1 introduction
Controls the table widely to apply in the industrial control and so on many domains, this article introduces has the computer communication function intelligence warm to control the table is one kind of novel Process Automation Instrumentation warm, it take monolithic integrated circuit AT89C52 as the core, uses the voltage/frequency transformation technology and RS-485 correspondence connection chip MAX487, has the measuring accuracy to be high, the reliability is good, the resistance to interference strong, may realize merits and so on computer network control, but widely applies in professions and so on metallurgy, spinning and weaving, chemical industry, medical service. It has - 200~ 500°C the scope temperature survey and the automatic control, is old style controls the table warm the substitution product, the market prospect is broad.
2 system hardware designs
System hardware diagram as shown in Figure 1.
After the system puts through the 220V alternating current supply, through the voltage-stabilizer circuit (7805, 7905) produces ±5V the d C operation power source, satisfies in this system the integrated circuit job requirement, system’s telemetric circuit starts to work: Cannot enter the situation which in the personnel or is not easy enter, through temperature sensor platinum resistance PT100 and operational amplifier OP07 will be measured that the temperature the change transforms the voltage signal, carries on the V/F transformation by LM331 to carry on the frequency counting for the signal impulse input to the 89C52 T0 mouth, this counter impulse frequency namely had reflected measured the temperature the size, the system carries on the PID operation, if measured that the temperature and the system hypothesis temperature do not tally, control output circuit’s electromagnetic type relay attracts according to PID in the computed result transmitted light pair TIL117 gathers, carries on the temperature control the control, simultaneously various extension telephones’ communication mouth MAX487 and the main engine carry on the data communication and the transmission, by main engineThe input parameter may carry on all controls the table warm the temperature hypothesis. The system establishes temperature data storage in watch-dog chip X25045, when simultaneously the examination temperature surpasses hypothesis temperature certain value the system carries on the warning. This system uses one piece 8155 to take 8 LED nixietubes and 4 keyboard’s connection, simultaneously the display system hypothesis temperature and the examination temperature value, 4 keyboards are: The position chooses, the increase, the decrement, the function.
2.1 temperature examinations and signal amplifying circuit
This system uses platinum resistance PT100 is the temperature element, PT100 has the stable property, the oxidation resistance ability to be strong and measuring accuracy higher merit. Bridge circuit which is composed of PT100 and the resistor because the temperature change will cause the platinum resistance’s resistance number change transforms for voltage signal input the amplifier. Because must install through the connecting lead is surveying the scene the platinum resistance to turn on the control bench, to reduce the lead wire resistance the influence, uses the three diagram.
The signal amplifying circuit transports by the integration puts OP07 to be composed, the OP07 noise peak - peak value for 0.6μV, the syntype restrains compares CMRR>106dB, OP07 the base pin function: IN and IN- are the signal differential motion input ends, 1, 8 feet are the zero alignment ends, 6 feet are the out-ports.
2.2 voltage/frequency switching circuit (LM331)
In controls and in the measurement system, generally transforms the component by AD the electric quantity or the non-electrical quantity through the sensor and pretage electric circuit turning on following electric circuit processing. This system uses V/F switch LM331 to transform the temperature signal change into frequency code processing.
LM331 is the performance price scaled height of burst which, the peripheral circuit American NS Corporation produces simple, may the single power source power supply, the low power loss integrated circuit. The LM331 dynamirange width reaches 100dB, the operating frequency lowers to the 0.1Hz fashion has the good linearity, the digital resolution reaches 12. The LM331 output driver uses the collecting electrode opening form, therefore may comes the nimble change output pulse through the choice logic electric current and the external connection resistance the logic level, by adaptive TTL, DTL and CMOS and so on different logic circuit. LM331 may work between 4.0V~40V, the output may reach as high as 40V, moreover may prevent VCC to short-circuit.
In this system, LM331 will output the frequency code turns the TTL level to give monolithic integrated circuit’s P3.4 mouth to take T0 the counter impulse. This switching circuit linearity is good, antijamming ability, the output range above 10Hz~10kHz, is advantageous in enhances system’s measuring range.
LM331 main base pin function:
RC: Reference current input end; CO: Electric current out-port; FO: Frequency out-port; CI: Voltage input end.
