Question and Answer on General Trends in Vibration
Interview with Dave Lally
This month we decided to get an interview on general trends of vibration with Vice President of Engineering at PCB Piezotronics, Dave Lally. Dave has spent +20 years at PCB, with past experiences including Senior Design Engineer, Product Manager of Industrial Monitoring Instrumentation and Marketing Manager. I asked Dave what his observations were on...
Q (Mike): ...the general trends in vibration sensors over the last decade?
A (Dave): Customers continue to request higher performing sensors at a lower price. While that demand places additional strain on sensor manufacturers like PCB, it simply represents another instance where competition and capitalism is alive and well.
Specifically, to answer your question, customers are requesting smaller and lighter weight sensors. This trend typically requires the use of titanium, which is lightweight, durable and welds easily enough to create hermetic packages, as the material of choice for sensor housings. To maintain high sensitivities and a low noise floor, high output piezoceramics are inertially loaded with a specially-formulated and extremely dense tungsten based alloy to create miniature, yet highly sensitive, vibration sensing elements.
If that doesn’t make matters complicated enough, since our world’s insatiable demand for more data often results in higher channel density on the test structures, customers routinely request integrated triaxial accelerometers rather than mounting 3 individual sensors to obtain the orthogonal vibration measurements. From the customer’s perspective, this tactic minimizes mass loading of the structure as well as simplifying cabling since ICP® triaxial sensors will normally use a single cable containing 4 miniature conductors (x,y,z and ground). From a manufacturer’s viewpoint, it requires a robust sensor design process, which includes tools such as Finite Element Analysis, to insure that everything can be stuffed into a tiny package while maintaining durability and important performance parameters, such as sensitivity, frequency response and noise floor. The challenges seem to never end…
Q (Mike): Anything new?
A (Dave): New materials and technologies continue to fuel engineers with ideas to create new product offerings. For example, advancements in MEMS technologies have allowed design engineers to create “nearly indestructible” high g shock accelerometers capable of measuring extreme impact events on the order of 100,000 g’s! While the process to produce the sensor is complex and beyond what can be covered in this interview, I think it is just fascinating to think that silicon wafers can be mechanically etched to repeatedly create mechanical structures which move up and down only 1 micron (about 1/100th the thickness of a human hair) before hitting mechanical stops intended to allow sensor survivability during intense pyroshock events. At the same time, this miniature gap that the sensing element moves within also serves to provide squeeze film damping reducing the amplification (or Q) of the device at resonance further increasing its durability. New MEMS devices have also seen widespread acceptance for low frequency and long duration measurements. With the advances in MEMS miniaturization, it is now possible to create highly sensitive elements and couple them with a custom ASICs to accurately provide a reasonably priced measurement solution for applications like ride quality. While there is still no measurement technology that can touch the combined bandwidth and dynamic range of piezoelectric sensors, MEMS sensors certainly have found their place in the measurement world today.
Q (Mike): What about wireless? Is it ready for primetime in dynamic sensing?
A (Dave): Wow! People have been talking about wireless solutions for nearly two decades now. Is the measurement world ready for wireless sensors in primetime? While I guess that the answer is “yes they are ready”, but unfortunately, too many technology hurdles still exist today which prevent more widespread acceptance.
First and foremost are the size considerations previously discussed. Today, it just isn’t feasible to stuff everything (sensing element, electronics, wireless capability and batteries) into the small sensors typically requested by automotive, aerospace or R&D customers. Additionally, the classic characteristics of wireless sensing are also still an issue for many customers. There simply isn’t a wireless sensor that is able to strike the perfect balance between battery life, size/weight, overall measurement bandwidth and signal transmission distance to meet the majority of measurement applications. Add wireless transmission protocol, IT security concerns and the lack of standardization for formatting the resulting digital data and it becomes clear why there isn’t more widespread acceptance.
With all of that being said, while there may not yet be a “silver bullet,” there are a number of niche applications which are large enough to attract businesses to design and sell customized wireless sensors. For example, PCB has developed a wireless sensor for machinery monitoring / predictive maintenance applications. Why does it work there? Well, for one thing, size typically isn’t an issue. The sensor itself weighs nearly 1 lb (0.5 kg). While that alone would preclude its use for automotive NVH applications, it is hardly noticeable when mounted on the enormous machines found in paper/steel mills, power plants and chemical factories.
Secondly, wireless sensing can take advantage of an economic issue as they eliminate expensive cable runs which can range anywhere from $100/ft in a typical plant to $3,000/ft in a nuclear power generation facility. Our 670 Series Sensors use a proprietary extended range rf technology which eliminates the necessity for “repeaters” in most applications. This technology can transmit over long distances (between ¼ and ½ mile in most plants) with very low power consumption. At 3 measurements per day, the battery life is between 7-10 years! Additionally, to continue to minimize power consumption and maximize signal transmission distances, while the sensor does not send complete time waveforms or FFTs, it does have built in DSP to provide rms high frequency acceleration levels for detecting incipient bearing failures, rms velocity levels up to ~2 kHz for “balance of plant” problems, true peak acceleration for identifying impulsive failures and crest factor for an overall fault severity indication. This type of remote monitoring is intended to provide a signal to the maintenance engineer that some type of problem exists and further on-site evaluation of the machine is warranted. So while wireless sensing may not yet be for everyone, there are certainly target applications which can take advantage of this technology.
Feel free to leave a comment or question. We'd be happy to ask Dave Lally for help with answers. Thank you.