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.

                                                          

 We can clearly see from the graph that the output obtained was linear.

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.

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 3^rd and 4^th 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“.

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.

 The above table shows the value of hysteresis that was observed during the experiment. This might be due to wiper shifting due to cable stretch change.

Drift

The values have been observed to change with respect to time.

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)