A practical workflow for selecting and configuring electrodynamic shakers and modal exciters for structural validation, environmental qualification, and modal analysis. Covers force sizing, controller and amplifier matching, fixturing, and test categories from bench-top to high-force water-cooled systems.
What’s Covered
Shakers vs. modal exciters
Both shakers and modal exciters apply controlled mechanical force to a structure, but they’re tools for different jobs.
- Shakers are sized for systems-level vibration qualification, driving an entire DUT or assembly to specified vibration levels per a profile (sine, random, shock, or mixed)
- Modal exciters are smaller and optimized for clean force injection at a single point during modal analysis, where the goal is to identify resonant frequencies, damping, and mode shapes rather than reproduce a service environment
Use the wrong tool and you either fail the test (an under-sized shaker can’t reach required levels) or contaminate the result (a large shaker introduces too much mass and stiffness change for clean modal data).
Force sizing for vibration testing
Required peak force is roughly the product of total moving mass (DUT plus fixture plus armature) and required peak acceleration, with margin for amplifier headroom and resonance.
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Calculate moving mass
Sum the DUT mass, fixture mass, and the shaker’s armature moving mass. The armature mass is published on the shaker datasheet.
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Determine required peak acceleration
From the test profile, sine peak, random grms with crest factor, shock peak, or modal force levels.
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Compute force = mass × acceleration
Add a safety margin (commonly 30–50%) for amplifier headroom, line voltage variability, and DUT resonance peaks where the shaker may need to push harder.
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Verify against shaker rating
Check both peak force and rms force ratings, they limit different aspects of test capability. Random tests with high crest factors push the rms limit; shock tests push peak.
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Confirm displacement and velocity limits
Low-frequency tests are often limited by stroke (displacement) rather than force. Velocity limits set the mid-band capability.
Resonance is where shakers earn their keep. Off-resonance, the impedance is dominated by mass and the force requirement is predictable. Near resonance, the shaker fights against amplification of input motion and may need substantially more headroom than a simple mass-acceleration calculation suggests.
LDS and HBK shaker / exciter families
| Family | Force class | Best fit |
|---|---|---|
| LDS V101 / V201 / V406 / V450 / V455 | Permanent magnet, low force | Component-level testing, lab benchtop, calibration |
| LDS V555 / V650 / V721 / V780 | Air-cooled, low to medium force | Subsystem qualification, broad lab use |
| LDS V8 / V830 / V850 / V875 / V8750 | Medium-force shakers | Component and small-assembly qualification |
| LDS V8900 / V9 / V964 / V984 / V994 | High-force water-cooled | Aerospace and large-assembly environmental qualification |
| HBK Type 4808 | Permanent-magnet exciter | Larger modal applications, in-situ excitation |
| HBK Type 4809 / 4810 | Small vibration exciters | Bench-top modal, calibration |
| HBK Type 4824 / 4825-4826 / 4827-4828 | Modal exciters | Modal analysis, transfer function measurement |
Browse the full shakers and exciters catalog for current specifications and accessories.
Amplifiers and vibration controllers
The shaker is one component of a complete vibration test system. Two more matter just as much.
Amplifiers
The amplifier converts controller output into the high-power drive signal that moves the armature. It must match the shaker electrically (impedance, voltage, current) and provide enough power to reach required force levels with margin.
- Linear amplifiers (LPA series), clean signal, lower efficiency
- Switch-mode (SPA-K, DPA-K series), higher efficiency, modern default for most installations
- Compact (HPA-K), smaller systems and modal applications
Vibration controllers
The controller generates the drive signal in response to feedback from a control accelerometer mounted on the shaker armature or DUT. Modern compact controllers (Comet USB, Laser USB) handle sine, random, shock, and mixed-mode tests with closed-loop control. Multi-shaker / MIMO setups require larger controller systems.
Fixturing, slip tables, and head expanders
Fixture design is one of the highest-impact decisions in any vibration test. A poor fixture changes the boundary conditions of the DUT, introduces unintended modes, and can absorb much of the energy that should reach the part.
- Slip tables (HBT, LPT) extend the shaker’s force into a horizontal plane for in-line and lateral testing
- Head expanders distribute force over a larger mounting surface for assemblies bigger than the armature
- Thermal barriers protect the shaker from thermal-cycle test articles
- DC centering units (Type 1056) maintain armature alignment for tests with significant DC offset
Validate fixture stiffness with a sine sweep before live testing. The first few resonances should be well above the test band, or the fixture itself becomes part of the response.
Sine, random, shock, mixed, and modal
| Test type | What it characterizes | Typical shaker requirement |
|---|---|---|
| Sine sweep | Resonances, frequency response | Moderate force, very stable amplifier |
| Random | Broadband qualification, real-world environment | High rms force, high crest-factor capability |
| Shock | Survival under transient events | Peak force capability, sharp pulse fidelity |
| Sine on random | Combined narrow-band + broadband | Both rms and peak headroom |
| Modal (impact / shaker) | Modes, damping, transfer functions | Modal exciter, clean force, low mass loading |
FAQ
How do I choose between a permanent-magnet and an electrodynamic shaker?
Permanent-magnet shakers are simpler, lower-cost, and ideal for component-level and bench-top work up to moderate force levels. Electrodynamic shakers with field coils provide higher force, larger displacement, and more flexibility, necessary for environmental qualification of subsystems and assemblies. The choice is driven by force, displacement, and frequency requirements.
Can I use a shaker for modal analysis?
Technically yes, but a modal exciter (HBK 4824 / 4825-4826 / 4827-4828) is purpose-designed for clean force injection with minimal mass loading. Using a large shaker for modal analysis introduces enough mass and stiffness change at the attachment point to contaminate the data. For dedicated modal work, use an exciter.
What’s the difference between rms and peak force ratings?
Rms force is the long-term sustained capability, the limit for random and steady sine tests. Peak force is the short-duration capability, the limit for shock and high crest-factor random. Random tests with high crest factor push the rms limit; shock tests push the peak limit. Both must be sufficient for the test profile.
Do I need water cooling on a shaker?
Above roughly 30 kN of rms force, water cooling becomes the practical default. Air-cooled shakers handle most lab and component-level work; water-cooled is reserved for high-force, long-duration aerospace and large-assembly testing.
Does Durham Instruments support full system installation?
Yes. Durham Instruments supplies complete vibration test systems, shaker, amplifier, controller, fixturing, and accelerometers, and supports installation, commissioning, and operator training. Contact our team for a turnkey system specification.
Specifying a vibration test system?
Send the test profile, DUT mass, and frequency range, Durham Instruments will return a sized shaker / amplifier / controller package with fixturing options.