Miniature Sensors for Testing Small Structures

Measurement Considerations

Most dynamics people think of structural testing as the system vibration study of large structures like automobiles, aircraft or civil structures. However, the ever increasing push for smaller, lighter, and more powerful in the mobile electronics world (phones, disk drives, subnotebooks, etc.) creates a unique set of challenges for structural test engineers. First and foremost is often simply finding space to locate a response accelerometer! This article discusses a number of other specialized measurement considerations for ultra-small or ultra-light test structures.

Accelerometer manufacturers have introduced a host of miniature sensors over the last few years. Some of the very smallest/lightest accelerometers are available in packages as light as a scant 0.16 grams. There are also extremely small ICP accelerometers in both single axis and triaxial packages that power from industry standard constant current power sources.

When selecting a miniature accelerometer, cable considerations are also important. Due to the extremely light mass of the sensor and structure, cable forces can affect the measurement if allowed to carry load. Select the lightest cable practical for your application and be sure to secure a strain relief service loop in the cable. The trade-off in selecting an ultra-light cable is often pitted against long durability of the cable, signal path and connector. A heavier cable generally lasts longer, but will carry more cable forces. Hair thin stranded 36 AWG wire minimizes any cable force but may last only a test or two. If a heavier gauge cable is sometimes desired, designs must resort to an integral cable. This eliminates the connector and keeps mass to a minimum but no longer allows for field repair.

Mass loading is often quoted as a concern on small structures. Rules of thumb exist such as keeping under 10:1 ratios, but these aren’t quite intuitive as it refers to dynamic mass. If measurements are for modal modeling, the key is not only to reduce the sensor mass, but also to ensure a CONSISTENT mass distribution rather than roving (New Developments in Multi-Channel Test Systems) accelerometers. Consistent mass distribution of fixed arrays of small accelerometers ensure measurement of consistent modal frequencies which, in turn, ensure more accurate modal parameter estimates from global parameter estimation algorithms.

A final consideration is mounting method. It is most often impractical to drill and tap mounting holes, as well as the fact that mounting studs would add more weight, so adhesive mounts are the most common choice. Be extra careful to follow the manufacturer's guidelines for mounting and removal as the case and connectors on some miniature accelerometers are very fragile. A specialized removal tool is included by some manufacturers.

As you can see, the options for miniature accelerometers have grown and so have the considerations for their proper use and care. Please don’t hesitate to contact us here at The Modal Shop or your PCB Group Field Application Engineer if we can help you with any additional questions.