Sensitivity testing involves making predictions of system response to environmental disturbance. The significance and importance of the system sensitivity estimates is very often overlooked. This gets companies in trouble and is one of the top reasons why we are called upon to provide timely solutions. Most teams are focused on getting their product to work and get it out the door. An engineering team will often not know how to quantify what to expect in the customer environment and how to test for those expected disturbances. We design hundreds of sensitivity tests and can explain the important concepts to consider in designing the tests and making the right measurements that allow for sound estimates of system response. This is an important expertise we use in our vibration, acoustic and magnetic field consulting services.
Sensitivity Testing and Analysis
The Specification: Contract With Customer - The frequency dependent sensitivity estimates must often be translated in to a specification for the customer. The vibration, acoustic, and magnetic field specification serves as an understanding between the manufacturer and the customer. If the tool has a performance problem and the site no longer meets the specification then the customer may take responsibility to analyze the issue and fix the facility problem. If the tool has a performance problem due to a facility disturbance and yet the levels are within the specifications, then the tool manufacturer may be asked fix the tool, or take the tool back.
We have found that the worst scenario is when the specification is too lax and the disturbance becomes an intermittent problem. At this point the customer has gone through all the preparation work, installed the tool, and gone through training, only to find that the tool does not perform well and that the manufacturer is to blame because they really didn't know the sensitivity of their product. On the other hand, a specification that is too hard to meet may cause the customer to choose a competitor's product that has a less stringent specification.
Thus, we feel that the best approach is to create a realistic specification that will accommodate the widest variety of installation sites and yet still provide confidence that it can be used to reject the sites that really would result in poor tool performance and unhappy customers. To accomplish this, thorough testing must be done on multiple tools to insure that the sensitivity measured is representative of the actual population of manufactured tools, given system variations, critical sensitive structures and processes.
Sensitivity testing for the following is explained below:
Floor Vibration Sensitivity Testing - Floor vibration sensitivity testing, analysis, and specification development require an understanding of the way vibration propagates in slab-on-grade floors and elevated floors systems. Elevated floor systems behave very differently that slab-on-grade floor systems. We have a number of methods for estimating this sensitivity where the floor is massive and stiff and vibration sources are assumed to be far from the tool. We have more involved tests and calcs for nearby sources that produce gradients. Estimates of horizontal floor vibration sensitivity and sensitivity to floor rotation are more challenging and we have methodologies that have proven to produce sound estimates of horizontal and rotational sensitivity to both upper floor, and slab-on-grade floor vibration. In a nut shell, the floor vibration sensitivity is an estimate that involves the predicted (or measured) floor dynamics at the customer site and a very sound experimentally derived estimate of the tool (or system) sensitivity.
Floor Vibration Specification Developed By Response Dynamics Using On-Site Testing and Analysis
The floor vibration sensitivity for upper floors often requires other methods of testing and present a different set of challenges. We have performed testing on many types of upper floor systems and understand the unique dynamics of these systems. An upper floor is both more flexible, and more resilient, and the coupling of the floor system is very important to consider.
Often customers will ask if they can just put their tool on a shake table. The answer depends on how well the shake table can control off axis response to the inertial forces of the payload, and how sensitive the system is to the other axes. Note that the floor often serves as a structural member, often unintended. The floor does not rotate as easily as a shake table and thus the response of the system under test to vertical vibration on a shake table will be very different than when it is supported by a slab-on-grade floor, and different again when mounted to an upper floor span where rotation and vertical displacement are coupled.
We enjoy this work and are happy to talk about specifics regarding isolation systems, various considerations involving various floor systems, and the inherent sensitivity of the product given its structure.
Acoustic Noise Sensitivity - Sensitivity to acoustic noise is very important and is often not well understood. Acoustic noise problems are often misattributed to floor vibration. Indeed, the "Achilles Heal" of many high resolution tools, with good vibration isolation, is the mechanical vibration driven by acoustic noise. The structural dynamics and geometry of the isolated structure determine its acoustic sensitivity. We understand this puzzle and have the tools and experience to engineer solutions.
Acoustic Sensitivity Frequency Response Functions,
On-Site Testing Performed By Response Dynamics to Determine Acoustic Enclosure Design Requirements
Acoustic sensitivity testing is often not performed, and when it is, it is often done poorly. Often a total SPL level is given that may match the tool manufacturing site, with no consideration to the frequency dependence of the tool's sensitivity.
Response Dynamics has developed testing methodologies to give the best estimates of acoustic sensitivity we have seen in our 30-plus years of work. We understand the challenges to testing in a semi-reverberant environment, and how to mitigate and take into account the many acoustic room modes that will effect the testing.
We have performed these tests on many high resolution tools including:
Scanning Electron Microscopes
Atomic Force Microscopes
Precision Timing Electronics
Magnetic Field Sensitivity Testing - Sensitivity to stray magnetic fields is important for tools the using ion beams for process and inspection. We have instrumentation to measure AC and DC fields from nano-Gauss levels to hundreds of Tesla. We design the right custom coils, amplifiers and geometries for the testing space and application. We enjoy discussing magnetic field applications and have the equipment and expertise to perform testing quickly and produce meaningful results.
Magnetic Field Lines of a Toroid, Scanning Electron Microscope Image Showing Stray Magnetic Field Disturbance
We have been working with magnetic field issues for sensitive tools for many decades from cutting edge development of scanning electron microscopes to active cancellation systems for MRI tools, to site surveys for specification compliance, debugging, and tool magnetic sensitivity testing.
We have worked for many years on the development of new scanning electron microscopes (SEMs). A SEM image disturbance will often be caused by a combination of environmental vibration, acoustic noise, and stray magnetic fields, as well as on board sources, such as fan noise, fan imbalance, stage transients, stage control related disturbances (servo instability) and field from stray grounding currents.
These mechanical disturbances are filtered by the structural dynamics of the system and combine with magnetically induced disturbances of the electron beam and result in very confounding image problems.
We have made the task of untangling these issues our work for many decades. Our extensive structural dynamics and acoustics experience is indispensable to our ability to sort out cause and effect from these very different disturbance source types. A very firm grasp of basic physics allow us a broad range of problem solving ability on real systems (see Diagnostic Testing for more details).
We provide site analysis to insure that magnetically sensitive tools will perform well prior to installation. We typically look at floor and pedestal vibration, acoustics noise, and magnetic fields when performing a site survey for a SEM, or electron beam based tool (see Site Evaluation for more details).
In the last 10 years MRI installations have become more common place and we have performed numerous site surveys for these sensitive imaging tools. In some instances we have created custom active cancellation systems to correct a magnetic field problem that could not be resolved by other means. Many spaces where a SEM or MRI tool can be placed have unique source and/or steel framing geometries that necessitate a custom cancellation geometry to effectively cancel a disturbing field.
We consider the concepts of magnetic flux path analysis and source geometry to debug and troubleshoot magnetic field issues. Mu metal shielding, flux containment, electrical current routing, and active cancellation are the tools we use to mitigate magnetic disturbances.