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Proportional control

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In the proportional control algorithm, the controller output is proportional to the error signal, which is the difference between the setpoint and the process variable. In other words, the output of a proportional controller is the multiplication product of the error signal and the proportional gain.
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In proportional control, the power output is always proportional to the (actual versus target speed) error. If the car is at target speed and the speed increases slightly due to a falling gradient, the power is reduced slightly, or in proportion to the change in error, so that the car reduces speed
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tells us that the spring applies a corrective force that is proportional to the object's displacement. While this will tend to hold the object in a particular location, the absolute resting location of the object will vary if its mass is changed. This difference in resting location is the offset
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gradually and reaches the new target point with very little, if any, "overshoot", which is much smoother control than on–off control. In practice, PID controllers are used for this and the large number of other control processes that require more responsive control than using proportional alone.
2378:, full power would again be applied. It can be seen that this would obviously result in poor control and large variations in speed. The more powerful the engine, the greater the instability; the heavier the car, the greater the stability. Stability may be expressed as correlating to the 171:. On–off control will work where the overall system has a relatively long response time, but can result in instability if the system being controlled has a rapid response time. Proportional control overcomes this by modulating the output to the controlling device, such as a 2334:, the proportional gain, is very high, the proportional band is very small, which means that the band of controller output over which the final control element will go from minimum to maximum (or vice versa) is very small. This is the case with on–off controllers, where 2238:
adjustment, if possible) to eliminate offset error for expected conditions, when a disturbance (deviation from existing state or setpoint adjustment) occurs in the process, corrective control action, based purely on proportional control, will result in an offset error.
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A drawback of proportional control is that it cannot eliminate the residual SP − PV error in processes with compensation e.g. temperature control, as it requires an error to generate a proportional output. To overcome this the
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error. Over a range of operating conditions, proportional control alone is unable to eliminate offset error, as it requires an error to generate an output adjustment. While a proportional controller may be tuned (via
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The proportional band is the band of controller output over which the final control element (a control valve, for instance) will move from one extreme to another. Mathematically, it can be expressed as:
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The clear advantage of proportional over on–off control can be demonstrated by car speed control. An analogy to on–off control is driving a car by applying either full power or no power and varying the
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Consider an object suspended by a spring as a simple proportional control. The spring will attempt to maintain the object in a certain location despite disturbances that may temporarily displace it.
1305: 2015: 1598: 1218: 1949: 2374:, to control speed. The power would be on until the target speed is reached, and then the power would be removed, so the car reduces speed. When the speed falls below the target, with a certain 1007: 272: 2302: 1869: 1797:{\displaystyle \lim _{t\to \infty }y(t)=\lim _{s\,\searrow \,0}\left(s\times {\frac {k_{CL}}{\tau _{CL}s+1}}\times {\frac {\Delta R}{s}}\right)=k_{CL}\times \Delta R=y(t)|_{t=\infty }} 746: 934: 792: 572: 537: 338: 1489: 1532: 1416: 1012: 2204:{\displaystyle \lim _{t\to \infty }y(t)=\lim _{s\,\searrow \,0}\left(s\times {\frac {k_{c}}{s(s+1)+k_{c}}}\times {\frac {\Delta R}{s}}\right)=\Delta R=y(t)|_{t=\infty }} 449: 1443: 2359: 2332: 602: 367: 631: 398: 498: 473: 301: 136:
in which a correction is applied to the controlled variable, and the size of the correction is proportional to the difference between the desired value (
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is an early example of proportional control. The balls rise as speed increases, which closes the valve, reducing speed until a balance is achieved.
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term (I) to eliminate the residual offset error by integrating the error over time to produce an "I" component for the controller output.
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at a level which avoids instability, but applies correction as fast as practicable by applying the optimum quantity of proportional gain.
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meaning there is no offset in this system. This is the only process that will not have any offset when using a proportional controller.
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as a ratio with the span of the instrument. This span is in the same units as error (e.g. C degrees) so the ratio has no units.
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Flow control loop. If only used as a proportional controller, then there's always an offset between SP and PV.
