Ten secrets to selecting and implementing sensors in industrial applications

July 11, 2013
The successful application of sensors depends on selecting the right technology for the application, the variables of the product being sensed and the conditions in the operating environment.
1. Measuring range. When choosing a sensor (pressure, temperature, analog, etc.), the measuring range should directly correspond with the physical measuring range in order to obtain the most accurate reading and optimum sensor lifespan. For example, to measure 0-10 psi pressure range, a pressure transducer with a sensing range of 0-10 psi is most suitable. Same concept applies for voltage or resistance analog signals.

2. Weather watch. Be aware of environmental conditions when installing equipment. As an example on one project, the intake air ducts on air handler units were installed facing southwest, with no grating or gooseneck. The ducts faced Lake Ontario and would be susceptible to lake-effect snow. If the ducts filled with snow, it would impair the operation of the air handler and the sensors within the ductwork.

3. Load cells important. No one gives load cells the credit they deserve when it comes to level measurement in tanks. They work great for batch measurement applications when using a hinged tripod tank mounting arrangement: two legs with hinges, the third with a load cell in compression. Use a panel meter with discrete output to give an output when the set point is reached. This also works well for providing a low-level alarm in applications. The load cell and controller can make it simple to automate the process in the future.

4. Need flexibility. When choosing a sensor, consider whether it provides the flexibility required, such as features that adapt to changing product. In this case, consider capacitive sensors, which are sensitive to more colors and materials than others, and in many cases are less expensive than ultrasonic sensors.

5. Select for technology, conditions. The most common mistakes in sensor applications include failing to select the most appropriate sensing principle/technology for the task, and failing to consider the full range of expected operating conditions. For example, although a capacitive proximity sensor can detect metals, in general an inductive proximity sensor would be a better choice. Ambient light, temperature, dirt, vibrations or other conditions can affect sensor performance. An example of failing to consider operating conditions would be to install a sensor rated for a maximum temperature of 70 degrees C in an area where temperatures can reach 85 degrees C or higher. In this case, the sensor may experience premature failure or may exhibit unstable operation, such as locking on or locking off. Additionally, sensor life can be extended in harsh environments when used with accessories that are designed to protect the sensor.

6. Digital lowers costs. If linear or proportional output sensor devices are needed, avoid analog field devices to minimize cost and support issues for shielded cables and grounding. When available, digital output equivalents are usually worth the additional purchase price for overall lower cost of ownership.

7. Faulty cables. When replacing a "bad" sensor in an existing field application, be sure to also check the condition of the quick-disconnect cable or cord set. In many cases, the pins or sockets are weakened or corroded, leading to intermittent operation. Replacing only the sensor may temporarily restore electrical contact, but it will certainly fail again because the root cause, a faulty mating connector, has not been corrected. Depending on the cost of the sensor being replaced, this can be a pretty costly situation when, in fact, a relatively inexpensive replacement connector cable could have solved the problem in the first place.

8. Intelligent sensors. Consider adopting intelligent sensors that can be scaled, calibrated or configured remotely in order to shorten changeover time, automate sensor re-configuration and facilitate remote sensor diagnostics.

9. Advance warning. An important trend in sensors is to upgrade or augment discrete on-off sensors with sensors that provide a continuously variable output signal, either analog or digital. Continuous sensor output signals enable much higher levels of operational sophistication, such as fault prediction and statistical process control. Through analysis of sensor data, machine controllers can alert operators of impeding problems before they actually become a problem. An optical analog sensor, for example, may measure the bend angle on a stamped part. If the bend angle begins to drift beyond a given +/- tolerance, the sensor will detect the change and the machine controller tolerance limits can alert the production team before bad parts are produced.

10. More sensor tips. If changes are required, ensure proper calibration and document it in the software changes in the DCS. A built-in display unit for local monitoring of process data is recommended. Follow selection guides and material. Install the sensor in a proper rack/enclosure if the dust level is high. Put proper tags at the site to guide servicing. After commissioning, put the instrument on continuous monitoring/trending to make sure readings are correct and the calibration is accurate before handing it off to operations personnel.

Operating environment critical factor

Don't overlook critical details about environmental conditions when choosing a photoelectric sensor. Typical selection criteria include sensing range, electrical output, connection type, etc. However, in order to get the best results, sometimes users need to go beyond these basic criteria and include other critical details. These factors often end up as some of the most important considerations:

1. Are there sudden temperature changes? Condensation can build up on the lens. Some sensors are more immune to internal condensation than others.
2. Is it an extremely dusty environment? This can be particularly challenging if the sensor is mounted looking up. Consider mounting the sensor to look down. Another option would be to select an infrared LED sensor, which is better at seeing through dust and fog than a standard red LED sensor. Sometimes users will even direct an air purge at the lens to prevent accumulation of dust or particles.

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