多点温度检测系统的设计

1、多点温度检测系统设计附录D外文文献In troduct ion of Programmable con trollersFrom a simple heritage, these remarkable systems have evolved to not only replace electromechanical devices, but to solve an ever-increasing array of control problems in both process and non process in dustries. By all in dicatio ns, these

2、microprocessor powered gia nts will continue to break new ground in the automated factory into the 1990s.HISTORYIn the 1960s, electromecha ni cal devices were the order of the day ass far as con trol was concern ed. These devices, com monly known as relays, were being used by the thousa nds to con t

3、rol many seque ntial-type manu facturi ng processes and sta nd-al ong machi nes. Many of these relays were in use in the transportation industry, more specifically, the automotive industry. These relays used hundreds of wires and their interconnections to effect a control solution. The performance o

4、f a relay was basically reliable - at least as a sin gle device. But the com mon applicati ons for relay pan els called for 300 to 500 or more relays, and the reliability and maintenance issues associated with support ing these pan els became a very great challe nge. Cost became ano ther issue, for

5、in spite of the low cost of the relay itself, the in stalled cost of the panel could be quite high. The total cost including purchased parts, wiring, and installation labor, could range from $30$50 per relay. To make matters worse, the con sta ntly cha nging n eeds of a process called for recurri ng

6、 modificati ons of a con trol pan el. With relays, this was a costly prospect, as it was accomplished by a major rewiri ng effort on the pan el. I n additi on these cha nges were sometimes poorly docume nted, caus ing a sec on d-shift maintenance ni ghtmare mon ths later. I n light of this, it was n

7、ot un com mon to discard an entire control panel in favor of a new one with the appropriate components wired in a manner suited for the new process. Add to this the un predictable, and pote ntially high, cost of maintaining these systems as on high-volume motor vehicle production lines, and it becam

8、e clear that something was needed to improve the control process to make it more reliable, easier to troubleshoot, and more adaptable to cha nging con trol n eeds.That someth in g, in the late 1960s, was the first programmable con troller. This first system wan developed as a specific resp onse to t

9、he n eeds of the major automotive manu facturers in the United States. These early controllers, or programmable logic controllers (PLC), represented the first systems that 1 could be used on the factory floor, 2 could have there logicchanged without exte nsive rewiri ng or comp onent cha nges, and 3

10、 were easy to diag nose and repair whe n problems occurred.It is interesting to observe the progress that has been made in the past 15 years in the programmable con troller area. The pion eer products of the late 1960s must have bee n confusing and frightening to a great number of people. For exampl

11、e, what happenedto the hardwired and electromechanical devices that maintenance personnel were used to repairing with hand tools? They were replaced with computers disguised as electronics designed to replace relays. Even theprogram ming tools were desig ned to appear as relay equivale nt prese ntat

12、i ons. We have the opport unity now to exam ine the promise, in retrospect, that the programmable con troller brought to manu facturi ng.All programmable controllers consist of the basic functional blocks shown in Fig. 10. 1. W6 ll exam ine each block to un dersta nd the relati on ship to the contro

13、l system. First we look at the cen ter, as it is the heart ( or at least the brain ) of the system. It consists of a microprocessor, logic memory for the storage of the actual con trol logic, storage or variable memory for use with data that will ordin arily cha nge as a fun cti on power for the pro

14、cessor and memory. Next comes the I/O block. This fun cti on takes the con trol level sig nals for the CPU and con verts them to voltage and curre nt levels suitable for connection with factory grade sen sors and actuators. The I/O type can range from digital (discrete or on / off), an alog (con ti

15、nu ously variable), or a variety of special purpose smart I/O which are dedicated to a certain application task. The programmer is shown here, but it is n ormally used only to in itially con figure and program a system and is not required for the system to operate. It is also used in troubleshooting

16、 a system, and can prove to be a valuable tool in pinpointing the exact cause of a problem. The field devices shown here represent the various sensors and actuators connected to the I/O. These are the arms, legs, eyes, and ears of the system, in clud ing push butt on s, limit switches, proximity swi

17、tches, photose nsors,thermocouples, RTDS, positi on sensing devices, and bar code reader as in put; and pilot lights, display devices, motor starters, DC and AC drives, sole no ids, and prin ters as outputs.No single attempt could cover its rapidly changing scope, but three basic characteristics can

18、 be exam ined to give classify an in dustrial con trol device as a programmable con troller.(1) Its basic internal operation is to solve logic from the beginning of memory to some specified point, such as end of memory or end of program. Once the end is reached, the operati on beg ins aga in at the

19、beg inning of memory. This sca nning process con ti nues from the time power is supplied to the time it it removed.(2) The programming logic is a form of a relay ladder diagram. Normally open, normally closed con tacts, and relay coils are used within a format utilizi ng a left and a right vertical

20、rail. Power flow (symbolic positive electron flow) is used to determine which coil or outputs are energized or dee nergized.(3) The mach ine is desig ned for the in dustrial en vir onment from its basic con cept; this protect ion is not added at a later date. The in dustrial en vir onment in cludes

