What Is Proximity Sensing
When you use a smartphone that has a touch screen, you interact with the phone via the screen, which senses touch. So a smart phone uses touch sensing to interact with the interface. Proximity sensing is similar, but instead of touching the interface, the user approaches it, either with a finger or their hand. There are different kinds of technology that allow proximity sensing; our products, such as the Touch Board, Pi Cap and Light Up Board, use capacitive sensing to create a capacitive sensor, but you can also use an inductive sensor, a photoelectric sensor or an ultrasonic sensor.
Capacitive proximity sensor
There are three electronic components of an electric circuit that are ubiquitous: resistors, inductors and capacitors. A capacitor in its simplest form consists of two plates that store a charge when a voltage is applied across them. The amount of charge a capacitor is able to store depends on the size of the plates, the distance between them and the dielectric constant of the material between the plates. Storing charge forms an electric field and a capacitive proximity sensor uses the electric field to detect an object. Capacitive sensors are able to detect any material that can interfere with the electric field, such as an organism, a metal object and fluids.
There are two common types of capacitive sensing. Mutual capacitance and self-capacitance. Mutual capacitance is used on touch screens, where there is an array of capacitor plates that are all linked with each other through their electric fields. When the user touches the screen, they are changing the electric field, which is registered as a touch.
Self-capacitance is the method we use in our technology. Here, each electrode acts a capacitive plate that is initially forming an electric field with the earth when powered on. When the user approaches the sensor, the user’s finger or hand take over and change the electric field. In both cases, mutual and self-capacitance, there is a change in the electric field that is used for detection, but generally, self-capacitance is used for proximity sensing.
Touch sensing is binary, so the device can register either “interface is being touched” or “interface isn’t being touched anymore”, just like a switch. Proximity sensing allows for more gradual sensing as well as binary sensing. So you can set the sensors to register an event at, for example, 5cm sensing distance and make a distance light switch. Or you can make it gradual and increase the brightness the closer you get to the hand. Proximity sensing also allows the user to embed the sensor behind different surfaces and by touching the surface, the user interacts with the device.
Inductive proximity sensor
Inductive sensing uses inductors to detect an object. An inductor in its common form consists of a wire wound into a coil around a core and when a current flows through the wire, it generates a magnetic field. Inductive sensors are similar to capacitive sensors. Capacitive sensors use voltage to create an electric field, whereas inductive sensors use current to create a magnetic field. Where in a capacitive sensor a change in the electric field was used to detect proximity, in an inductive sensor any change in the magnetic field is registered as an event.
This magnetic field can only be changed by metals, so the only sensing object inductive sensors can detect are metal objects, not fluids or an organism. When a metal target enters the magnetic field of the sensor, the field generates small currents, known as eddy current, on the object’s surface, which in turn oppose the magnetic field. This change is registered and allows to detect a nearby metallic object. Because they are able to detect only metal objects, an inductive proximity sensor is also a magnetic proximity sensor.
Not being able to detect fluids, makes inductive sensors suitable for wet environments, where the user needs to detect metal objects, the most common example being a traffic sensor. Inductive proximity sensors have about the same sensing distance as capacitive proximity sensors.
Photoelectric proximity sensor and ultrasonic proximity sensor
A photoelectric sensor uses light to detect nearby objects and surfaces. They generally consist of an emitter and receiver; the emitter’ output being a light beam with the receiver receiving the light. Depending on how much light reaches the receiver, it is able to distinguish the target object’s position or a change in the surface. Most emitters in a photoelectric sensor either use infrared (IR) light from an IR LED or visible light. There are three methods: through-beam, reflective and diffuse, each having their own benefits and disadvantages. Photoelectric sensors have a large sensing range, from a couple of millimetres to several metres. Ultrasonic sensors are of a similar sensor type as the photoelectric sensor, but instead of light, they output a sound wave to distinguish the position of the target. Both of these sensors are ideal for long-distance sensing, compared to the capacitive and inductive sensors.
Each proximity sensing technology has its own advantages and disadvantages, depending on the use case, such as distance and material that require to be sensed. Capacitive and inductive are great for short distances, whereas inductive can only sense metal photoelectronic and ultrasound for larger distances.
We use capacitive sensing within our technology to create smart surfaces, by embedding them within the environment. If you want to explore proximity and capacitive sensing, our development kits and electronics provide an easy starting point for prototyping smart surfaces. To explore more community projects make sure to check out our Blog and Resources pages, where you can find more inspiration and information on how to use our products.
If you have any questions or want us to feature your project, feel free to contact us at firstname.lastname@example.org.