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Value analysis of capacitive technology over float based methods for liquid level sensing

Thu, 05/22/2014 - 12:31pm
Anirban Sengupta and Akshay Dixit, Cypress

In one sense, technological advancement is about enabling people to take things for granted. Imagine youre driving at 80 mph on a highway and the car immediately ahead stops suddenly. Instinctively your foot goes to the brake pedal, and your car transmits the force from your foot to the brakes through fluid or brake oil. If the brake oil is low, this could result in a malfunctioning of the cars braking system and injury or death.

In the consumer world, malfunctions dont often result in death but they can impact the user experience.  For example, automatic monitoring of the milk and water level in the reservoir of your coffee machine protects your morning from an unpleasant coffee experience. Similarly, monitoring of the color coming out of a printer cartridge saves you from discovering youre out of ink at a critical moment. Effective liquid level sensing and monitoring is a key element of a safer and more convenient life.

Measuring liquids
Traditionally, liquid level sensing has been done using mechanical floats that float on the surface of the process liquid but sink to the bottom of the head-space vapor. These floats are good indicators of locating the fluid surface but are difficult to use to measure the actual fluid level. Unfortunately, for years developers have focused not on improving the sensitivity and accuracy of liquid level measurement but rather on improving the float. There have been improvements made by replacing the pulleys and levers accompanying primitive mechanical floats with magnetic sensors and electronic switches. At the end of the day, however, the basic problem remains: how does a system measure liquids continuously?

Consider two popular techniques for liquid level measurement in use today: magnetic Reed sensors and Hall Effect sensors.  A magnetic Reed sensor uses a magnetic float that floats on the liquid surface and a Reed switch fitted on the side of the vessel. When the liquid level reaches the Reed switch, the magnetic float causes the switch to toggle. Detection of the toggle communicates that the liquid level has reached the Reed Switch. Thats it. If the vessel is 100mm long and the Reed switch is located at 20mm, the data that can be derived is the fact that the liquid level is at 20mm. We can, of course, connect multiple Reed switches (maybe at 40mm, 60mm and 80mm) but then the cost of the system multiplies linearly.

Alternatively, the Reed Switch can be replaced by a Hall-effect sensor. This sensor can interact with the float without being in contact with the liquid, making it a safer and more convenient option in comparison to a Reed switch. However, it also does not overcome the 20mm problem stated above.

While both of the above solutions can improve sensitivity, they do so at a linearly increasing cost, thus forcing a trade-off between sensitivity of measurement and cost. Moreover, the presence of a movable float in contact with the liquid introduces the possibility of contamination.

On a broad level, there are two kinds of measurement that are commonly required when working with liquids: point-level and continuous-level.  Point-level measurement determines if the liquid has reached a particular level or not.  This suffices when there is a reservoir connected to the liquid column measured. Each time the point level is reached, the control system ensures that the liquid level is restored either by pouring in more or stopping the pour. In systems like automobiles, however, the reservoir is not a part of the liquid system (i.e., we still have to drive down to the gas station). In such cases it is critical to measure the liquid level on a continuous basis or at least at discrete level. In these applications, float-based systems become expensive to implement while still providing only sub-optimal measurement capabilities.

Level sensing through capacitive measurement


Capacitive sensing offers an effective and inexpensive way of measuring liquid level accurately. The basic principles of capacitance used in touch screens are applied in a different way.

For parallel plate capacitors the equation of capacitance is as follows:

C = E0ErA /d

E0 = permittivity of free space (8.85 X10^-12 F/m)

Er = Relative permittivity of di-electric placed between plates

A= Area of plate                        

d = Distance between plates

The capacitance is directly proportional to the dielectric constant and inversely proportional to the distance between the plates. When there is no liquid in the container, the total capacitance or initial capacitance is due to air only. When the liquid starts rising, the total capacitance of system changes since there are effectively two different capacitors now, attached in series: the Liquid Capacitor (Cl) and Air Capacitor (Ca). As the length of the liquid column increases and the air column decreases, the Cl and Ca values change, and thus so does the total capacitance. By measuring the total capacitance (or change in capacitance), the level of liquid in the container can be computed using a simple algebraic equation or look-up table.

For practical applications, limitations may arise due to viscosity changes of the fluid, container shape, and so on. However, it is possible to implement multiple level sensing using multiple IOs from the same capacitive sensing controller.  At each level of the container where a measurement is required, a metal trace is placed which in turn is connected to an IO of the capacitive sensing controller.

How does this compare to traditional float methods?  Consider the example described above with the 100mm column. If we need measurements with an interval of 10mm, then using the Reed Switch method requires 10 different switches in addition to the magnetic float that must be placed in the liquid and a separate MCU to monitor the liquid level. With a capacitive sensing approach, the system only needs 10 low-cost traces and a single 10 I/O capacitive sensing controller.  A separate MCU is not required because this functionality is integrated into the controller.

 

The benefits of using capacitive sensing (already discussed above) can be summarized as follows:

1. Non-invasive solution (non-contact with process fluid)

2. Better resolution of measurement

3. Wear-tear free due to the absence of moving parts

4.  Low price.

Capacitive sensing technology continues to gain momentum in many markets, both consumer and industrial. Just as it has eliminated the buttons from phones and created a whole new user experience, it is similarly positioned to remove complicated, expensive, and primitive systems used for liquid level measurement.

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