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Developing a liquid level control/delivery system

Posted: 16 Jan 2013 ?? ?Print Version ?Bookmark and Share

Keywords:pressure sensors? data acquisition? delta-sigma ADC?

Hydrostatic pressure produced by the water column at the bottom of the controlled liquid reservoir uses the trapped air in the measurement tube to produce the same amount of pressure on the sensor. At its output, the pressure sensor produces a pressure equivalent voltage that is measured and digitized by the MAX11206 ADC, processed by the integrated MAXQ622 microcontroller, and finally sent to a PC though the USB cable. The PC-based control-and-dispense GUI then sends a delivery request to the DAS, which activates the valve-driver PCB to deliver a certain amount of the liquid predefined by the software. The DAS also provides control signals to the pump-driver PCB to turn on/off so an constant liquid height is maintained.

Precision and resolution
For a system like this, we must account for the density of the liquid if we want to dispense by weight. In general, liquid density varies with changes in temperature. For example, the density of water increases between its melting point at 0C and +4C, reaching a standard value of 999.972 (practically 1000) kg/m3 at +4C. At room temperature, +22C, the density of water is 997.774kg/m3. All measurements in this article were done at room temperature around +22C, 3C, where water density varies around 0.1%. Note that this is below the targeted precision for the DAS referenced in this article. For a typical MPX2010 fullscale range of 10kPa, the water height equivalent is 1.022m.

We start by calculating the fullscale voltage swing that we will see from the pressure sensor when the maximum pressure for this sensor, PFS10kPa, is applied. Note that 10kPa translates to a water height of 1m.

VFS = VFST (VDD/VPST)

Where:
VFS: Fullscale voltage swing when excited by VDD
VPST: Typical excitation voltage
VFST: Fullscale sensor voltage swing when excited by VPST
VDD: Excitation voltage

Equation 1

Since we are exciting this pressure sensor with a VDD of 3.3V instead of the typical VPST of 10V, we will only see a swing of VFS = 8.25mV instead of VFST = 25mV.

VFS = 25mV (3.3/10) = 8.25mV (fullscale span at 3.3V)

Equation 2

From this equation we know how much of the ADC's range we require: 8.25mv to measure up to the 1000mm level of water. Note that in this setup the ADC does have a range of 3.3V. In fact, we are not using the full range of 1000mm for this sensor. We are only going to the height of 480mm, which will translate to a pressure range of approximately half of maximum range of 10kPa. To keep it simple, we will just multiply by 0.48 to obtain the new fullscale voltage swing.

The MAX11206 used in this design is a 20bit delta-sigma ADC suitable for low-power applications that require a wide dynamic range. It has an extremely low input-referred RMS noise of 570nV at 10sps. We know that the noise-free resolution (NFR) is about 6.6 RMS noise. In this case it will be 2.86?V. (This is also sometimes called the flicker-free code.) The noise-free codes present in the range can be found by dividing the ADC range used by the input-referred noise-free bits size:

Where HFS is the measurement resolution of height. Therefore:

Equation 3

The estimated fullscale resolution at 0.056% is more than sufficient to achieve the DAS's target precision of 1% in this reference design. This proves that the ADC can directly interface with a new compensated silicon pressure sensor without additional instrumentation amplifiers.

Figure 2: Calculation of dispensing volume.

Calibration and calculation
In the current design example liquid is located inside two concentric cylindrical walls. Dispensing volume can be calculated using a linear function based on the two-point calibration, as shown in figure 2.

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