Noise Troubleshooting

Calculators and Tools

There are two different noise sources that typically cause trouble in vibration testing systems. Ground loop noise and radio frequency (RF) noise can cause a variety of problems in a vibration testing system. RF noise can be addressed by using the jumpers on the front of the VR9500. By placing these jumpers in the connected position the input ground is tied directly to the chassis ground, bypassing the input ground fuse that convert EMI to a voltage. This will reduce the effects of the EMI that is introduced into the system by the input cable.

Ground loop noise is typically a 50/60 Hz spike and is caused when the different components of the system have a different ground potential. This potential difference causes current flow between the components, which is then amplified through the amplifier and sent to the shaker. Often this noise can be felt or heard on the shaker head when the system is powered on. In order to determine the source of the noise issue complete the following steps:

  1. Power down the amplifier, blower, and any hydraulic pumps.
  2. Disconnect the drive cable between the controller and the amplifier.
  3. Disconnect the accelerometers from the controller
  4. Turn off Accel Power in VibrationVIEW, but keep the accelerometer sensitivity set. This will ensure all noise comparisons are done at the same level.
  5. Switch to system check mode. Set the sample rate to 32768 and the number of lines to 32768.  This will give .5 Hz resolution out to 16 kHz.
  6. Bring up a waveform graph and a spectrum graph
  7. Terminate the controller inputs (short the Input pin to the Input shell either with a 50-75 Ohm BNC Terminator or jumper). This will give you the base noise floor of the controller.
  8. Save a data file called ‘base noise floor’
  9. Make a copy of the traces on the waveform and spectrum graphs
  10. Connect the accelerometers to the controller and turn on accel power if needed.
  11. Mount the accelerometers on the shaker. Use an isolation base or some other means to guarantee there is no electrical connection between the accelerometer and the shaker.  Some accelerometers claim to be isolated, but that is achieved by a thin Anodized layer that is easy to scratch and does not last long.
  12. Save a data file called ‘accelerometers connected’
  13. Compare the signal levels to the base noise floor traces.
  14. Sources of noise can include
    • The base noise floor of the accelerometer
    • Emitted interference from other equipment picked up by the accelerometer cable
    • Vibration on the shaker. Either coming through the floor or through the air.
  15. Make a copy of the traces on the waveform and spectrum graphs
  16. Power on any hydraulic pumps
  17. Compare the signal level to the ‘accelerometers connected’
  18. Save a data file called ‘hydraulic pump on’
  19. Make a copy of the traces on the waveform and spectrum graphs
  20. Power on any blower
  21. Compare the signal level to the ‘accelerometers connected’
  22. Save a Data file called ‘blower on’
  23. Make a copy of the traces on the waveform and spectrum graphs
  24. Power on the field to the shaker
  25. Save a data file called ‘field on’
  26. Make a copy of the traces on the waveform and spectrum graphs
  27. Sources of noise can include:
    • If the field is not stable, the armature can vibrate
    • Emitted interference from the field that is picked up by the accelerometer cable.
    • To tell the difference between the two, the following steps can be taken:
      • Feel the shaker head to see if it is vibrating
      • Disconnect the accelerometer from the shaker head but keep the cable in the same location.
      • Short the accelerometer shell to the ground screw on the back of the VR9500 unit. If the source is emitted interference from other equipment, this will reduce it.  If this is the case, there are jumpers that can be installed to bypass the protective fuse and short the shell to ground.  Contact Vibration Research for details.
  28. Put a terminator on the input for the amplifier (Where the controller is usually connected)
  29. Power on the amplifier
  30. Turn the gain all the way down to reset.
  31. Save a data file called ‘Amp on gain reset’
  32. Make a copy of the traces on the waveform and spectrum graphs
  33. Turn the gain all the way up to 100%
  34. Save a data file called ‘Amp on gain 100’
  35. Any noise generated at this stage comes solely from the amplifier. Contact the amplifier manufacturer for further guidance.
  36. Make a copy of the traces on the waveform and spectrum graphs.
  37. Turn the amplifier gain down to reset
  38. Connect the drive of the controller to the input of the amplifier
  39. Turn the gain up to 100%
  40. Save a data file called ‘Controller connected’
  41. Sources of noise include:
    • A ground loop between the controller and the amplifier
    • Amplifier emissions picked up by the accelerometer cable
  42. The most likely cause of noise is a ground loop between the controller and the amplifier. The following steps can be taken to remove this ground loop.
    • Power the controller from the outlet on the amplifier provided for controller.
    • Power the controller and the amplifier from the same outlet
    • Remove the ground pin from the controller power cable and connect the ground screw on the VR9500 to an appropriate ground point on the amplifier.
    • If the amplifier has differential inputs, use the differential output adapter cable and differential output mode on the VR9500.
  43. Submit the following information to Support@VibrationResearch.com for further assistance.
    • Vibration Research Controller Serial Number
    • Manufacturer and Description of Shaker
    • Manufacturer and Description of Amplifier
    • Noise Diagnosis Data Files
    • Base noise floor
    • Accelerometers connected
    • Hydraulic pump on (if applicable)
    • Blower on
    • Field on
    • Amp on gain reset
    • Amp on gain 100
    • Controller connected
    • A detailed description of any other troubleshooting

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