Vibration Testing Equipment & Specifications
Vibration testing is a complex method of testing used to determine an object’s integrity. This method of testing involves exposing an object to specific vibration profile within a controlled testing environment. These vibration profiles mimic actual conditions the object will face during use (i.e. testing automotive parts for function and safety prior to distribution). Obtaining accurate data is vital for this form of testing; thus, there’s specific vibration testing equipment and specifications required to accurately perform this test.
This article provides information on how vibration testing is conducted on a technical level. Specifically, the article discusses the equipment/specifications used for testing and shows results from a mock test conducted by the globally accredited vibration testing laboratories at JTL America.
Continue reading to learn more.
Equipment Involved in Testing
To help readers better understand the subject matter, here’s a list of the equipment used in testing with a brief description of their function.
Vibration Controller – A vibration controller is used to produce an input profile that imitates a wide range of vibration inputs. The controller uses multiple frequency and amplitude (acceleration, velocity, or displacement). Real world inputs are road inputs, airborne inputs like wind or noise, fluid borne inputs, powertrain inputs, as well as the path that the vibration follows.
Amplifier – Amplifiers are used to adjust the required electrical current. In this case, they are used to send voltage to the electrodynamic shaker based upon the vibration profile requirements.
Electrodynamic Shaker (ED Shaker) – ED Shakers accurately emulate real world conditions based upon the signal sent from the vibration controller after the signal is enhanced by the amplifier.
Accelerometer – The control accelerometer is a measuring device. In this case, the device is used to obtain feedback from the Electrodynamic Shaker. This feedback is used to send new information back to the vibration controller to further improve the accuracy of the test.
How Vibration Tests Are Conducted
Vibration testing is a closed loop operation. This means that signals are passed back and forth from one piece of equipment to another in a circular pattern. This reciprocal communication between various pieces of equipment is necessary for clean and accurate data during testing.
Here’s a step-by-step demonstration on the complete closed loop operation and how it works on a technical level.
1. The vibration controller sends the information to the amplifier.
2. The amplifier send the current to the ED shaker based on the vibration profile requirements.
3. The shaker reacts and the control accelerometer sends this response back to the vibration controller.
4. The vibration controller adjusts based on feedback from the control accelerometer and sends the new requirements to the amplifier.
5. The amplifier sends the new current to the shaker.
6. The control accelerometers send the new information back to the vibration controller.
This communication loop happens very quickly but could take several tries, or iterations to get to full vibration level. Also, if there are equipment issues or concerns, the process may become even more tedious.
Vibration Testing Equipment Verification
In this section, we’re going to describe the importance of testing and verifying equipment prior to conducting a test. We’re also going to show a visual example of a mock test we conducted at our vibration testing labs in Livonia, Michigan
First off, it’s absolutely vital to test your equipment for proper configuration. A clean set up in the beginning will ensure clean data in the end.
Let’s say you suspect that an accelerometer or cable that is not performing. Perhaps the accelerometer and cable just finished a 300hr thermal vibration with high humidity. Maybe the accelerometer cable was saturated with humidity, or an accelerometer cable pins are now dirty. Sometimes this can be something as simple as a bad connection where all you need to do is clean your connection points and continue testing.
On the other hand, the problem may stem from a bad accelerometer or cable. The point is, we never assume and always test our equipment.
Anti-Resonance channel 6 on bullnose
(red arrow)
Recently I ran a sine sweep and captured an anti-resonance. I didn’t like this result, so I placed another accelerometer next to it for a sanity check.
Bullnose Anti-Resonance at 900Hz
I added a second accelerometer at the same location.
Channels 5 and 6 Anti-Resonance at 900Hz
Here was my response at this location. Channel 6 was not moved. Channel 5 was placed on a block next to channel 6. This clearly shows that the anti-resonance is real. Both signals are identical.
Now if we ran the original sweep and channel 6 looks like this in the graph below. Then we may have a bad accelerometer or cable.
The ground loops in this graph (60Hz, 120Hz, 240Hz, etc.) show a good indication of the issue. For example, channel 6 loses energy after 5Hz and channel 6 should read 0.5g. At this point it would be best to try a different accelerometer or cable. If the new accelerometer and or cable/corrects this issue, then the bad accelerometer/cable should be removed and sent back to the supplier for evaluation. The equipment manufacture will most likely run X-ray CT scanning on the accelerometer and or cables to diagnose the issue.
X-Ray CT Scanning
At JTL America we offer X-ray CT scanning. This is a nice tool for checking accelerometers as well as cables.
These photos show what a good accelerometer and cable should look like under X-ray scan. The microdot center pin connection and shielding has no bad connections.
(Accelerometer courtesy of Dytran)