PIC16C773-I/SP [ETC]
IC-8-BIT MCU ; IC- 8-BIT MCU\n
| 型号： | PIC16C773-I/SP |
| 厂家： | 
ETC |
| 描述： | IC-8-BIT MCU
|
| 文件： | 总4页 (文件大小：182K) |
| 中文： | 中文翻译 | 下载： | 下载PDF数据表文档文件 |
TB044
Sensing Air Flow with the PIC16C781
R7 is heated by R6. If moving air cools R6, the amount
of heat transferred to R7 is reduced. The resistance of
R7 falls with the temperature. As R7 resistance falls,
the voltage drop across R7 also falls. The Op Amp out-
put is directly proportional to the voltage across R7.
When the Op Amp output goes below VR, the compar-
ator output goes high. The PSMC responds to the high
comparator output by supplying drive pulses to Q1,
thereby heating up R6. The temperature rise of R6
overcomes the cooling effect of moving air, and heat is
transferred to R7 closing the loop.
Author:
Ward Brown
Microchip Technology Inc.
INTRODUCTION
Programmable Switch Mode Controllers (PSMC) are
not just for switching power supplies. This technical
brief describes how to use the PIC16C781 PSMC in
combination with the Integrated Operational Amplifier,
Digital-to-Analog Converter (DAC), and gated timer to
construct a thermally operated air flow sensor with min-
imum external components.
The PSMC is configured for pulse skipping. The control
loop generates pulses until the temperature of R7, and
the corresponding resistance, is high enough to disable
the pulse drive. At equilibrium, the number of drive
pulses match the heating requirement to keep the volt-
age at R7 equal to the voltage at R6.
Theory of Operation
Air flow is detected by the cooling effect of air move-
ment across a heated resistor. The circuit schematic is
shown in Figure 1. R5 and R7 are thin film platinum
Resistance Temperature Detectors (RTD). These are
essentially thermistors with a very linear temperature
response. The flow sensor is comprised of R6 and R7.
The bias on R7 is intentionally set below the bias on
R5. R6 and R7 are thermally linked so that when R7 is
heated by R6, the resistance of R7 increases. As R7
resistance increases, the voltage across R7 also
increases until it matches the voltage across R5, at
which time the Op Amp output will shut down the Pro-
grammable Switch Mode Controller (PSMC) and cease
heating R6. As moving air cools R6, more power is
required to heat the R6-R7 pair to maintain the same
R7 resistance and voltage.
The DAC output is used to adjust the equilibrium point
in still air by varying the bias on R7. At high bias levels,
less heat is required by R6 to reach the equilibrium
resistance level. Low required heating in still air means
that there is plenty of headroom in the potential drive
output, but this also means less variation due to cooling
and thus low sensitivity. At low bias levels, more heat is
required by R7. Greater heat means the effect of cool-
ing is greater and, in turn, higher sensitivity. There is a
limit to the drive available to R6 so that if the bias level
is low enough the equilibrium resistance and voltage
cannot be obtained. In other words, at low bias levels
there is better sensitivity but less head room in potential
heating drive. It was determined empirically that a good
bias point is obtained when the Op Amp output is 100
mV below VR when R6 heating is inhibited.
Changes in ambient temperature conditions are com-
pensated by two voltage dividers, R2-R5 and R1-R7.
R2 and R5 form a voltage divider between the Op Amp
output and the Op Amp inverting input. Similarly, R1
and R7 form a voltage divider between the variable
DAC reference and the non-inverting Op Amp input.
Since R5 and R7 are identical RTD's, resistance varia-
tions due to self heating, as well as changes in the
ambient conditions, cancel out at the Op Amp inputs.
The power being delivered to R6 is proportional to the
cooling effect of moving air. This power is measured by
counting the average time that the R6 driver is enabled.
The PIC16C781 has an integral Timer1 count enable
input (Timer1 Gate). By connecting the PSMC output to
the Timer1 Gate input, Timer1 will count only when the
PSMC output is low. Average PSMC drive time is deter-
mined by clearing Timer1 then using Timer0 to wait a
fixed period and reading Timer1 at the end of that
period. Since the gate is low true, higher counts indi-
cate that less power is being delivered to R6.
R6 heat is controlled by a closed loop comprised of:
• R7 Voltage
• Op Amp
• Comparator
• PSMC
A 10-segment LED bar graph is used to display relative
air flow. The circuit shows how to drive ten segments
with five outputs. Each microcontroller output is tied to
two segments. When the output is high, one LED is
• R6 driver Q1
2002 Microchip Technology Inc.
DS91044A-page 1
TB044
driven. When the output is low, the other LED is driven.
When the output is high-impedance, neither LED is
driven.
FIGURE 1:
CIRCUIT SCHEMATIC FOR SENSING AIR FLOW WITH PIC16C781
TP3
RB0
INTERNAL PIC16C781
SHOWN FOR REFERENCE
DAC
VR
RB1
T1G
TIMER1
VR
R1
100K
RB7
+
-
PSMC1A
PSMC
RA0
RA1
+
RB6
OPA
C1
1000 pF
-
RB3
R2
TP1
100K
+5 VDC
CYNTEC Part #SE102
Thermally
coupled
R7
1 K
RTD
R5
1 K
RTD
R6
51
TP2
R3
Q1
2N2222
2.2K
+5 VDC (ANALOG)
D1
+5 VDC
+5 VDC
470Ω(TYP)
PIC16C781
RA2
VDD
AVDD
+
C5
10 µF
+
C3
0.1 µF
C2
10 µF
0.1 µF
C4
RA6
RA7
RB2
RB4
1
9
RA0
RA1
RB0
RB1
2
7
10
19
20
+5 VDC
RA2
RB2
RB3
RB4
RB5
R8
47 K
8
RA3
R9
470 Ω
3
11
12
13
RA4
4
MCLR
17
18
RA6
RA7
RB6
RB7
C6
0.1 µF
Analog and Digital
Grounds connect
at single point
14
RN1
5
6
VSS
AVSS
RESET
SW
RN2
Bar Graph
up or down to compensate for the offset and, after the
six second settling time, another measurement is
taken. This process repeats until the desired R7 bias
level has been obtained.
Zeroing and Calibration
The integral DAC makes automatic zeroing of the R7
bias current possible. While this process is in progress,
the sensor should be in still air (no air flow). One LED
flashes as a calibration-in-progress indicator. When the
LED stops flashing, air flow may be resumed and mea-
surements can begin. The first task after power-on
initialization is to calibrate the Op Amp offset using the
built-in Op Amp calibration utility of the PIC16C781.
After Op Amp calibration, the DAC is initially set for
about 3.0 volts output. The RTD temperatures are
allowed to settle for 6 seconds, then the average
PSMC drive time is measured using Timer1 and the
Timer1 gate input. If the measured value is within plus
or minus one display resolution of the expected zero
value, then the zeroing routine is exited and measure-
ment and display commences. If the measured value is
outside of the expected window, the DAC is adjusted
SUMMARY
This technical brief demonstrates how temperature
changes resulting in milliohm differences can be mea-
sured quickly and accurately using only the built-in
peripherals of the PIC16C781. This is the first of the
®
mixed-signal PICmicro microcontrollers with integral
DAC, operational amplifier, comparators, PSMC and
gated timer inputs which, when used in harmony, make
such measurements possible.
Source code for this application is available for free.
Download it from the Microchip web site.
DS91044A-page 2
2002 Microchip Technology Inc.
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DS91044A - page 3
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2002 Microchip Technology Inc.
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