How to Choose Sensors for Industrial Machine Monitoring?

Sensors as the Foundation of Predictive Maintenance

Imagine this situation: you have the best artificial intelligence algorithms, the most advanced analytical software, and an experienced team of data scientists. But if the data entering the system is imprecise, incomplete, or distorted, all this advanced technology is useless. That's exactly why sensors are often called the eyes and ears of a predictive maintenance system - without them, the system is blind and deaf.

Choosing the right sensors, their proper installation in appropriate locations, and professional calibration is the absolutely crucial first step of every implementation. Errors made at this stage will affect the entire project, impacting prediction quality and the credibility of the whole system. That's why it's worth dedicating adequate time and attention to properly planning this element.

In this article, we'll discuss in detail the main types of sensors used in industrial machine monitoring, selection criteria tailored to different applications, and installation best practices that will ensure long-term monitoring system reliability.

Vibration Sensors - The Most Important Parameter in Machine Monitoring

Vibrations are undoubtedly the earliest and most valuable indicator of mechanical component degradation. Bearing problems, rotor imbalance, mechanical play, gear wear - all these faults generate characteristic vibration patterns long before actual failure. Proper vibration signal analysis allows detecting problems weeks or even months in advance, giving sufficient time to plan repairs.

Moreover, each type of fault generates a unique vibration "signature" - specific frequencies and amplitudes that an experienced analyst or advanced AI algorithm can recognize. This makes vibration monitoring not only a tool for detecting problems but also for diagnosing them - we can not only say that something is wrong but also precisely identify what it is.

Types of Vibration Sensors

Where to Mount Vibration Sensors - Key Location Principles

Vibration sensor location is fundamentally important for data quality. The principle is simple: the sensor should "see" the element it's monitoring. Every layer between the sensor and the vibration source - whether rubber, plastic, or even air - dampens and distorts the signal, which can lead to incorrect interpretations.

For bearings, which are one of the most common sources of problems in rotating machines, sensors should be mounted as close to the load zone as possible, both in the radial direction (perpendicular to the rotation axis) and axial direction (parallel to the axis). This allows detecting both bearing problems and rotor imbalance or axial load issues.

For gearboxes, sensors should be placed on the housing at the input and output, always on rigid structural elements that don't dampen vibrations. Avoid mounting on covers or plastic elements that can introduce artifacts into the signal.

For electric motors, the standard approach is mounting sensors on bearings on both the drive side and the opposite side. This allows detecting problems with both the motor itself and the components it drives.

A principle worth remembering: The sensor should "see" the element it's monitoring. Every layer between the sensor and the vibration source dampens and distorts the signal. If you can't mount the sensor directly on the element you want to monitor, make sure the vibration transmission path is as short as possible and passes through rigid structural elements.

Temperature Sensors - Early Failure Warning

Temperature is the second key parameter in industrial machine monitoring. Overheating of bearings, motors, or hydraulic systems often precedes failure by hours or even days, giving sufficient time to react. Moreover, temperature increase is often easier to detect than vibration changes, especially in initial degradation stages.

Types of Temperature Sensors - Choosing the Right Technology

Several temperature measurement technologies are available on the market, each with its advantages and limitations. Thermocouples offer the widest measurement range - from -200°C to even +2000°C - and are resistant to harsh environmental conditions, making them ideal for industrial applications with extreme temperatures.

RTDs (Resistance Temperature Detectors), such as Pt100 or Pt1000, are characterized by high accuracy - even ±0.1°C - and excellent long-term stability. They're ideal for applications where measurement precision is required, such as production process control or monitoring critical components.

NTC thermistors (Negative Temperature Coefficient) offer fast response to temperature changes and low price, but have a limited range - usually up to about 150°C. They're excellent for applications where temperature isn't extreme but response speed is important.

IR sensors (non-contact) allow temperature measurement without physical contact with the object, making them ideal for moving elements, elements under voltage, or hard-to-reach places. Their main disadvantage is the need to know the surface emissivity coefficient, which can affect measurement accuracy.

Typical Measurement Points - Where to Place Temperature Sensors

The choice of temperature measurement location depends on what we want to monitor. Bearing housings are an obvious choice - bearing temperature increase is one of the earliest problem signals. Motor windings can be monitored through service openings, which allows detecting winding overheating before it leads to failure.

Oil in lubrication systems should be monitored because its temperature can indicate problems with the lubrication pump, contamination, or insufficient cooling. Similarly, coolant temperature can indicate cooling system problems that could lead to machine overheating.

