The Risks of Phase-Matching and Teed Drives in Multi Shaker Single Axis Vibration Testing
Higher Force Testing Using Multi Shaker Test Set-Ups
Mechanical vibration testing has become ubiquitous across numerous industries for decades—aerospace, defense, and automotive, for example. The growing demand to test larger assemblies and structures necessitates the use of shaker systems capable of higher forces. However, as a shaker’s size increases (approximately 40 K lbf to 60 K lbf) their frequency range can diminish. The reason for this reduction in performance is that higher force shakers require larger and heavier armatures, which result in a lower first resonance frequency, thus reducing the useful bandwidth of the vibration shaker table. Employing multiple, smaller shakers is another logical path forward. But before you take the first steps on that path, careful consideration needs to be made when selecting the vibration controller used to drive the coupled multi shaker setup.
Challenges with Multi Shaker Control
A coupled two shaker system is perhaps the most deceptive shaker system to control – it seems simple enough… however, most vibration controllers will fall short when trying to control this system.
Often, manufacturers will sell single-shaker controllers masquerading as a MIMO controller, with a “phase-matching” algorithm or “dual loop” control that does not properly address the complexities of a MIMO test configuration.
In some cases, a single shaker control algorithm is used by simply teeing (and/or inverting) the drive to/between two shakers, or with a “phase-matching” piece of electronics. In each case, the results are sub-par and there is a risk of damaging the shaker system.
The Engineering Case for True MIMO Vibration Control to Avoid Damaging the Device Under Test (DUT) and Shaker
A MIMO controller designed specifically to address ALL the following phenomenon is critical to ensure a multi shaker test can be run safely without causing damage:
- Differences in the shakers and amplifiers
- Control (or null) rotations
- Shakers fighting each other
- Cross coupling between both shakers and all control accelerometers
- Structural resonances, off-center mass, and overturning moments
The following sections will detail some challenges and dangers that arise when an improper controller is used on a multi-shaker system.
1. Inability to Account for Differences in Shaker and Amplifier
Several controllers will use a “phase matching” control scheme to synchronize two control loops, or piece of hardware which is intended to “counteract” the phase differences between two shakers. Such algorithms or hardware always fall short, as these phase (and amplitude!) differences between different shakers are very complex – even for two shakers of the same make/model. Phase matching hardware and algorithms are typically designed to prevent shakers from fighting each other at low frequencies – where dynamics are very simple and predictable – but they do not work well at higher frequencies once modes of the table/test article add complexity to the mix.
The proper way to counteract the structural resonances and electrodynamics of any shaker/amplifier used in a dual shaker setup is to do so with a MIMO control scheme; whose measurements and system identification encompass and adapt for this. Relying on a piece of phase matching hardware or patch-work algorithm to “phase match” different shakers is both unreliable and unnecessary.
2. Inability to Control (or null) Rotations
For certain dual shaker configurations, such as a “Push-pull” shown below, it can easily be seen that moving the two shakers out of phase would be destructive and could damage the test article (or the shakers, if the test article was stiffer). The armatures moving out of phase with each other amounts to rigid body rotation; and needs to be carefully measured and controlled out to avoid damage to the test article or shakers.
If a controller uses an Average control scheme to try and control a dual shaker system – it is ignoring these potentially damaging rotations. Average control is often used to hide the inability to properly control multiple shakers – but it is actually dangerous here, as traditional average control schemes completely disregard phase information. Particularly at low frequency, this will damage the shakers and/or test article.
To properly control rotation, the controller must require the phase between the two control channels (or an angular signal) to be defined as a target reference profile; and must use the MIMO system identification to determine how to drive the shakers in a way which prevents any rotation. There is no guesswork in a MIMO controller’s ability to control this channeling configuration of shakers.
3. Inability to Prevent Shakers Fighting Each Other
One very serious risk when driving two shakers together is the fact that they can – at any frequency – output equal and opposite forces which cancel each other out. These are common when the two shakers are in a push/pull configuration (shown below) but can happen in other configurations as well.
In such a test, it is possible for the two shakers to be outputting an equal and opposite force which cancels each other out; producing no vibration and overstressing the shakers/amplifiers/table. This scenario can easily cause the amplifiers to be overdriven.
