Vibration testing is continuously evolving throughout time. The requirements of testing large and heavy devices demand shakers with larger sizes and higher force ratings. On the other hand, with the increase of size and force ratings comes the cost of lowering the usable frequency range of a shaker system. Shakers with 50,000 lbf force typically operates up to 2 kHz. Larger shakers can have usable frequency ranges of less than 1.5 kHz. The main reason for this frequency reduction in performance is that higher force requires larger armature, and this results in a lower first resonance frequency, thus reducing the useful shaker frequency range. Intuitively, the larger the shaker, the higher the price tag.
A good solution to this type of situation is the dual shaker system. With two shakers, the force rating can be doubled, which is obvious. Geometrically, two shakers can be arranged to excite the device under test within a certain distance away of the shakers. This will address the issue when testing a long structure. To set up the dual shaker testing environment, it will require an additional table or fixture complexity.
The following photo illustrates the vertical dual shaker arrangement, with the flexible fixture on two ends of the device under test.
Figure 1. Vertical dual shaker testing system
Another typical dual shaker arrangement is the horizontal push-push, as shown in the following illustration. One big slip table is connected rigidly to both shakers through driving bars. With this setup, both shakers needs to drive in phase at all times.
Figure 2. Horizontal push-push of dual shaker testing system
Another horizontal dual shaker arrangement is the push-pull case. Two shakers are arranged opposite to each other; thus, originating the push and pull name. Also, this is a rigid connection situation.
Figure 3. Horizontal push-pull of dual shaker testing system
The advanced MIMO VCS control system has made dual shaker single axis testing more practical. Some older technology uses a single shaker controller to drive both shakers, which is possible with a constant phase shift device. In practice, this depends on the dynamic response and an unbalanced load, in which the fixed constant phase shift is obviously inadequate. Also, the single drive amplitude is inadequate as well. A superior solution is provided by a MIMO shaker controller that uses the 2X2 transfer function matrix to drive each shaker with an individual magnitude and phase signal across the frequency bandwidth. Spider MIMO VCS control provides the control for such dual shaker testing.
Figure 4. MIMO Sine control, in phase
The MIMO vibration control for dual shaker single axis testing is the same whether the configuration is for vertical testing of a long device or a slip table for horizontal testing. There is also no difference in the control strategy when driving a slip table in either a push-push or push-pull configuration.
The Spider MIMO VCS controller is capable of Random, Sine, Shock, and Time Waveform Replication testing.
For more details regarding the Spider MIMO VCS controller, read: https://www.crystalinstruments.com/mimo-vibration-control-overview