DJB Instruments

You are here: Products Instrumentation


E-mail Print PDF

Data derived from piezo-electric transducers is in the form of an electric charge analogous to some dynamic physical parameter, and must be further processed in order to extract information. The initial task comprises charge to voltage conversion, thus presenting the source signal in amplified, low impedance format.

This process may be further partitioned into charge to current and thence current to voltage conversion for enhanced long distance signal transmission. The broad band data contained in these signals relates to the dynamic behaviour of machinery and structures, fluid flow, turbulence, together with spurious information relating to additional environmentally induced affects and noise.

Further frequency selective processing may therefore be needed to extract specific information for display and control. By way of illustration, vibration data derived from a rotating machine will contain information correlating to bearing condition. This may be extracted from a wideband signal by bandpass filtering and used as a basis for wear diagnosis.

The vast range of applications of analysis, display and control of piezo-electric transducer derived dynamic information gives rise in turn to a multiplicity of analogue and mixed format (analogue/digital) instrumentation options. A specific choice may be somewhat subjective, involving judgements in such areas as vendor support, but cost/benefit criteria are likely to predominate.

The modular instrumentation described in this section is, in essence, a partitioning of the hardware means of extracting realtime information relating to vibration and related physical phenomena. Partition bands comprise transducer interface, wave filtering including intergration, display and control. Within each band individual modules offer an additional high degree of flexibility, thus overall is the capabilty of meeting a wide diversity of vibration measurement/control requirements with minimal hardware redundancy and customisation.

04 Series Instrumentation

Comprises a comprehensive family of analogue piezo-electric signal conditioning modules based upon a 2U (3 1/2") height front panel. Module panel width is dictated by the requirements of front panel controls and is an integer multiple of the standard single width (1.4"). The 04 series comprises tranducers interface, wave filtering, level display and comparator modules, together with housings for up to twelve single width panels. The process of module specification is simply that of partitioning the measurement task in terms of a string of module functions. At its most trivial this would comprise just one module. Attention should be given to environmental factors that effect choice of transducer amplifiers and interface amplifier.

Transducer Interface Amplifiers

Charge amplifiers convert piezo-electric charge to low impedance voltage; conversion gain is unaffected by transducer connecting lead capacitance ; transducer charge sensitivity entered as gain control parameters calibrates (normalises) output ; additional range amplifier increases controllable dynamic range to 90 dB (1V/pC max); range comprises low noise, extended low frequency variant CA/04/N, CA/04/NL, CA/04/NH, CA/04/EH, (0.1Hz min), CA/04/D, CA/04/DH, (differential input), and CA/04/V which includes an integrator for conversion of periodic acceleration to velocity. Long distance transmission of piezo-electric signals is subject to noise degradation that may be sufficient to seriously impair data integrity, amplification local to the transducer mitigates the problem. W/04 interface two wire charge/voltage (QVC) transducer signal boosters respectively, combining co-incident booster power source and signal extraction, together with normalising ranging and fault Diagnosis.

Wave Filters, Integrators

All filters have unity passband gain, integrators include a high pass noise limiting filter, Resultant integrator output (velocity, displacement) is valid for steady and quasi steady state periodic signal inputs only, fundamental frequency > filter corner frequency. Signal integrator VM/04 and double integrator DM/04 convert periodic acceleration to velocity, displacement format respectively, have 50dB switched gain variation, and are available in a wide range of scaling options. Filters include HP/04 highpass, LP/04 lowpass, BP/04 fixed frequency bandpass, and TF/04 manually tuneable bandpass. Both HP/04 and LP/04 are switch tuneable over a 50:1 frequency range ; BP/04 is intended as an application specific noise reduction filter. ; TF/04 allows manual spectral analysis.


Analogue/digital meters for signal levels display, level comparator for vibration severity, detection/control. Analogue meters MU/04, MU/04/E and MU/04/L display true rms level. MU/04/L also includes logarithmic compressor giving optional 60dB dynamic range readout. DD/04 and variants are autoranging 3.5 digit true rms LCD displays with extended low frequency response, PM/04 is a peak hold meter with digital readout. LA/04 level comparator switches state when vibration severity exceeds setpoint for a specific time interval. Timeout constraint prevents spurious activation.C

Input and Inter Module Switching

Input and inter module switching is catered for by the IS/04 and OS/04 range of switch modules.

Module Housings

These contain module backplane, rear panel mounted input/output signal connectors, power supply. Modules may also have intermediate outputs, may be cascaded. All outputs, not only final output(s), are, subject to availability of space, observable. Power supply options include 230/110V. 50/60 Hz mains, combined mains/internal rechargeable battery. External DC supplies 12/15V @ 24/28V via DC/DC converters. Housings accomodate from 2 to 12 single width modules.