Smart wireless sensors for rotating machines are deployed in thousands of installations worldwide. But producing a cost-effective and easy-to-deploy sensor solution for motors and pumps operating in hazardous areas, is a challenge. These are areas where the presence of flammable vapor or gases requires special precautions to prevent the risk of explosion. The lack of such sensors has limited the range of machinery that could be remotely monitored and has left a huge gap in the ability to gain meaningful information on the health and performance of motors that operate equipment such as pumps, fans, and compressors. It has also resulted in an increased safety risk for operators who need to venture into hazardous areas of plants to carry out condition monitoring of equipment.

However, a new generation of smart sensors has been developed by ABB, which are designed especially for rotating equipment operating in explosive atmospheres. This enables operators in industries like chemical, oil, and gas to benefit from cost-effective condition monitoring in a wide variety of demanding applications.

Functions of Smart Sensors for Hazardous Areas

Smart sensors for hazardous areas monitor key parameters of assets such as motors and pumps and provide detailed insights into their performance and health. This includes gathering information on power and energy consumption, which enables the identification of energy- saving opportunities, for potential cost savings of up to 10%. The sensors can also indicate when specific maintenance operations are required, such as when a bearing needs regreasing.

The data collected by the smart sensors can help predict potential failure, so that preemptive action can be taken before a breakdown occurs. This can prevent unexpected downtime and reduce maintenance costs, as predictive maintenance is far cheaper and less disruptive than reactive maintenance.

Using smart sensors also allows equipment installed in difficult or dangerous-to-access locations to be safely monitored from a distance. As a result, employees have to enter these hazardous locations less frequently, increasing their safety.

Figure 2. ABB smart sensor for hazardous locations. (Photo courtesy of ABB)

This new generation of ABB hazardous duty smart sensors — Ability™ — incorporates a microprocessor that is both faster and more robust than previous designs. It also has more working memory and more internal storage memory, as well as a battery life of up to three times longer than many competing designs. It can communicate with smartphones, tablets, PCs, and plant gateways using low energy Bluetooth or WirelessHART. A new antenna design ensures reliable communication over distances of up to several hundred meters in line-of-sight. The new sensor also has greater sensitivity to small changes in the condition of the equipment, including advanced warning of bearing damage.

Designing for Hazardous Areas

Smart sensors for hazardous areas face several design challenges. They must measure high-frequency vibrations, demonstrate physical resilience against harsh and hazardous ambient conditions, and have a long life.

While many other sensors measure only vibration and temperature, the Ability Smart Sensor also measures high-frequency vibrations, magnetic fields, temperature, and acoustics. As a result, it can measure the speed and rotation of motors with extreme accuracy.

To earn qualification to operate in hazardous areas, sensors must meet specific design requirements and comply with a large number of standards. For instance, they must ensure that an internal short circuit in the battery will not cause heating that could ignite gas. When battery-powered sensors are short circuited during testing they must reach a temperature no higher than 135°C, the T4 temperature class standard.

The sensor enclosure must not have any components that generate heat or sparks. To ensure this, sensors must be filled with conductive material to prevent sparks or the spread of heat if the enclosure is compromised. Furthermore, the sensor must be able to sustain stresses that arise from its environment. This smart sensor was designed to operate in a temperature range of -40°C to +85°C, as this is typical for most industrial electronic components. However, it can actually operate in a range of -70°C to +130°C. This was determined through Highly Accelerated Life Tests (HALTs), which use cycles of high and low temperature and a combination of high vibrations and extreme temperatures to test the resilience of the sensor.

Vibration

Figure 3. Hazardous duty sensor testbed. (Photo courtesy of ABB)

Vibration sensors are becoming commonplace in consumer electronics and industrial automation — but creating a high-quality vibration sensor is no easy task. For example, it is essential to stop resonances occurring anywhere in the sensor body from impacting the transducers that pick up vibrations from the machine being monitored. Many aspects make the problem more difficult to solve — positioning, mounting brackets, and method of attachment to cite a few. Doing so at the lowest possible cost is a complex tradeoff to solve.

