If you’ve ever been involved in large-scale vibration testing—think seismic simulation for aerospace components, fatigue testing for heavy industrial machinery, or modal analysis for bridge prototypes—you know that a single vibration exciter can only do so much. To replicate real-world forces accurately, you need multiple exciters working in perfect sync. But here’s the thing: getting several shakers to move in unison, at the right frequency, amplitude, and phase, is not just a technical challenge—it’s the difference between a reliable test that delivers actionable data and a waste of time, money, and materials. As someone who’s spent over a decade as a vibration exciter supplier, working with labs, manufacturers, and test engineers across 17 countries, I’ve seen firsthand what goes wrong when synchronization breaks: test specimens fail prematurely, data is invalidated, and even equipment gets damaged. Today, I want to break down exactly how we (as a supplier, and as test teams) can ensure multiple vibration exciters stay in sync, no matter the test requirements. vibration exciter
Let’s start with why synchronization matters. At its core, synchronization means each exciter produces a vibration signal with identical frequency, matching amplitude, and a fixed phase relationship relative to one another. For small setups—say two exciters testing a small automotive subframe—misalignment might show up as a slightly wobbly reading. But for large structures: a 10-exciter array testing a full airplane fuselage, for example, even a 1-degree phase shift across one exciter can create a “beat frequency” that amplifies vibrations in unexpected spots, leading to test results that don’t match real flight conditions. I remember a client in Germany a few years back who was testing a wind turbine tower with four exciters. They skipped proper sync setup, and during a 2-hour fatigue test, a node point on the tower developed a stress crack that wasn’t present in finite element analysis (FEA) models. When they brought us in to troubleshoot, we found one exciter was 12 degrees out of phase with the others—enough to create a secondary vibration that the FEA didn’t account for, because the software assumed perfect synchronization. That’s the kind of mistake that costs companies months of delays and hundreds of thousands of euros. So this isn’t just a technical detail; it’s make-or-break for test validity.
First on the list: invest in a dedicated master control system, not a generic signal generator. I’ve seen teams try to sync multiple exciters using off-the-shelf function generators or even basic audio interfaces, and it never works for long. Generic generators create a single signal, but they can’t compensate for the inherent delays or drift in each exciter’s hardware. Modern vibration control systems—like the ones we design and supply with our high-force shakers—use a digital signal processing (DSP) core that locks all exciters to a single 10 MHz reference clock. Wait, why 10 MHz? Because that’s a frequency that’s high enough to capture even the fastest vibration signals (most tests run between 1 Hz and 20 kHz, so 10 MHz is 500x the maximum frequency) but low enough to avoid introducing jitter. Jitter is tiny variations in the timing of each cycle—think of a clock that’s off by a nanosecond here and there. At 20 kHz, a 1-nanosecond jitter causes a phase error of 0.007 degrees, which is negligible, but if your reference clock is only 1 MHz, that jumps to 0.07 degrees—still small, but enough to accumulate across 10+ exciters over a 10-hour test.
Our control systems also use a technique called “phase-locked loop (PLL) cascading.” Here’s how that works: the master control sends a reference signal to each exciter’s local driver, and each driver locks to that signal using a PLL. The key is that we calibrate the PLL in each exciter during manufacturing to have less than 0.1 degrees of inherent phase error at full force. That means when the master sends a signal, each exciter adjusts its own internal timing to match the reference, so even minor differences in the exciter’s armature mass or coil resistance don’t throw it off. I always advise clients to avoid daisy-chaining reference signals between exciters, by the way—each connection adds a tiny amount of signal delay, and if one cable is longer than another (even by a few feet), that creates a phase shift. All our exciters are designed to accept a wired or wireless reference input, but we strongly prefer hardwired for critical tests—wireless can introduce latency if the network is congested, which is common in large test facilities with dozens of pieces of equipment running at once.
Next: pre-test calibration and phase verification. Even with a top-tier control system, you can’t just plug in exciters and start a test. I recommend a full sync check before every test sequence, especially if you’re using the setup for different specimens or changing test frequencies. Here’s the step-by-step process we train our clients on during our on-site installation training: first, mount all exciters to a rigid test base (we always advise a steel or concrete base that’s at least 10x the total weight of all exciters to avoid base vibration contamination). Then, attach a small accelerometer to each exciter’s armature—not the test specimen, not the base—because that’s where the actual output vibration is. The accelerometer sends data back to the master control, which calculates the phase difference between each exciter relative to the master. We set our control system to automatically adjust the phase of each exciter in 0.01-degree increments until all are within ±0.05 degrees of the master. That might sound overly precise, but for a test running at 100 Hz, a 0.05-degree phase shift is less than 1 microsecond—way below the threshold that would affect test data.
I once worked with a client in Canada who was testing a heavy-duty mining truck frame with eight exciters. They skipped this pre-test calibration because they were “short on time” and had to meet a deadline. Halfway through the 72-hour fatigue test, the frame’s rear crossmember failed catastrophically, sending shrapnel that damaged three exciters (costing $120,000 in repairs) and took the test offline for two weeks. When we analyzed the phase data from their control system, we found the phase difference between the leading four and trailing four exciters had drifted to 1.2 degrees over the first 12 hours, and by 36 hours, it was at 4.7 degrees. That drift came from two things: the exciters’ coil temperature increasing as they ran (higher temperatures change the coil’s inductance, which affects the exciter’s response time) and the power supply voltage dropping slightly due to the test facility’s grid load as other equipment turned on. Pre-test calibration also includes setting a “phase drift alarm” in the control system—our systems alert the test engineer immediately if any exciter’s phase drifts more than 0.5 degrees from the set point, so they can stop the test and adjust before damage occurs.
