ECE2019 Sensors, Signals, and Systems A 2011

Lab #4: Inductive Sensing

Introduction

This lab involves construction of capacitors and measurement of behavior when change in inductance is used as to sense the presence / absence of metallic objects in the magnetic field of an inductive loop.

Materials you will need from TA (to be returned at end of lab period):

Traffic Loop board

30 meters 26AWG solid wire


Prelab

All of your prelab work is to be done in your laboratory notebook and will be checked by a lab TA prior to the start of lab.

P1. Measuring Inductance

In similar fashion to the RC step response from lab 3, one way to measure inductance is to apply a voltage step to an LR circuit with a known resistance R and observe the step response in the time domain. Figure 1 shows one approach using a lowpass LR circuit; the delay tD from the VSTEP transition to the 50% point of the vC waveform can be used to determine the capacitance if R is known. Figure 2 shows another approach using a highpass LR circuit; in this case the pulse width tW at the 50% point of the vR waveform can be used to determine the inductance if R is known.

Figure 1. Step response of LR lowpass.

Figure 2. Step response of LR highpass.

P2. For the circuit of Figure 1, determine the delay time tD for the component values given.

Find a general equation providing L, given R and tD.

For the circuit of Figure 2, determine the pulse width time tW for the component values given.

Find a general equation providing L, given R and tW.


Lab

L1. Loop Inductance

To construct the inductor, wind 30 meters of 26 AWG copper wire on the scale model traffic detector loop form, as shown in Figure 3. It should take around 30 turns.

Be sure to leave enough wire free at each end of the loop so you can wire the inductor leads to your breadboard while allowing room for the ECE2019 Model Vehicle to roll over the loop.

Figure 3. Scale model traffic detector loop.

Using the procedure from prelab, measure the inductance. To avoid excessive power dissipation in the resistor, use a 1V peak value for the square wave step size.

Use both highpass and lowpass techniques.


L2. Inductive Traffic Sensor

While monitoring the step response waveform on the oscilloscope, drive the ECE2019 Model Vehicle (a triumph of mechanical engineering) back and forth over the loop as shown in Figure 4 below.

Figure 4. Approaching the traffic light.

Does the inductance value increase or decrease? By what percentage does the value change?

You can measure the inductance change in either the high-pass and low-pass method.

For the lab signout, show the TA your measured times and calculated inductance values with and without the “vehicle” present.


Lab Writeup

Note: Both Lab 3 and Lab 4 results should be combined in a single writeup.

W1. Inductance Measurement

Present your calculations from part P1, as well as your measured results from part L1. Comment on any interesting aspects of the measured data.

W2. Inductive Traffic Sensor

Present your measured results from part L2. Comment on any interesting aspects of the measured data, in particular the change in inductance from the vehicle-absent to vehicle-absent condition. How big a change is this? How easy is it to detect? Did the inductance increase or decrease, and why?

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