Selecting the right sensor for the application is crucial for measuring machine vibration. The sensor signal is the input for the protection or monitoring system. A false sensor leads to a false input signal, which affects the quality and accuracy of the monitoring and protection functions.
Considerations when selecting the correct sensor for the application:
- What is the vibration level and frequency range?
- Are the resonance frequencies of the application coinciding with those of the sensor?
- Are there size and weight considerations?
- What cable length is required?
- What is the temperature range?
- Is it an explosion hazard area?
- Are strong electromagnetic fields present?
- Is the environment corrosive?
There are three types of sensors for vibration monitoring: acceleration sensors, velocity sensors and displacement sensors.
Acceleration sensors are most commonly used to measure casing vibration, in either acceleration or velocity (after integration) measurements. They measure low to high frequencies in single, dual and triaxial directions, and are available in a variety of general purpose and application-specific designs.
Two main sensor technologies are used: Piezoelectric and MEMS (microelectromechanical systems). The piezoelectric accelerometer is reliable, versatile and unmatched for frequency and amplitude range. A MEMS sensor is a semiconductor that offers lower accuracy on frequency and amplitude, but with improved power consumption. A MEMS sensor can measure static and dynamic acceleration.
Velocity sensors are used to measure a frequency range of 1 – 1000 Hz. The measurement principle is based on a coil moving around a magnet, generating a voltage that is proportional to the movement. The sensors are suitable for vibration monitoring and balancing applications on rotating machinery. This type of sensor has a lower sensitivity for vibrations with high frequencies than accelerometers, and is therefore less susceptible to overload.
In some applications, piezoelectric acceleration sensors can also be used to generate a velocity signal, by integrating the acceleration signal.
Displacement sensors, also known as non-contact proximity sensors, are used to measure shaft position or movement without physical contact in order to determine the internal movement and clearance on bearings. Proximity sensors measure the movement (vibration) of a shaft relative to a sleeve bearing or other supporting (fixed) parts. This type of sensor is used to measure a frequency range of 1 to 1500 Hz and low amplitudes, typical for sleeve bearing applications.
A non-contact proximity sensor consists of three parts: a probe, an extension cable and a driver. Together they create a tuned oscillation frequency which is disturbed by the metal surface of the target. All three parts are matched and can only be replaced by identical components, or the calibration of the system will become invalid.
Expert observation on wireless sensors
In recent years, wireless acceleration sensors with built-in digital signal processing have found their way onto the condition monitoring market. The major benefits of these wireless technologies are the greatly reduced infrastructural requirements and the automated (basic) analysis. But this comes at a price. To minimise battery usage, signal resolution and data throughput are reduced, resulting in low resolution data, while battery management and sensor logistics are becoming increasingly important, especially on large quantity installations. Power consumption of each sensor is influenced by its environment and application. Replacing batteries in the field is often complex, due to hazardous area restrictions, reachability and risk of pollution of the battery chamber.
Wireless sensors as they are today, are definitely of value for certain applications, but they do have their limitations. It is important to set the right expectations and understand the impact of a battery powered solution on data quality and sensor maintenance and logistics.