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Projekt „ISSNBS“
- 1 -
DAADDeutscher Akademischer Austausch DienstGerman Academic Exchange Service
Direct Sensor to Microcontroller Direct Sensor to Microcontroller InterfacingInterfacing
Presented by: Zivko Kokolanski, M. Sc.Presented by: Zivko Kokolanski, M. Sc.
E-mail: [email protected]
Research director: Prof. Dr. Cvetan Gavrovski
Nis, November 2010
Projekt „ISSNBS“
- 2 -
DAADDeutscher Akademischer Austausch DienstGerman Academic Exchange Service
OUTLINE
• Comparison of programmable signal conditioning
techniques• Introduction to direct sensor-C interface• Single point calibration technique• Sources of measurement uncertainty• Experimental results and discussion• Effective number of resolution bits• Conclusions
Projekt „ISSNBS“
- 3 -
DAADDeutscher Akademischer Austausch DienstGerman Academic Exchange Service
SensorProgrammable
signal condit.circuit
C
A/D Sensor OSCILLATOR
C
A/D converter based programmable measurement system
Oscillator based programmable measurement system
• Conversion of the measurement quantity in to proportional electrical signal and use of an A/D converter
• Techniques based on measurement of frequency, period or duty cycle
C
Sensor
Cref,Rref
Direct sensor-microcontroller interface
Projekt „ISSNBS“
- 4 -
DAADDeutscher Akademischer Austausch DienstGerman Academic Exchange Service
tp1 tp2
VTL
Po
Pi
Rx
C
Rp
“HZ”
CRt pp 1
“1”
tl
rxp VV
VVCRt
0
02 ln
“0”
“In”
NT0tp2
f0
DIRECT SENSOR-C INTERFACE BASED ON MEAS. OF DISCHARGING TIME OF RC CIRCUIT
Projekt „ISSNBS“
- 5 -
DAADDeutscher Akademischer Austausch DienstGerman Academic Exchange Service
tl
rxp VV
VVCRtN
0
02 ln
xkRN
CONSTANTS
BUT:• The constant k is not very stabile• The output resistances and leakage currents of the microcontroller ports cause gain, offset and nonlinearity errors• Therefore direct sensor-microcontroller interface in practice is realized by using some calibration method
DIRECT SENSOR-C INTERFACE BASED ON MEAS. OF DISCHARGING TIME OF RC CIRCUIT
Projekt „ISSNBS“
- 6 -
DAADDeutscher Akademischer Austausch DienstGerman Academic Exchange Service
Po1
Pi
Rx
C
Po2
Rc
Rp
Vc
t
Vth
V0
tp tx tp tc
tl
rc
tl
rx
c
x
VVVV
CR
VVVV
CR
tt
0
0
0
0
ln
ln
cc
xx Rtt
R
SINGLE POINT CALIBRATION
Projekt „ISSNBS“
- 7 -
DAADDeutscher Akademischer Austausch DienstGerman Academic Exchange Service
• Reference freq. uncertainty,
• Quantization uncertainty,
• Uncertainty in the triggering
point,
• Program related errors ,
• Uncertainty of the reference
element
They are small for short meas. intervals.
rr CRprogtrigqfxt /0
Every digital system makes ±1 sample error.
Errors are smaller for higher f0.
2
/
22
T
nsnitn TV
VV
Better reference
element.
Dynamic element matching technique.
Noise from the mC during the execution of the program cycle.
SOURCES OF MEASURMENT UNCERTAINTY
Projekt „ISSNBS“
- 8 -
DAADDeutscher Akademischer Austausch DienstGerman Academic Exchange Service
Po
P
i C
Px
Rx
Pc
Rc
R0
Po
Pi C
Px
Rx
Pc
Rc
R0
Vr
+-
K
Schmitt trigger circuit Comparator circuit
Without external circuit
Schmitt trigger
Comparator with divider
Comparator with TL431
(tx)[s] 0,36 0,16 0,19 1,34
u(tx)/tx x10-3 0,32 0,14 0,17 1,18
IMPROUVING THE MEASUREMENT UNCERTAINTY BY USING ADITIONAL ELECTRICAL CIRCUITS
Projekt „ISSNBS“
- 9 -
DAADDeutscher Akademischer Austausch DienstGerman Academic Exchange Service
0,00
1,00
2,00
3,00
4,00
5,00
6,00
7,00
8,00
9,00
0,0 2,0 4,0 6,0 8,0 10,0
u(t x)
/t x
x10-
4
t[ms]
7
8
9
10
11
12
13
14
15
0,1 0,22 0,47 1 2,2 4,7 10
Резо
луци
ја
C[F]
M
ENOB, n=1
ENOB, n=5
Relative standard measurement uncertainty
Effective number of bits
• PIC16F877
• MSP430F2274
• ATmegaAVR128
EFFECTIVE NUMBER OF RESOLUTION BITS
Projekt „ISSNBS“
- 10 -
DAADDeutscher Akademischer Austausch DienstGerman Academic Exchange Service
CONCLUSIONS• Direct sensor-C interface is alternative approach for conditioning the
resistive and capacitive sensors without use of A/D converter,• Single point calibration is the simplest calibration technique that cancels
the contribution of the unstable parameters,• The output resistances and leakage currents of the microcontroller ports
cause gain, offset and nonlinearity errors,• One of the uncertainty sources is the microcontroller “program” noise
while execution the program cycle, • This uncertainty can be improved by using Schmitt Trigger or
comparator circuit (one extra resolution bit),• The ENOB when using Schmitt Trigger circuit at time constant of 4ms
was approximately 11.5/12.3 bits• Although this approach achieves relatively low measurement resolution
(arround 13 bits max) and low sampling speed (arround 30 sps),its performances are remarkable considering the simplicity and the cost
Projekt „ISSNBS“
- 11 -
DAADDeutscher Akademischer Austausch DienstGerman Academic Exchange Service
THANK YOU FOR YOUR ATTENTION!
Practical demonstration…