? freescale semiconductor, in c., 2006. all rights reserved. an1668 rev 2, 11/2006 freescale semiconductor application note 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 c onservation 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 requ irements, 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 mp x10 (or the mpx12) is 10 kpa (40 ? h 2 o) full-scale pressure range device. it is uncompensated for temperatur e 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 diff erential amplifier (see figure 1 ) so it can be interfaced to an a/d and to obtain the desired range. since the mpx10/mpx12 sensor s 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 accommoda te 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-sca le 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). be cause expensive and time consuming calibration, te mperature 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 considerat ions 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 se ction must be designed carefully in order to meet the target accuracy requirement. the MPX2010 compensated sensor has the following specifications shown on ta b l e 1 .
an1668 sensors 2 freescale semiconductor the sensor system errors is ma de up of the sensor errors, amplifier errors and a/d errors. in other words, table 2 shows the MPX2010 with the errors converted to %v fss . the expected maximum root mean squared error of the sensor is = 7.19% fs. with auto-zeroing, the offset calibration, temperature effect on offset and offset stabilit y is reduced or eliminated, = +/- 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 (mpxv 4006g + 8 bit a/d) with auto-zero is 3.1% fs. table 1. MPX2010 specifications characteristic min typ max unit pressure range 010kpa supply voltage 10 16 vdc supply current 6ma full scale span 24 25 26 mv offset * 11mv sensitivity 25 mv/kpa linearity * 11%v fss pressure hysterisis 0.1 %v fss temperature hysterisis ( * 40 to 125 c) 0.5 %v fss temperature effect on span * 11%v fss temperature effect offset (0 to 85 c) * 11mv input impedance 1300 2550 w output impedance 1400 3000 w response time (10% to 90%) 1 ms warm-up 20 ms system sensor 2 amplifier 2 adresolution 2 + + 1 () = sensor spancal 2 lin 2 phys 2 thys 2 tcs 2 offcal 2 tco 2 offstab 2 + ++ + + + + 2 () = sensor spancal 2 lin 2 phys 2 thys 2 tcs 2 + + + + 3 () = table 2. MPX2010 span, offset and calculated maximum rms error* span errors (converted to %v fss ) symbol error value note unit span calibration spancal 4 %v fss linearity lin 1 %v fss pressure hysterisis phys 0.1 %v fss temperature hysterisis thys 0.5 %v fss temperature effect on span tcs 1.5 %v fss offset errors (converted to %v fss ) offset calibration offcal 4 %v fss temperature effect on offset tco 4 %v fss offset stability offstab 0.5 %v fss calculated maximum rms errors rms error no compensation* 7.19 % fs with auto-zero 4.42 % fs * this assumes the pow er supply is constant.
an1668 sensors freescale semiconductor 3 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 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. 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 f capacitor in it's feedback loop (in parallel with r4) to reduce noise. the amplifier output can be characterized with the equation below: equation 4 is the differential gain of the amplifier and equation 5 is the resulting offs et 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 clos ed-loop gain of the amplifier is increased. furthermore, th ere 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 a ccount 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., offs et calibration, span calibration, tcs, tco), power supply and the inherent errors of the amplification circuit. the of fset and span errors greatly determines the resolution of t he system (which adds to the system error). even though t he system offset error can be 2 3 6 x1 3 12 4 r+s1 r+s2 + v cc s + s ? pressure sensor +v cc v ref 5 u1b r1 r2 u1a r4 r3 1 7 v out_fs + ? ? + (4) gain r4 r3 ------- - 1 + = (5) voffset vref r2 r1 ? r1 r3 ? -------------------- - ?? ?? vscm r2 r4 ? r1 r3 ? -------------------- - ?? ?? 1 ? ? = (6) vout = (s+ - s-) gain + voffset (7) where (s+ - s-) = sensor differential output + sensor offset
an1668 sensors 4 freescale semiconductor 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 sp an 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 spec ification and component values for the system bas ed on MPX2010 sensor. 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 not ed mpxv4006g sensor has better overall accuracy. the system reso lution is very good because of its large span (4.6 v versus 3.0 v typical). 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, th e system amplifie r 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 solu tion is geared to wards high volume manufacturing because trimming, compensation and 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 m anufacturability the system. references [1] benchmark of washing machine mechanical sensor, jack rondoni, freescale semico nductor, 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. table 3. MPX2010 sensor system values MPX2010 sensor design parameter description value units v cc reg power supply 5 v differential gain gain 433 v/v vout_fs full scale span 3.02 v v ref offset reference 0.66 v parts list u1a,u1b mc33272 op-amp r1 gain resistor 39.2k ? r2 gain resistor 90.9 ? r3 gain resistor 909 ? r4 gain resistor 392k ? r + s1 level shift resistor 1k ? r + s2 level shift resistor 150 ? x1 MPX2010 table 4. performance comparison between MPX2010 and mpxv4006g solution error contribution MPX2010 solution error (fs = 600 mm h 2 o) mpxv4006g solution error (fs = 612 mm h 2 o) % fs mm h 2 o % fs mm h 2 o max sensor error 7.19433.0018 system resolution (a/d + amplification) 1.30 8 0.80 5 system error (sensor + a/d + amplification) 7.3 44 3.10 19 system error with auto-zero 4.6 28 t 3 t 19
an1668 sensors freescale semiconductor 5 notes
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