What is an Accelerometer? A Guide for Vibration Analysis

Definition: The Core of Vibration Measurement

An accelerometer is a transducer (or sensor) that converts mechanical motion, specifically acceleration from vibration or shock, into a proportional electrical signal. It is the most widely used sensor in the field of predictive maintenance and condition monitoring. By measuring the acceleration of a machine’s components, an accelerometer provides the raw data needed to diagnose a wide range of mechanical and electrical faults, from bearing defects to unbalance and misalignment.

How Do Accelerometers Work? The Piezoelectric Principle

While several types of accelerometers exist, the vast majority used for industrial machinery monitoring are based on the piezoelectric effect. Here’s a simplified breakdown of how they work:

1. Piezoelectric Crystal: Inside the accelerometer is a small mass attached to a piezoelectric crystal (often a type of ceramic like PZT).
2. Applying Force: When the machine vibrates, the sensor housing moves with it. Due to inertia, the internal mass resists this motion and exerts a force on the crystal.
3. Generating a Signal: The piezoelectric crystal has a unique property: when compressed or stressed, it generates a small electrical charge (a voltage) that is directly proportional to the force applied.
4. Output: This voltage signal is then conditioned by internal electronics and transmitted via a cable to a data collector or monitoring system. The output signal is an analog representation of the machine’s acceleration at that point.

Types of Accelerometers

Different applications call for different types of accelerometers, each with its own strengths.

General Purpose Accelerometers

These are the workhorses of industrial monitoring. They typically have a sensitivity of 100 mV/g and a frequency range suitable for most common machinery like pumps, motors, and fans (e.g., from 2 Hz to 10 kHz).

MEMS Accelerometers

Micro-Electro-Mechanical Systems (MEMS) accelerometers are silicon-based sensors. They are very small, low-power, and cost-effective. While traditionally less sensitive than piezoelectric types, modern MEMS sensors are improving rapidly and are common in portable electronics, automotive applications, and some lower-cost condition monitoring systems.

Piezoresistive Accelerometers

These sensors are used for shock testing and measuring low-frequency motion. They can measure down to 0 Hz (DC acceleration), which is useful for applications like measuring the constant acceleration in a centrifuge.

High-Frequency Accelerometers

Designed specifically for detecting high-frequency events, such as early-stage gear and bearing defects. They have a smaller mass and a higher resonant frequency, allowing them to accurately measure vibrations up to 20 kHz or more.

Key Specifications and Selection

When choosing an accelerometer, engineers consider several key parameters:

• Sensitivity (mV/g): A higher sensitivity produces a stronger signal, which is better for measuring low-level vibrations. 100 mV/g is a common standard.
• Frequency Response: The range of frequencies the sensor can accurately measure. This must match the expected fault frequencies of the machine.
• Temperature Range: The sensor must be able to withstand the operating temperature of the machine surface where it is mounted.
• Mounting Method: The way the sensor is attached to the machine (stud mount, adhesive, magnet) significantly impacts the accuracy of high-frequency measurements. A stud mount provides the best connection and frequency response.

Applications in Condition Monitoring

Accelerometers are fundamental to nearly all vibration analysis tasks, including:

• Predictive Maintenance Programs: Collecting routine vibration data to trend machine health and predict failures.
• Fault Diagnosis: Identifying specific issues like unbalance, misalignment, looseness, and bearing wear.
• Acceptance Testing: Verifying that new or repaired machinery meets vibration specifications.
• Modal Analysis: Studying the natural frequencies and mode shapes of a structure.

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