Pressure Sensors - Monitoring Hydraulic and Pneumatic Systems

Pressure monitoring is crucial in hydraulic systems, pneumatic systems, and lubrication systems. Pressure is a direct indicator of these systems' condition - its changes can indicate problems with pumps, valves, leaks, or contamination.

What to Monitor - Key Measurement Points

Lubrication oil pressure is one of the most important parameters to monitor. Pressure drop can indicate problems with the lubrication pump, leaks in the system, or contamination blocking flow. Pressure increase can indicate blocked lubrication channels or valve problems.

Pressure in hydraulic systems should be monitored both at the pump output and at various points in the system. Pressure fluctuations can indicate pump wear, valve problems, or system leaks. Pressure stability is crucial for proper hydraulic machine operation.

Pressure differential across filters is an excellent indicator of filter condition. Pressure differential increase means the filter is clogging and needs replacement. Monitoring this parameter allows planned filter replacement instead of waiting for failure caused by a blocked filter.

Data Transmission - Wired or Wireless?

Choosing between wired and wireless data transmission from sensors is one of the key decisions when designing a monitoring system. Each approach has its advantages and limitations, and the choice should be dictated by application specifics, reliability requirements, and project budget.

Wired Solutions - Reliability Above All

Wired data transmission solutions offer the highest reliability - there's no risk of signal loss due to radio interference, range problems, or battery depletion. This makes them ideal for critical machines where any data loss could lead to incorrect decisions.

An additional advantage is no need for battery management - wired sensors are powered through the cable, eliminating problems related to battery replacement and ensuring operational continuity. Moreover, wired solutions offer higher bandwidth, which is important for applications requiring high sampling frequency, such as high-frequency vibration analysis.

The main disadvantage is cabling cost - especially for machines distributed over a large area or in hard-to-reach places. Cable installation can be time-consuming and costly, and in some cases may require machine construction modification.

Standard wired protocols include 4-20mA (classic industrial standard), Modbus RTU (popular communication protocol), and HART (protocol enabling bidirectional communication via 4-20mA signal).

Wireless Solutions - Flexibility and Easy Installation

Wireless solutions offer significant benefits in terms of installation ease and flexibility. Wireless sensors can be installed in places where cable routing would be impractical or impossible, opening monitoring possibilities for machines that were previously inaccessible.

Low deployment cost - especially for distributed monitoring of many machines - makes wireless solutions often more economical than wired ones. There's no need to run cables, which significantly reduces installation costs.

The main disadvantage is the need for battery management - wireless sensors require periodic battery replacement, which can be problematic for a large number of devices. Additionally, wireless solutions can be susceptible to radio interference, especially in industrial environments with many machines and electrical devices.

Limited bandwidth can be a problem for applications requiring high sampling frequency, but for most predictive monitoring applications where data is transmitted every few minutes, this isn't a significant limitation.

Popular wireless protocols include WirelessHART (specially designed for industry), LoRaWAN (long range, low power consumption), Wi-Fi (high bandwidth but higher power consumption), and Bluetooth Low Energy (low price, short range).

Sensor Selection Criteria

1. Measurement Range

Match the sensor range to expected values with a 20-30% safety margin. Too wide a range = lower resolution.

2. Accuracy and Repeatability

For prediction, repeatability (whether the sensor shows the same thing under the same conditions) is more important than absolute accuracy.

3. Environmental Conditions

Consider ambient temperature, humidity, dust, presence of aggressive chemicals. Choose the appropriate IP protection rating.

4. Sampling Frequency

For vibration analysis, you need at least 2x the frequency of the highest component (Nyquist theorem). For bearings rotating at 3000 rpm, you need at least 10 kHz.

5. Compatibility

Make sure the sensors are compatible with your data acquisition system, communication protocols, and analytical software.

Common Installation Mistakes

• ❌ Mounting on vibration-damping elements (rubber, plastic)
• ❌ Cable too long between sensor and transducer
• ❌ Lack of labeling and documentation of measurement points
• ❌ Installation in places exposed to impacts or flooding
• ❌ Skipping calibration and validation after installation

GC AIT Summary

Sensor selection is a technical-economic decision that benefits from expert guidance. The most expensive sensor isn't always the best - as GC AIT engineers often emphasize, the key is matching to the specific application, environmental conditions, and analytical system requirements.

If you're planning to implement a monitoring system in your plant, contact us. We'll help select the optimal solution and conduct professional installation.