Freescale MPXM2010水位参考设计

        Freescale 公司的MPXM2010是硅压力传感器，具有高度精确以及和所加压力成正比的线性电压输出，单片内集成了应力测量仪和薄膜电阻网络。

       采用激光调整，以得到精密测量，失调校准和温度补偿，广泛应用呼吸病症诊断，空气运动控制，控制器以及压力交换等。本文介绍了压力传感器MPXM2010的主要性能以及水位参考设计的主要特点，主要元件如MCU，显示器等性能以及，压力传感器应用框图和传感器放大电路图。

      The MPXM2010 device is a silicon piezoresistive pressure sensor providing a highly accurate and linear voltage output directly proportional to the applied pressure. The sensor is a single, monolithic silicon diaphragm with the strain gauge and a thin-film resistor network integrated on-chip. The chip is laser trimmed for precise span and offset calibration and temperature compensation.

      MPXM2010主要特性：

      ·Temperature Compensated Over 0°C to +85°C
      ·Available in Easy-to-Use Tape & Reel
      ·Ratiometric to Supply Voltage
      ·Gauge Ported & Non Ported Options
      ·MPXM2010应用案例：
      ·Respiratory Diagnostics
      ·Air Movement Control
      ·Controllers
      ·Pressure Switching

                                       图1.全集成压力传感器MPXM2010框图

        水位参考设计：

       The Water Level Reference design continuously monitors water level and water flow using the temperature compensated MPXM2010GS pressure sensor in the low cost MPAK package, a dual op–amp, and the MC68HC908QT4, 8–pin microcontroller. This system uses very few components, reducing the overall system cost. This allows for a solution to compete with a mechanical switch for water level detection but also offer additional applications such as monitoring water flow for leak detection, and the other applications for smart washing machines.

        水位参考设计主要特性：
       Demonstrate Water Level Monitor plus additional features such as water flow        monitoring and leak 
       Pressure Sensor - MPXM2010 
       MPAK Package
       Sensitivity 2.5 mV / kPa
       Pressure Rating 10kPa (Max)
       Microprocessor MC68HC908QT4
       40K Bytes of in-application reprogrammable Flash and 128 Bytes of RAM
       High performance, easy to use, HC08 CPU
       4 Channel 8-bit analog to digital converter
       8-pin DIP or SOIC packages

图2. 水位参考设计外形图

                                                      图3.压力传感器应用框图

      The pressure sensor family has three levels of integration . Uncompensated, Compensated and Integrated. For this design, the MPXM2010GS compensated
pressure sensor was selected because it has both temperature compensation and calibration circuitry on the silicon, allowing a simpler, yet more robust, system circuit
design. An integrated pressure sensor, such as the MPXV5004G, is also a good choice for the design eliminating the need for the amplification circuitry.

        The height of most washing machine tubs is 40 cm, therefore the water height range that this system will bemeasuring is between 0.40 cm. This corresponds to a pressure range of 0.4 kPa. Therefore, the MPXM2010GS was selected for this system. The sensor sensitivity is 2.5 mV/kPa, with a full-scale span of 25 mV at the supply voltage of 10 VDC. The full-scale output of the sensor changes linearly with supply voltage, so a supply voltage of 5 V will return a full-scale span of 12.5 mV.

        (VS actual / VS spec) * VOUT full-scale spec = VOUT full-scale (5.0 V/ 10 V) x 25 mV = 12.5 mV Since this application will only be utilizing 40 percent of the pressure range, 0.4kPa, our maximum output voltage will be 40 percent of the full-scale span.
VOUT FS * (Percent FS Range) = VOUT max 12.5 mV * 40% = 5.0 mV
The package of the pressure sensor is a ported MPAK package. This allows a tube to be connected to the sensor and the tube is connected to the bottom of the tub. This isolates the sensor from direct contact with the water. The small size and low cost are additional features making this package a perfect fit for this application.
Microprocessor
       To provide the signal processing for pressure values, a microprocessor is needed. The MCU chosen for this application is the MC68HC908QT4. This MCU is perfect for
appliance applications due to its low cost, small eight-pin package, and other on-chip resources. The MC68HC908QT4 provides: a four-channel, eight-bit A/D, a 16-bit timer, a
trimmable internal timer, and in-system FLASH programming.
       The central processing unit is based on the high performance M68HC08 CPU core and it can address 64 Kbytes of memory space. The MC68HC908QT4 provides 4096 bytes of user FLASH and 128 bytes of random access memory (RAM) for ease of software development and maintenance. There are five bi-directional input/output lines and one input line shared with other pin features.

       The MCU is available in eight-pin as well as 16-pin packages in both PDIP and SOIC. For this application, the eight-pin PDIP was selected. The eight-pin PDIP was chosen
for a small package, eventually to be designed into applications as the eight-pin SOIC. The PDIP enables the customer to reprogram the software on a programming board and retest.

      Display
       Depending on the quality of the display required, water level and water flow can be shown with two LEDs. If a higher quality, digital output is needed, an optional LCD interface is provided on the reference board. Using a shift register to hold display data, the LCD is driven with only three lines outputted from the microcontroller: an enable line, a data line, and a clock signal. The two LEDs are multiplexed with the data line
and clock signal Multiplexing of the microcontroller output pins allows communication of the LCD to be accomplished with three pins instead of eight or 11 pins of I/O lines usually needed. With an eight-bit shift register, we are able to manually clock in eight
bits of data. The enable line (EN) is manually accepted when eight bytes have been shifted in, telling the LCD the data on the data bus is available to execute.

       The LEDs are used to show pressure output data by displaying binary values corresponding to a pressure range. Leak detection, or water-flow speed, is displayed by blinking a green LED at a speed relating to the speed of water flow. The red LED displays the direction of water flow. Turning the red LED off signifies water flowing into the tub. Turning the red LED on signifies water flowing out of the tub, or alternatively,
there is a leak.

       Digital values for water height, rate of water flow, and calibration values are displayed if an LCD is connected to the board 。This system is designed to run on a 9.0 V battery. It
contains a 5.0 V regulator to provide 5.0 V to the pressure sensor, microcontroller, and LCD. The battery is mounted on the back of the board using a space saving spring battery clip.

       Converting Pressure to Water Level
       Hydrostatic pressure being measured is the pressure at the bottom of a column of fluid caused by the weight of the fluid and the pressure of the air above the fluid. Therefore, the hydrostatic pressure depends on the air pressure, the fluid density and the height of the column of fluid.

P= Pa + g h
where P = pressure
Pa = pressure
= mass density of fluid
g = 9.8066 m/s^2
h = height of fluid column
To calculate the water height, we can use the measured pressure with the following equation, assuming the atmospheric pressure is already compensated for by the
selection of the pressure sensor being gauge:
h = P \ g

       所用元件列表：

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