Part Ⅴ
Sensors and Transmitters
In a feedback control system, the elements of a process-control systemare defined interms of separate functional parts of the system . The four basic components of controlsystems are thesensors, transmitter , controller , and final control elements . Thesecomponents per form the three basic operations of every control system: measurementdecision, and action.
Sensors and transmitters perform the measurements operation of control system. Thesensor produces a phenomenon, mechanical, or the like related to the process variable that itmeasures. The function of transmitter in turn is to convert the signal from sensor to the formrequired by the final control device. The signal, therefor e, is related to the process variable.
Two analog standards are in common u se as a means of representing the range ofvariables in control systems. For electrical systems we use a range of electric current carriedin wires , and for pneumatic systems we use a range of gas pressure carried in pipes . Thesesignals are used primarily to transmitvariable information over some distance, such as to andfrom the control room and the plant .Fig .5 . 9 shows a diagram of a process- controlinstallation where current is used to transmit measurement data about the controlled variableto the control room, and gas pressure in pipes is used to transmit a feedback signal to a valve to change flow as the controlling variable .
Fig .5 .9 Electrical current and pneumatic pressures are the most common means of information transmitter in the industrial environment
Current signal The most common current transmission signal is 4 to 20 mA . Thu s , in the preceding temperature example, 20℃ might be represented by 4 mA, and 120℃ by 20 mA, with all temperatures in between represented by a proportional current . The gain is
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That is , we can say that the gain of sensor/ transmitter is ratio of the span of the output to the span of input .
Current is used instead of voltage because the system is then les s dependent on load . Voltage is not used for transmission because of its susceptibility to changes of resistance in the line .
Pneumatic signals The most common standard for pneumatic signal transmitter is 3 to 15 psi . In this case, when a sensor measures some variable in a range it is converted into a proportionalpressure of gas in a pipe . The gas is usually dry air .The pipe may be many hundreds of meters long , but as long as there is no leak in the system the pressure will be propagated down the pipe . This English system standard is still widely used in the U .S ., despite the move to the SI system of units . The equivalent SI range that will eventually be adopted is 20 to 100 kPa.
The two cases presented show that the gain of the sensor/ transmitter is constant over its completeoperating range . For most sensor/ transmitter this is the case; however , there are some in stances , such as a differential pressure sensor used to measure flow, when this is not the case . A differential pressure sensor measures the differential pressure ,h, across an orifice . This differential pressure is related to the square of the volumetric flow rate F . That isF2 ah .
The equation that describes the output signal form an electronicdifferential pressure transmitter when used to measure volumetric flow with a range of 0～Fmaxgpm is
MF = 4 + 16 F2/ ( Fmax )2 Where MF = output signal in mA
F = Volumetric flow
From this equation the gain of the transmitter is obtained as follows:
K′r = dMF/ d F = 216 F/ ( Fmax )2
with a nominal gain
K′T = 16/ Fmax
This expression shows that the gain is not constant but rather a function of flow . T he greater the flow is , the greater the gain . So the actual gain varies from zero to twice the nominal gain .
This fact results in a nonlinearly in flow control system . Nowadays most manufactures offer differential pressure transmitters with built-in square root extractor s yielding a line r transmitter .
The dynamic response of most sensor/ transmitter s is much faster than the process . Consequently , their time constants and dead time can often beconsidered negligible and thus , their transfer function is given by a pure gain . However , when the dynamics must be
considered , it is usual practice to represent the transfer function of the instrument by a first-orderor second-order system: G( s) = K / ( T s + 1)
or G( s) = K / ( T2 s + 2 Tξs + 1) WORDS AND TERMS
3 .1 Numerical Control
Numerical control is a system that uses predetermined instructions to control a sequenceof manufacturing operations. The instructions are coded numerical values stored on sometype of input medium, such as punched paper tape , magnetic tape, or a common memory for program storage . The instructions specify such things as position ,direction , velocity , and cutting speed . A partprogram contains all the instructions required to produce a desired part . A machine program contains all the instruction s required to accomplish a desired
process . Numerical control machines per form operations such as boring , drilling , grinding , milling , punching , routing , sawing , turning , winding ( wire ) , flame cutting , knitting ( garments ) , riveting , bending , welding , and wire processing .
Numerical control ( NC) has been refer red to as flexible automation because of the
relative ease of changing the program compared with changing cams , jigs , and templates . The same machine may be used to produce any number of different parts by using different programs . The numerical control process is most justified when a number of different parts are to be produced on a particular machine: it is seldom used to produce a single par t
continually on the same machine . Numerical control is ideal when a part or process is defined mathematically . With the increasing u se of computer- aided design (CAD) , more and more processes and products are being defined mathematically . Drawings as we k now them have become unnecessary ―a part that is completely defined mathematically can be manufactured
by computer-controlled machines . A closed-loop numerical control machine is shown in Fig . 5 .13 .
The NC process begins with a specification ( engineering drawing or mathematical
definition ) that completely defines the desired par t or process . A programmer uses the
specification to determine the sequence of operations necessary to produce the par t or carry out the process . The programmer also specifies the tools to be used , the cutting speeds , and the feed rates . The programmer uses a special programming language to prepare a symbolic program . APT ( Automatically Programmed Tools ) is one language used for this purpose . A computer converts the symbolic program into the part program or the machine program . In the past , the pa r t or machine program was stored on paper or magnetic t ape . The numerical control machine operator fed the tape into the machine and monitored the operation . If a change was necessary , a new tape had to be made . Now, it is possible to store the program in a common database with provision for on-demand distribution to the numerical control machine . Graphicterminals at the matching center allow operators to review programs and make changes if necessary .
The x position controller moves the work piece horizontally in the direction indicated by the + x a r row . The position controller moves the milling machine head horizontally in the direction indicated by the + yarrow . The z position controller moves the cutting tool
vertically as indicated by the + z arrow . The following actions are involved in changing the x-axis position .( 1 ) The control unit reads an instruction in the program that specifies a
+ 0 .004-inch ( in .) change in the x position . ( 2 ) The control unit send s a pulse to the
machine actuator . (3 ) The machine actuator rotates the lead screw and advances the x- axis position + 0 .001 in . (4 ) the position sensor measures the + 0 .001-in . measured motion and sends another pulse . Steps ( 1 ) through ( 5 ) are repeated until the measured motion equals the desired + 0 .004 in .
Computerized numerical control ( CNC ) was developed to utilize the storage and
processing capabilities of a digital computer . CNC uses a dedicated computer to accept the input of instructions and to perform the control functions required to produce the part . However , CNC was not designed to provide the information exchange demanded by the recent t r end toward computer-integrated manufacturing (CIM) . The idea of CIM is to“ get the right information- to the right person- at the right time- to make the right decision .”“I t link s all aspects of the business-f rom quotation and order entry through engineering , process planning , financial reporting , manufacturing , and shipping-in an efficient chain of production .”
Direct numerical control ( DNC ) was developed to facilitate computer-integrated
manufacturing . DNC is a system in which a n umber of numerical control machines are
connected to a central computer for real- time access to a common database of part programs and machine programs . General Electric used a central computer connected to DNC machines through a communication s network in the automation of its steam turbine-generator
operation s (STGO) .“A typical turbine-genera tor consists of more than 100 , 000 parts , some
自动化专业英语PartⅤ-Ⅵ 课文原文内容