ABB's first approach to the new sensor used a large pressed-steel plate at the bottom of the sensor to transmit the vibrations of the monitored asset in as direct a manner as possible. The sensor had two electronic boards, one glued to the metal plate (itself screwed to the asset) and another connected to the plate and that board using only a flexible cable. This first version of the metal plate delivered poor performance with respect to self-resonance. Resonance forces from the body of the sensor propagated to the pressed metal base and were picked up by the vibration transducers.

By using a fine-grained model of the sensor and metal plate, however, many alternatives were simulated, resulting in a metal plate that not only completely sticks to the machine without propagating vibration forces from the body of the sensor to the location of the vibration transducers, but also maintains cost targets.

Batteries

Batteries are a special problem for wireless sensors operating in hazardous areas. Replaceable or rechargeable batteries are undesirable because:

  • Replaceable batteries can increase the cost of the sensor to the point where it makes more sense to simply change the entire sensor — and get new electronic components with higher performance into the bargain.

  • There is a risk that the user would compromise the hazardous area protection status by inserting the new batteries incorrectly.

  • Ingress protection against dust and water could also be compromised if the batteries are not replaced correctly. ABB's goal was to design an embedded system with a design life of up to 15 years as well as providing a reliable indication of remaining battery life.

That was difficult for at least three reasons:

  • To limit battery internal leakage current, the temperature experienced by the battery must be moderate.

  • To prevent the soldered pads from breaking, vibration forces from the battery and the sensor must not propagate to the interface between the two.

  • The sensor's power consumption must be kept low, even with a large battery installed.

In the new smart sensor, the battery and its soldered pads are enclosed in a battery holder that is separated from the primary heat sources by an air gap, which protects it from the heat coming from the monitored asset.

To evaluate the temperature-dependent leakage current of the battery, the sensor measures battery temperature during operation and estimates the corresponding leakage current based on a proven battery model.

Figure 4. Motor-mounted hazardous duty sensors. (Photo courtesy of ABB)

In a further battery charge measurement tactic, the firmware uses a point system to calculate the charge consumed by normal sensor operations. Most of the time, the sensor is in a deep sleep and consumes very little power, but when the sensor awakens, its power consumption ramps up. The sensor records how much time each battery-consuming operation takes — for example, the duration of Bluetooth chip activity. From the durations and power curves of the operations, the consumed charge is calculated and subtracted from initial battery capacity. Based on a rolling average value of the consumption, remaining lifetime is estimated and published. This approach captures actual battery usage rather than relying on a predefined battery lifetime and assumed power consumption levels, which are often inaccurate.

Future Proofing

The new Smart Sensor's firmware and software have two major goals: to support the creation of different types of sensors in the future and to allow a deployed sensor to be reconfigured for monitoring various asset types. As an example of the latter goal, a new sensor could be reconfigured on-the-fly to be used either as a motor sensor or a pump sensor — or for any other asset type, characterized by selecting predefined or custom-made machine profiles. The sensor's firmware can be easily adapted to new requirements due to a flexible software architecture that decouples individual components from the hardware/operating system platform. These components communicate using a publish-subscribe middleware. Overall, the firmware is organized as a software product line, and thus it is suitable for the creation of new sensor variants based on the same underlying platform.

As with all wireless sensors, cybersecurity features are critical. The Ability Smart Sensors include secure key exchange for Bluetooth communication with out-of-band pairing, Bluetooth encryption, user authentication, role-based access control, and secure firmware update.

Looking to the Future

The heart of Industry 4.0, the Industrial Internet of Things (IIoT), and Predictive Maintenance (PdM) is a pervasive network of sensors that provide high-quality, fine-grained data. Without wireless sensors that are safe to operate in hazardous areas, these networks would be incomplete. In fact, safe monitoring of machinery in hazardous areas, is more critical than any other because it has to be kept safely isolated from its environment.

This article was written by Teijo Karna, Global Product Manager, ABB Ability™ Smart Sensor. You can contact Mr. Karna at This email address is being protected from spambots. You need JavaScript enabled to view it. or com or visit here .


Sensor Technology Magazine

This article first appeared in the September, 2020 issue of Sensor Technology Magazine.

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