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Motorola Sensor Device Data
For More Information On This Product,
Go to: www.freescale.com
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.
SEMICONDUCTOR APPLICATION NOTE
F
Freescale Semiconductor, Inc.
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