset and full–scale

span. This means that the end user must temperature com-

pensate as well as calibrate the full–scale offset and span of

the device. The output of the device must be amplified using

a differential amplifier (see Figure 1) so it can be interfaced to

an A/D and to obtain the desired range.

Since the MPX10/MPX12 sensors must be calibrated, the

implications of this device being used in high–volume pro-

duction is expensive. Because the offset and full–scale out-

put can vary from part to part, a two–point calibration is

required as a minimum. A two point calibration is a time con-

suming procedure as well as possible modification to the pro-

duction line to accommodate the calibration process. The

circuitry must also accommodate for trimming, i.e., via trim-

pots and/or EEPROM to store the calibration data. This adds

extra cost to the system.

The MPX2010 is a 10kPa (40” H2O), temperature compen-

sated, offset and full–scale output calibrated device. A differ-

ential amplifier like the one shown in Figure 1 should be used

to amplify its output. Unlike the MPX10 or MPX12, this device

does not need a two–point calibration but auto–zeroing can

improve its performance. This procedure is easily imple-

mented using the system MCU.

The MPXV4006G is a fully integrated pressure sensor spe-

cifically designed for appliance water level sensing applica-

tion. This device has an on board amplification, temperature

compensation and trimmed span. An auto–zero procedure

should be implemented with this device (see Application Note

AN1636). Because expensive and time consuming calibra-

tion, temperature compensation and amplification is already

implemented, this device is more suitable for high volume pro-

duction. The MPXV4006G integrated sensor is guaranteed to

be have an accuracy of +/–3 % FS over its pressure and tem-

perature range.

For washing machine applications where low cost and high

volume productions are involved, both the MPX2010 and

MPXV4006G are recommended. Both solutions can be used

in current vertical axis machines where the water level in the

600 mmH2O or 24 “ H2O range. In the following, a comparison

is made between MPX2010 and MPXV4006G in terms of

system and performance considerations to help the customer

make a decision.

EXPECTED ACCURACY OF THE MPX2010

SYSTEM SOLUTION

The MPX2010 compensated sensor has an off the shelf

overall RMS accuracy of +/–7.2 % FS over 0 to 85

°C tempera-

ture range.

Auto–zeroing can improve the sensor accuracy to +/– 4.42

% FS. However, since this sensor does not have an integrated

amplification, its amplifier section must be designed carefully

in order to meet the target accuracy requirement. The

MPX2010 compensated sensor has the following specifica-

tions shown on Table 1.

MOTOROLA

SEMICONDUCTOR APPLICATION NOTE

Freescale Semiconductor, Inc.

For More Information On This Product,

Go to: www.freescale.com

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