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Sensors for industrial machines are used to detect a variety of process signals for a given application. A few examples of process signals are, material color, pressures, flow, fluid levels, temperature, and human presence.
The most common type of sensors are:
- Proximity Sensor: Used to detect an object without physically contacting the object.
- Capacitive – Useful for detecting levels within a glass or plastic container
- Inductive – Useful for detecting metal objects
- Magnetic – Useful for sensing magnetic fields
- Photo-eye – Detects the amount of light reflected from a material
- Pressure Sensor: Provides feedback on the process of pressure, enabling safety shutoffs if pressures exceed a set point, and process alarms to prevent poor quality due to low pressure levels in the case of a pressing application.
- Flow Sensor: Similar to pressure sensors. Detects the presence or absence of flow.
- Level Sensor: Allows for process control based on the level in a vessel. Consider a tank which must remain full. A level sensor could be used to provide the feedback to automatically open a valve to let more fluid in and close when the required level is reached.
- Temperature Sensor: Detects the process temperature. Sensors can be as simple as a thermocouple, to more sophisticated infrared imaging sensors. Consider an oven that must maintain a fixed temperature. A sensor can be place within the oven and read by the control system to vary the gas valve actuation to increase or decrease the flame height to maintain a fixed temperature.
Sensors can be defined further by their type of output. The simplest sensors output a simple discreet on/off signal based on the sensed condition. More sophisticated sensors can output an analog value which varies based on the intensity of the detected condition. Sensors can also communicate with the machine control using specific communication protocols which enable the viewing of sensing intensity, lens contamination levels, and even the ability to remotely program the sensors sensing parameters.
Sensors are often the core element of the equipment, which with proper use can create more efficient equipment operation, improved quality, and safer interaction with operators by controlling critical process parameters.
Because sensors play such a critical role, there are some important points to consider when selecting and using them for your application:
- Use the highest quality sensors your budget will allow. Nowadays, proximity sensors are commodity items. Some manufacturers will take steps during the assembly process to help ensure higher overall quality. Some things to look for are fully encapsulated circuits to prevent failures from vibration and more robust sensor faces to resist failures from impacts.
- Consider the environmental conditions. Often manufacturers will provide alternatives to fit the application, and these should be considered. Weld spatter immune housings, extended temperature ratings for hot or cold environments, wash down ratings to prevent damage from direct fluid sprays are some examples.
- Consider the sensing conditions. If the target is non-ferrous metal (aluminum), the sensing ranges will be reduced. For example, if using a photo-eye sensor, a through beam sensor may work better depending on the condition being detected.
- Proper mounting matters. Mounting a proximity sensor where it could inadvertently become the axis hard stop is not good design. Try to mount the sensor so the sensing flag passes the sensing zone perpendicular to the sensor face. If using a photo-eye consider the accumulation of debris on the sensor face. Try to mount the sensor so gravity allows particles to fall free.
- Select the proper sensor output based on the controller. When it comes to solid state devices, such as PLC transistor outputs and sensors, the concept of PNP and NPN configurations can be easily confused. The difference between these two configurations is simply the direction in which current flows.
- Proper sensor cabling is equally important. Consider the following:
- High Flex Cables exist to prevent premature failure of the cable due to repeated bending.
- Cable Routing is critical to avoid cuts, crushing, snags, and erratic sensing due to electrical noise.
- Minimum Bend Radius. In some case, depending on the cable construction, too tight of a bend radius can damage the inner conductors.
- Sensor Termination. If the sensor failed, how time consuming would it be if the cable was not detachable? Consider using a sensor with a detachable cable to allow for faster assembly and maintenance.
- If electrical noise could be a problem due to signal type or inability to route the cable clear from an electrically noisy area, shielding can be used to prevent the noise from deteriorating the sensor signal.
- Shield Termination. Follow the manufacturer’s recommendation for termination of the shielding. It is often recommended to only terminate one end of the cable shield to ground.
- Don’t rely on basic sensors for operator safety. There are sensors designed for safety, which are tested to preform within a specific condition. These sensors typically have redundant contacts and work in conjunction with more sophisticated safety controllers to create a complete safety solution.
- Always read the full datasheet. The datasheet is a critical read to understand the proper application and limitations of the sensor.
- Contact the sensor manufacturer. Manufacturers often have support specialists to provide application and design guidance, best practices, and troubleshooting support.