Abstract: Introduced one vacuum freeze-drying tester’s temperature controller design, take AT89C52 as the control core, selects the PT100 sensor, MC14433A/D, completes using the increase type PID algorithm to the electrical machinery implementing agency motion control, thus realizes the temperature accuracy control.
Key words: Temperature/monolithic integrated circuit /PT100/MC14433
1 introduction
The vacuum freeze-drying technology in the low pressure, under the vacuum condition causes the material moisture content to sublimate, is in the dry area of technology the technology content high, involves the aspect of knowledge broad one kind of technology, because its dry product has the rehydrate performance to be good, the luster maintains, the nutrient content damages lacks, the product weight lightly, to be advantageous carries merits and so on transportation, easy long-term preservation, surpasses other dry products far in the quality, causes it to become the dry engineering research and the development front.
The vacuum freeze-drying technology has the different experiment and the temperature request regarding the different application situation. This system test object for some biological material, uses the liquid nitrogen to take the refrigerant, the request temperature control setting value for - 60℃, the temperature smallest resolution for 0.5℃, the deviation requests ≤±2℃. The request can the real time display current and the hypothesis temperature value, and may through the keyboard adjustment hypothesis temperature value.
2 system hardware system design and essential circuit design analysis
The hardware system uses atmel Corporation’s at89c52 monolithic integrated circuit is the control core, its interior includes 8kb e2prom and 256b ram, can meet the general need, but does not need to expand the exterior program memory, the system clock uses 12mhz. Controller’s main hardware architecture diagram as shown in Figure 1.
Figure 1 controller’s hardware architecture
2.1 temperature sensor electric circuit
The consideration request’s control precision is high, the temperature sensor selects the pt100 platinum thermal resistance temperature sensor. the pt100 temperature measurement precision is high, the stability is good, very suitable - 200~ 650℃ temperature range precision measuring, because sensor resistance rt and between the temperature t relations are misalignment, to eliminate the misalignment to the temperature measurement precision influence, design misalignment compensating circuit, and considered that uses the four-wire system connection to eliminate the lead wire resistance the influence. As shown in Figure 2, in the chart the mutual conductance amplifier’s mutual conductance is g, may enable the output belt voltage to obtain a 1/(1-grt) scale-up factor, the suitable selection g value may fall the misalignment error to a very small scope. the g value may determine through the calibration test that in this design takes g=0.38ma/v through the trial survey, the measuring error may achieve is smaller than 0.1℃ the precision.
Figure 2 pt100 temperature transmitting instrument model electric circuit
In Figure 2, 4 wire’s resistances are ra, rb, rc, rd. Then rb may regard as is a a1 difference mold input impedance part, but rc may regard as is a a1 output impedance part. After the closed loop, rb and the rc influence may eliminate. Uses the voltage buffer, causes only then to look toward left from d spot input impedance ri is enormous, also uses the current supply electric circuit to cause from c spot direction to look toward left output impedance rd is enormous, like this eliminated the lead wire resistance to the temperature measurement the influence.
2.2 a/d transformations
Because the vacuum drying tester is the liquid nitrogen conduction cooling by radiation, is the typical big lag temperature system, simultaneously experiments the request to have the very high curacy control request, therefore selects the double integral a/d switch, because the double integral the repeated integrals time comparison is long, therefore a/d conversion rate is slow, but the precision is very high, moreover to periodic change’s unwanted signal integral is zero, the resistance to interference is very good. Very suits the control condition which this system faces.
Selects
position double integral a/d switch mc14433 (precision to be equal to 11 binary numbers), has the precision to be high, the resistance to interference good and so on merits, the speed approximately is 1~10 /s, mc14433 a/d switch is transformed the voltage range is 199.9mv. After transforming, divides 4 times by the bcd code’s form to send out. mc14433 electric circuit structure drawing as shown in Figure 3.
Figure 3 mc14433 electric circuit structure drawing
the mc14433 a/d transformation result is the dynamic time sharing output bcd code, q3~q0 is thousand, hundred, ten, units place bcd code, but the ds1~ds4 pin output respectively is thousand, hundred, ten, units place gating signal.
Figure 4 mc14433 is the 2.5v precise composite voltage datum source, after potentiometer differential pressure uses the voltage reference as a/d transformation. the du end and eoc look carefully the company, namely the choice conversion mode, each time transforms the result to deliver continuously to the output register or. eoc is the output symbol signal which a/d transformation ended. at89c52 read transformation result uses the interrupt mode.
Figure 4 mc14433 and at89c52 connection hardware interface
2.3 keyboard/display system design
Demonstrated that the function and the input function are the monolithic integrated circuit system’s man-machine connections, is its most basic constituent. Because the monolithic integrated circuit at89c52 i/o mouth quantity is limited, simultaneously must consider to data bus and address bus, therefore the i/o mouth resources are tight, must realize the demonstration and the input function must expand the i/o mouth. The commonly used i/o expansion chip has ttl, the cmos latch, the buffer chip and 8255a, 8155 and so on, considered the system expands to overall system’s stability and the operating efficiency influence, selects 8155 carries on the expansion, selects the common cathode led nixietube demonstration temperature, realizes the actual temperature real time display, simultaneously can demonstrate the initial setting temperature value. The hypothesis temperature value interrupts the input by the keyboard, and real time display.
