Multishaker testing has been growing in popularity over the past several years. This testing is done not only in the automotive industry, but across the board in the Aerospace, Defense, and even Electronics industries. Along with the increased use, more standards are being written around multishaker testing. The most notable standard to now include multishaker testing is the MIL-STD 810G, Method 527.
There are three primary reasons to do multishaker testing: (1) better real-world reproduction of vibration, (2) large or massive test articles, and (3) reductions in test time.
A more accurate simulation of the vibration seen in actual use enables engineers to better produce failures that they will see in the field, and consequently have confidence in the survivability of products and equipment.
Extremely large test articles often pose problems for single shaker testing. The payload may require complicated or near impossible fixtures to be tested with a single shaker. Very heavy test objects may even require more force than a single shaker can provide.
An additional benefit that is increasingly being realized is the reduction in test time that one sees from doing more than one linear axis of motion at a time. Traditional development, qualification, and screening vibration testing has always been single axis testing. Being able to better reproduce failures along with reducing test time makes simultaneous three degree of freedom an attractive alternative.
The current state of multishaker vibration control has now made it not only possible, but beneficial, to use two shakers instead of a single large shaker when testing large payloads. In additional to the higher achievable force, a dual shaker provides less concern about issues such as payload placement and upper end frequency performance. The flexibility provided by this arrangement also makes it possible to use the two shakers independently for component level testing at even higher frequencies.
Typical applications for multi shaker testing include earthquake testing and simulation, weapons system vibration qualification, seat vibration (both with and without a person in the seat), spacecraft and satellite qualification, automotive durability testing, automotive component testing, squeak and rattle testing, and general purpose electronics testing.
The case of the electronics testing is an interesting one. Studies being undertaken at the Center for Advanced Life Cycle Engineering (CALCE) are showing that 6 DOF shakers are better at exciting resonances and reproducing failures in certain types of PCBs than repetitive shock testing (a.k.a. HALT). Over time, this could represent a significant shift in test methodology for electronics testing.
Although many shakers are available in off-the-shelf configurations, currently most multi degree of freedom shaker systems are custom built per requirement. Considerations regarding the shaker type (electrodynamic vs. servo hydraulic), the table requirements, reaction mass requirements, and couplings are very important to the design of these systems. It is always recommended to work with reputable professionals in building MDOF vibration test systems.
A proper multishaker vibration controller is required to accurately and effectively control multishaker tests. All degrees of freedom that are not physically constrained should be controlled. For MDOF tests, a multishaker vibration controller is required because all of the actuators affect all of the test points in a rigid or flexible body. For multishaker single axis tests, additional issues such as impedance mismatching between shakers, load imbalance, and structural dynamics make a true multishaker controller very necessary for maintaining control without undergoing painstaking efforts of complex fixtures, couplings, and system matching.
Data Physics controllers use a pretest method for system identification to generate the first drive signal of a test. In multishaker testing, this pretest phase is more critical than in general single shaker tests, and requires careful evaluation. The FRF and multi coherence measurements provide valuable information to determine controllability of the test and preferred control points.
Careful consideration of sensor location is required in both field data acquisition and lab simulation. As many users have found, a mistake in location or direction can make running a test not possible. Additionally, it is important to be sure the data points are spatially located to adequately define the degrees of freedom in the test.