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application note
General measurement::
Noise Measurement
Vibration Measurement
Machine Health Monitoring
Measurement using bridge circuit:
Ground Loop Causing Problem in Measurement

Ground Loop resolved by isolation
50 Hz interference in measured data; input stage damaged in the equipment.....
you may have experienced similar problems in measurement when a few instruments are connected together; or when transducers are installed on a AC powered machine.
A typical measurement problem can easily be illustrated by the example of large machinery or a production line. The following figure depicts a situation where two measurement points (e.g. two motors in a paper mill) are 50 meter apart from each other. A user would most likely place the measurement system in the middle and run cables to the two measurement points.
Typically both devices under test (DUT) are connected to ground and the data acquisition system (DAQ) too is grounded by its mains connection. One would assume that the potential of each ground connection is identical. This assumption is unfortunately not always true, mainly due to load-switching combined with inappropriate ground wiring (too small wire diameter, bad connections, etc.). The potential of the ground connections at the different measurement points can change for a short period of time.

Even though these differences may occur only for a short period of time and with only a few volts potential difference, the effect produced is still significant. When two points with different potential are connected using low-impedance cable (e.g. measurement cable), a current starts to flow and the potential difference is equalized. Assuming a voltage difference of only 1 Volt and a measurement cable with a resistance of 0.1 Ohm, a current of 10 Amps will flow.

This phenomenon is called ground loop and has the potential to damage measurement equipment and DUT as well as to interfere with sensitive measurements. 
One way of avoiding ground loop is using isolated transducer with casing isolated from the signal ground. For full story and different solutions to ground loop problem, please visit Isolation Improves Measurement Result.... at HBM website

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Use of Dynamic Trigger in Noise Monitoring

During the day, the sound of a barking dog may be masked by background noises such as passing cars, whereas during the night, when the background noise is lower, this would more likely be perceived as an annoyance.

When performing unattended noise monitoring using a fixed trigger level which is appropriate for  capturing the higher level events, the dog barks would be missed since they are below the trigger level, as shown in the figure below.

Fixed Trigger Level

Dynamic triggering is a technique in which event triggering tracks the background level; and the event trigger level is equal to the background level plus an offset.

Using the dynamic trigger method, we can select to track the L90 level (background noise) and trigger a noise event when the measured level exceeds the L90 level plus an offset, e.g. 15 dB. This permits the capture of lower level noise events which occur during periods of low background noise, as shown in figure below.

LARGE DC OFFSET After Integration from Acceleration into Velocity or Displacement

You may have experienced problem in large DC offset after integrating accelerometer signal due to the DC offset in the accelerometer. The problem is even more annoying when integating 2 times to displacement. When Overall RMS or Crest Factor are calculated, the DC offset completely distorts the values.

By using High Pass Filter after integration, the DC offset can be removed in subsequent post-processing calculation - RMS, Crest Factor, FFT..... An example is illustrated in the workflow diagram below (using nCode GlyphXE software.)

nCode GlyphXE workflow
Reciprocating Machinery Protection

Typical vibration monitoring techniques that are used on rotating machinery have been unsuccessfully applied to reciprocating engines and compressors for many years. The reason is that many typical faults on reciprocating machinery are characterized by mechanical looseness, which results in impacting or shock events in the machine. Since impacts generally have little effect on the overall vibration level, these faults are not detected at an early stage. As a result, abnormalities are not diagnosed until damage has occurred and it is too late to take simple corrective measures.

The monitoring technology of Reciprocating Machine Protector (RMP) is based upon the detection and counting of mechanical shock events that occur in or near the machine‚Äôs cylinder assembly. The RMP compares the impact vibration against two predefined threshold vibration levels, Alert (low) and Alert (high).
The diagram below uses triangles to indicate impacts over Alert (low). Impacts over Alert (high) are shown as circles, which are significant shocks caused by mechanical looseness. 
IMI Reciprocating Machne Protector

The frequency of occurence of shocks over Alert(high) and Alert(low) are counted separately as N(high) and N(low) respectively. Weighings are applied to these counts but with greater weighting on N(high) due to its higher severity level. Then the two weighted counts are summed up to give the Reciprocating Fault Index (RFI), which is a better and more reliable indication of machine health than the conventional impact transmitters based only on vibration level.

