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A manual switch enables an operator to interact with a machine. If, for example, an operator sees a problem on a manufacturing line, he could move a switch to stop the line. Or, think of a light switch in your home. If you (the operator) want the light turned on, you have to move the switch. A sensor can be thought of as an automatic switch. In a factory, a sensor can be used to detect a problem on the line and stop the line automatically. Or, in your home, a sensor could be used as a security device to detect an open window or door. Sensors have contributed significantly to recent advances in manufacturing technology. Using a sensor makes a process or system more automated and removes the need for human operators to monitor and control the situation. The three main categories of sensors are limit switches, proximity sensors and photoelectric sensors. Let’s take a moment to look at each type of sensor. Limit Switch A limit switch is an electromechanical device. A part of the limit switch, called an Actuator, is placed in the path of an oncoming object, such as a box on a conveyor. When the object contacts the actuator, the contacts in the limit switch are opened (or closed, depending on the limit switch’s design) to stop (or start) the flow of current in the electrical circuit.
Figure 2. Limit Switch with Standard Roller Lever Proximity Sensor This type of sensor uses an electromagnetic field to detect when an object is near. There is no physical contact between the object and the sensor. Inductive proximity sensors detect only metal objects. Capacitive proximity sensors can sense both metallic and non-metallic objects. Think of a manufacturing process where the alignment of a part is critical. A proximity sensor can be used to make sure the part is aligned within a certain tolerance. If the part is not properly aligned, the proximity sensor will be triggered. This type of sensor is generally used to sense at distances less than one inch.
Figure 3. Proximity Sensor Types Photoelectric Sensor This type of sensor uses light to detect the presence or absence of an object. A Thru-Beam photoelectric sensor uses two devices (a light source and a detector) facing each other. Detection occurs when an object blocks or breaks the beam of light passing between them [Figure 4]. A Diffuse Reflective sensor emits a light beam that must be reflected back to it by the target object itself for detection to occur [Figure 5]. A Retro-Reflective sensor emits a light beam that is reflected back to the sensor from a retroreflector. When an object blocks the beam between the sensor and the retroreflector, detection occurs [Figure 6]. We’ll cover more on these types of photoelectric sensors later in this module. Most electric garage door openers include a photoelectric sensor for safety reasons. If the photoelectric sensor’s beam is broken (by a child for example) as the door is going down, the sensor signals the door opener to reverse the direction of the door. Although environmental factors can affect photoelectric sensors, these devices have a long sensing range. The objects they detect can be of any material.
Figure 4. Thru-Beam
Figure 5. Diffuse Reflective
Figure 6. Retro-Reflective Sensor Comparison Each of the three sensor categories has its strengths and weaknesses. The chart below provides you with a comparison:  As a conveyor moves the stacked boxes onto a turntable, a sensor detects the boxes in position and tells the machine to start the turntable and index the wrapping material. Another sensor monitor the play out of wrapping material to detect an empty spool and alert set-up personnel. Once the operation ends, the wrapped boxes move on to their shipping destination.
Figure 7. The Sensor “Sees” the Box and Tells the Wrapping Machine to Begin Operating Thanks to sensors, the repetitive and tedious work done in this factory is handled precisely and reliably by machinery and systems working together.
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