Introduction to Multi-Axis Vibration Testing
March 29, 2018
Multi-axis vibration testing is becoming standard in the automotive industry and others. A vehicle experiences vibrations along many axes when it travels; therefore, it makes sense for automotive engineers to test products along more than one axis.
In the past, engineers ran tests along one axis at a time. To do so, they would mount the product to a single-axis shaker and test the product along one axis. Then, they would take the product off the shaker, re-orient and remount the product, and re-test on a second axis. Finally, they would repeat the process on a third axis. This is a sequential, single-axis method for testing along more than one axis.
Now, engineers can test products along multiple axes at once. In addition to being time-efficient, multi-axis vibration testing brings products to fatigue faster. A product may pass in a sequential, single-axis test but fail in a more realistic multi-axis test of the same time duration. For this reason and others, multi-axis vibration testing is becoming increasingly popular. This lesson will discuss multi-axis test configurations, concerns with multi-axis vibration testing, and control methods for multi-axis vibration.
Types of multi-axis vibration tests
There are many ways to configure a multi-axis vibration test. The two key characteristics of any configuration are the number of shakers and the number of axes/degrees of freedom. Let’s consider some examples.
A MESA (multi-exciter/single-axis) configuration is composed of two or more shakers that move in the same direction along one axis. This configuration is used to test oversized products that require a shaker on each end. The shakers are often synchronized and use the same test profile (see Figure 1.)
A four-post configuration has four shakers moving along the same axis. This method is often used for full vehicle testing. During such, a shaker is placed under each wheel. Four recorded field data files from each wheel are played back (see Figure 2). Then, the recorded vibrations are played back simultaneously as if the vehicle was in motion.
Watch Vibration Research controlling a four-post shaker at TUV America here.
In recent years, the automotive industry has also witnessed a rising interest in other multi-axis shaker configurations. The three-axis configuration belongs to the MEMA (multi-exciter/multi-axis) class. This method involves at least three shakers and motions along the X, Y, and Z axes simultaneously (see Figure 3.)
Three-axis testing is primarily used for component or sub-system testing. It is accomplished by random vibration testing along each axis using identical or individualized test profiles. The three-axis configuration creates a more realistic test compared to traditional single-axis testing. As we mentioned previously, a vehicle experiences vibration from many directions simultaneously; the three-axis configuration accommodates the three linear directions of motion.
Although a three-axis configuration incorporates the linear directions of motion, it does not accommodate the three rotational directions of motion: roll, pitch, and yaw. A six degrees of freedom (6-DOF) system handles all six: X, Y, Z, roll, pitch, and yaw. There are numerous ways to configure a 6-DOF system.
Additional Automotive Testing
Four-post testing is one of many multi-axis methods used in the automotive industry. Another common application is squeak and rattle testing. No automobile driver likes a squeaky seat, but a seat must experience a particular set of vibrations along more than one axis before it emits a noise. This real-world scenario can only be simulated with multi-axis testing.
The advantages of multi-axis testing are clear. Multi-axis vibration testing brings products to fatigue failure faster and is more realistic than sequential, single-axis testing. However, there are several concerns with multi-axis configurations.
Issues with Multi-Axis Configurations
Multi-axis configurations raise a number of concerns, including the coupling of multiple shakers to a table. Ideally, a shaker along one axis should be able to transfer its motion to a table without affecting the other axes of motion. When the coupling is set up properly, a shaker should move the table along one axis without causing significant motion in the other two axes.
Cross-axis motion occurs when excitation along one axis excites another axis or axes. An example would be a slip table moving side to side when it should only be moving forward and back. While we cannot completely eliminate cross-axis motion, proper coupling methods can minimize its effect.
A common coupling method employs hydrostatic bearings, which allow connections to pivot freely and permit the transmission of motion from shaker to table. Sliding bearings are another option. Whatever the coupling method, the goal is the same: to transmit motion without exciting the other axes and to let the table move as freely as possible when excitation is applied.
Resonances are a significant concern in multi-axis testing. In cross-axis motion, resonances are more difficult to control because excitation along several axes can potentially excite a resonance along another axis or axes. The Sound & Vibration article, Defining the Global Error of a Multi-Axis Vibration Test, describes the potential for differences in excitation levels during multi-axis testing as well as ways to account for the differences. As mentioned in the article, the reference profiles must be designed so that resonances are minimized not just along one axis, but along several (see Figure 1. MIL-STD-810G common-carrier test profiles.)
Accelerometers also raise concerns in multi-axis testing. Transverse sensitivities produce more noticeable effects because the system is intentionally moving in all directions. In addition, accelerometer placement must be considered with respect to both the number of accelerometers and their location. Most tables or fixtures aren’t perfectly rigid; as such, accelerometer location matters (see Figure 4.) Multiple accelerometers are often used in conjunction with one another to determine motion in a particular degree of freedom. In such cases, the effects of their placement are magnified.
Even the design of the head expander—i.e. table—is affected by multi-axis configurations, as resonances must be minimized not just along one axis but several.
Controlling Multi-Axis Vibration
There are several methods for controlling simultaneous multi-axis vibration. Matrix control is one of the most popular.
Matrix control applies to a MIMO (multiple input/multiple output) system that is typically over-determined, meaning there are more accelerometers than mechanical degrees of freedom. When employing matrix control, the transfer function matrix should contain the terms that define how each direction of motion affects the other directions. An example would be a matrix that defines how excitation in the Y-axis affects the X-axis, the Y-axis (i.e., the gain along the Y-axis), and the Z-axis.
Matrix control receives information from the accelerometers and determines the vibration along each mechanical degree of freedom. With knowledge of the transfer function matrix and the system response, it is able to compute the drives necessary for control. Natural dangers—such as when a matrix is not able to be inverted or when the matrix inverse is very large—may occur, but a method exists to accommodate these events (refer to MIL-STD-810G.) Matrix control theory is comprehensive and takes into account the relationship of each axis with every other axis. It defines simultaneous control of all the shakers as an inter-related set, rather than each shaker being controlled independently.
Extremal control is another simpler control method. In multi-axis vibration testing, extremal control is similar to extremal control in single-axis testing. The controller controls off of the accelerometer with the maximum (or minimum) value. Unlike matrix control, one control loop cannot account for all of the axes simultaneously. Rather, a three-axis system requires one independent control loop per axis of motion. The extremal method is used to control each separate control loop.
With either control method, reasonable control tolerances for multi-exciter testing need to be addressed.
This article only briefly addresses multi-axis configurations, concerns, and available control methods. It is not an exhaustive account but rather a brief overview of multi-axis vibration testing. Test engineers can use this overview to identify topics of interest for specific testing situations.