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 H
2
O
(24 “ H
2
O ~ 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 H
2
O, 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 “ H
2
O) 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" H
2
O), 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 H
2
O or 24 “ H
2
O 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.
Table 1. MPX2010 Specifications
Characteristic
Pressure Range
Supply Voltage
Supply Current
Full Scale Span
Offset
Sensitivity
Linearity
Pressure Hysterisis
Temperature Hysterisis (*40 to 125°C)
Temperature Effect on Span
Temperature Effect Offset (0 to 85°C)
Input Impedance
Output Impedance
Response Time (10% to 90%)
Warm-Up
*1
*1
1300
1400
1
20
*1
0.1
0.5
1
1
2550
3000
24
*1
25
1
Min
0
10
6
25
26
1
Typ
Max
10
16
Unit
kPa
Vdc
mA
mV
mV
mV/kPa
%V
FSS
%V
FSS
%V
FSS
%V
FSS
mV
W
W
ms
ms
The sensor system errors is made up of the sensor errors,
amplifier errors and A/D errors. In other words,
With auto-zeroing, the offset calibration, temperature effect
on offset and offset stability is reduced or eliminated,
εSystem
=
εSensor
2 +
εAmplifier
2 +
εADResolution
2
(
1
)
εSensor
=
SpanCal 2 + Lin 2 + Phys 2 + Thys 2 + TCS 2
(
3
)
Table 2
shows the MPX2010 with the errors converted to
%V
FSS
. The expected maximum root mean squared error of
the sensor is
εSensor
=
2
2
2
2
2
2
2
2
SpanCal + Lin + Phys + Thys + TCS + OffCal + Tco + OffStab
= +/- 4.42% FS.
The sensor error is calculated using the full-scale pressure
range of the device, 0 to 85°C temperature and 10 V
excitation.
In comparison with the MPXV4006G solution, the expected
accuracy of the system (MPXV4006G + 8 bit A/D) with
auto-zero is 3.1% FS.
(
2
)
=
±
7.19% FS.
Table 2. MPX2010 Span, Offset and Calculated Maximum RMS Error*
Span Errors (converted to %V
FSS
)
Span Calibration
Linearity
Pressure Hysterisis
Temperature Hysterisis
Temperature Effect on Span
Offset Errors (converted to %V
FSS
)
Offset Calibration
Temperature Effect on Offset
Offset Stability
Calculated Maximum RMS Errors
No Compensation*
With auto-zero
* This assumes the power supply is constant.
OffCal
Tco
OffStab
4
4
0.5
RMS Error
7.19
4.42
%
FS
%
FS
%V
FSS
%V
FSS
%V
FSS
Symbol
SpanCal
Lin
Phys
Thys
Tcs
Error Value
4
1
0.1
0.5
1.5
Note
Unit
%V
FSS
%V
FSS
%V
FSS
%V
FSS
%V
FSS
AN1668
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Sensors
Freescale Semiconductor
AMPLIFIER SELECTION AND AMPLIFIER
INDUCED ERRORS
A differential amplifier is needed to convert the differential
output of the MPX2010 sensor to a high level ground-
referenced (single-ended). The classic three-op amp
+V
CC
R2
R+S1
V
REF
R+S2
R1
2
–
instrumentation amplifier can be used. However, it requires
additional components (3 op-amps and possibly a split power
supply). An instrumentation topology shown in
Figure 1
requires only a single supply and only 2 op-amps and 1%
resistors.
R4
1
R3
3
+
U1A
6
–
5
+
U1B
7
V
OUT_FS
+V
CC
3
X1
1
2
4
S–
Pressure Sensor
S+
Figure 1. MPX2010 Amplifier Circuit
The circuit uses a voltage divider R+S1 and R+S2 to
provide the reference (level shift), U1A and U1B are non-
inverting amplifiers arranged in a differential configuration with
gain resistors R1, R2, R3, and R4. Note that U1B is the main
gain stage and it has the most gain. It is recommended to
place a 0.015
µF
capacitor in it's feedback loop (in parallel with
R4) to reduce noise. The amplifier output can be
characterized with the equation below:
R4
-
Gain = -------
+ 1
R3
R2
⋅
R1
R2
⋅
R4
-
-
Voffset = VREF
⎛
--------------------
⎞
–
VSCM
⎛
--------------------
⎞
⎝
R1
⋅
R3
⎠
⎝
R1
⋅
R3
⎠
(4)
–
1
(5)
Vout = (S+ - S-) Gain + Voffset
(6)
where (S+ - S-) = Sensor differential output + Sensor offset
(7)
Equation 4 is the differential gain of the amplifier and
equation 5 is the resulting offset voltage of the amplifier.
The above equations assume that the amplifier is close to
ideal (high A
OL
, low input offset voltage and low input offset
bias currents). Since an ideal op-amp is hard to come by, the
customer should select an op-amp based on cost and
performance. Below are some points to keep in mind when
selecting an op-amp and designing the amplifier circuit.
Note that the ratio R2*R4/R1*R3 controls the system offset
as well as the common mode error of the amplifier.
