Table of Contents

Scanning LDV System

CAML owns a single-point Polytec PDV-100 Laser Doppler Vibrometer(LDV). A small additional system has been designed and built by Mark Rau to allow the single-point LDV to be used as a scanning LDV. This page describes the setup and operation of that system.

When used as a single-point system, the LDV is normally mounted on a camera tripod. In the context of measuring string instrument admittances, the instrument is typically excited with one of our miniature impact hammers (PCB Model 086E80 or B&K Type 8203), either of which can be mounted in a pendulum mechanism, a motorized impact hammer system, or simply handheld.

For the scanning system, the LDV must be attached to a plate, together with the scanning hardware and a microcontroller housing (see images below), and used with a motorized impact hammer system. The measurement process is controlled through a Matlab script that coordinates moving the laser to a desired measurement point, initiating one or more hammer strikes, simultaneously recording the hammer force and LDV signals, possible averaging of signals, and saving of results to a specified directory. The laser beam is then moved to another position and the process repeated.

The scanning system works by reflecting the LDV laser beam off two mirror galvonometers, each of which is attached to a motor control system that allows the mirrors to be rotated to desired angles. Because the resulting distances to the object being measured will vary with position, it is not possible to achieve the same refinement of focus as normal (thus leading to a lower signal to noise level). The system can identify strong vibrational peaks but will be less useful for discerning admittance notches (regions of minimal vibration).

The signal acquisition is currently designed to make use of our National Instruments USB-4431 device, though it could be handled with a calibrated audio interface as well.

Setup

The setup of the system involves:

  1. Attaching the LDV to the wooden plate that holds the motorized mirror assembly and the microcontroller housing. Note that the laser beam is redirected 90 degrees with respect to the LDV housing (the normal beam direction without the scanning system).
  2. Connecting the USB cable from the scanning microcontroller box to a USB-A port on the computer.
  3. Connecting the USB cable from the motorized hammer system to a USB-A port on the computer.
  4. Connecting the USB cable from the NI USB-4431 to the computer (currently through a USB-A to USB-C adaptor, since the CAML laptop only has two USB-A ports).
  5. Connecting the hammer signal cable to the first channel of the NI card using a BNC adaptor.
  6. Connecting the LDV signal output to the second channel of the NI card using a BNC cable.
  7. Attaching and plugging in the power adaptor for the motorized hammer system.
  8. Attaching and plugging in the power adaptor for the LDV.
  9. Attaching and plugging in the power cable for the scanning microcontroller box (but do not power on until all other components are on).
  10. Positioning the object to be measured at an appropriate distance such that the scanning area covers the region of interest.

Operation

The motorized mirror system is controlled by a Teensy 3.6 microcontroller. The system is designed to receive serial commands that specify vertical and horizontal positions within the range 0-4095. A horizontal position value of 0 corresponds to the left-most position and a value of 4095 corresponds to the right-most position. A vertical position value of 0 corresponds to the bottom-most position and a value of 4096 corresponds to the top-most position.

The motorized hammer system consists of a stepper motor that is controlled by an Arduino microcontroller. The system has an attached trigger button that can be used to manually trigger the hammer, though the hammer can also be triggered using a serial command message of “s1”. The system also includes a rotary potentiometer that can be used to adjust the pullback level.

The measurement process is currently controlled using a Matlab script, as we have significant experience controlling the NI data acquisition card using the Matlab data acquisition toolbox. That said, the process could also be controlled through a different environment, such as python, if a library exists to interface with the NI card (or if a normalized audio interface is used instead). NI drivers only exist for the Windows operating system, thus requiring the use of a Windows computer if using the NI card for data acquisition.

Mark has created a short video describing the setup and operation of the scanning LDV, available at this link.

The Matlab scripts to control the scanning are provided here:

The Teensy sketch for the controller is provided here: