Inductive and Capacitive Proximity Sensors

The world is becoming more connected than ever before, thanks to the continued rise of the Internet of Things. At the core of this rising, connected world, is monitoring systems, which use arrays of sensors to interact with the surrounding environment. Monitoring systems can be seen in industrial environments, which monitor product quality and process efficiency, and even in bathrooms, allowing faucets to switch on and off without touch. You can thank proximity switches for these object sensing capabilities.

Proximity sensors vary in the way they function. These contactless sensors use an acoustic (ultrasonic) signal to determine when an object is within the sensor’s field of view, or they use the electromagnetic field in some way. Common proximity sensors use infrared light to detect an object, while others use the self capacitance or inductance of an object to generate an electrical signal.

Applications of Proximity Sensors

Inductive and capacitive proximity sensors operate under a similar concept as capacitive touch sensors. These sensors detect nearby objects by measuring some disturbance in the surrounding electromagnetic field. Unlike traditional sensors, which require a mechanical switch or lever, proximity sensors can be used to accurately detect an object without requiring contact. A proximity sensor that is designed or adjusted for a short-range can also be used as a touch switch.

Proximity sensors are ideal for use in dirty or wet environments thanks to their contactless operation mode. They are less sensitive to dust, moisture, vibration, and noise, and their solid-state based internal circuitry makes them in applications where vibration can interfere with traditional contact switches. As such, proximity sensors tend to have a long functional life and are highly reliable due to the lack of mechanical parts and lack of physical contact with the target object.

Inductive and Conductive Proximity Sensors

Outside the domain of ultrasonic or infrared sensing, the most common proximity sensors are:

• Inductive proximity sensors: These sensors are used for contactless detection of metal objects, both ferrous and non-ferrous, and use a coil and an AC signal to generate an oscillating electromagnetic field near the sensing element. When a metal object is nearby, the coil induces an eddy current, which then induces a back EMF in the coil, which dampens the field amplitude. The rise or fall of such oscillation is identified with a Schmitt trigger circuit. The sensitivity of these sensors depends on the specific metal being sensed.
• Capacitive proximity sensors: These sensors are used for detecting conductive or insulating objects, making them much more flexible than inductive proximity sensors. These sensors operate by measuring the impedance of the sensor as a highly stable oscillating signal propagates through the sensing element. When an object is brought nearby, the object and the sensing element have some mutual capacitance, which changes the impedance of the sensing element. This change in impedance can also be detected with a Schmitt trigger, ADC, or other thresholding circuit.

A capacitive proximity sensor

Choosing a Proximity Sensor

Proximity sensors or switches can be designed for accurate operation under either a high-speed operating environment or low-speed environment. They can provide analog or digital output, and they use a typical operating voltage from 0 to 10 V DC with low current. When selecting a proximity sensor, some things to consider are:

• Material being detected: Different proximity sensors are optimized for sensing different materials. Inductive sensors are typically chosen for distinguishing between metals and nonmetals.
• Form factor and sensing distance: These sensors come in a variety of shapes and sizes. These sensors can also be shielded or unshielded. An unshielded proximity sensor typically provides longer range, while a shielded sensor typically allows flush mounting.
• Sensitivity: This is primarily related to the sensitivity of the thresholding circuit and the strength of the AC signal used in the sensing element. As mentioned above, sensitivity is also related to the material being detected.

Types of Omron Sensors

Omron, E2J Series

The Omron E2J is a set of capacitive sensors with two main variants: Omron E2J-W20MA and the Omron E2J-W10MA. The E2J-W10MA provides an adjustable sensing distance ranging from 4 to 10 mm, while the E2J-W20MA supports an adjustable sensing distance ranging from 8 to 20 mm. They both have a response frequency of 70 Hz and an IP66 rating. Typical power supply for the sensors is 24 V DC, and when mounted on a robotic hand or in some handling system, you can expect to be able to detect LCDs, Wafers, PDPs, and other products.

Omron E2J-W20MA capacitive proximity sensor. From the E2J-W20MA datasheet.

Omron, TL-N20MD

The Omron TL-N20MD inductive proximity provides a sensing distance of 20 mm. The response frequency is 300 Hz, and it supports a power supply from 12 to 24 V DC. With an operating temperature from - 25 to 70°C (with no icing or condensation) and IEC 60529 IP67 ratings, this proximity sensor can be deployed for a wealth of applications requiring high sensing distance. Other models with lower sensing distance are also available, although these models carry longer response time.

Omron TL-N20MD inductive proximity sensor. From the TL-N20MD datasheet.

Omron, E2K-C25ME1

Continuing in the quest for proximity sensors with high sensing distance, the Omron E2K-C25ME1 is an unshielded capacitive proximity sensor capable of providing accurate object detection up to 25 mm. It supports a wide range of power supply voltages from 10 to 40 V DC. The response frequency is 70 Hz and the sensor is IP66 rated. The Omron E2K-C25ME1 finds its place in applications requiring high sensing distance and fast response time.

Omron E2K-C25ME1 capacitive proximity sensor. From the E2K-C25ME1 datasheet.

Future of Proximity Sensors and Selecting the Right Component

Proximity sensors will continue to be instrumental in an evolving range of applications. The proximity sensors shown here are just a few examples of the options you can find on the market. Browse our parts inventory, specs, and manufacturers to find the components you need for object detection and other applications.

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