2.4 communication system designs
The monolithic integrated circuit system needs to carry on the communication with superior pc machine. Selects a more popular serial communication. The serial communication divides into the synchronized communication and the asynchronous communication two communication ways. Is the same as a result of the synchronized communication way to the communication both sides’ clock rate request, but guarantees this point with difficulty regarding the monolithic integrated circuit and between pc machine, therefore the system communication selects the asynchronous communication method. Moreover the pc machine serial port’s level is the rs-232 level, but the mcs-51 monolithic integrated circuit serial port’s level is the ttl level, if wants to realize between both’s correspondence, needs to add the level switch electric circuit between them, in the use is not very convenient. Therefore uses max232 to replace. between the pc machine serial port and monolithic integrated circuit’s connection mode is 3 zero moderm way generally, namely only picks txd, rxd and gnd and so on 3 basic data lines direct connected. Monolithic integrated circuit the temperature which transforms real-time gathering delivers the demonstration at the same time, every other 5 minutes deliver the current temperature value the pc machine memory, provides the printing temperature variation curve. As expansion, establishment, and may by the pc machine transmission initial setting temperature, receive the demonstration by the monolithic integrated circuit.
2.5 output controls and warning
Monolithic integrated circuit’s output control electrical machinery work, to avoid the strong jamming, selects has electro-optical isolation function thyristor mc3041. Moreover, to guarantee the workpiece quality, considered in the circuit design Canada supposes the alarm circuit, when the temperature is lower than - 80℃, the green light emitter diode is lightened; When the temperature is higher than - 30℃, the red light emitter diode is lightened.
2.6 system antijamming design
The antijamming technology is mainly on the hardware and software’s antijamming technology, uses the good earth on the hardware, selects aspects and so on appropriate primary device. This system after was considering fully on hardware’s antijamming, has designed a series of software antijamming measure: (1) in sensor data acquisition processing, uses the value law digital filtering; (2) establishes the software trap, will catch forcefully with the bootstrap flies the procedure to course the replacement entry point address randomly, makes a mistake specially in here procedure turn to the procedure carries on processing the procedure, causes the procedure to bring around to the correct course. (3) “watch-dog” technology: Uses the procedure surveillance technology, said that “the watch-dog” technical (watch-dog), avoids the procedure entering “the endless loop”. “watch-dog” unceasingly monitor program shuttle service time, if the discovery time surpasses the known hypothesis time, then thought that the system fell into “the endless loop”, then the force procedure returns to the 0000h entrance, in a 0000h place arrangement section of error handling procedure, causes the systems operation to bring around to the correct course. Designs 2 timers, one is the short timer, one is the long timer, and the respective independence, the short timer typical watch-dog works equally likely, it guarantees in the ordinary circumstances the watch-dog to have the quick reaction rate, the long timer nearly fixed time is bigger than cpu to carry out main loop procedure time, uses for to prevent to guard the door the expiration.
3 control algorithm design
According to this system’s temperature control precision ≤0.5℃, and considered that the error accumulates to control system’s influence, selects the increase type pid control algorithm, namely refers to the monolithic integrated circuit digit controller’s output is only controls the quantity the increase
, its control mathematical formula is:
(1)
(1) in the formula:
, 
respectively is the proportion, the integral, the differential coefficient;
For basic deviation, expression current observed value and hypothesis goal difference.
(2)
This algorithm’s merit includes: (1) computer output increment, therefore the misoperation is quite small. (2) in the mathematical formula does not need to accumulate, the control increase’s
determination only (when system uses constant sampling period with the recent 3 times, k takes 3 times) sampling value related, thus obtains the quite good control effect quite easily through weighting processing.
4 system software designs
The system software design uses the modulation design method, by the master routine module, the function realizes the module and the operating control module and so on three big modules is composed.
4.1 master routine modules
First assigns the pid algorithm in the procedure the parameter value, then through the circulation demonstration current temperature, waits for the interrupt, and establishes the keyboard external interrupt is the high priority, enables the master routine real-time response keyboard processing, software hypothesis timer t0 to be 10s fixed time, when does not have the pressed key, should every other 10s respond 1 time, uses for to gather the temperature sensor the temperature signal which transforms after a/d. Establishment timer t1 is the t0 nesting interrupt, the starting value provides by the pid algorithm subroutine. Uses for to carry out to the motor control as well as the alarm circuit and so on. Flow chart as shown in Figure 5.
Figure 5 master routine flow chart
4.2 functions realize the module
The function realizes the module mainly by a/d transformation subroutine, the interrupt processing subroutine, the keyboard processing subroutine and the demonstration subroutine and so on is composed. And the main interrupt subroutine is: (1) the t0 interrupt subroutine as a result of temperature change’s hysteresis quality, the monolithic integrated circuit interior establishes the 8s timer interruption, low priority, when this interrupt response, the monolithic integrated circuit completes the following work: a/d transformation and data acquisition, digital filtering, judgment temperature whether outside field, demonstration temperature, calculated temperature deviation, transfer increase pid algorithm and output control quantity. (2) t1 interrupt subroutine t1 timer interruption nesting during the t0 interrupt, for high priority interrupt. the t1 starting value fixed time provides by the pid algorithm subroutine, the t interrupt response time uses in outputting to the electrical machinery the control signal.
4.3 operating control module
Subroutine diagram as shown in Figure 6. Mainly completes the increase type pid algorithm the operation, calculates the output control increase, and △u(k) establishes t1 according to the output control increase the value fixed time, controls electrical machinery’s movement.
Figure 6 subroutine flow chart
5 concluding remark
After the experimental movement, this system can the basic reliability service, coordinate the software algorithm design, but the effective addressing temperature big lag control precision not precise question, obviously increases the temperature survey result precision, and may avoid because of the control electrical machinery implementing agency lag, the excessive execution affecting the survey accuracy, can thus lovely complete the test objective.
