Sensors play a crucial role in control systems, acting as the eyes and ears that allow us to monitor and manage various processes. Whether it’s controlling industrial equipment or automating tasks, sensors are essential. As technology enthusiasts and engineers, it’s important to have a good understanding of the different types of sensors available. In this article, we will explore two broad categories of sensors: digital sensors and analog sensors.
Contents
Digital Sensors
Digital sensors are the most commonly used sensors in the industrial world. These sensors provide on/off signals, making them ideal for dividing things into two states – true and false, on and off. Let’s take a closer look at a few examples of digital sensors.
Mechanical Switches
Mechanical switches are one of the oldest types of sensors and are still in use today. They consist of a contact that touches another contact when something reaches a specific position. However, they have become less popular due to their large size compared to other sensors.
Proximity Sensors
Proximity sensors, also known as “proxes,” are used to detect close metal objects using magnetic fields. They have replaced limit switches in position sensing applications in many environments due to their reliability and compact size.
Optical Sensors
Optical sensors, on the other hand, use light to detect objects. They have a longer range compared to proximity sensors but can be susceptible to dirt and other environmental factors. These sensors are commonly used in applications where the exact position of the target is not critical.
Capacitive Proximity Sensors
Capacitive proximity sensors are similar to proximity sensors but are specifically designed to detect non-conductive materials. However, they are highly sensitive to contamination and historically have been less dependable.
Ultrasonic Proximity Detectors
Ultrasonic proximity detectors use high-frequency sound waves to detect solid objects. They can solve sensing problems that other sensors can’t, but they are also susceptible to environmental conditions and dirt.
Auxiliary contacts are part of a relay and provide information about whether the relay is turned on or off. Pushbuttons, on the other hand, sense the operator’s actions.
Digital sensors are still widely used in the industrial world, as most control systems are designed around on/off signals. They are the “bread and butter” of a controls engineer’s life.
Analog Sensors
Analog sensors convert variable physical quantities into signals that the control system can understand, typically in the form of voltage or current. These sensors are essential for measuring temperature, pressure, humidity, distance, speed, and more. Let’s take a closer look at how different physical quantities are measured using analog sensors.
Temperature Sensors
Temperature sensors can be either thermocouples or Resistance Temperature Detectors (RTDs). Thermocouples produce millivolt-range signals, while RTDs rely on variable resistances. RTDs are generally more accurate and repeatable, making them a preferred choice when possible.
Signal Conversion
Analog signals, including temperature signals, are converted into more universally understood forms, such as voltage or current, before being connected to the control system. The most commonly used signal standard is the four to twenty milliamp signal, known for its noise immunity and other favorable characteristics.
By utilizing a variety of analog sensors and processing their signals through hardware and software, we can monitor and control various industrial processes. From the operator’s button presses to the levels of liquid in a tank and the pressure and temperature of steam in a boiler, these sensors enable precise control and automation.
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FAQs
Q: Are digital sensors more reliable than analog sensors?
A: Both digital and analog sensors have their strengths and weaknesses. Digital sensors, with their on/off signals, are often more reliable for systems that require simple binary information. Analog sensors, on the other hand, provide more precise and continuous measurements. The choice between the two depends on the specific application requirements.
Q: Can digital and analog sensors be used together in a control system?
A: Yes, it is common to use both digital and analog sensors in a control system. Digital sensors are often used for discrete sensing, while analog sensors are employed for continuous measurements. By combining the strengths of both types of sensors, we can achieve more comprehensive control and monitoring capabilities.
Conclusion
Sensors are the eyes and ears of control systems, enabling us to monitor and manage various industrial processes. Digital sensors provide on/off signals and are widely used in the industrial world. Analog sensors convert variable physical quantities into signals that the control system can understand. By understanding the different types of sensors available, we can design more effective control systems and automate processes with precision.