A setup similar to that shown above was run with a teed (inverted) drive going to the two shakers. The shakers were identical and had amplifiers from the same manufacturer. Below were the control and drive results (average control shown in Red). The Average that was being controlled to looks good, however, there are deviations in each of the individual control channels (each was on one side of the slip table). Note, the drive level (analogous to the overall energy output by the amplifiers) is 27mV RMS per shaker; 54mV RMS total:
The controller was then configured to run a MIMO control algorithm and drive the shakers independently.
Above: Teed (inverted) drive going to two shakers, Push/Pull
Not only was the control performance better with the MIMO control loop (both controls within +-6dB across the test band) – but the overall drive required to do so was only 5.4 and 12mV RMS: 17mV RMS total compared to 54mV RMS for the split drive with average control. All this additional energy was going into the shakers fighting each other – overstressing the test article and table; and the single shaker controller had no way to even know it was happening; let alone prevent it. Since feedback accelerometers do not provide a way to know when the shakers are fighting each other, it is very easy to damage an amplifier when not using a proper MIMO controller.
4. Cross Coupling Between Both Shakers and all Control Accelerometers
In any coupled multi-shaker configuration, there will be cross coupling between each shaker and both control channels. For example, in Figure 1‑4, driving one shaker will cause a response at both accelerometer locations A1 and A2; and depending on the frequency of excitation the responses at A1 and A2 (when driving only one shaker) can vary wildly in amplitude and phase. This is the nature of the system.
A properly engineered MIMO control algorithm should be able to control in this manner: control on both the individual control accelerometers, and include all cross coupling between both shakers and both control accelerometers in its control computations. Other incomplete controllers will try to use an Average control to hide the fact that they cannot control on your two control points, or cannot keep the two control points in phase; and the result is that the levels are not uniform across your two control points.
5. Structural Resonances, Off-Center Mass, and Overturning Moments
Above: Dual shaker push/push cantilevered and off-center mass.
It is not uncommon for one shaker to require different drive levels than the other shaker because of an off-center CG or a structural resonance. An off-center CG or a structural resonance can also to an overturning moment that will require the shakers to be driven with wildly different drive amplitude or phases. Take for example the idealized Figure 1‑4 shown above. While such an ideal setup is unlikely to be put into practice, it is very likely that practical considerations will force a test structure to be mounted off-center to some degree. It is also very likely for a test structure to have a structural resonance similar to first bending mode of the cantilevered arm (mass M2) which will go into resonance during the test and impart an overturning moment onto the system. To keep both A1 and A2 moving in-phase, the off-center mass will require the shakers to be driven with different drive levels; and the cantilevered arm going into resonance will require the shakers to be driven with both different amplitudes and phases – changing with frequency as the arm goes in and out of resonance.
A controller running a sub-standard control algorithm such as a “teed drive” or a “phase compensated” control loop will not be able to counteract these changing dynamics. A teed drive has no ability to treat the shakers differently; and the phase interactions and cross coupling are very complex for phase compensation hardware or algorithms to try and synchronize multiple control loops. A proper MIMO control algorithm, however, will encompass the effects of both of these in its system identification phase; and be able to keep both controls at the same amplitude and phase through the test.
Complete Solutions to MIMO control problems
The insights above point out the numerous shortcomings when using single-shaker control algorithms (“phase” compensated or teed/inverted single-shaker control loops) to control multiple shakers and illustrate how this can easily lead to damage to your shaker system.
The Data Physics Multi-Shaker Controllers use a MIMO system identification procedure which numerically quantifies the relationship between each shaker and each control accelerometer before running the test profile. With this information, our controllers can predictively and intelligently counteract all the common issues discussed in this article.
Data Physics is the industry leader in MIMO vibration control and continues pioneering the field with the current release of our MIMO control applications on our 900 Series platform.
Data Physics MIMO controllers include all commonly run test modules.
- Random
- Sine
- Shock/SRS
- Mixed Mode (Sine/Random on random)
- Time Replication
Data Physics MIMO controllers also include important features such as notching/limiting (in Sine or Random), recording time data to disk, MS Word-based reporting, and more.
For better test results, you need better control.
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