ST AN2679 APPLICATION NOTE

AN2679

Application note

Smart inductive proximity switch

Introduction

The STEVAL-IFS006V1 inductive proximity switch demonstration board is designed based
on the principle of metal body detection using the eddy current effect on the HF losses of a
coil. It consists of a single transistor HF oscillator, an ST7LITEUS5 microcontroller and the
TDE1708DFT intelligent power switch. The board is a compact and cost-effective solution
for an inductive proximity sensor designed for simplicity and for a wide temperature range
and supply voltage variations. Other board features include:

■ Great flexibility: the MCU firmware can be modified depending on application

requirements

■ Sensitivity and hysteresis adjustment

■ In-circuit programming and debugging capabilities

■ Analog and digital temperature compensation

■ PNP and NPN sensor functionality configurations

■ Indicator status LED

■ Overload and short-circuit protection

■ GND and Vs open wire protection

■ Compact design

■ Supply voltage: 6 V to 48 VDC

■ Temperature range: -25 °C to +85 °C

Figure 1. Smart inductive proximity switch demonstration board

July 2008 Rev 1 1/16

www.st.com

Contents AN2679

Contents

1 Sensor overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

2 Sensor circuit description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

2.1 Initial configuration and jumper settings . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

2.2 Output driver configurations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

2.3 Application schematic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

2.4 Bill of materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

2.5 ICC connector . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

3 Software implementation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

4 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

5 Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

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AN2679 List of tables

List of tables

Table 1. Initial configuration and jumper settings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

Table 2. Low side (NPN) output driver configuration jumper settings. . . . . . . . . . . . . . . . . . . . . . . . . 7

Table 3. High side (PNP) output driver configuration jumper settings . . . . . . . . . . . . . . . . . . . . . . . . 8

Table 4. Bill of materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

Table 5. Diagnostic LED blinking modes (power up self-test) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

Table 6. Diagnostic LED blinking modes (normal operation) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

Table 7. Document revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

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List of figures AN2679

List of figures

Figure 1. Smart inductive proximity switch demonstration board. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

Figure 2. Smart inductive proximity switch block diagram . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

Figure 3. Initial configuration and jumper settings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

Figure 4. Low side (NPN) output driver configuration jumper settings. . . . . . . . . . . . . . . . . . . . . . . . . 7

Figure 5. High side (PNP) output driver configuration jumper settings . . . . . . . . . . . . . . . . . . . . . . . . 8

Figure 6. Smart inductive proximity switch schematic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

Figure 7. Inducing the demonstration board self-test. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

Figure 8. Oscillator amplitude vs. temperature (MCU pin 3) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

Figure 9. D2 voltage vs. temperature (MCU pin 5) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

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AN2679 Sensor overview

1 Sensor overview

Proximity switches are generally used to sense the position of a moving object in
manufacturing processes. Typically, they utilize an oscillator driver circuit in combination with
an induction tank circuit. The tank circuit includes an induction coil as a means for sensing
the presence of an object such as metal. The magnetic field induces eddy currents in a
conductive object which enters within the generated magnetic field. The oscillation
amplitude is attenuated due to the energy drawn from the induction coil. The amount of the
attenuation is directly related to the distance between the metal object and the induction
coil.

A typical inductive proximity switch employs a ferrite cup core as the sensing element. It
allows the flux field to be focused in front of the cup and to further increase the sensing
distance. The oscillator typically operates between 100 kHz and 800 kHz, where the eddy
current losses are significant.

Some benefits of the MCU approach compared with a traditional solution are:

● more reliable operation thanks to the sensor self-diagnostics

● cheap and easy sensor trimming in the production line

● digital temperature compensation

● linearization of the sensor characteristic

● simple implementation of an analog or PWM output

Figure 2. Smart inductive proximity switch block diagram

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