21、un reliable AC power, high temperatures (0 to 60 degree Celsius), extremes of humidity, vibrations, RF noise, and other similar parameters.Gen eral applicati on areasThe programmable con troller is used in a wide variety of con trol applicati ons today, many of which were not economically possible j

22、ust a few years ago. This is true for two general reasons: 1 there cost effectiveness (that is, the cost per I/O point) has improved dramatically with the falling prices of microprocessorsand related components, and 2 the ability of the controller to solve complex computatio n and com muni catio n t

23、asks has made it possible to use it where a dedicated computer was previously used.Applications for programmable controllers can be categorized in a number of different ways, in clud ing gen eral and in dustrial applicati on categories. But it is importa nt to un dersta nd the framework in which con

24、 trollers are prese ntly un derstood and used so that the full scope of prese nt and future evoluti on can be exam in ed. It is through the power of applicati ons that con trollers can be seen in their full light. Industrial applications include many in both discrete manufacturing and process in dus

25、tries. Automotive in dustry applicati ons, the gen esis of the programmable con troller, continue to provide the largest base of opport uni ty. Other in dustries, such as food process ing and utilities, provide current development opportunities.There are five general application areas in which progr

26、ammable controllers are used. A typical installation will use one or more of these integrated to the control system problem. The five general areas are expla ined briefly below.Descripti onThe AT89C51 is a low-power, high-performance CMOS 8-bit microcomputer with 4K bytes of Flash programmable and e

27、rasable read only memory (PEROM). The device is manu factured using Atmel s higde nsity nonv olatile memory tech no logy and is compatible with the in dustry-sta ndardMCS-51 instruction set and pinout. The on-chip Flash allows the program memory to be reprogrammed in-system or by a conven ti onal no

28、nv olatile memory programmer. By comb ining a versatile 8-bit CPU with Flash on a monolithic chip, the Atmel AT89C51 is a powerful microcomputer which provides a highly-flexible and cost-effective solution to many embedded con trol applicati ons.Function characteristicThe AT89C51 provides the follow

29、i ng sta ndard features: 4K bytes of Flash, 128 bytes of RAM, 32 I/O lines, two 16-bit timer/counters, a five vector two-level interrupt architecture, a full duplex serial port, on-chip oscillator and clock circuitry. In addition, the AT89C51 is designed with static logic for operation down to zero

30、frequency and supports two software selectable power saving modes. The Idle Mode stops the CPU while allowing the RAM, timer/counters, serial port and interrupt system to continue functioning. The Power-down Mode saves the RAM contents but freezes the oscillator disabling all other chip functions un

31、til the next hardware reset.Pin Descripti onVCC : Supply voltage.GND: Ground.Port 0:Port 0 is an 8-bit ope n-dra in bi-directi onal I/O port. As an output port, each pin can sink eight TTL in puts. Whe n 1s are writte n to port 0 pins, the pins can be used as highimpeda nee in puts.Port 0 may also b

32、e con figured to be the multiplexed loworder address/databus duri ng accessesto external program and data memory. In this mode P0 has internal pullups.Port 0 also receives the code bytes duri ng Flash program min g,a nd outputs the code bytes duri ng programverificati on. Exter nal pullups are requi

33、red during programverification.Port 1Port 1 is an 8-bit bi-directional I/O port with internal pullups.The Port 1 output buffers can sink/source four TTL inputs.When 1s are written to Port 1 pins they are pulled high by the internal pullups and can be used as in puts. As in puts,Port 1 pins that are

34、exter nally being pulled low will source curre nt (IIL) because of the internal pullups.Port 1 also receives the low-order address bytes during Flash programming and verification.Port 2Port 2 is an 8-bit bi-directional I/O port with internal pullups.The Port 2 output buffers can sin k/source four TT

35、L in puts.Whe n 1s are writte n to Port 2 pins they are pulled high by the internal pullups and can be used as in puts. As in puts,Port 2 pins that are exter nally being pulled low will source curre nt, because of the internal pullups.Port 2 emits the high-order addressbyte duri ng fetches from exte

36、r nal program memory and duri ng accesses to exter nal data memory that use 16-bit addresses. In this application, it uses strong internal pullupswhen emitting 1s. During accesses to external data memory that use 8-bit addresses, Port 2 emits the contents of the P2 Special Function Register.Port 2 a

37、lso receives the high-order address bits and some control signals during Flash programming and verification.Port 3Port 3 is an 8-bit bi-directional I/O port with internal pullups.The Port 3 output buffers can sin k/source four TTL in puts.Whe n 1s are writte n to Port 3 pins they are pulled high by

38、the internal pullups and can be used as in puts. As in puts,Port 3 pins that are exter nally being pulled low will source current (IIL) becauseof the pullups.Port 3 also serves the functions of various special features of the AT89C51 as listed below:Port 3 also receives some control signals for Flas

39、h programming and verification.RSTReset in put. A high on this pin for two machi ne cycles while the oscillator is running resets the device.ALE/PROGAddress Latch Enable output pulse for latching the low byte of the address during accessesto exter nal memory. This pin is also the program pulse in put (PROG) duri ng Flash program ming.ln normal operation ALE is emitted at a constant rate of 1/6 the oscillator frequency, and may be used for external timing or clocking purposes. Note, however, that one ALE pulse is skipped during each ac