From: DigitalPlease on
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.
From: Tauno Voipio on
DigitalPlease wrote:
> 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.

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
From: Vladimir Vassilevsky on


Tauno Voipio wrote:

> DigitalPlease wrote:
>
>> 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.
>
>
> 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?

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
http://www.abvolt.com
From: Tim Wescott on
On 06/03/2010 11:18 AM, Tauno Voipio wrote:
> DigitalPlease wrote:
>> 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.
>
> 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?

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
From: mook johnson on

"DigitalPlease" <mrperfectionest(a)gmail.com> wrote in message
news:5aafaf2e-ad18-419b-8141-54734805c857(a)w3g2000vbd.googlegroups.com...
> 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.

Could it be that there is some analog filtering before the A2D that causes a
5 degree phase shift at 1.9Khz?