Third: account for the load on the test specimen. This is a mistake a lot of new test engineers make—they assume the synchronization set with no load on the exciters will hold once the specimen is mounted. But every test specimen has a different mass, stiffness, and damping, and that load changes how each exciter responds. Let’s use a simple example: if you have two exciters, each rated for 10 kN of force, mounted on a 100 kg steel plate with uniform mass distribution, synchronization is straightforward. But if you mount a 1,000 kg concrete beam on the same plate, the exciter closer to the beam’s heavier end will have to exert more force to stay in sync, and its armature will deflect slightly more than the other. That deflection creates a small phase shift, because the exciter’s driver has to account for the load. Modern control systems use real-time force feedback to adjust each exciter’s output as the test runs—our systems measure the force from each exciter’s load cell 10,000 times per second, and adjust the phase and amplitude of each signal to compensate for changes in load. For complex structures like airplane wings, which have variable stiffness along their length, we recommend using modal testing first to map the structure’s natural frequencies, then calibrate each exciter’s sync pattern to match the mode shape. That way, you’re not just forcing all exciters to move the same way—you’re forcing them to move in the exact pattern that replicates real-world vibration.
Fourth: regular maintenance and monitoring of exciter components. Synchronization issues often creep in over time because of worn or damaged parts in the exciters. The most common culprits are the slip rings and brushes in larger exciters (over 50 kN force). Slip rings transfer power and control signals from the stationary body of the exciter to the moving armature, and as they wear, they create electrical noise and signal delay. A slip ring with 0.2 mm of wear can cause a phase shift of 2 degrees at 10 kHz—enough to throw a large test off. We recommend inspecting slip rings every 500 hours of operation, and replacing brushes when they wear down to 20% of their original length. Another common issue is the armature’s suspension springs: if a spring is corroded or stretched, it will change the armature’s natural frequency, leading to phase drift over the test. We include a maintenance checklist with every exciter we sell, and our service team offers quarterly checkups for clients running 24/7 tests, to catch these issues before they affect synchronization.
I also want to address a common myth: that more exciters automatically mean better test results. That’s not true if they’re not synced properly. A client in Brazil once contacted us asking to add 12 more exciters to an existing 8-exciter array for testing a offshore oil rig module. When we came in to upgrade the setup, we found their control system wasn’t rated for 20 exciters, and their reference clock was only 1 MHz. We recommended upgrading to our 20-exciter DSP control system with a 10 MHz clock, and calibrating the new exciters to match the existing ones. Without those upgrades, adding more exciters would have created even more synchronization errors, because the old system couldn’t process the extra data fast enough. So when planning a multi-exciter setup, it’s important to work with your supplier to size the control system and reference clock for the number of exciters and the test frequency range you need—trying to force more exciters into an underrated system is a recipe for failure.
Finally: work with a supplier that offers end-to-end sync support. As an exciter supplier, we don’t just ship you a shaker and a manual—we provide on-site installation, calibration, and training to make sure your multi-exciter setup is properly synchronized. We’ve seen too many clients buy cheap exciters from overseas, skip the calibration step, and end up with test data they can’t trust. Our exciters are designed with sync in mind: each unit comes pre-calibrated at our factory to within ±0.03 degrees phase error, so when you set up your array, you only need to adjust for your specific test setup, not fix inherent manufacturer errors. We also offer custom sync patterns for specialized tests, like seismic simulation, where the sync needs to match real earthquake waveforms, or acoustic testing, where phase alignment is critical to avoid sound cancellation.
At the end of the day, synchronization of multiple vibration exciters isn’t a one-time setup—it’s an ongoing process that requires the right hardware, careful calibration, and proactive maintenance. I’ve been doing this for long enough to know that even the smallest oversight can lead to big problems, but with the right approach, you can get data that’s accurate, reliable, and gives you the confidence you need to design better products.
If you’re setting up a multi-exciter test system and want to make sure your synchronization is on point, or if you’ve been dealing with sync issues that are throwing off your tests, we’re here to help. Reach out to our team to discuss your specific needs, whether that’s sizing a new setup, calibrating an existing array, or upgrading your control system.
Spare Parts References
- Harris, C. M. (2004). Harris’ Shock and Vibration Handbook (5th ed.). McGraw-Hill.
- International Organization for Standardization. (2018). ISO 10816-3: Mechanical vibration – Evaluation of machine vibration by measurements on non-rotating parts – Part 3: General industrial machines.
- Newland, D. E. (1993). An Introduction to Random Vibrations, Spectral & Wavelet Analysis (3rd ed.). Dover Publications.
- American Society for Testing and Materials. (2020). ASTM E756-20: Standard Test Method for Measuring Vibration-Damping Properties of Materials.
- Bendat, J. S., & Piersol, A. G. (2010). Random Data: Analysis and Measurement Procedures (4th ed.). John Wiley & Sons.
Xinxiang Fengda Machinery Co., Ltd.
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