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Difference between revisions of "NVGate Torsional"
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Add section: setting Pulse/rev and Missing teeth for a fractional gap (measured guide); fix duplicated paragraph
(Add section: setting Pulse/rev and Missing teeth for a fractional gap (measured guide); fix duplicated paragraph) |
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<font color="#0070C0"><u>''''' Piano tape junction for fractional missing pulse management'''''</u></font> | <font color="#0070C0"><u>''''' Piano tape junction for fractional missing pulse management'''''</u></font> | ||
The empty area (missing teeth) length at the piano tape junction must be included between 1 and 3 teeth. The number of missing teeth must be set to the | The empty area (missing teeth) length at the piano tape junction must be included between 1 and 3 teeth. The number of missing teeth must be set to the '''integer part''' of the gap length: in this example (gap of 2.x teeth): 2. See [[#Setting Pulse/rev and Missing teeth for a fractional gap|Setting Pulse/rev and Missing teeth for a fractional gap]] below. | ||
Note: the total number of teeth includes the missing teeth. By example, an encoder with '''24 pulse/rev''' and '''1 missing teeth''' must be set as : | Note: the total number of teeth includes the missing teeth. By example, an encoder with '''24 pulse/rev''' and '''1 missing teeth''' must be set as : | ||
[[Image:Torsional_Setup_With_MissTeeth.png|700px|none]]<br> | [[Image:Torsional_Setup_With_MissTeeth.png|700px|none]]<br> | ||
[[Image:front_end_22.png|framed|none]] | [[Image:front_end_22.png|framed|none]] | ||
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'''It is recommended to set the instrumentation in order to limit the number of missing pulse below 3s''': Indeed long interval without pulses will lead to a strong order 1 artefact in the instantaneous speed signal due to interpolation of sample. Remaining below 3s for the missing pulse provide correct measurements. | '''It is recommended to set the instrumentation in order to limit the number of missing pulse below 3s''': Indeed long interval without pulses will lead to a strong order 1 artefact in the instantaneous speed signal due to interpolation of sample. Remaining below 3s for the missing pulse provide correct measurements. | ||
====Setting Pulse/rev and Missing teeth for a fractional gap==== | |||
With a piano tape, the gap at the junction is never a whole number of teeth: it is, for example, 2.4 teeth, never exactly 2 or 3. NVGate handles such a fractional gap perfectly, '''provided that the two settings are chosen as described below'''. A wrong choice gives no warning message, and the error on the speed can be small enough to go unnoticed. | |||
[[File:Torsional_piano_tape_gap.png|800px|none]] | |||
<font color="#0070C0">''Piano tape with P = 58 pulses present per revolution and a gap of g = 2.4 teeth''</font> | |||
=====What the two settings mean===== | |||
{|class="wikitable" style="width:100%;" | |||
!Setting!!Meaning | |||
|- | |||
|'''Pulse / rev''' (N)||Total number of tooth positions on one revolution, '''missing teeth included'''. | |||
|- | |||
|'''Missing teeth''' (m)||Number of missing teeth (the gap). | |||
|- | |||
|'''N − m'''||Number of pulses '''actually present''' on one revolution (P). This is the decisive value: it must match the real number of pulses. | |||
|} | |||
=====Setting rule===== | |||
{|class="wikitable" style="width:100%;" | |||
|style="background:#E8F0FA;"| | |||
# '''Count P''', the number of pulses actually present on one revolution (black stripes of the tape, or pulses between two gaps on a recording). | |||
# '''Measure g''', the gap length in teeth: '''g = (gap interval ÷ normal interval) − 1'''. | |||
# '''Missing teeth = integer part of g'''. If g is within 0.1 of a whole number, use that whole number. | |||
# '''Pulse/rev = P + Missing teeth''' — not the "nominal" number of stripes of the tape. | |||
|} | |||
'''Example''': 58 pulses present and a gap interval 3.4 times the normal interval → g = 2.4 → '''Missing teeth = 2''' and '''Pulse/rev = 58 + 2 = 60'''. | |||
=====How to measure P and g on site===== | |||
# Run the shaft at a roughly constant speed and record the Ext. sync. track for a few revolutions. | |||
# On the recording, read the normal interval between two pulses and the interval across the gap. Compute g = (gap interval ÷ normal interval) − 1. Example: 3.4 ms ÷ 1.0 ms − 1 = 2.4 teeth. | |||
# Count the pulses between two gaps: this is P. | |||
# Check the result: at constant speed the Tors signal must be flat (no spike once per revolution), and its mean value must equal the speed given by another sensor (or by the same input in Tach mode). | |||
=====What happens with a wrong setting===== | |||
The following results were measured with NVGate 18.00.003 on an OR35, with synthetic pulse trains whose pulse instants are exactly known (58 pulses present per revolution, 1200 RPM constant, gap g from 0.6 to 4.0 teeth, 63 cases). Results are identical online (Ext. sync. input) and in post-processing (Signal Op.). | |||
[[File:Torsional_missing_teeth_map.png|900px|none]] | |||
<font color="#0070C0">''Error on the mean speed (%) depending on the actual gap g and on the settings. Colour = worst of mean and peak error. Left: Pulse/rev = P + m. Right: Pulse/rev kept at the nominal value 60.''</font> | |||
{|class="wikitable" style="width:100%;" | |||
!Setting!!Example (g = 2.4, P = 58)!!Result | |||
|- | |||
|style="background:#5EF86D;"|'''Missing teeth = integer part of g''', Pulse/rev = P + m||m = 2, Pulse/rev = 60||Correct in all tested cases (g = 1.0 to 4.0): mean error < 0.002 %, peak error < 0.035 % | |||
|- | |||
|Missing teeth rounded up||m = 3, Pulse/rev = 61||Correct only when the decimal part of g is large enough (1.7 → 2, 2.4 → 3 are correct, but 1.3 → 2 gives −57 %) | |||
|- | |||
|style="background:#FBBCB7;"|One tooth less than the integer part||m = 1, Pulse/rev = 59||Speed '''too high by +1.6 to +2.9 %''', with no visible spike — the most misleading error | |||
|- | |||
|style="background:#FBBCB7;"|One or two teeth too many||m = 4, Pulse/rev = 62||Speed too low by −2 % up to '''−68 %''' (speed divided by two) | |||
|- | |||
|style="background:#FBBCB7;"|Missing teeth = 0 although there is a gap||m = 0, Pulse/rev = 58||Mean speed nearly correct, but a '''spike of 37 to 65 %''' once per revolution | |||
|- | |||
|style="background:#FBBCB7;"|Pulse/rev kept at the nominal value whatever m||Pulse/rev = 60, m = 3||Correct only if Pulse/rev − m equals P. Otherwise +0.7 to +3.6 %, or worse | |||
|} | |||
=====Limits===== | |||
* The gap must be at least 1 tooth long, i.e. the gap interval must be longer than twice the normal interval. With a shorter gap (g = 0.6 tested), no setting gives a correct speed (spikes of 37 to 39 %). | |||
* Near a whole number (g = 1.95, 2.0, 2.05), setting that whole number is correct on both sides; setting the lower whole number fails as soon as the gap exceeds it (g = 2.05 set to 1 → +1.8 %). | |||
* In post-processing, when the ''Tach'' option of Signal Op. is used, check that the ''Missing teeth'' value of the created tachometer (''Tachometer > Ext. tach. 1'') is the same as the Tors one. | |||
===Tutorial === | ===Tutorial === | ||