EE3901/EE5901 Sensor Technologies

Practical 4 — Eddy current sensors

Equipment

You will need the following items:

1. An eddy current sensor and amplifier circuit (Figure 1),
2. Conductive surfaces (steel or aluminium sheets),
3. Thin plastic shims (RS Components stock number 681-407),
4. Bench power supply, and
5. Multimeter.

Experimental measurements

1. Connect the sensor to the amplifier.
2. Set up and connect a DC power supply to the amplifier circuit. The amplifier requires a $-24$ V supply. Please pay careful attention to the voltage polarity to avoid damaging the amplifier. The pinout is:

Pin	Meaning	Comments
OUT	Voltage output	Measure with multimeter.
COM	Common (i.e. the zero volt reference for both OUT and -24V)	Connect to the +24 V side of your DC supply, and also to the multimeter ground.
-24V	Negative voltage power supply	Connect to the 0 V side of your DC supply.

3. Remove the white plastic cover from the eddy current sensor.
4. Choose a type of conductor (steel or aluminium). Monitor the voltage on the amplifier OUT pin when you bring the sensor in proximity to the conductor.
5. Place one of the plastic shims between the eddy current sensor and the conductor. Gently press the sensor into the shim so that the distance to the conductor is controlled by the thickness of the shim. Notice that the voltage will vary depending upon the shim’s thickness.
6. Complete the table below with your measured voltages:

Shim colour	Thickness (inches)	Thickness (mm)	Voltage (V)
Blue	0.002	0.05
Green	0.003	0.08
Orange	0.004	0.1
Brown	0.005	0.13
Transparent white	0.0075	0.19
White	0.010	0.25
Red	0.015	0.38
Yellow	0.02	0.51

Note that your voltages will depend upon the type of conductor that you selected.

7. When you are finished, reattach the white plastic cover to the sensor, and return the equipment to the box where you found it.

Data analysis

The amplifiers have an excitation frequency of $f=125$ kHz. Look up material properties for your choice of conductor and calculate the skin depth. How does this relate to the conductive pieces you were using?

Plot your measurements (i.e. voltage vs distance from conductor) and comment on the functional form.

Fit a linear function to your measurements to obtain the transfer function for distance measurements. Hint: you can use Excel or Matlab or another software package with curve fitting functionality.

Assessment criteria

You will be awarded a binary mark (1 or 0) based upon completion of the task. To receive a mark of 1, you must:

1. Plot the measured transfer function.
2. Obtain a trend line that matches the measurements.

If you are not able to complete the task within the scheduled practical session, you have 7 days to complete the above items and submit evidence (photos, etc) by email to your lecturer.