Control System Components
Introduction
This project focuses on the analysis and comparison of
Linear Variable Differential Transformer or LVDT (DC/DC) against Cable Position
Transducer or CPT. In this project, an experiment was done on the CPT to find
its accuracy, drift, and hysteresis. And to compare it with the manufacturer.
Types of LVDT
AC/AC
DC/DC
Frequency Based
Working of LVDT
LVDT is an electrical transducer that is used to measure
displacement. They have low hysteresis and excellent repeatability. LVDT
converts linear displacement into proportional electrical signals containing
phase for direction and amplitude.
The LVDT has 3 solenoid coils placed in the tube. The center
coil is primary and the other two are secondary. A ferromagnetic core is inside
these solenoids which move forward and backward due to which a voltage is
induced in the secondary coil and the voltage output is created. The LVDT is
designed with long slender coils to make the output voltage essentially linear
over displacement up to several inches long.
LVDT can be used as an absolute position sensor. Even if the
power is switched off, when restarted it shows the same measurement. The
biggest advantage of LVDT is repeatability. The sliding core does not touch the
inside of the tube the movement of the core is frictionless and accurate.
DC/DC LVDT vs AC/AC LVDT
The advantages of using DC LVDT over AC LVDT are
1. Pre-calibrated
analog and/or digital output
2. eliminate reliance on amplification equipment
3. Integrated error compensation
4. lower overall system cost
5. setup time reduced
Technical specifications of LVDT: - NEWTEK SENSORS NT-HE-750
Series
1. welded hermetically sealed steel housing
2. measurement from 0.1 to 20 inches
3. linearity +/-0.25% of full range max
4. Plug & Play 0-10VDC
Electrical specification
Input power: - 11- 30 VDC
Full scale output: - 0 – 10 VDC
Linearity: - +/- 0.25% of Full range Max
Hysteresis: - <0.01% of full range
Repeatability: - <0.01% full range
Mechanical Life: - 20 million strokes
Comparison with CPT (firstmark controls) series 150
subminiature
Supply Voltage: - 35 VDC max
Hysteresis: - ±0.001%
Non-Linearity: - ±0.25%
Mechanical Life: - 5
million shaft revolutions min
Experiment: Analysis of CPT to obtain values of resistance relative to the distance of the cable
Setup: - this experiment consisted of using a C-clamp to fix
the CPT on the flat surface. The cable was tied to the scriber. The 3 terminals
of the CPT out of which the input was covered up with the tape so that it would
not ground. A multimeter was set to measure the resistance. The hole of the cable
at the CPT housing was made sure to be perpendicular to the cable knot.
Analysis: The experiment was initiated with an initial value of 4.583KΩ. below is the graph of values that were obtained.
|
Displacement
(Inches) |
Resistance |
|
0 |
4.583KΩ |
|
0.1 |
4.398KΩ |
|
0.4 |
3.574KΩ |
|
0.5 |
3.294KΩ |
|
0.7 |
2.737KΩ |
|
1 |
1.897KΩ |
|
1.1 |
1.616KΩ |
|
1.6 |
445.6Ω |
But when the scriber arm was released back to the zero
position of the cable a sag was observed. And the CPT cable reached the end at 0.002”
which showed that the cable knot was displaced during the experiment.
A second trial was conducted by tying the knot correctly.
|
trial
2 |
Column1 |
|
Displacement
(In) |
resistance |
|
0 |
4.59KΩ |
|
0.1 |
4.438KΩ |
|
0.4 |
3.613KΩ |
|
0.5 |
3.338KΩ |
|
0.7 |
2.782KΩ |
|
1 |
1.946KΩ |
|
1.1 |
1.666KΩ |
|
1.6 |
374.2Ω |
From the above Table, it can be observed that the values vary a lot if the string is loosened by 0.002”.
Error value of 19% was observed between resistance values when
the cable was displaced by 1.6”.
To mitigate this problem a 3rd and 4th trial were conducted in which the 0 on the scriber was set when the string was lifted at 0.05” and was limited to maximum displacement to 1.2“.
|
trial 3 |
Column1 |
|
displacement |
resistance |
|
0 |
4.571 KΩ |
|
0.1 |
4.315 KΩ |
|
0.4 |
3.492 KΩ |
|
0.5 |
3.208 KΩ |
|
0.8 |
2.362 KΩ |
|
1 |
1.815 KΩ |
|
1.2 |
1.255 KΩ |
|
trial
4 |
Column1 |
|
displacement |
resistance |
|
0 |
4.577
KΩ |
|
0.1 |
4.316
KΩ |
|
0.4 |
3.492
KΩ |
|
0.5 |
3.208
KΩ |
|
0.8 |
2.37
KΩ |
|
1 |
1.814
KΩ |
|
1.2 |
1.255
KΩ |
And from the resistance values obtained we can see that the difference between both the resistance values is highly accurate.
Hysteresis
Hysteresis was calculated using a method in which we will use positions A, B, C, and D. Measurements should first be taken in the order of A, B, C, and D. After those measurements, the direction should be reversed, and measurements should be taken in the order of D, C, B, and A. The error can be calculated by comparing the measurements at each point.
|
Displacement |
Resistance |
Resistance2 |
Hysteresis error |
|
0 |
4.218 |
4.222 |
-0.004 |
|
0.1 |
3.952 |
3.944 |
0.008 |
|
0.5 |
2.836 |
2.836 |
0 |
|
0.8 |
2.012 |
2.005 |
0.007 |
|
1 |
1.451 |
1.446 |
0.005 |
|
1.2 |
0.893 |
0.892 |
0.001 |
|
Avg hysteresis error |
0.002833333 |
Drift
The values have been observed to change with respect to time.
|
Drift with
time |
resistance
at multiple time intervals |
30 sec |
1 minute |
2 minutes |
|
distance
selected (Inch) |
initially |
|||
|
0.2 |
3.676 |
3.672 |
3.672 |
3.671 |
|
0.5 |
2.837 |
2.833 |
2.833 |
2.832 |
|
1 |
1.454 |
1.45 |
1.449 |
1.448 |
|
1.2 |
0.894 |
0.891 |
0.891 |
0.89 |
From the above table, we observe that there is a major
difference with values when taken initially and at 30 seconds. This can be due
to the wiper shifting or due to temperature.
Conclusion
As the results obtained from the experiment show that the
CPT is prone to the movement of the cable and if it moves even by a fraction the
entire reading gets flawed and changes the output by 19%. To mitigate this
problem two methods can be used. The first is to lift the string pot at a certain
small distance and avoid maxing out the cable length which would disturb the
tied knot. The other method is to use a slot or a place to hold the cable on
to. Also, special care needs to be taken about how much hysteresis error
can be compensated from the Arduino which registers all the displacement
values.
As it is a mechanical type of contact sensor it has various
types of errors that could affect the readings like FREQUENCY RESPONSE, LIFETIME,
CABLE TENSION ON APPLICATION, CABLE INTERFERENCE, CATENARY CURVE ERROR EFFECTS.
Errors like drift and hysteresis could be eliminated by the use
of LVDT which as it is has no physical contact to create resistance.
References
NT-HE-750
Datasheet.pdf (newteksensors.com)
String
Potentiometer and String Encoder Engineering Guide.pdf (firstmarkcontrols.com)
Non
Contacting Linear Position Sensors | Variohm
What
is a LVDT? | linear variable differential transformer (omega.co.uk)
How
sensors work - DC-LVDT displacement transducer (sensorland.com)
Industrial
Sensors: Why Use an AC LVDT versus a DC LVDT Linear Position Sensor? (controldesign.com)
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