Already before the two young engineers Per V. Brüel and Viggo Kjær graduated as Masters of Electrical Engineering in 1939, they had decided to start a company together. However, they felt it was necessary to learn more and get some practical experience before actually going ahead, so P.V. Brüel became assistant to professor P.O. Pedersen in the newly created laboratory of acoustics while V. Kjær worked as a development engineer in the Danish radio industry.
In 1942 the company Brüel & Kjær was formed. The first instruments were analyzers and generators for the audio frequency range, but already in 1943 the development of vibration transducers started and the first ones were sold.
Early Bender Constructions
Per V. Brüel followed the development within the acoustical field in the Journal of the American Society of Acoustics (J.A.S.A.) until it became unavailable in Europe due to the war.
In one of the last issues, a description of a vibration pick-up using piezoelectric crystals had been given by Benjamin Baumzweiger, and Brüel saw the possibilities in measuring and analysing vibrations as well as sound.
The first sketch of an accelerometer is seen in figure 1. This became Type 4301, of which the first one was sold in 1943, and it was rapidly developed further into Types 4302 and 4303 (figure 2). These accelerometers were all based on Rochelle salt crystals that could be grown from dissolved chemicals. Afterwards, they had to be cut to the right shape and finally joined by glue to form the desired benders.
Ceramics and the Importance of Housing
The main drawbacks of the Rochelle salt-bender accelerometers were that the crystal was destroyed if subjected to:
- Temperatures above 50-55 °C, even for short periods
- Damp air above 85% relative humidity for more than very short periods
- Dry air below 30% relative humidity for extended periods
- Tensile or compressive loads above 15 MN/sq.m
The first of these problems poses severe difficulties, for example, when leaving the transducer in a hot car. The second and third problems were to a large extent overcome by good encapsulation, but the low strength cannot be improved. This meant that the accelerometers could easily be destroyed if dropped.
In the beginning of the fifties it was reported that a new polarised ferroelectric ceramic material, Barium Titanate (BaTiO3), could be used in accelerometers. Shortly after P.V. Brüel and C. G. Wahrman designed a new accelerometer based on BaTiO3, and the new generation of accelerometers, Types 4306 and 4307, were released in 1955.
As can be expected when looking at the cross sectional drawing, the ingenious thought of letting the housing act as protection and a spring gave a very good signal to mass ratio, but also let all external forces act directly on the mass, leading to all kinds of erroneous outputs (however none of the hundreds of customers complained about this!). When this was realised, a project for rapid replacement was started, and already the year after the first accelerometers Type 4308 were sold. Here the housing was made stiff, and an internal spring between the housing and the mass was used to decouple the mass from the housing and provide preload.
Shortly after another new material, the lead titanate zirconate (often now called PZT) became available, and from 1957 the accelerometer family Types 4308-9-10-11 became available.
Centre Bolt and Unigain®
The sixties were a time of rapid expansion for Danish industry, and new accelerometers Types 4332, 4333, 4334, and 4335 were all introduced in 1964. Although they were still of the same basic design, a number of improvements were made.
In the early types, all resonance frequencies were low, and simple clamping of the benders worked well. However, as soon as the resonances were pushed up towards 20 kHz, it became more and more dependent on the contact surfaces between the ceramic discs and the metal parts. Layers of lead that filled the voids between the surfaces were used for some time, also providing some damping of the resonance peak thus reducing the risk of overloading the preamplifiers.
When the operational temperatures and the demand for stability were increased, lead was not a good solution anymore. Therefore the quality of the ceramic discs was improved by surface lapping and vacuum deposition of electrodes. This gave high quality surfaces.
Stainless steel and titanium housings were used, and the machining of the surfaces was improved. A new sintered material comprised of tungsten bonded together by copper was introduced for seismic masses, reducing the volume by a factor of two, compared to the previously used brass.
The data supplied with each transducer was increased considerably, not only was an individual frequency response curve supplied, but also voltage and charge sensitivity, capacitance, and their dependence on temperature up to 250 °C was measured and documented for each accelerometer.
In 1966 the line was complemented by a triaxial unit, Type 4340, which lived for many years, and the miniature Type 4336. For the latter, one of the limitations of the construction principle soon became obvious. When the housing became small and less stiff, the external forces from cable, mounting and sound created noticeable signals at the output.
This led to the consideration of the single ended compression configuration, and before the end of the decade a new miniature accelerometer, Type 4344, was introduced together with two larger ones, Types 4339 and 4343. These types also had a new unique feature, later baptised Uni-Gain®; a sensitivity adjusted to the nominal value ±2%. Simultaneously a new hermetic connector based on alumina and glass sealing was introduced.
The sensitivity adjustment was a rather tedious procedure involving measurement of sensitivity, machining off the seismic mass to reduce its weight, ageing and recalibration until the sensitivity was inside the narrow tolerances. No other manufacturer to date did this on a standard type, without taking into account the adjusted built-in preamplifiers.
The DeltaShear® Decade
At the beginning of the seventies a lot of effort was put into improving the stability and repeatibility of the piezoceramics, the result being the still very popular material called PZ23. Simultaneously, the older types were substituted by types using a centre bolt attached at the top, thus giving very low base-strain sensitivity, though at the expense of a lower resonance frequency.
With the advent of the ANSI S2.11-69 standard, it was also a major task to measure all parameters on the transducers, and it became clear in that process that the compression type transducers had several drawbacks compared to the shear types, which started to be marketed heavily. The main areas of concern were the base strain sensitivity and the temperature transient sensitivity, which could often be seen as a problem when shakers had to be controlled at lower frequencies.
