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Phase shift in cabling?

I'm working with linear hydraulic actuators which have multiple LVDTs (for the ram and main control valve) and I have a spec for allowed phase shift from primary to secondary for the transformers.

I'm trying to make sense of the Labview code I've been given for measuring this parameter and have found an unexplained hardcoded offset value related to this phase measurement. My analog skills are down so I feel like I'm missing something obvious.

The setup is: The excitation board is connected to the actuator via a ~25ft cable. Excitation is ~1900Hz at 5V. If I probe either near the excitation board or near the actuator, I get similar measurements for a couple of different parts that are in the 3-7 degrees range, well within the +- 10 degree spec. The problem is, the hardcoded offset is

5 degrees so some parts pass and some parts fail.

I assume the cabling can impart some sort of delay thanks to capacitance/inductance of the line and perhaps this is where the offset is coming from, but I'd like to understand if this is correct and how I can characterize this to calibrate my measurement.

Reply to
DigitalPlease
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Come on ...

The free space delay is roughly 1 ns / ft. In most common cables it will be about 1.5 time as much. For your cable, it is less than 40 ns there and another back. How big phase shift is this at 1900 Hz?

--

Tauno Voipio, MSEE (and OH2UG for nearly 50 years)
tauno voipio (at) iki fi
Reply to
Tauno Voipio

The stray capacitance of a cable can be substantial: for OP's 25 ft it could be in ~nanofarad range. This capacitance combined with impedances and crosstalks could make all kinds of peculiar phase shifts.

Vladimir Vassilevsky DSP and Mixed Signal Design Consultant

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Reply to
Vladimir Vassilevsky

That logic would apply if the source were impedance matched to the cable. But a typical LVDT has tons of leakage inductance and series resistance in its transformer -- it is, after all, a sensor, not a power conversion device. All this leads to a fairly high source impedance.

So a more accurate model is to look at the cable capacitance working against the LVDT source impedance -- do this, and you'll see that the phase shift can get severe.

--
Tim Wescott
Control system and signal processing consulting
www.wescottdesign.com
Reply to
Tim Wescott

Could it be that there is some analog filtering before the A2D that causes a

5 degree phase shift at 1.9Khz?
Reply to
mook johnson

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I've pulled the part off the stand and hooked a function generator to the primary and am measuring with a fairly nice Tek. The numbers I was quoting earlier were for a single leg of the secondary. The current setup is using three probes, one on the primary, Va, and Vb. I then use the built in math function to generate Va-Vb and measure the phase relative to that. I get the secondary about 8 degrees leading unloaded or with a voltage divider of 5k/65k (its target application has this on the output) on each leg.

Is this the appropriate way to measure phase shift or should I load the output in some way? Calls/emails to the mfg about how they are doing it haven't been returned yet.

Reply to
DigitalPlease

LVDTs

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offset is

causes a

Set up a test jig as close to actual use as possible, measure at the same places as actual use.

Reply to
JosephKK

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