Glossary term

Modal Shaker Test

Controlled-force modal test method that uses an electrodynamic or hydraulic shaker, stinger and force measurement to estimate structural frequency response functions.

Branch: Mechanical Engineering
Glossary type: method
Content: Glossary term
Updated: Jun 25, 2026
Revision: v1.0.1 · reviewed

Definition

method

A modal shaker test is an experimental modal test that uses a controlled shaker force input and measured structural response to estimate frequency response functions and modal parameters.

Modal shaker testing uses an electrodynamic, hydraulic or inertial shaker connected to the test article through a stinger, load cell or force transducer. The excitation may be sine, swept sine, random, burst random or another controlled signal. Because the input is sustained and measurable, shaker testing can improve low-frequency energy, repeatability and nonlinear screening compared with impact-only tests, but it can also disturb the structure through shaker mass, stinger stiffness, side loads, control-loop behaviour and boundary-condition changes.

A modal shaker test excites a structure with a controlled measured force and records the structural response. The force is usually measured close to the structure with a load cell or force transducer, not inferred only from the shaker command signal.

For a force input $F(\omega)$ and measured response $Y(\omega)$ , the main data product is a frequency response function:

\displaystyle H(\omega)=\frac{Y(\omega)}{F(\omega)}

The response may be acceleration, velocity, displacement, strain, pressure or another dynamic measurement. The value of the test depends on both the measured FRF and the evidence that the shaker did not alter the dynamic behaviour being measured.

Engineering Role

Modal shaker testing is useful when impact testing cannot provide enough input energy, when low-frequency modes need sustained excitation, when the structure is large, when nonlinear response must be explored with controlled amplitude, or when a repeatable input is needed for multiple averages and multiple response locations.

Typical hardware includes:

an electrodynamic, hydraulic or inertial shaker;
a stinger or flexure that transmits mainly axial force;
a load cell or force transducer at the structure side of the drive path;
response sensors such as accelerometers, laser vibrometers or strain gauges;
a signal analyzer, controller or data acquisition system;
fixtures, soft supports or service boundary conditions that match the test objective.

Common excitation choices are:

Excitation	Typical use	Main caution
Stepped sine	high signal-to-noise ratio at selected frequencies	slow test and possible nonlinear amplitude dependence
Swept sine	resonance search and qualification-style screening	sweep rate can bias damping or miss transient effects
Random	broadband FRF estimation with averaging	input spectrum must remain adequate across the band
Burst random	reduced leakage for lightly damped structures	burst timing and windowing must be documented
Chirp	quick broadband excitation	force spectrum and coherence still need review

Shaker testing is often preferred over an impact hammer when the target modes are low in frequency, the structure is heavily damped, the required response is small, or a test team needs controlled repeatability. It is not automatically better. A poorly attached shaker can add mass, stiffness, damping, side load and control-loop artifacts.

Shaker Interaction and Acceptance Gates

A shaker test should prove that the input system is measuring the structure, not dominating it. The load cell should measure force at the structure side of the drive path whenever practical, because the controller command and shaker armature force can differ from the force actually entering the test article.

Useful acceptance checks include coherence over the decision band, repeatability across averages, force spectrum adequacy, no response clipping and a drive-level sensitivity check. A simple nonlinearity screen compares two FRFs measured at different force levels:

\displaystyle \delta_H(f)=\frac{|H_{high}(f)-H_{low}(f)|}{|H_{low}(f)|}

If $\delta_H$ is large near a target mode, the structure, fixture, joint or shaker attachment may be amplitude dependent. The response can still be useful, but it should not be reported as one linear FRF without stating the drive level.

The stinger should transmit mostly axial force. A small angular misalignment $\theta$ creates an approximate transverse component:

\displaystyle \frac{F_\perp}{F}\approx\sin\theta

That side force can matter on light panels, aircraft surfaces, slender frames and brackets. Alignment, stinger flexibility, fixture stiffness and shaker mass should therefore be part of the setup evidence, not informal lab notes.

An engineer plans a shaker modal test on a machine support frame. The required frequency band is:

5\leq f\leq 200\ \text{Hz}

The data acquisition settings are:

f_s=1024\ \text{Hz}

T=16\ \text{s}

The Nyquist frequency is:

\displaystyle f_N=\frac{f_s}{2}=\frac{1024}{2}=512\ \text{Hz}

The anti-alias low-pass filter is set at $400\ \text{Hz}$ , so the filter cutoff is above the required $200\ \text{Hz}$ band and below Nyquist. This is a reasonable measurement setup, provided the filter roll-off is appropriate for the analyzer.