These parameters are especially important for very small transducers, and the first shear construction was introduced in a 0.5 gram transducer type 8307. This used a cylindrical ceramic element (inspired by the competition), and the exotic material Beryllium was used for the base in order to reduce weight.
However, the gluing and clamping used in 8307 was not thought to be feasible for larger accelerometers, and many different configurations were proposed and some tried, but finally the idea of using flat plates around a prismatic centerpost kept in place by a clamping ring matured. The construction required extremely good surfaces, a tight control on the dimensions, a high strength material for the ring and a practical way to put on the ring with the right tension.
In 1974 the patent on the construction, later known as the DeltaShear®, was obtained and the work to transform as many of the accelerometers in the program as possible to this concept got started. The construction has been in use ever since, and many hundreds of thousands have been produced. Today it is regarded as one of the classic accelerometer constructions, also used by some of our competitors.
Towards the end of the seventies the development of electronics reached a stage where it became possible to make small very stable thickfilm preamplifiers to be placed inside normal size accelerometers.
Although these modifications have drawbacks in lowering the maximum temperature, reducing the dynamic range and decreasing the reliability, the market liked the lower channel price and the reduction of cable and noise pick-up problems.
A number of system configurations were considered before the line-drive system was decided upon. This uses a constant line voltage to supply the power to the transducer, which then modulates the current proportional to the input signal. This is basically the same system, that was, and still is, used for most temperature and pressure transmitters. The big advantage of such a system compared to a voltage-modulated system is its much higher immunity to external disturbances.
An extremely compact screw-on (5 gram) preamplifier Type 2644 was released first, and later the same technology was used inside a number of accelerometers.
Because of a number of very large projects taking most of the development capacity the number of input possibilities was not very large, and the simpler and less expensive voltage modulating systems became a defacto industry standard, marketed under a number of different names.
Towards the end of the decade and in the nineties the line-drive system did however get some success. This was in critical monitoring systems requiring very low noise levels and high performance.
FROM Δ TO Θ
Around the turn of the nineties a long recession and loss of eastern markets changed the situation for Brüel & Kjær. A number of smaller companies got more attention because price became a much more important issue in the market.
This was especially true in the modal analysis market in the auto industry, so an effort was made to address this challenge. A critical review of the transducer constructions and new possibilities were undertaken, and a new concept called the ThetaShear®, because it looked like the Greek letter Θ, was born. This was basically an inverted DeltaShear® construction, the seismic mass being in the middle, but with a greatly reduced number of parts. This reduces production costs, and without sacrificing too much of the inherent good parameters of the DeltaShear®, makes it well suited for the modal market. To make it light and still not too expensive, aluminum was used for the housing. Nowadays, titanium is the preferred material for the housings, as seen in the ThetaShear® accelerometers Type 4507 and Type 4508.
Due to the systems used it is required to have high sensitivities for small transducers, and low levels have to be measured, calling for low noise built-in amplifiers. To fulfil this requirement it has been necessary to develop dedicated ASICS (Application Specific Integrated Circuits) with specially designed input transistors giving much better performance than the normal MOSFETs.
The latest addition to the construction concepts is the unique OrthoShear®, which has evolved from the ThetaShear® and permits the use of one seismic mass and one cylindrical piezoelectric element to make measurements in three orthogonal directions as indicated by the name. Examples of devices using OrthoShear® are Type 4506 and Type 4524.
The first decade of the 21st century was one where microelectronics really began to shake the world. For accelerometers this had three main impacts:
- Microprocessors provided serious number-crunching power that could be used to solve finite-element models (FEM)
- Microchips were incoroprated inside the accelerometer casing - not just ASICS
- Micro electro-mechanical systems (MEMS) could be used to construct transducers on chips
Using the computing power offered by PC’s, once the realm of mainframes, it was now possible for Brüel & Kjær engineers to construct and solve finite element models(FEM) in their own lab. This has led to the creation of a huge database of transducer design data, and, through verification of the FEM with measured parameters of built transducers, the ‘quality’ of the mathematical models constantly increases, and less prototypes are needed.
The key feature of the IEEE 1451 standard set, introduced in the nineties, was the definition of Transducer Electronic Data Sheets (TEDS). Realised in practice in this decade, TEDS is a memory device integral within the transducer casing which digitally stores transducer identification, calibration, correction data, measurement range, manufacture-related information, etc. Communication with the TEDS chip is via the signal cables, and during final testing and initial calibration Brüel & Kjær burns in the specific data for every TEDS equipped accelerometer before they leave the factory.
By seeing new technological possibilities and providing innovative new constructions, Brüel & Kjær has played an important role in the history of accelerometers. But we don't envisage our role ending now. Our transducer research and development team continuously scour the technological landscape and horizon for ideas and methods that may be useful for transducer evolution, or possibly revolution. For example, the recent massive advances made in microelectronics, battery technology and wireless communication bode well for the future of transducers and acquisition systems.
At Brüel & Kjær, transducers are a core part of our business. They always have been, for more than 67 years – almost a lifetime! The quality of our transducers is world renowned. It is the result of our unique experience and knowledge, backed up by meticulous testing and quality control. In this article, our aim was to give you an insight into the history of accelerometers as seen from our point of view, and to give you a feel for the heritage that is part of every tried and tested transducer that leaves our factory in Denmark.