RFI = N (high) * Weighting (high) + N (low) * Weighting (low) + constant

Go to top of page...                    reading full paper of IMI RMP....                        Download RMP Data Sheet  
Diagnosis of slow rotating machine
container crane
cable car
coal mill
Diagnosis of Slow Rotating Machine

Rotating Machine for crane, cable car, escalator, mill...may run in slow speed that makes vibration analysis a challenging job.

The followings are the measurement requirements in general:
  • In order to capture enough cycles of the signal, long sampling duration may be needed (e.g. 16 seconds)
  • High sampling rate (e.g. 50kHz) is used to capture the vibration impact due to e.g. bearing fault (which means 900,000 data samples in 16 sec)
  • To remove the dominated low frequency signal due to rotor, gearbox.., High Pass Filtering is needed (e.g. 500 Hz HP)
Case Story:
A slow rotating rotor (<40 rpm) driven by electric motor through gearbox was found defective in bearing, which leads to high impacts in vibration due to change in  tooth meshing (impacts are detected in figure below using ACMT <Adash Compressed Time>, which can meet the above measurement requirements).
ACMT - bearing failure
Below Figure : Time Signal of good bearing using ACMT <Adash Compressed Time>
ACMT - good bearing
source : ISSN 1392 - 1207. MECHANIKA. 2011. 17(1): 71-77
 Monitoring Bearing Condition by gENV
gENV envelope acceleration
Fault in rolling element bearing will generate vibration spikes, that can be deteced in early stage by measuring Peak value of the bearing vibration (acceleration). However, Peak value is extremely sensitive; and its low repeatability sometime leads to false alarm and wastes time in investigation.

Other methods of detections using bearing resonance frequency to amplify the vibration have similar issue as the Peak method.

gENV represents the energy of the Envelope Acceleration of the bearing vibration, that plays a balance between early fault detection and repeatability. By trending the value of gENV, bearing fault can be detected in early stage so that unscheduled breakdown and secondary damage can be avoided.

Why 3-wire is preferred than 2-wire in 1/4 bridge measurement?

For 1/4 bridge measurement, if 2 wires are connected to the strain gage (refer to the diagram below), the resistances of the 2 cables become part of 1/4 bridge circuit. The cable resistance can cause big offset (especially for long cable) which can be compensated in the amplifier, but not the measurement error due to resistance change in varying temperature.
2-wire in 1/4 bridge measurement
By using 3 wire circuit (refer to diagram below), the change in cable resistance will cause the same variation on both 1/4 bridges (green and yellow); and the balance of resistance maintains. Therefore the ouput voltage (Umeas) is independent of the cable resistance and its variation due to temperature.
3-wire in 1/4 bridge circuit
There is some loss of sensitivity due to the cable resistance, but can be compensated by increase of excitation voltage through feedback control in amplifier.

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Why 6-wire is preferred than 4-wire in full bridge measurement?

Long cable to strain gauge sensor causes measurment error as 'excitation' votage drops due to resistance of long cable. By using 'Sense' leads which do not carry current, the amplifier measures the feedback voltage in real-time; and increases the excitation voltage until the 'sense' voltage reaches the level required. This ensures accuracy in measurement especially when cable resistance changes with varying temperature.
Below is an example of 6-wire full bridge measurement circuit. For quarter-bridge measurement, 4-wire circuit is preferred for the same reason.
6-wire for full bridge measurement
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Why using Carrier Frequency Amplifier in bridge measurement.

In industrial environment, there are strong and frequent electromagnetic interference from the surroundings - line voltage and its harmonics, high frequency pulse... or even cellular phone, Carrier Frequency measurement circuit can systematically and effectively mask the intereference frequencies. By means of Amplitude Modulation, amplifier filters and measures only the signal within the narrow sidebands around the carrier frequency, so interferences, e.g. due to thermal voltages in the circuit are fully eliminated.
Carrier Frequency Amplifier avoid interference in bridge measurement
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HBM Reference Book - Introduction to measurement using Strain Gage
HBM Introduction to Strain Gage Measurement
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