Selection of Vibration and Acceleration Sensors (Based on Their Operating Principle)

The increasing demands for accuracy in measurements are met by modern analog and digital measuring instruments, but only if the other elements of the measuring chain also meet those requirements. Therefore, the accuracy of the measurement often depends on the correct measurement setup, proper cabling, and sensor selection. Vibration and acceleration measurements are very common, and their accuracy primarily depends on the measurement principle, that is, the selection of the sensor type. The following article presents the sensor types currently available on the market and their key characteristics.

Every vibration measurement is based on acceleration sensing, with the mathematical relationship: a = dv/dt = d²s/dt², where a ... acceleration v ... velocity t ... time s ... distance From a measurement perspective, there are fundamentally three types of acceleration: 1. linear acceleration: a = dv/dt » constant 2. sinusoidal vibration acceleration: a = dv/dt » periodic, a(t) = x*sinwt 3. shock: a = dv/dt where a(tto) = 0 Fundamentally, each type of acceleration requires a specific sensor type and sensing principle. The operation of traditional sensors is based on Newton's law, which, expressed in an equation, takes the following form:

F = M * a, where a ... acceleration F ... force M ... mass

From this equation, the functional elements of traditional acceleration sensors can be derived: The unit to be measured is acceleration. This acceleration acts on a seismic mass, which exerts a force on the sensor element. The sensor element converts this force into an electrical signal. Based on their operation principle, there are several sensor types. These are as follows: 1. Piezoelectric sensors with quartz- or ceramic-based sensing elements These sensors utilize the charge-generating property of quartz- or ceramic-based sensing elements. Due to the finite resistance (charge) of the sensing element, these sensors can only be used for dynamic accelerations (vibrations) and shock measurements. The output signal is electrical charge (for Charge Mode sensors) or voltage (for ICP-type sensors, which have a built-in charge amplifier). Both types have special - partially overlapping - application areas. 2. Piezoresistive sensors with strain gauge-based sensing elements In this type, the sensing element is parallelly attached to a spring system within the sensor. Its resistance changes under mechanical stress. With proper excitation, the output signal is voltage. Piezoresistive sensors can be used for measuring linear acceleration, vibration, and shock. 3. Capacitive acceleration sensors Here, the seismic mass acts as a spring on a capacitor, whose capacitance changes due to the force. The built-in capacitance bridge in the sensor provides an output voltage signal proportional to acceleration. Sensors based on this principle are excellent for measuring linear accelerations and relatively slow (low-frequency) vibrations. 4. Inductive acceleration sensors These sensors are based on a differential transformer, where the core acts as a seismic mass and changes the inductance under acceleration. Sensors constructed in this way can be used for measuring linear accelerations and slow (low-frequency) vibrations. The selection of a sensor or sensor type for measurement is primarily based on the type of acceleration to be measured (linear acceleration, vibration, shock). Additionally, considering environmental factors (temperature, contamination, etc.) and adhering to the maximum size and weight of the sensor are equally important to avoid changes in vibration values and natural frequencies of the vibrating system. Furthermore, the expected measurement range should be taken into account in terms of both acceleration values and frequency range. Based on the listed selection criteria, the most suitable sensor type for solving the current measurement task can be relatively easily identified. In accordance with the accuracy requirements of the measurement, the following sensor parameters should also be considered:

• linearity and resolution of the sensor
• sensitivity frequency response
• sensitivity to temperature changes
• lateral acceleration sensitivity
• measurement error due to bending (Base Strain)
• measurement error due to sudden temperature changes

The following table contains the limit values of sensor types available on the market.

	piezoelectric sensor with charge output	ICP-type piezoelectric sensor	piezoresistive acceleration sensor	capacitive acceleration sensor	inductive acceleration sensor
measurement acceleration range (g)	100,000	100,000	200,000	600	50
frequency range (Hz)	0.2 ... 900,000	0.2 ... 50,000	0 ... 150,000	0 ... 1,000	0 ... 1,000
minimum weight (g)	0.1 ... 0.15	0.1 ... 0.15	1	10 ... 15	15 ... 20
temperature range (°C)	-270 ... +650	-270 ... +200	-25 ... +95	-55 ... +120	-10 ... +60
power supply	---	current generator	voltage generator	unregulated voltage	carrier frequency generator
evaluation	charge amplifier	capacitive coupling	bridge amplifier	voltage meter	carrier frequency measuring amplifier
Base Strain (g/µinch/inch)	0.0005 ... 0.1	0.005 ... 0.1	2 * 10 -5	( < 0.5 % )	---
lateral sensitivity (%)	< 5	< 5	< 3	0.005g/g	0.003g/g

For the ease of implementation of measurements, it is advantageous to use sensors that are hermetically sealed (if the sensor/cable connector is properly designed to be insensitive to environmental contaminants) and ground-independent (to avoid large measurement errors caused by ground loops).

Rahne Eric (PIM Ltd.) www.pim-kft.hu, www.gepszakerto.hu  

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