Non-contact sensors. What are they and how do they work?

Sensors have always been all around us, but they are becoming more ubiquitous as automation and the Internet-of-Things (IoT) becomes more widespread in our home and working lives.

New sensor applications are being developed all the time and being used in increasingly critical installations, where reliability and dependability are not only desirable but essential. Autonomous cars, for example, will need sensors that have the utmost reliability to gain user acceptance.

One way to improve the dependability of sensors is to eliminate mechanical failure by using non-contact technologies, such as capacitive and inductive sensing typically used for liquid level and position measurement.

Here we describe the basics of these two technologies and how they can be applied.

How does capacitive level sensing work?

To explain capacitive level sensing, we must first understand the concept.

A capacitor is formed by two electrodes, electrically insulated from each other. The electrodes themselves must be conductive and are typically made from metal. They can be any shape, although two parallel plates are easiest to visualise.

Capacitors have the ability to store energy in an electric field between these electrodes when a voltage or ‘potential’ is applied to the circuit. The property of capacitance relates the amount of energy stored in this field to the applied voltage or potential.

By placing non-conductive material between the electrodes, the ability for the capacitor to store energy increases and so the capacitance increases. This material between the electrodes is referred to as the ‘dielectric’.

The key property of dielectric materials is the amount of charge that can be stored.

As a dielectric liquid is introduced between the electrodes of the capacitor, the capacitance changes proportionately and liquid level can be determined.

To measure variations in the capacitance, electric energy flowing into and out of the electrodes is measured as the voltage or potential is varied. This flow of energy is created by connecting the electrodes to an alternating current measurement circuit. The greater the energy flow to the electrodes, the greater the capacitance, meaning more dielectric between the electrodes.

For level sensor calibration, reference measurements at empty and full tank levels must be taken. Generally, the dielectric constant value of the liquid being measured is required, to enable calibration of the sensor at its ‘full’ level. With the empty and full outputs set, liquid level sensing comes down to relating the sensor output to these values.

Capacitor plates can also be designed as a concentric tube and rod, with the advantage of reduced interference as well as improved mechanical stiffness and robustness, as engineered in Gill’s liquid level sensors.

Gill’s capacitive liquid level sensor are often constructed from aluminium, stainless steel or carbon fibre allowing Gill to cater for harsh applications from fuel tanks in Formula 1 race cars, to oil transmission systems in mining and industrial machinery.

What are the principles of Induction position sensing?

Induction position sensors are capable of measuring linear, angular, rotary and non- uniform movement across measurement zones expanding from 20mm to 100mm and beyond.

The principle of induction position sensing is a 3 step process.

Gill’s Induction sensors use a series of coils which measure the position of a metallic target, referred to as the activator. The activator can be mounted to or machined into, the moving part of the application.

In the first step, the coil generates a magnetic field, which penetrates the activator, causing it to produce its own opposite field.

Next, the coil stops generating its magnetic field and measures the activator’s induced field.

Finally, this measurement of the induced field is compared across the series of coils, to calculate an absolute measurement of the activator position.

This measurement principle allows inductive position sensors to be used for a wide range of measurement types, with no inherent mechanical limit or need for mechanical control.

The constant air gap between the sensor and activator ensures the sensor will not deteriorate through use and, unlike Hall Effect technology, induction technology is unaffected by the presence of permanent magnets or nearby electro-magnetic interference and will not drift with temperature, a key benefit.

Inductive sensors are used in a versatile range of applications throughout industry to measure the positional movement of gearboxes, pedals and mechanical arms, to name a few.

Inductive sensors are non-contact, fully sealed position sensors designed for extreme conditions and harsh environments. This solid-state construction removes any potential interference from dust, dirt and liquid ingress to provide a range of sensors for demanding applications with highly accurate and durable measurement.

With ever increasing pressure on costs, the need for accurate and reliable monitoring equipment, is fundamental to maximising productivity and reducing mechanical down time and system failure.

Capacitive and Inductive sensors are a key technical and knowledge specialism at Gill Sensors & Controls, who have extensive experience in developing commercially innovative and successful sensing solutions using these technologies. If you would like to learn more, and how Gill may be able to provide a solution to your need visit us at www.gillsc.com or email at info@gillsc.com.