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Thread: FYI Galvo Bench Testing.

  1. #1
    mixedgas's Avatar
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    Default FYI Galvo Bench Testing.

    Before reading, note the following requires a well equipped electronics lab to achieve the results required.
    This is not something you can do without fairly precise gear and advanced planning. This was brought on by receiving a "gift" of three used 6800s
    in varying states of wear.

    Steve's Bearing test:

    Requirements:

    Precision adjustable bench supply capable of at least 4 amps and 30 volts for both supply rails.
    Supply shall have voltage and current meters for each rail. **

    Dual trace, DC accurate, Oscilloscope
    Lab Grad signal generator capable of sine, square and triangle waves.
    Small bench laser, such as a HENE.
    Piece of poster paper you can roll up and keep.
    Galvo amp that is paired with and tuned for galvo under test (GUT)
    Tape measure.
    4 digit or better Digital Voltmeter
    Optional lab grade audio spectrum analyzer.

    Caveats:
    A. The test method assumes no tuning changes are made to the amplifier during use, or that a bench amplifier of known tuning is dedicated to the test.
    B. The tester must ensure that the test conditions are repeatable. This especially applies to the signal generator used. In this case a programmable Direct Digital Synthesis
    waveform generator was used.




    1. Properly aim the test laser at the GUT and aim it onto a piece of poster paper a known, measured, distance (3 meters at least)
    The GUT must be rigidly mounted and properly heatsinked on the test bench.

    2. Ensure the galvo amp is properly heatsinked. Connect the galvo amp to the power supplies. Set the supply voltages to a precisely known voltage within the operating range of the Galvo amp, verify this voltage with the DVM and note it for your records. Ground the inputs to the amp, enable the laser and set the
    Galvo to be centered on the poster paper. Mark zero with a pencil.

    3. Input a small signal precision 30 HZ square wave and note the current consumption of the amplifier at resting (no scan), 8 degrees, and full angle.
    At this time you should be monitoring the position sensor feedback signal on the scope, looking for ringing on the leading edge of the square wave.
    Mark where the dots land on the piece of poster paper with a pencil and label them with the galvo's serial number. Switch the signal generator to sine and triangle and look for distortion on the position sensor outputs. Place the command signal on the lower trace
    for comparison. Measure the peak to peak voltage of the test waveform. Take a camera picture or screen shot of the position sensor waveforms. Record the settings of the oscilloscope.

    4. Switch to a high frequency sine wave, say 70-120 Hz and repeat data taking, measuring supply currents.

    5. If a lab grade spectrum analyzer is available, apply a scaled position sensor signal to the input and record the spectral data. Note the settings of the test device and insure its input is not saturated.

    6. If possible without damaging amplifier, remove the output fuse, rotate the mirror by hand, and watch the GUT position sensor voltage on the scope
    while noting the "feel" of the bearings. (Subjective test, not quantifiable)

    7. Roll up the calibration poster as well as the results sheet and retain them for future use.

    Rationale: As the bearings wear, the current required to scan a known waveform increases. A precise measurement and test scheme allows the determination of the wear. The average current used by the galvo is directly proportional to the health of the galvo. If done upon receiving the new galvo/amp pair, a reference point is established. The use of precision instruments enables the tester to see the wear over time. Care is required to use a good power supply setup with a high degree of "stiffness" and regulation for the test. A cheap switcher commonly used in projectors may have poor regulation.

    Examples: Cambridge 6800 in decent shape, scanning a small angle square wave. Results: positive rail, 140 mA at 24.1 VDC, negative rail, 120 mA at -24.1 VDC. The 20 mA difference in the rails is the position sensor LED current. Same model 6800 with a bad bearing, 220 mA positive rail at 24.1 VDC and 200 mA negative rail at -24.1 VDC.
    Third 6800, labeled "distorted" by user. Shows a marked "bump" on the triangle wave positive slope as the error in the bearing causes a need for more current at the damaged point. The same galvo showed a lack of symmetry on the peaks of a sine wave at 70 Hz.

    To conclude, Using a calibrated bench setup enables characterization of a new scanner/amplifier pair when new. Significant bearing failure may be quantified by the increase in amplifier current during the use of the device. Repeatable bearing errors were evident in the position sensor output that may not be visible during the scanning of a test pattern, were readily visible on the oscilloscope. Future work allowing the measurement of peak currents at specific points of a waveform may allow further insight into the condition of the Galvo.

    ** A current sensing resistor and low pass filter design will be published later in this thread for power supplies without the average current measurement function.


    Steve
    Last edited by mixedgas; 04-08-2014 at 07:02.
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  2. #2
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    Default

    This is superb; many thanks Steve for taking the time to publish this FOC.
    Looking forward to the next post.

    Dan
    - There is no such word as "can't" -
    - 60% of the time it works every time -

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