If I were to explain capacitive proximity sensors in a nutshell, that would be: they are designed to detect the presence of nearby objects without any physical contact. They have evolved since I studied them back in college in the 70s, from narrow application in industrial automation systems into broader use. It amazes me how technology continues to grow.
Today, they can now be found in consumer and communications products ranging from ATM and vending machines to security systems to leading-edge personal computers. Proximity sensors are used in the most advanced mobile handsets to disable screens when a user is on a call, preventing accidental hang-up or muting.
However, applying proximity sensors in these new applications is no longer an electrical challenge. Using them now in my job requires mechanical expertise as well as experience in optical system design, in which engineers must combine light sources with sensors that effectively detect the light.
In this article, I would be telling you the interesting and innovative applications for proximity sensors, describe some of the important criteria for selecting the best sensor, and explain how designers must combine electrical, mechanical, and optical skills to complete a successful design.
Most electronic components accept an electronic signal and provide an electronic output. On the other hand, a proximity sensor works in tandem with an inductive sensor. The sensor commonly signals me to drive and monitor both proximity and heat. It then reflects the infrared signal off of a nearby object. The time delay between send/receive and the brightness of the received signal can be measured to discover the distance of something from the proximity sensor.
It is the setup I always use with optical operations. However, what you need to consider are the following: Is there a lens over the proximity sensor? Is there anything, like the dark glass on a cell phone face, for example, that can hinder the reception of light? Is there any scattering or leakage from the emitting IR LED that makes it to the receiver and tricks it into believing there is an object nearby?
With a proximity sensor, the amount of light will be related to the distance of the object from the system. If something is close, the more light will be reflected in the sensor. If something is far, less of the light is reflected in the sensor. That could help explain why the first proximity sensors in the market are designed for signalling when an object is centimetres from the sensor. It is a convenient distance to allow a cell phone to know when the user has brought the device near his or her face to talk on the phone. The addition of a proximity sensor allows the phone to turn off the screen during this time, saving power and prolonging battery life.
Should you choose to work with two or more sensors, say with a photoelectric sensor, always remember that you should place sensors as close as possible so that you don’t cause false triggers. One solution is to add another structure. The designer might place a small barrier or wall between the IR LED and the sensor. It would be best if this barrier was as tall as possible. In the cell phone example, there is typically a certain distance available between the circuit board and the cover glass. That would limit the size of the barrier that could be used, as long as the barrier is made of something that light cannot permeate. The other dimensions of the barrier can be set by the ease of use or cost of the material.
What I am trying to say is that the knowledge of mechanical engineering and optical knowledge must accompany the electrical design that predominates in devices like cell phones. Simulating these systems is equally tricky, taking into account the optical properties and physical locations of all of the materials involved.
Also, most cell phone companies want their devices to be simple to use. For example, they don’t want to calibrate the proximity system for users with different skin or hair colours.
The use of proximity sensors goes beyond cell phones. A customer once told me that if we built a proximity sensor that reliably worked in the range of 10 m, he would put them on the tips of every plane he used to avoid bumping the door on the way out of the hangar. Proximity sensors have been used as burglar devices where an ambient light sensor or tripwire could be fooled. Robotics students at universities are using them to give devices the ability to react to approaching people or when they near an obstacle. The truth is that proximity sensors are just beginning to reach their strides. New uses are being discovered all the time. The popularity of these new uses will determine whether proximity sensors of the future will strive for distance, resolution, lower power, or something new.
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