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Use capacitive sensing for reliable liquid level measurement (Part 2)

Posted: 13 Aug 2012     Print Version  Bookmark and Share

Keywords:liquid level  Base lining  Temperature drift 

As mentioned in part 1, point level measurement uses one or more sensors placed at discreet levels on the tank based upon the application's need to know the liquid level. The system uses the on/off state of the various sensors to deduce the liquid level.

For example, figure 1 shows a tank and PCB with one sensor attached to it at the end for tank empty detection. Tank Empty is determined by the sensor ON/OFF status—when the tank is filled with a conductive liquid, the sensor will be ON and when tank is empty the sensor will be OFF.

Figure 1: Capacitive sensor for empty level detection.

While the concept behind this approach appears simple, there are several issues, which need to be resolved while measuring liquid compared to detecting a human finger. Some of these issues include:

 • Base lining
 • Temperature drift
 • Tank thickness
 • Liquid viscosity
 • Conductive object interference

Base lining
To make any measurement, there has to be a reference point. If that reference is not correct, the system will provide a false measurement. For the capacitive sensing algorithms discussed in this article, a baseline serves as the reference point. In a user interface implementation, for example, the system assumes that a finger is present at power on and the baseline is initialized based upon parasitic capacitance.

Establishing the baseline requires a different approach when it comes to liquid level measurement. Consider if there is no liquid at power on, then the sensor will be OFF indicating the tank is empty. Now, if tank is later filled with liquid, the liquid will add capacitance and the sensor will turn ON indicating tank is not empty. However, when there is liquid in the tank at power on, since the raw counts measured at power on are used as baseline reference, the sensor will be reported as OFF indicating that the tank is empty when in fact it is not.

A straightforward solution to this problem is to have a reliable reference. This can be achieved using a virtual sensor. A virtual sensor is a sensor that has similar characteristics as the sensor being used to detect the liquid but it is not placed in direct contact with the liquid or through overlay. Since this sensor takes a reading that is not affected by liquid, the raw counts of the virtual sensor can be used as the reference baseline for the actual sensor.

Let CX be the virtual sensor and tank empty detection sensor capacitance (without liquid). Let CL be the capacitance added by the liquid.

Without liquid

Virtual Sensor capacitance = CX

Tank empty detector sensor capacitance = CX

With liquid

Virtual Sensor capacitance = CX

Tank empty detector sensor capacitance = CX+CL

The difference in the capacitance (CL) of the tank empty detector sensor and virtual sensor is measured to determine whether the tank is empty or not upon turning on the system.

Designing a reliable virtual sensor can be challenging for system layout designers since the capacitance of the virtual sensor has to match the capacitance of the actual sensor for the baseline to be of value. To achieve this, the virtual sensor should have the same dimensions as the actual sensor. The trace length to each sensor should be the same as well, although the trace length can be varied to accommodate other discrepancies in the system and match the capacitances. The number of vias should be the same for both sensors and, to reduce the overall parasitic capacitance, the number vias should be limited to no more than 3. In addition, the virtual sensor and actual sensor should be placed on different layers.

Figure 2: Board with virtual sensor (front view: left, side view: right).

Temperature drift
Temperature drift can impact several system parameters, including the capacitance to be measured, the Cmod value, and the IDAC current, which is used to excite the sensor. As a result of these variations, the raw counts will also increase or decrease because of temperature (figure 3). At power up, the system assumes the raw counts are 'X' counts, so the baseline is 'X'. An increase in temperature, however, will increase the current raw count to 'X+Y'. If Y is greater than the ON threshold, then the sensor will be reported as ON even though liquid is not present. Similarly, if the raw counts decrease due to temperature, then the sensor will be reported as OFF in the presence of liquid.

Figure 3: Change in raw count due to change in temperature, X-axis temperature and Y-axis raw counts.

Since the virtual sensor has the same characteristic as that of the actual sensor, the effect of temperature will be the same on both sensors (i.e., the differential capacitance between the two sensors cancels the effect of temperature drift). For this reason, the system must regularly update the reference baseline to reflect any changes in the operating environment.

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