Key word: Air gage; Pneumatic transmitter; V/F switch; Monolithic integrated circuit
1 introduction
The air gage is for easy to examine using the pressure cell the testing barometric pressure direct transform, the transmission electric current or the voltage signal, then passes through the following electric circuit to process and to carry on the real time display again one kind of equipment. In which core part is the pneumatic transmitter, it in the surveillance pressure size, operating pressure change as well as physical parameter’s aspects and so on survey is playing the influential role. Utilizes basically is time the dependence different altitude barometric change gains the barometric pressure value in air gage’s pneumatic transmitter.
The meteorology research indicated that the barometric pressure along with increases highly in the vertical direction reduces. For example in the underlying bed, rises 100m 熎 to press every time then reduces 10hPa; In the 5~6km upper air, will increase 100m highly every time, the barometric pressure will then reduce 7hPa; But when highly further increases, namely after 9~10km upper air, will increase 100m highly every time, the barometric pressure will then reduce 5hPa; Similarly, when in the air has the down current, the barometric pressure will increase; If in the air has time the ascendant current, will affect will reduce in the air column base barometric pressure. Generally is called the function in the unit area sky air column’s weight the atmospheric pressure.
2 air gage’s structures
This article studies air gage structure as shown in Figure 1. And the pneumatic transmitter will use for to measure that the barometric pressure will transform into the voltage signal; May the voltage signal conversion which outputs the pneumatic transmitter have with the V/F switch certain frequency signal impulse; In order to receive this signal impulse with the monolithic integrated circuit, and pulse number which obtains according to the unit interval/unit time, calculates the barometric pressure value which based on the voltage and the frequency linear relationship corresponds, finally demonstrates under the monolithic integrated circuit control by LED.
This air gage can in pneumatic transmitter’s linearity range the correct measurement corresponding barometric pressure value. What needs to explain, its observed value is the absolute atmosphere value. This article studies air gage’s technical specification is as follows:
* measuring range: 300hPa~1050hPa;
* measuring accuracy: 0.1%FS(20℃);
* demonstration precision: 0.1%, realize by 4 8 section of LED demonstrations;
* operating temperature scope: 0~85℃;
* supply voltage: 9V.
3 systems realize
In the system construction process, needs to consider stable, complex degree, construction cost and debugging factors and so on difficulty degree. Shown in Figure 1 in the diagram each part is one unit electric circuit, may complete the respective function. Between module not complex signaling, and disturbs are very few, thus the system whole is quite stable.
3.1 pneumatic transmitters
The pneumatic transmitter occupies the core position in the air gage. When design may according to the measuring accuracy, the measuring range, the temperature compensation, the survey absolute atmosphere value and so on several performance indices select the pneumatic transmitter.
What because this air gage demonstrated is the absolute atmosphere value, thus needs to select the survey absolute atmosphere value the pneumatic transmitter. Meanwhile for the simplified circuit, enhances the stability and antijamming ability, requests this pneumatic transmitter to have the temperature compensation.
Therefore, the author selects Motorola the MAX4100A pneumatic transmitter to survey the absolute atmosphere value. This sensor’s temperature compensation scope for - 40~ 125℃; The pressure scope is 20kPa~1050kPa; The output voltage signal (Vs=5.0V) the scope is 0.3~4.65V; The measuring accuracy is 0.1%VFSS, simultaneously when 20kPa~1050kPa has the good linearity, outputs the relations to be as follows specifically:
Vout=Vs (0.01059 P-0.1528) ±Error
In the formula, Vs is the working voltage, P is the atmospheric pressure value, Vout is the output voltage.
3.2 V/F transformations
The V/F component’s function is transforms the pulse train which the input voltage peak-to-peak value the frequency and the input voltage peak-to-peak value is proportional. Although V/F itself has not been able to calculate makes the quantizer, but will add on the timer and the counter later may also realize a/D transformation. Its prominent characteristic is transforms the analogue voltage antijamming ability, but the long-distance range transmission and can the direct input computer’s pulse train, thus through surveys V/F the output frequency to realize a/D transformation function.
Considered the peripheral circuit realizes the difficulty degree and the corresponding performance index, the author has selected the LM331 voltage/frequency transformation chip. This component used the temperature compensation to be able the crack reference circuit, thus had the extremely good temperature stability, was most greatly warm floats for 50ppm/℃, simultaneously this component’s pulse output might be compatible with any logical form; LM331 may the list, the double power source power supply, the voltage range be 5~40V; Full scale scope 1Hz~100kHz; The biggest misalignment error is 0.01%. Shown in Figure 2 is in this system the LM331 peripheral circuit. In this electric circuit, based on the LM331 pressure frequency transformation relations are:
fo=K Vi
And, K=Rs/(2.09 Rt Ct RL) 牐 Rs=Rs1 Rs2
In fact, in electric circuit’s Rs mainly uses in regulation circuit’s translation gain 煟 remote, the Ct, RL typical value respectively be 6.8kΩ, 0.01pF and 100kΩ, the K value may decide by designer. In this design, takes K=2000, Rs=28.424 gluttonously kΩ 熤 is considered the monolithic integrated circuit part use measured the frequency method measured fo can guarantee the frequency code the measuring accuracy. Because Rs, RL, Rt and electric capacity Ct the immediate influence fo transformation result. Therefore, has certain request to these part’s parameter, when the design should choose suitably according to the conversion accuracy. Electric capacity CL to transforms although finally does not have the immediate influence, but should choose leaks the electric current small capacitor. With resistance R1, the electric capacity C1 composition low pass filter, may reduce in the input voltage the interfering impulse, increases the conversion accuracy.
