Sensor Types and What Each Is For

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Sensor Types and What Each Is For

Choose IEPE, charge or piezoresistive for a stated test
Say when a force gauge, strain gauge or laser instrument is the right channel

The IEPE Accelerometer

A piezoelectric element with a small amplifier built into the case
Powered by a constant current down the same coaxial cable that brings the signal back
One manufacturer's figures: 2 to 20 mA from 18 to 30 V; output on a bias of 8 to 12 V, about 5 V either side

Fixed Range, Fixed Low End

The range is the full-scale output over the fixed sensitivity
100 mV/g reaches 5 V at 50 g; 10 mV/g reaches it at 500 g
The low-frequency limit is set by a discharge time constant chosen at manufacture
The temperature limit is the electronics', and varies by model

The Bias Is the First Diagnostic

Inside its window: the sensor is powered and its amplifier alive
Near the supply: an open circuit; near zero: a short
It cannot say whether the sensitivity is right, the sensor mounted, or the axis correct

The Charge Accelerometer

The same element with no electronics in the case; an external charge amplifier
Hotter than an IEPE sensor will stand, and a range set for the test
The cost is the cable: low-noise, short, held still, connectors clean and dry

The Piezoresistive Accelerometer

A resistor bridge on a flexure: it reads acceleration down to zero hertz
Needs bridge excitation and an input that accepts the steady part; lower sensitivity
Method 516.8 requires it where shock response below 2 Hz is wanted

Compression and Shear

Compression: the element stacked under preload on the base, so base bending and base temperature make output
Shear: the element loaded sideways, isolated from the base; both effects much smaller
A bending plate or a moving temperature asks for shear; the datasheet says which design it is

Triaxial Sensors and Transverse Sensitivity

Three elements, three sensitivities, three rows in the channel table
The most common fault: an axis assigned to the wrong channel
Transverse sensitivity: 5 percent or less for 514.8, 7 percent or less for 516.8
Orientation recorded against a feature of the article and photographed

Other Channels a Test Is Handed

A force transducer between the fixture and the article: the channel force limiting runs on
A strain gauge: stretch at a point, closer to what breaks; usually a monitor
Laser vibrometer, displacement transducer, laser displacement gauge: no mass added

The Sensor-Selection Table

One row per channel type: lowest frequency, temperature, range, shock, what it needs from the rack, the test it is for
A printable page for the lab kit

Type	Lowest frequency read	Temperature limit set by	Range	Shock	Needs from the rack	Chosen for
IEPE	a fixed cutoff, its time constant	the electronics: datasheet	fixed at manufacture	survives its rating; zero shift possible	constant current, 2 to 20 mA	most channels on most tests
Charge	a fixed cutoff, set by the chain	the element: far hotter; the converter stays outside the chamber	set at the charge amplifier	survives its rating; zero shift possible	charge amplifier or in-line converter; low-noise cable	the hottest chamber tests
Piezoresistive	zero hertz	datasheet	fixed at manufacture	survives; no zero shift; some damped	bridge excitation; DC-coupled input	shock where response below 2 Hz matters
Capacitive	zero hertz	datasheet	fixed at manufacture	datasheet	a DC supply; DC-coupled input	long-stroke tests at a few hertz
Force transducer	a fixed cutoff (piezoelectric)	datasheet	fixed (IEPE type) or set at the amplifier (charge type)	datasheet; it carries the article's load and preload	constant current or charge amplifier	force limiting: the interface force
Strain gauge	zero hertz	the gauge and its adhesive: datasheet	set at the amplifier	datasheet	bridge completion and excitation	stress at a point, as a monitor
Laser vibrometer	datasheet	not on the article	set on its own controller	not mounted: nothing to survive	its own controller; a voltage input	light articles; surfaces nobody may touch
Laser displacement gauge	zero hertz; its top is a few hundred hertz	not on the article	fixed by its measuring range	not mounted: nothing to survive	its own supply; a voltage input	shock displacement on the calibration load; body motion at the low end
Displacement transducer (LVDT or proximity probe)	zero hertz	datasheet	fixed by its stroke	datasheet	its own conditioner and excitation	the shaker's stroke against its rating

Datasheet marks a value that varies by model: the datasheet for the sensor in hand is the only number that counts. Zero shift is the piezoelectric element's failing after a severe shock.

Figure 1.2. The sensor-selection table. Rows: IEPE, charge, piezoresistive, capacitive, force transducer, strain gauge, laser vibrometer, laser displacement gauge and displacement transducer. Columns: lowest frequency read (zero hertz or a fixed cutoff), temperature (set by the electronics, the element or the converter, with "datasheet" where it varies by model), range (fixed or set at the amplifier), shock (survives, zero shift, damped), what it needs from the rack (constant current, charge amplifier, bridge excitation), and the test it is chosen for.

Decide

Choose the sensor type from the test, not the drawer: lowest frequency, temperature at the sensor, peak level
Shock below 2 Hz, or a zero-shift risk: piezoresistive
Hotter than the IEPE datasheet allows: charge, with its converter outside
Everything else starts as IEPE; a bending surface or a moving temperature asks for shear

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