cd00221230

AN2863
Application note
Smart inductive proximity switch with SPT01-335DEE
1
Introduction
The STEVAL-IFS006V2 demonstration board shows an inductive proximity switch 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, a ST7LITEUS5 microcontroller, an
intelligent TDE1708DFT power switch, and an SPT01-335DEE triple Transil™ array. The
board demonstrates a very simple compact and cost-effective solution of an inductive
proximity switch with regard to a wide temperature range, supply voltage variation, and
noise immunity in industrial environments.
Features
■
Metal body detection using the eddy current effect on the HF losses of a coil
■
Good flexibility: MCU firmware can be modified depending on the application
requirements - sensitivity and hysteresis adjustment
■
In-circuit programming and in-circuit debugging capabilities
■
Analog and digital temperature compensation
■
PNP (high-side) sensor functionality configuration
■
Complete Transil protection of the output and power supply lines
■
Overload and short-circuit protection
■
GND and Vs open wire protection
■
Indicator status LED
■
Compact design
■
Supply voltage: 7 V to 32 V DC
■
Temperature range: –25 °C to +85 °C.
Figure 1.
February 2011
Smart inductive proximity switch board
Doc ID 15277 Rev 1
1/13
www.st.com
Contents
AN2863
Contents
1
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
2
Sensor overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
3
Sensor circuit description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
3.1
Initial configuration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
3.2
ICC connector . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
4
Software implementation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
5
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
6
Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
2/13
Doc ID 15277 Rev 1
AN2863
List of tables
List of tables
Table 1.
Table 2.
Table 3.
Table 4.
Bill of material . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Diagnostic LED blinking modes (power up self test). . . . . . .
Diagnostic LED blinking modes (normal operation) . . . . . . .
Document revision history . . . . . . . . . . . . . . . . . . . . . . . . . .
Doc ID 15277 Rev 1
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3/13
List of figures
AN2863
List of figures
Figure 1.
Figure 2.
Figure 3.
Figure 4.
Figure 5.
Figure 6.
Figure 7.
4/13
Smart inductive proximity switch board . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
Smart inductive proximity switch block diagram . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
Initial configuration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
Application schematic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
Demonstration board self test inducing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
Oscillator amplitude vs. temperature (MCU pin 3) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
D2 voltage vs. temperature (MCU pin 5) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
Doc ID 15277 Rev 1
AN2863
2
Sensor overview
Sensor overview
Proximity switches are generally applied to sense the position of a moving object in
manufacturing processes. Typically, they utilize an oscillator drive 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 comes within the generated magnetic field. The oscillation
amplitude is attenuated due to the energy drawn from the induction coil. The amount of the
oscillation 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 further prolong the sensing
distance. The oscillator usually 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 include:
Figure 2.
●
More reliable operation thanks to the sensor self diagnostics
●
Easy and cheap sensor trimming in the production line
●
Digital temperature compensation
●
Linearization of the sensor characteristic
●
Easy realization of an analog or a PWM output.
Smart inductive proximity switch block diagram
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Doc ID 15277 Rev 1
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5/13
Sensor circuit description
3
AN2863
Sensor circuit description
The sensor HF oscillator circuit is based on the Colpitts oscillator type which has a simple
circuit configuration, produces a very clean sinusoidal wave signal, and is capable of
oscillating in a wide range of frequencies. The resonant circuit made up of the inductor L1
and capacitors C12, C9, and C8 determine the frequency of the oscillations according to
Equation 1. The circuit actually oscillates at a slightly lower frequency due to the coupling
capacitor C10, junction capacitances of the transistor Q1, and other stray capacitances.
Equation 1
1
f = -------------------------------------------------------------C C ⎞
⎛
8 9--------------------⎟
2π L 1 ⎜ C 12 + C
⎝
8 + C 9⎠
The oscillator employs a transistor Q1 operating in common base configuration which
derives its feedback from the capacitor divider C9 and C8. Resistors R3 and R6 set its bias
point and diode D2 temperature stabilizes it.
The oscillator signal amplitude is further detected by diodes D4 and D5 and filtered by
capacitor C13. Together with C10, this circuit acts as a charge pump, therefore the full range
of the ST7 ADC converter (0 V - 5 V) is used.
Another function of the diode D2 is temperature sensing. The voltage across a diode
operated at constant current is linear in a very large range of temperatures and reduces with
an increasing temperature with approximately –2 mV/K (see Figure 7). With the ST7 10-bit
ADC converter, the temperature can be measured with the resolution of approximately
2.5 °C which is enough for the overall correction of the sensor temperature variations
(see Figure 6).
The output power stage of the sensor is equipped by the intelligent TDE1708DFT power
switch integrating overload and short-circuit protection, GND and Vs open wire protection,
+5 V linear regulator, indicator status LED driver, and many other functions.
The triple Transil array SPT01-335DEE realizes the overall protection of the sensor. This
device was specially developed for 24 V proximity sensors. It provides a very compact
solution for the efficient protection of bus, output power switch, and power supply reverse
blocking. Demagnetization of an inductive load is another key feature.
3.1
Initial configuration
Figure 3 shows the initial demonstration board configuration in relation to the power supply
and the load. Compared to the previous version of this board (STEVAL-IFS006V1) this new
board only supports the more frequently used PNP (high-side) output driver configuration,
although the intelligent TDE1708DFT power switch and the triple Transil array SPT01335DEE support both. The reason for this is the number of necessary jumpers needed for
functionality change.
