What is an Accelerometer? A Guide for Vibration Analysis
An accelerometer is a transducer (or sensor) that converts mechanical motion — specifically the acceleration produced by vibration or shock — into a proportional electrical signal. It is by far the most widely used sensor in predictive maintenance and condition monitoring. By measuring how quickly a point on a machine is changing velocity, an accelerometer supplies the raw data that lets an analyst diagnose a wide range of mechanical and electrical faults — from bearing defects to unbalance and misalignment.
1. Definition: The Core of Vibration Measurement
Acceleration is the natural quantity to measure on rotating machinery because the dynamic forces that damage a machine — centrifugal force from a heavy spot, impact from a spalled bearing race — scale with acceleration. An accelerometer responds directly to those forces, which is why it sits at the heart of nearly every modern vibration analyzer and data collector.
One of the practical strengths of the accelerometer is that its acceleration signal can be electronically integrated once to give velocity (mm/s), and twice to give displacement (µm). A single, well-mounted sensor therefore covers all three classic vibration units, letting the analyst pick whichever best reveals a given fault.
2. How Do Accelerometers Work? The Piezoelectric Principle
Although several physical principles exist, the overwhelming majority of accelerometers used for industrial machinery are based on the piezoelectric effect. The working sequence is straightforward:
- Piezoelectric crystal: Inside the sensor a small seismic mass is attached to a piezoelectric element — typically a ceramic such as PZT, or in premium sensors a precision-cut quartz crystal.
- Applying force: When the machine vibrates, the housing moves with it. By inertia, the internal mass resists that motion and exerts a force on the crystal — equal, by Newton’s second law, to mass times acceleration.
- Generating a signal: A stressed piezoelectric crystal produces a tiny electrical charge directly proportional to the applied force, and therefore to the acceleration.
- Pato: Internal electronics condition this charge and pass it down a cable to a data collector or monitoring system as an analog representation of the acceleration at that point.
How that charge is conditioned defines two common families. A charge-output sensor delivers the raw charge to an external charge amplifier and tolerates very high temperatures. Far more common in industry is the IEPE (or voltage-mode) type, which builds the amplifier into the sensor and outputs a low-impedance voltage that travels well down ordinary two-wire cable. The most robust designs use a shear construction, which isolates the crystal from base bending and thermal transients.
3. Types of Accelerometers
Different applications call for different sensors, each with its own strengths.
General-Purpose Accelerometers
These are the workhorses of industrial monitoring. They typically offer a sensitivity of 100 mV/g and a frequency range suited to most common machinery such as pumps, motors and fans — roughly 2 Hz to 10 kHz.
MEMS Accelerometers
Micro-Electro-Mechanical Systems (MEMS) accelerometers are silicon-based, very small, low-power and cost-effective. Historically less sensitive than piezoelectric types, modern MEMS devices are improving rapidly and are common in portable electronics, automotive systems, wireless monitoring nodes and lower-cost condition-monitoring installations.
Piezoresistive Accelerometers
Used for shock testing and low-frequency motion, these sensors respond all the way down to 0 Hz (DC acceleration), which makes them useful for measuring the steady acceleration in a centrifuge or the slow ride motion of a vehicle.
High-Frequency Accelerometers
Designed to catch high-frequency events such as early-stage gear and bearing damage, these sensors use a smaller seismic mass and a higher resonant frequency, allowing accurate measurement to 20 kHz or beyond — the range where techniques like envelope analysis and the shock pulse method live.
4. Key Specifications and Selection
When choosing an accelerometer, engineers weigh several parameters:
- Sensitivity (mV/g): Higher sensitivity yields a stronger output, better for resolving low-level vibration; 100 mV/g is the common industrial standard.
- Frequency response: The band over which the sensor reads accurately. It must span the expected fault frequencies of the machine, with a comfortable margin below the sensor’s own natural (resonant) frequency.
- Temperature range: The sensor must withstand the surface temperature where it is mounted; a temperature sensor is often co-located for combined monitoring.
- Mounting method: How the sensor attaches — stud, adhesive or magnet — strongly affects high-frequency accuracy. A stud mount per ISO 5348 gives the best coupling and the widest usable bandwidth; a magnet is convenient for route work but lowers the upper frequency limit. Poor mounting can introduce a false mounting resonance that masquerades as a machine fault.
You can estimate the bandwidth penalty of a given attachment with the Accelerometer Mounting Resonance Calculator before committing to a mounting scheme.
5. Applications in Condition Monitoring
Accelerometers underpin nearly every vibration analysis task, including:
- Predictive maintenance programs: collecting routine data on a route to trend machine health and forecast failures.
- Fault diagnosis: pinpointing unbalance, misalignment, looseness and bearing wear from the vibration spectrum.
- Acceptance testing: verifying that new or repaired machinery meets vibration specifications such as ISO 20816 (the modern successor to ISO 10816).
- Modal analysis: studying the natural frequencies and mode shapes of a structure.
Field balancing is one of the most demanding of these jobs, because it needs both the amplitude and the phase of the once-per-revolution vibration. A portable two-channel instrument such as the Balancet-1A takes its two accelerometers, locks them to a tachometer pulse, and measures the 1× amplitude and phase directly in the machine’s own bearings at operating speed — turning the raw accelerometer signal into the influence coefficients and correction weights needed to balance the rotor on site.
