In this article, we tell you more about IEPE and charge accelerometers (also known as acceleration meters, accelerometers, or accelerometers) and how these are used for vibration measurements on rotating machines.
For measuring vibrations, there are various methods, usually done via a piezoelectric sensor, or an accelerometer. An accelerometer is a sensor that converts the dynamic vibrations of a physical object into a voltage. Accelerometers are full-contact transducers that are directly attached to the element where vibration is expected. Examples include ball bearings and gear transmissions. This versatile sensor can be used for both high and low-frequency vibration measurements. The advantages of an accelerometer are its linearity over a broad frequency range and its wide dynamic range.
How does an accelerometer work?
Most accelerometers use the piezoelectric effect, meaning that the crystal in the sensor emits an electrical voltage under pressure. The acceleration of the housing is transmitted to a calibrated mass (seismic mass), after which proportional pressure is exerted on the piezoelectric crystal. The external voltage on the crystal generates electrical resistance with high impedance proportional to the applied force and, therefore, proportional to the acceleration.
How do I choose the right accelerometer?
Because accelerometers are so versatile, they come in various forms, sizes, ranges, and other specific properties. By forming an idea of the signal expected to be measured in combination with environmental factors (such as temperature and explosion sensitivity), specific models can be considered. Of course, you can always call us for advice when choosing the right sensor, but we will explain below what to consider.
Vibration amplitude
The maximum amplitude, or the maximum range of the vibration you are measuring, determines the required sensor range. When a vibration is measured with a sensor that does not match the range, the output becomes an unreliable signal. Typically, accelerometers used to monitor high-frequency vibrations have lower sensitivity and mass.
Sensitivity
Sensitivity is an important parameter when choosing the right accelerometer. Sensitivity is the correlation between the vibration and voltage at a reference frequency. The exact sensitivity is determined by calibration and is usually listed in the calibration certificate provided with the sensor. Sensitivity also depends on the (expected) frequency. Ultimately, full calibration over the entire usable frequency range is required to determine how sensitivity correlates with frequency.
Generally, an accelerometer with low sensitivity is used to measure signals with high amplitude/frequency, and an accelerometer with high sensitivity is used to measure signals with low amplitude/low frequency. Another aspect is the self-resonant frequency of the sensor, and it can be stated that the maximum measurable frequency should always be a factor of two below the self-resonant frequency of the sensor.
Number of axes
In terms of measurement direction, there are two types of accelerometers. The most common accelerometer measures vibration in one direction. This type is often used to measure mechanical vibration levels on installations and machines. The second type is a triaxial accelerometer. This accelerometer can create a 3D representation of the acceleration.
Weight
Accelerometers must weigh significantly less than the structure being monitored. If the mass of the object to which the accelerometer is attached is significantly increased by the sensor, it may affect the vibration characteristics. This can lead to inaccurate measurement data. In general, the mass of the accelerometer should not exceed 10% of the weight of the object. There is a specific solution for every application.
Mounting
Another consideration when choosing the right accelerometer is how the sensor can be mounted on the surface. There are four types of mounting options:
- Handheld
- Magnetic
- Adhesive
- Screw mount
Screw mounting is by far the best method, but it requires drilling a hole in the machine, which is only done when permanent installation is needed. Alternatively, the sensor can be glued, though this provides a less effective transfer characteristic than screw mounting.
The other mounting methods are mainly intended for temporary installations. Each mounting method affects the accelerometer in some way. In general, the “looser” the mounting, the lower the measurable frequency limit. Adding mass to an accelerometer, such as in magnetic mounting, lowers the resonant frequency, which can affect the accuracy and limits of the usable frequency range of the accelerometer.
Environmental factors
When choosing an accelerometer, environmental factors such as maximum operating temperature, exposure to harmful chemicals, and humidity must be considered. Most accelerometers can be used in hazardous environments due to their robust and reliable construction. For additional protection, industrial accelerometers can be made from stainless steel, making them resistant to corrosion and chemicals.
A charge accelerometer is suitable for extreme temperatures. Since these accelerometers do not have built-in electronics, the maximum operating temperature is only limited by the sensor element and the materials used in construction. However, since these accelerometers lack built-in conditioning and signal amplification, they are sensitive to environmental interference, requiring low-noise cabling. If there is too much environmental noise, an inline signal converter or an IEPE sensor with a built-in signal amplifier should be used.
Humidity specifications are determined by the type of sealing that an accelerometer has. Common seals are made of glass or epoxy. Most of these seals can withstand high humidity levels and prolonged exposure to excessive moisture.
Cost
Although the prices of charge mode and IEPE accelerometers are comparable, IEPE accelerometers have significantly lower costs when used in multi-channel systems, as they do not require special low-noise cables and charge amplifiers. Additionally, IEPE accelerometers are easier to use as they are easier to maintain.
Proximity probe vs accelerometers
Another type of sensor used to measure machine vibrations is the proximity probe, or distance sensor. Unlike accelerometers, which measure acceleration to detect vibration, proximity probes are non-contact sensors that measure the distance to an object. These sensors are primarily used to measure shaft vibrations in large, heavy rotating machines. A steam turbine is one of the typical machines where proximity probes are used to measure shaft vibrations. The focus is mainly on measuring the vibrations of the shaft within the journal bearing. Due to the damping effect of the oil film in which the shaft is positioned and the heavy casing, shaft vibrations can rise significantly without significantly increasing the housing vibrations. Therefore, in such situations, it is better to use proximity probes that directly measure shaft movement. Often, a combination is used.