2.3 CPU and peripheral circuits
AT89C52 is the MCS-51 series monolithic integrated circuit which ATMEL Corporation produces, the built-in 8K character electricity saving cleans the programmable EEPROM internal program memory and 256 byte RAM, the internal program memory space can meet needs of this system program memory, may omit outside the piece the EPROM program memory and the address latch, causes the circuit structure to be simple and direct.
TXD, RXD, P1.5, P1.6 with MAX487 DI, RO, DE unite separately carry on the data communication control, the P1.0~P1.4 mouth, RESET meet to X25045ALE, the P0 mouth, P2.0, P2.1 meet to 8155. P3.4 carries on the pulse counting the mouth counter T0 input termination to the LM331 frequency out-port. P1.7 mouth for light pair TIL117 control end.
The system data storage and the fail safe part is composed of X25045, X25045 is one kind of serial communication 512 byte EEPROM, simultaneously has at the same time the watch-dog and the power source monitoring function. X25045 has three kind of programmable watch-dog cycles, when on the electricity and VCC are lower than the detection threshold, the output reset signal, the X25045 output repositions the high level to be effective, its replacement out-port is direct and 89C52 replacement end connection.
X25045 base pin function: Selects patches or strips of land as worth saving for seed the input; SO: Serial output; SI: Serial input; SCK: Serial clock input; WP: Writes the protection input; RESET:: Replacement output.
2.4 communication mouths (MAX487)
This system uses RS-485 connection chip MAX487 to take the communication mouth. MAX487 is MAXIM Corporation produces uses in RS-485 and the RS-422 correspondence difference main line low power transceiver, it includes a driver and a receiver, has the driver/receiver to enable the function, the input impedance is 1/4 load (≥48kW), the nodal point number is 128, namely each MAX487 driver may actuate 128 standard loads. The MAX487 driver designs limits the slope way, causes the output signal border not as for excessively steep, avoids in the transmission line having excessively many high frequency components, thus has restrained the disturbance phenomenon effectively. When MAX487 receiving sensitivity for ±200MV, namely receiving end’s difference voltage ≥ 200MV, the receiver outputs for the high level, ≤-200MV when the receiver outputs for the low level, is situated between ±200MV between when the receiver outputs for the indefinite condition, therefore, once some node receiver in time main line free time, transmission line opening or short circuit has the low level, will cause the serial receiver not to be able to find the outset position, thus causes the correspondence to be unusual, for this reason, this system has made processing on the hardware: MAX487 A, the B out-port affixes on pulls, under pulls the resistance, guaranteed when sends out the valid data all receivers can receive the complete data.
The MAX487 data transfer rate is 0.25Mbps, the quiescent operation electric current for 120μA, the 5V single power source work, in this system, MAX487 selects the half-duplex operation method, during various nodes’ correspondence takes the transmission medium through a pair of twisted pair line, because of twisted pair line characteristic impedance for 120Ω, therefore system, in the MAX487 beginning end and the terminal meet a 120Ω resistance to reduce on respectively the line the transmission signal reflection. Is far as a result of the main engine and the extension telephone great distance, but on the extension telephone system the electricity or the replacement frequently do not complete in the identical time, if is at the transmission mode in this time some MAX487, will take the correspondence main line to cause other extension telephones to be unable with the main engine to carry on the correspondence, this system affixes light pair TIL117 in the 89C52 P1.6 mouth and between the MAX487 DE end, has guaranteed on the system when the electricity replacement the MAX487 DE end is “0″, effective addressing this question.
MAX487 main base pin function: RO: Receiver out-port; : The receiver outputs enables the end, is “0″ when RO is enabled; DE: The driver outputs enables the end; DI: Driver input end; A: Receiver synchronism input end and driver synchronism out-port; B: Receiver opposition input end and driver opposition out-port.
3 control software’s design
This system software uses the modular design, is composed of the master routine and the subroutine and the interrupt service. Master routine flow chart as shown in Figure 2, the main subroutine includes: Demonstrates the subroutine; Keyboard scanning subroutine; PID interpretative subroutine. As space is limited, concrete program slightly.
4 concluding remark
This intelligence controls the table measuring accuracy to be high warm, the stable property is reliable, not only may substitute old style controls the table warm, but may also realize the computer network highly effective data management, is in the modern industry control domain the practical intellectualized measuring appliance.