Figure 3
3.3 monolithic integrated circuits
This air gage realizes the plan to use monolithic integrated circuit’s P1 mouth and the P3 mouth part of as well as an interrupt source, a timer and a counter. Therefore, the author has selected the ATMEL AT89C2051 monolithic integrated circuit, this component and 89C51 are compatible, have 2kB to be possible to duplicate program dodge save, the 2.7V~6V working voltage scope, 128Byte internal RAM as well as two I/O mouth (P1, P3), 2 16 counter/timers and 6 interrupt sources, and may the direct drive LED output, simultaneously have the programmable serial communication mouth. Moreover, this monolithic integrated circuit also has the volume to be small, price low status characteristic.
3.4 LED demonstrations
Single LED is the display unit which constitutes by 7 section of light emitter diodes. Some 10 pins, correspond to 7 sections, a decimal point and two public ends. In the display circuit, these light emitter diode has two seed grafting law: Altogether anode connection and common cathode connection. In this design needs to use 4 LED composition display unit, and uses the dynamic display mode.
Because uses 4 single LED to carry on the demonstration the segment to be quite complex, simultaneously monolithic integrated circuit’s port driving force also with difficulty guaranteed, but needs to join the special actuation chip. Therefore, the author has used 4 LED joined bodies, the interior already its corresponding Duan Jie good altogether anode LED, it has 12 pins, including 7 sections and 4 public ends, to enhance the nixietube brightness, may reign in the route selection to join a triode driving circuit.
By AT89C2051 control’s display circuit as shown in Figure 3. This display circuit needs to select appropriate resistance R and Ra, can guarantee that LED the brightness, oversized or too small is unable to let the LED normal demonstration. When design takes R is 4.7kΩ 煟 remote yu 510Ω is quite ideal. If consideration board wiring’s convenient, may use pastes the piece resistance and the row anti- saves the space. Moreover, may also use 74LS244 and the 74LS06 constitution actuation display circuit, but must add the limiting resistor like this similarly. Because 74LS06 is opens leaks the component, needs to lose the source to add on pulls the resistance.
4 softwares realize
Through the above design, then may calculate P through fo the size by to obtain the real-time barometric pressure value. After the hardware circuit design completes, may use the AEDK5196PH simulator’s simulation environment to carry on the simulation, and the available C51 language compiles the disposal procedure. Its main routine flow as shown in Figure 4.
Procedure hypothesis: T0 is a timer, the basic time base fixed time is 50ms. T1 is a counter, after the internal interrupt 0 may guarantee T0 fixed time full 500 ms on the read this time the counter value, calculates the barometric pressure value. Like causes T1, T0 to work in the way 1, and delivers the font code in the P1 mouth, simultaneously available P3.0~P3.3 makes the position route selection, its corresponding function is so as follows:
(1) timer T0 interrupt function:
void timer0(void) interrupt 1 using 1
{uint x, y;
uint count_ pluse;
ET0=0; // closes the T/C0 interrupt
Tcount ; // interrupt number of times
if 煟 abundant disobeys zheng acid radical wisdom == 10) {
TR1=0; // stop counter counting
Tcount=0;
x=TH1;
y=TL1;
count_pulse=(x*256 y)*2;
ph=(uint) (10 * ((float) (count pulse 1520) /105.9 牔 // computation barometric pressure value
TH1 = 0×00; // heavy design number starting value
TL1=0×00;
}
TH0 = - 50000/256; // supposes the 50ms starting value again
TL0 = - 50,256%;
if (TL0! = 0) TH0–;
ET0=1;
TR1=1;
return;
}
This interrupt function mainly uses in completing the pulse the read and the barometric pressure value computation. ph is the global variable, may use for to preserve the barometric pressure value.
(2) in demonstrates in the function, carries on the first barometric pressure value according to the position separates and preserves the array, then delivered Duan Ma and the corresponding position elects to be possible to demonstrate the corresponding barometric pressure value. The concrete program is as follows:
void display (uint ph_in)
{uchar i=0;
uchar j=0;
uchar select_bit=0; // elects
do {
cur_buf[i]=ph_in;
i ;
j=i;
} while(ph_in=ph_in/10); // works as top digit for zero hour 熃 yao circulation
i=0;
select_bit=0xfe;
do
{P1=tab[*p];
P3=select_bit;
dl_ms();
select_bit= (select bit<<1) 1;
// from most right side starts to demonstrate, rotate left
p ;
i ;
} while(i<j);
p=cur_buf; // indicator homing
return;}
Thus, in master routine, so long as in the procedure first movement time carries on the initialization, then circulates the transfer demonstration function then to realize the real time display function again.
5 concluding remark
The author once used the pure hardware circuit to design the air gage. The practice indicated that as a result of temperature influence and hardware parameter limit, when real time display the stability is bad, and the precision is not high. But changed to the V/F transformation signal and the programming method realizes this survey completely to overcome the above shortcoming. The result indicated: This method has the precision to be high, the stability is good, the function easy to expand and so on merits, may provides one kind of new mentality for the instrument and the electronic products design.