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is very high and hence, for even a small error, the controller output is driven from one extreme to another.
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Constraints: In a real plant, actuators have physical limitations that can be expressed as constraints on
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is process transfer function; the input into the block is flow rate and output is tank level.
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was devised, which uses a proportional term (P) to remove the gross error, and an
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Offset error is the difference between the desired value and the actual value,
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may be bounded between −1 and +1 if those are the maximum output limits.
760:, which is the difference between setpoint and measured process output. 2411:. Upper Saddle River, New Jersey: Prentice Hall PTR. pp. 165–168. 2375: 2371: 1871:, which, when combined with the closed-loop transfer function, becomes 160: 114: 1009:. Combining this with the closed-loop transfer function above returns 1954:
Introducing a step change to the system gives the output response of
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Introducing a step change to the system gives the output response of
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which shows that there will always be an offset in the system.
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Proportional control compared to on–off or bang–bang control
1375:{\displaystyle \tau _{CL}={\frac {\tau _{p}}{1+k_{p}k_{c}}}} 144:, PV). Two classic mechanical examples are the toilet bowl 1814:
For an integrating process, a general transfer function is
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For a first-order process, a general transfer function is
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The proportional control concept is more complex than an
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control system used in something like an automobile
1002:{\displaystyle g_{p}={\frac {k_{p}}{\tau _{p}s+1}}} 936:are stable, then the closed-loop system is stable. 656:
may be too technical for most readers to understand
267:{\displaystyle P_{\mathrm {out} }=K_{p}\,{e(t)+p0}} 2353: 2326: 2296: 2203: 2009: 1943: 1863: 1796: 1592: 1526: 1483: 1437: 1410: 1374: 1299: 1212: 1138: 1001: 928: 891: 786: 740: 625: 604:as a unitless number. To do this, we can express 596: 566: 531: 492: 467: 443: 392: 361: 332: 295: 266: 2408:Process Control: Modeling, Design, and Simulation 2441: 2061: 2030: 1644: 1613: 752:, the setpoint, is the flowrate into a tank and 577:Qualifications: It is preferable to express 748:. From the block diagram shown, assume that 51:introducing citations to additional sources 797:The output as a function of the setpoint, 2297:{\displaystyle PB={\frac {100}{K_{p}}}\ } 2072: 2068: 1864:{\displaystyle g_{p}={\frac {1}{s(s+1)}}} 1655: 1651: 684:Learn how and when to remove this message 668:, without removing the technical details. 240: 2404: 2220: 1491:(standard practice is to make sure that 695: 199:This can be mathematically expressed as 113: 41:Relevant discussion may be found on the 1146:. Simplifying this equation results in 741:{\displaystyle {\mathit {g_{c}=k_{c}}}} 340:: Output of the proportional controller 2442: 1809: 939: 400:: Instantaneous process error at time 165:proportional–integral–derivative (PID) 159:system such as a bi-metallic domestic 666:make it understandable to non-experts 637:Development of control block diagrams 2400: 2398: 2250: 640: 303:: Controller output with zero error. 