AN1668 Datasheet, PDF (1/6 Pages) Freescale Semiconductor, Inc – Washing Appliance Sensor Selection

Freescale Semiconductor
Application Note
AN1668
Rev 2, 11/2006
Washing Appliance Sensor Selection
by: Ador Reodique
Sensor and Systems Applications Engineer
INTRODUCTION
North American washing machines currently in production
use mechanical sensors for water level measurement
function. 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
sensors (accuracy, linearity, repeatability) even at lower
pressure ranges. Benchmarks indicate that these
performance goals is difficult to achieve using current
mechanical pressure sensors.
In Europe, where energy conservation is mandated,
washing machine manufacturers have started to look at
electronic solutions 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 mm H₂O
(24 “ H₂O ~ 6 kPa) sensor with a 5% FS accuracy spec.
Certain appliances call for a lower pressure range especially
in Europe where horizontal axis machines are common.
SENSOR SOLUTIONS
For the typical 600 mm H₂O, 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
uncompensated for temperature and untrimmed offset and
full-scale span. This means that the end user must
temperature compensate 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
production is expensive. Because the offset and full-scale
output can vary from part to part, a two-point calibration is
required as a minimum. A two point calibration is a time
consuming procedure as well as possible modification to the
production line to accommodate the calibration process. The
circuitry must also accommodate for trimming, i.e., via
trimpots and/or EEPROM to store the calibration data. This
adds extra cost to the system.
The MPX2010 is a 10 kPa (40" H₂O), temperature
compensated, offset and full-scale output calibrated device. A
differential 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
implemented using the system MCU.
The MPXV4006G is a fully integrated pressure sensor
specifically designed for appliance water level sensing
application. This device has an on board amplification,
temperature compensation and trimmed span. An auto-zero
procedure should be implemented with this device (refer to
Application Note AN1636). Because expensive and time
consuming calibration, temperature compensation and
amplification is already implemented, this device is more
suitable for high volume production. The MPXV4006G
integrated sensor is guaranteed to be have an accuracy of
± 3% FS over its pressure and temperature 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 mm H₂O 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
temperature 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
specifications shown on Table 1.
© Freescale Semiconductor, Inc., 2006. All rights reserved.