The success of automated industrial processes relies on the seamless operation of numerous individual components to deliver high-quality end products. This article focuses on position sensors, a small yet crucial component in the world of industrial automation. In manufacturing and processing plants, position sensors assist in monitoring and controlling factory processes by confirming that key activities are carried out as expected. Specifically, their primary function is to detect the presence or absence of moving objects or "targets." When a target enters the predetermined distance of the position sensor, it sends a signal to the system, conveying the need to execute programmed functions, a feature widely utilized in pneumatic valve applications. When the target leaves the position sensor, a signal is emitted, indicating that the system should cease executing the pre-programmed function or switch to a new one.
In theory, the target could be anything, but for simplicity, this article will only discuss metal targets and the "mainstream" technologies that perceive the presence of metal targets. These technologies include mechanical limit switches, inductive proximity sensors, spring limit switches, and limit switches. Before delving into discussing different types of position sensors, it helps to understand common terms used by most sensor manufacturers.
Sensing Range: The distance from the sensing surface to the target that activates the switch.
Hysteresis: The distance between the activation and release points of the switch.
Reproducibility: The ability of the switch to consistently detect the same target within the same range during its switch lifespan.
Response Time: The amount of time between detecting the target and generating an output signal.
Mechanical Limit Switches:Mechanical limit switches detect the position of a target by direct physical contact. They do not require power to operate and can handle large current loads. As mechanical switches use dry contacts, they are not polarized or voltage-sensitive, making them immune to electrical issues like electrical noise, radiofrequency interference, leakage current, and voltage drops. However, these switches often include multiple moving parts (levers, buttons, bodies, bases, heads, contacts, terminals, etc.) that may require maintenance. Due to the physical contact with the target, their reproducibility can be poor, leading to wear on levers, arms, and even the target itself. Moreover, they are susceptible to inadequate protection against moisture, dust, and corrosion due to open points. Sealed contacts and hazardous area certifications can significantly increase costs.
Spring Limit Switches:Spring limit switches detect the position of magnetic targets by attracting them with a magnetic field. The switch contains two small metal prongs hermetically sealed in a glass tube, known as "reed elements." These elements are magnetic-sensitive and activate when a magnetic target approaches. Spring limit switches encompass almost all the advantages of mechanical switches, avoiding wear and tear issues by not relying on physical contact with the target. However, they require magnetic targets and are vulnerable to fragility as the reed elements can be fragile, the glass tube can be breakable, and the small metal tips can suffer from fatigue due to bending, leading to unreliable operation. High vibration applications may cause contact flutter and false signals due to the low contact pressure.
Inductive Proximity Sensors:Inductive proximity sensors detect the presence of metal targets by interfering with their energy field. They do not require physical contact and have no moving parts, resulting in lower maintenance needs. Their lack of moving parts also means they are immune to the influence of dust or dirt. There is a wide range of shapes and sizes available, making inductive proximity sensors highly customizable for various applications. Inductive proximity sensors operate with an external power source (electricity) and cannot handle large current loads. Additionally, they may be susceptible to electrical noise, radiofrequency interference, leakage current, and voltage drops. Extreme temperature fluctuations and moisture ingress can adversely affect inductive proximity sensors.
Limit Switches:Limit switches use a unique hybrid technology to detect the position of iron targets through an electromagnetic field. Rugged, durable, and with a long lifespan, poleless limit switches prove highly reliable even in harsh environments and challenging applications. They do not require physical contact or an external power source, eliminating potential wear and the need for electricity. Similar to mechanical switches, they are not polarized or voltage-sensitive, making them resistant to electrical issues. Furthermore, limit switches are impervious to dust, dirt, humidity, physical contact, and most corrosive or chemical influences. The majority of models are intrinsically safe and boast a broad operating temperature range. With sealed contacts and a fully metallic housing, poleless limit switches stand out as the ideal choice for explosive and watertight applications.
Position sensors play a crucial role in industrial automation processes. Several position sensor technologies are available, each with a unique set of performance characteristics. It's essential to select the appropriate sensor type for an application to achieve the desired reliability and performance.
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