6/13
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AN2863
3.2
Sensor circuit description
ICC connector
The board ICC connector offers in-circuit programming and in-circuit debugging capabilities
and therefore simplifies the firmware development. More information about ST7
development tools is available at www.st.com/mcu.
Figure 3.
Initial configuration
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Doc ID 15277 Rev 1
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Sensor circuit description
AN2863
Application schematic
AN2863
Table 1.
Sensor circuit description
Bill of material
Reference
Value
Description
Quantity
Supplier
Order code
U1
TDE1708DFT
Intelligent power switch
1
STMicroelectronics™
TDE1708DFT
U2
ST7LITEUS5
8-bit MCU
1
STMicroelectronics
ST7FLITEUS5U3
U3
SPT01-335DEE
Triple Transil array
1
STMicroelectronics
SPT01-335DEE
C1, C3, C11 10 nF
Capacitor
3
EPCOS
B37931A5103K0
C2
100 nF
Capacitor
1
EPCOS
B37941A5104K0
C4, C5
100 nF
Capacitor
2
EPCOS
B37931K0104K0
C6
10 µF / 6.3 V
Polarized capacitor
1
C7
10 nF
Capacitor
1
EPCOS
B37941A1103K0
C8, C13
1.5 nF
Capacitor
2
EPCOS
B37931A5152K0
C9
100 pF
Capacitor
1
EPCOS
B37930A5101J0
C10
47 pF
Capacitor
1
EPCOS
B37930A5470J0
C12
470 pF
Capacitor
1
EPCOS
B37930A5471J0
D1
Status LED
LED
1
D2
Diag. LED
LED
1
D3
BAS69-04W
Schottky barrier double
diode
1
STMicroelectronics
BAS69-04W
D4
1N4148
High conductance fast
diode
1
L1
80 µH
PS-core inductor
1
BOHEMIA ELECTRIC
BS361
P1
HS
Header, 1-pin
1
P2
L+
Header, 1-pin
1
P3
L–
Header, 1-pin
1
P4
Con ICC
ICC connector
1
Q1
BC857B
PNP transistor
1
R1
220 KΩ
Resistor
1
R2
5.6 KΩ
Resistor
1
R3, R4
1.5 KΩ
Resistor
2
R5, R6
10 KΩ
Resistor
2
Doc ID 15277 Rev 1
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Software implementation
4
AN2863
Software implementation
After the first startup following the firmware upload, the ST7 MCU performs a simple board
self test. It reads the oscillator amplitude level and voltage across the temperature sensing
diode D2 and checks whether these values are in a specific range (this state is indicated by
diagnostic LED D3 blinking, see Table 2). The oscillator amplitude level detected during this
test is also considered as an initial oscillator level when no metal object approaches the
sensing inductor L1 and its value is recorded in the Flash memory (address 0xfc00) using
the in-application programming (IAP) method. This value is later used for amplitude
reduction comparisons caused by metal objects.
Note:
The initial board self test procedure can be induced anytime by placing a jumper on the ICC
connector (pins 3-4, see Figure 5) and powering up the application.
Figure 5.
Demonstration board self test inducing
#ON)##
!-
During normal operation the MCU controls the sensor output based on the information
regarding the oscillator amplitude and the actual temperature. The main sensor part of the
firmware is realized in an auto-reload timer interrupt service routine. In equi-distant time
intervals the oscillator amplitude is sampled and its value is compared with two system
variables (ucUpperCompThreshold and ucLowerCompThreshold). One of these defines onto-off transition while the other defines off-to-on transition of the sensor state. The distance
between them determines the hysteresis. These threshold variable values are defined as a
percentage of the initial oscillator level recorded in the Flash memory and are further
modified depending on the temperature by a co-efficient from a lookup table.
Table 2.
Table 3.
Diagnostic LED blinking modes (power up self test)
LED status
Meaning
Blinking
Input values within limits
Constant
Error
Diagnostic LED blinking modes (normal operation)
Flashing style
❋
❋
❋❋
❋❋❋
10/13
❋
❋❋
❋
❋❋
❋❋❋
❋
Meaning
❋
Undertemperature
❋❋
Overtemperature
❋❋❋
Doc ID 15277 Rev 1
Ferrite approaching the coil
AN2863
Software implementation
Figure 6.
Oscillator amplitude vs. temperature (MCU pin 3)
/SCILLATOR
AMPLITUDE6
n n
4EMPERATURE #
!-
Figure 7.
D2 voltage vs. temperature (MCU pin 5)
$VO LTAGE6 4EMPERATURE #
!-
Doc ID 15277 Rev 1
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References
5
6
AN2863
References
1.
ST7LITEUSx datasheet.
2.
TDE1708DFT datasheet.
3.
SPT01-335DEE datasheet.
4.
BAS69-04W, see BAS69 datasheet.
5.
AN495 application note.
6.
EN60947-5-2; Low-voltage switchgear and controlgear - Part 5-2: Control circuit
devices and switching elements - Proximity switches”.
Revision history
Table 4.
12/13
Document revision history
Date
Revision
10-Feb-2010
1
Changes
Initial release.
Doc ID 15277 Rev 1
AN2863
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