Mismatches in these resistors will result in an offset and
common mode error which appear as offset. It is therefore
recommended to use 1% metal film resistors to reduce these
errors. Also, V
REF
source impedance should be minimized in
comparison with R1 in order to reduce common mode error.
Amplifier input offset and input bias currents can induce
errors. For example, an input offset (Vio) of the amplifier can
become significant when the closed-loop gain of the amplifier
is increased. Furthermore, there is also a temperature
coefficient of the input voltage offset which contribute an
additional error across temperature. If the input bias current of
the amplifier is not taken into account in the design, it can also
become a source of error. A technique to reduce this error is
to match the impedance the source impedance of what the op-
amp input pins sees.
It is important to note that high performance op-amps are
more expensive. An MC33272 op-amp has a low input offset
and low input bias current which is suitable for the two-op amp
amplifier design. We can see that there is a tradeoff between
accuracy and cost when designing a solution with the
MPX2010.
When designing a system based on the MPX2010, it is
important to take into account errors due to parametric
variation of the sensor (i.e., offset calibration, span calibration,
TcS, TcO), power supply and the inherent errors of the
amplification circuit. The offset and span errors greatly
determines the resolution of the system (which adds to the
system error). Even though the system offset error can be
AN1668
Sensors
Freescale Semiconductor
3
nulled out by auto-zeroing, these errors must be accounted for
when setting the system gain (refer to AN1556 for more
details). This forces the total span of the system to be smaller,
because we must reserve an extra headroom from the total
span to account for amplifier and A/D variations (i.e., amp. sat.
voltage, power supply variation, A/D quantization error, and
gain errors). If these errors are not accounted for, it could, for
example, result in non-linearity errors if the sensor span or
offset error causes the amplified output of the sensor to reach
the saturation voltage of the amplifier.
As an example, a MPX2010 sensor system is designed
which has a range of 600 mm H
2
O FS range with a
±5%
FS
RMS error. The system uses a +5.0 V
±5%
linear regulated
power supply, a MC33272 dual op-amp and a 1% resistors.
Table 3
shows the resulting specification and component
values for the system based on MPX2010 sensor.
Table 3. MPX2010 Sensor System Values
MPX2010 Sensor Design
Parameter
V
CC
Differential Gain
Vout_FS
V
REF
Parts List
U1A,U1B
R1
R2
R3
R4
R + S1
R + S2
X1
MC33272 Op-amp
Gain Resistor
Gain Resistor
Gain Resistor
Gain Resistor
Level Shift Resistor
Level Shift Resistor
MPX2010
39.2K
90.9
909
392K
1K
150
Description
Reg Power Supply
Gain
Full Scale Span
Offset Reference
Value
5
433
3.02
0.66
Units
V
V/V
V
V
Ω
Ω
Ω
Ω
Ω
Ω
Table 4. Performance Comparison between MPX2010 and MPXV4006G Solution
Error Contribution
Max Sensor Error
System Resolution (A/D + Amplification)
System Error (Sensor + A/D + Amplification)
System Error with Auto-Zero
MPX2010 Solution Error
(FS = 600 mm H
2
O)
MPXV4006G Solution Error
(FS = 612 mm H
2
O)
±
% FS
7.19
1.30
7.3
4.6
±
mm H
2
O
43
8
44
28
±
% FS
3.00
0.80
3.10
t3
±
mm H
2
O
18
5
19
t19
Note that the error due to system resolution is higher for the
MPX2010 solution (± 2 bit A/D accuracy). This is because the
MPX2010 span is limited as discussed above. Also, this
accuracy assumes that the amplifier does not induce
significant errors. As noted MPXV4006G sensor has better
overall accuracy. The system resolution is very good because
of its large span (4.6 V versus 3.0 V typical).
amplification is already on board. Besides the system
simplicity and using less component, the resolution and
overall accuracy of this solution is better than the MPX2010
solution. In some cases, less components can actually
improve the reliability and manufacturability the system.
REFERENCES
[1] Benchmark of Washing Machine Mechanical Sensor,
Jack Rondoni, Freescale Semiconductor, Inc. Internal
Document.
[2] Mechanical Sensor Characterization, Ador Reodique,
Freescale Internal Document.
[3] AN1551 Low Pressure Sensing with the MPX2010
Pressure Sensor, Jeff Baum, Freescale Application Note.
[4] AN1636 Implementing Auto-Zero for Integrated Pressure
Sensors, Ador Reodique, Freescale Application Note.
[5] AN1556 Designing Sensor Performance Specifications
for MCU-based Systems, Eric Jacobsen and Jeff Baum,
Freescale Application Note.
SUMMARY
Several washing machine solutions were examined. The
MPX10/12 solution can be expensive in terms of additional
support circuitry and the added time and labor involved during
the calibration procedure. The MPX2010 is good alternative
for high volume manufacturing because is already calibrated.
With this solution, however, the system amplifier design must
be chosen and designed carefully in order to minimize the
system error. This is a consideration when deciding to
implement a high accuracy solution with the MPX2010
because the cost of the system will go up.
The MPXV4006G solution is geared towards high volume
manufacturing because trimming, compensation and
AN1668
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Sensors
Freescale Semiconductor
NOTES
AN1668
Sensors
Freescale Semiconductor
5
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