Avoid Hazards and Downtime by Replacing Failing Sensors

Today’s industrial machines rely heavily on sensors, which serve the function of communicating information about presence, absence, size, level, position and movement to the automation controller. However, when sensors fail, it can negatively impact the safety of the plant, as well as product throughput and quality. For this reason, it’s important to know the signs of trouble so that you may replace faulty sensors before complete failure occurs. This blog will discuss common sensor types, signs of failure, and tips for selecting a suitable replacement. 

Sensors 101 

Sensors may be analog or digital, but with the rise of Industry 4.0, digital sensors have become more prevalent as more manufacturers want to collect and trend data for control and predictive maintenance purposes, so this blog will focus on digital sensor types. 

Digital sensor technology plays a variety of roles in industry. Sensors may provide information about the product being manufactured, deliver updates about the condition of the equipment, or feedback on the rotary position of the motor to ensure accurate positioning. Sensors may determine the position, distance, and proximity of objects. Some common applications for digital sensors include recognizing that a mechanism on a machine has reached a certain position, sensing an object on a conveyor, determining the fill level in a container on a filling machine or confirming that a robot gripper has picked up a part from an assembly line. Information, or output, from a digital sensor, is typically transmitted to the system’s PLC or to a PC. 

Because no two applications or industrial facilities are the same, there are a variety of sensor types available to suit the needs of the application. Common sensor types include: 

• Photoelectric sensors: Photoelectric sensors feature an emitter that generates an optical signal and a receiver that detects the signal. If an object interrupts or changes the return signal, the sensor will alert the system that an object is present. 
• Capacitive: Capacitive sensors detect the presence of objects or materials by monitoring a change in capacitance between a sensor field and the object of interest. 
• Inductive sensors: Inductive sensors feature a coil that emits a fluctuating electromagnetic field. An object passing through the field creates a disturbance that alerts the system to its presence. 
• Magnetic proximity sensors: Magnetic sensors include a switch that is controlled by a magnet. When an object nears the switch, it closes. 
• Ultrasonic sensors: Ultrasonic sensors employ ultrasonic sound waves to detect that an object is present or to determine the distance to an object. 
• Temperature and pressure sensors: These are commonly used in manufacturing and industrial applications to gather, transmit and trend information about temperature and pressure conditions within a machine, product or process. 

Signs of Trouble 

Sensors can fail for any number of reasons, including deterioration from age, damage from a dirty, harsh industrial environment and improper mounting. Whatever the reason, a failing sensor can adversely affect the safety of the facility and personnel, as well as productivity and product quality. 

Symptoms of failure will greatly depend upon the type of sensor and the application in which it is used. However, when a sensor fails to send a signal to a controller, equipment, processes or production may stop entirely or you may find that equipment moves or operates when it should not (for example, a conveyor moving despite a lack of product on it or not moving when it should be). If a sensor sends incorrect signals or transmits signals at the wrong time, movement may occur when it should not or vice versa or movement may not reach the designated target. Issues with temperature and pressure sensors may present as incorrect or unusual readings or alerts that don’t coincide with what is occurring in the process. 

Replacing a Failed Sensor 

When any of these issues are present, it is likely that you will need to replace the sensor. However, if premature failure occurs frequently, it might be a good time to consider replacing the sensor with a different type rather than swapping it out for the same model. 

When selecting a replacement sensor consider the conditions in which the sensor will operate, including high temperatures, dust or contaminants, and vibration or shock, which all tend to reduce the lifespan of sensors. In other cases, these environmental factors may prevent the sensor from properly doing its job (for example, if a sensor relies on positive contact between two switches and dust degrades the sensing path, it may not function properly). It is important to find a sensor designed to withstand environmental and operating issues that may be present in the application. If that is not possible, consider relocating the sensor in an effort to minimize exposure to these elements. 

Sometimes sensors fail because they are not the most suitable technology for the application. For example with Inductive Sensors, aluminum will be detected at shorter distances while the detection of ferrous metal will have a longer range. For this reason, it’s important to consider application factors such as the necessary target position and distance; the type of target (metallic or nonmetallic, solid or liquid, transparent or opaque, reflective or non-reflective); the working environment and whether it includes moisture or washdowns, has the potential for explosion or fire or is especially dirty or dusty. Performance requirements including sensitivity, resolution, accuracy, and repeatability are also important factors in sensor selection. Because sensor technologies are constantly improving, it may be helpful to ask your supplier if there is an option that better meets the needs of the application. 

Finally, if the application is critical or safety is a concern, it might be wise to incorporate redundancy into the feedback, or safety-rated sensors. If the control system receives similar signals from two different sensors installed to measure the same property, it will provide assurance that both sensors are functioning properly. Discrepancies between the readings would indicate an issue with one of the two sensors. 

In industrial equipment, it is essential to ensure that sensors provide reliable signals in order to prevent unsafe, throughput, or quality issues. Knowing the symptoms of a faulty sensor and what to consider when selecting a replacement are crucial to receiving dependable signals that will keep your facility safely up and running. 

For more information on sensor replacement or new sensing technologies, please contact a representative at JHFOSTER.