The Evolving Technology of Capacitive Sensors

In my last blog post, Sensing Types of Capacitive Sensors, I discussed the basic types of capacitive sensors; flush versions for object detection and non-flush for level detection of liquids or bulk materials.  In this blog post, I would like to discuss how the technology for capacitive sensors has changed over the past few years.

The basic technology of most capacitive sensors on the market was discussed in the blog post “What is a Capacitive Sensor”.  The sensors determine the presence of an object based on the dielectric constant of the object being detected.  If you are trying to detect a hidden object, then the hidden object must have a higher dielectric constant than what you are trying to “see through”.

Conductive targets present an interesting challenge to capacitive sensors as these targets have a greater capacitance and a targets dielectric constant is immaterial.  Conductive targets include metal, water, blood, acids, bases, and salt water.  Any capacitive sensor will detect the presence of these targets. However, the challenge is for the sensor to turn off once the conductive material is no longer present.  This is especially true when dealing with acids or liquids, such as blood, that adheres to the container wall as the level drops below the sensor face.

Today, enhanced sensing technology helps the sensors effectively distinguish between true liquid levels and possible interference caused by condensation, material build-up, or foaming fluids.  While ignoring these interferences, the sensors would still detect the relative change in capacitance caused by the target object, but use additional factors to evaluate the validity of the measurement taken before changing state.

These sensors are fundamentally insensitive to any non-conductive material like plastic or glass, which allows them to be utilized in level applications.  The only limitation of enhanced capacitive sensors is they require electrically conductive fluid materials with a dipole characteristic, such as water, to operate properly.

Enhanced or hybrid technology capacitive sensors work with a high-frequency oscillator whose amplitude is directly correlated with the capacitance change between the two independently acting sensing electrodes.  Each electrode independently tries to force itself into a balanced state.  That is the reason why the sensor independently measures  the capacitance of the container wall without ground reference and the capacitance of the conductivity of the liquid with ground reference (contrary to standard capacitive sensors).

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Up to this point, capacitive sensors have only been able to provide a discrete output, or if used in level applications for a point level indication.  Another innovative change to capacitive sensor technology is the ability to use a remote amplifier.  Not only does this configuration allow for capacitive sensors to be smaller, for instance 4mm in diameter, since the electronics are remote, they can provide additional functionality.

The remote sensor heads are available in a number of configurations including versions image2that can withstand temperature ranges of -180°C up to 250°C.  The amplifiers can now provide the ability to not only have discrete outputs but communicate over an IO-Link network or provide an analog output.  Now imagine the ability to have an adhesive strip sensor that can provide an analog output based on a non-metallic tanks level.

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Liquid Level Sensing: Detect or Monitor?

Pages upon pages of information could be devoted to exploring the various products and technologies used for liquid level sensing and monitoring.  But we’re not going to do that in this article.  Instead, as a starting point, we’re going to provide a brief overview of the concepts of discrete (or point) level detection and continuous position sensing.

 Discrete (or Point) Level Detection

Example of discrete sensors used to detect tank level
Example of discrete sensors used to detect tank level

In many applications, the level in a tank or vessel doesn’t need to be absolutely known.  Instead, we just need to be able to determine if the level inside the tank is here or there.  Is it nearly full, or is it nearly empty?  When it’s nearly full, STOP the pump that pumps more liquid into the tank.  When it’s nearly empty, START the pump that pumps liquid into the tank.

This is discrete, or point, level detection.  Products and technologies used for point level detection are varied and diverse, but typical technologies include, capacitive, optical, and magnetic sensors.  These sensors could live inside the tank outside the tank.  Each of these technologies has its own strengths and weaknesses, depending on the specific application requirements.  Again, that’s a topic for another day.

In practice, there may be more than just two (empty and full) detection points.  Additional point detection sensors could be used, for example, to detect ¼ full, ½ full, ¾ full, etc.  But at some point, adding more detection points stops making sense.  This is where continuous level sensing comes into play.

Continuous Level Sensing

Example of in-tank continuous level sensor
Example of in-tank continuous level sensor

If more precise information about level in the tank is needed, sensors that provide precise, continuous feedback – from empty to full, and everywhere in between – can be used.  This is continuous level sensing.

In some cases, not only does the level need to be known continuously, but it needs to be known with extremely high precision, as is the case with many dispensing applications.  In these applications, the changing level in the tank corresponds to the amount of liquid pumped out of the tank, which needs to be precisely measured.

Again, various technologies and form factors are employed for continuous level sensing applications.  Commonly-used continuous position sensing technologies include ultrasonic, sonic, and magnetostrictive.  The correct technology is the one that satisfies the application requirements, including form factor, whether it can be inside the tank, and what level of precision is needed.

At the end of the day, every application is different, but there is most likely a sensor that’s up for the task.