The frequency resolution is:

\displaystyle \Delta f=\frac{1}{T}=\frac{1}{16}=0.0625\ \text{Hz}

Two expected modes are separated by $0.50\ \text{Hz}$ . The number of frequency lines between them is:

\displaystyle N_{lines}=\frac{0.50}{0.0625}=8

Engineering comment: eight lines is usually enough for screening two separated peaks, although final damping estimates may still require longer records, curve fitting and checks for leakage.

The force transducer range is $500\ \text{N}$ and the planned peak force is $120\ \text{N}$ :

\displaystyle \frac{120}{500}=0.24

The accelerometer range is $50g$ and the expected peak response is $6g$ :

\displaystyle \frac{6}{50}=0.12

Both channels have margin against clipping. The engineer would still check that the measured signals are well above the noise floor and that the drive level does not change joint stiffness, contact state or damping.

At one frequency line near $42\ \text{Hz}$ , the measured RMS force is:

F_{rms}=32\ \text{N}

and the measured RMS acceleration response is:

a_{rms}=1.92\ \text{m}/\text{s}^2

The accelerance magnitude is:

\displaystyle \left|\frac{A}{F}\right|=\frac{1.92}{32}=0.060\ \frac{\text{m}/\text{s}^2}{\text{N}}

If the coherence at this line is $\gamma^2=0.97$ , the input-output relation is strong for that average. This does not prove the test is correct; it only supports the linear consistency of the measured force and response at that frequency.

Finally, the stinger alignment is checked. If the axial peak force is $120\ \text{N}$ and the stinger is misaligned by $4^{\circ}$ , a rough transverse force component is:

F_{\perp}=F\sin(4^{\circ})=120(0.0698)=8.4\ \text{N}

The transverse-force fraction is:

\displaystyle \frac{8.4}{120}=0.070=7.0\%

Engineering comment: a $7.0\%$ transverse component may be too high for a clean single-axis modal drive, especially on a light panel or flexible bracket. The engineer should improve alignment, use a longer or more flexible stinger, reduce lateral constraint, or document why the side load is acceptable for the objective.

Modal shaker test is not a frequency response function. The shaker test is the controlled excitation and measurement method. The FRF is the processed response/input data.

Modal shaker test is not impact hammer testing. A hammer test uses a short force pulse. A shaker test applies a controlled sustained or programmed input and can hold energy in a chosen band.

Modal shaker test is not modal analysis by itself. Modal analysis includes FRF estimation, curve fitting, mode-shape assembly, model correlation and validation decisions.

Modal shaker test is not a ground vibration test. A ground vibration test is an aerospace or large-structure modal campaign that may use shakers, hammers or other excitation. Shaker testing is one possible excitation method within such a campaign.

Modal shaker test is not operational modal analysis. OMA estimates modal parameters from ambient or service response without measured controlled input. Shaker testing is controlled-input testing.

Modal shaker test is not vibration qualification testing. Qualification tests often apply prescribed environmental vibration levels to prove durability or compliance. A modal shaker test is primarily a dynamic identification method.

Validation and Common Mistakes

A defensible modal shaker test states shaker type, force transducer location, stinger geometry, drive direction, attachment details, excitation signal, amplitude control, frequency band, force and response calibration, sensor layout, sampling rate, anti-alias filtering, windows, averaging, estimator, coherence, boundary condition, support condition, test configuration and uncertainty.

Common mistakes include:

using shaker drive current as force instead of measuring force at the structure;
letting shaker mass or rigid attachment change the boundary condition;
applying side load through a short, misaligned or over-constrained stinger;
driving too hard and changing joint contact, preload, friction or damping;
accepting high coherence while ignoring poor physical setup;
using a sweep rate that biases damping or fails to settle near resonances;
allowing force dropout near resonances or anti-resonances without review;
comparing shaker and hammer FRFs without checking drive point, direction, boundary condition and response units;
treating a qualification vibration test as if it automatically produced modal parameters.

REF

Disciplines

Modal Shaker Test

Definition

Engineering Role

Shaker Interaction and Acceptance Gates

Validation and Common Mistakes

See also

Engineering Role

Shaker Interaction and Acceptance Gates

Worked Example: Screen a Shaker Modal Setup

Distinction from Related Terms

Validation and Common Mistakes