18: 929:{\displaystyle {\mathit {g_{cl}}},} 13: 2196: 2162: 2142: 2040: 1995: 1789: 1755: 1719: 1623: 1578: 913: 909: 850: 846: 840: 836: 819: 815: 787:{\displaystyle {\mathit {g_{p}}},} 774: 770: 731: 727: 718: 714: 567:{\displaystyle P_{\mathrm {out} }} 558: 555: 552: 532:{\displaystyle P_{\mathrm {out} }} 523: 520: 517: 333:{\displaystyle P_{\mathrm {out} }} 324: 321: 318: 221: 218: 215: 14: 2471: 2428: 2395: 1603:Using the final-value theorem, 1484:{\displaystyle k_{p}k_{c}>-1} 1382:. For stability in this system, 645: 34:relies largely or entirely on a 23: 2216: 2020:Using the final-value theorem, 1527:{\displaystyle k_{p}k_{c}>0} 1445:must be a positive number, and 1411:{\displaystyle \tau _{CL}>0} 2185: 2180: 2174: 2117: 2105: 2069: 2054: 2048: 2037: 1970: 1964: 1922: 1910: 1855: 1843: 1778: 1773: 1767: 1652: 1637: 1631: 1620: 1553: 1547: 704:Proportional control dictates 620: 614: 420: 414: 387: 381: 251: 245: 140:, SP) and the measured value ( 16:Linear feedback control system 1: 2389: 2364: 803:closed-loop transfer function 700:Simple feedback control loop2 7: 2405:Bequette, B. Wayne (2003). 10: 2476: 444:{\displaystyle e(t)=SP-PV} 1438:{\displaystyle \tau _{p}} 190: 146:float proportioning valve 2450:Classical control theory 2355: 2328: 2298: 2226: 2205: 2011: 1945: 1865: 1798: 1594: 1528: 1485: 1439: 1412: 1376: 1301: 1214: 1140: 1003: 930: 893: 788: 742: 701: 627: 598: 568: 533: 494: 469: 445: 394: 363: 334: 297: 268: 123: 62:"Proportional control" 2380:power-to-weight ratio 2356: 2354:{\displaystyle K_{p}} 2329: 2327:{\displaystyle K_{p}} 2299: 2224: 2206: 2012: 1946: 1866: 1799: 1595: 1529: 1486: 1440: 1413: 1377: 1302: 1215: 1141: 1004: 931: 894: 789: 743: 699: 628: 599: 597:{\displaystyle K_{p}} 569: 534: 495: 470: 446: 395: 364: 362:{\displaystyle K_{p}} 335: 298: 269: 163:, but simpler than a 117: 2338: 2311: 2262: 2026: 1958: 1875: 1818: 1609: 1541: 1495: 1449: 1422: 1386: 1311: 1224: 1150: 1013: 948: 903: 809: 764: 708: 626:{\displaystyle e(t)} 608: 581: 543: 508: 481: 456: 408: 393:{\displaystyle e(t)} 375: 346: 309: 284: 206: 127:Proportional control 47:improve this article 2460:Control engineering 1810:Integrating process 940:First-order process 369:: Proportional gain 2351: 2324: 2294: 2227: 2201: 2077: 2044: 2007: 1941: 1861: 1794: 1660: 1627: 1590: 1524: 1481: 1435: 1408: 1372: 1297: 1210: 1136: 999: 926: 889: 801:, is known as the 784: 738: 702: 623: 594: 564: 529: 500:: Process variable 493:{\displaystyle PV} 490: 468:{\displaystyle SP} 465: 441: 390: 359: 330: 296:{\displaystyle p0} 293: 264: 124: 2418:978-0-13-353640-9 2382:of the vehicle. 2293: 2289: 2251:Proportional band 2152: 2134: 2060: 2029: 2005: 1939: 1859: 1729: 1711: 1643: 1612: 1588: 1370: 1295: 1208: 1134: 1131: 1078: 997: 884: 694: 693: 686: 150:fly-ball governor 120:fly-ball governor 112: 111: 97: 2467: 2423: 2422: 2402: 2360: 2358: 2357: 2352: 2350: 2349: 2333: 2331: 2330: 2325: 2323: 2322: 2303: 2301: 2300: 2295: 2291: 2290: 2288: 2287: 2275: 2237: 2232: 2210: 2208: 2207: 2202: 2200: 2199: 2188: 2158: 2154: 2153: 2148: 2140: 2135: 2133: 2132: 2131: 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For example, 516: 515: 511: 509: 506: 505: 482: 479: 478: 457: 454: 453: 409: 406: 405: 376: 373: 372: 353: 349: 347: 344: 343: 317: 316: 312: 310: 307: 306: 285: 282: 281: 241: 234: 230: 214: 213: 209: 207: 204: 203: 193: 108: 102: 99: 56: 54: 40: 28: 17: 12: 11: 5: 2473: 2463: 2462: 2457: 2452: 2438: 2437: 2430: 2429:External links 2427: 2425: 2424: 2417: 2393: 2391: 2388: 2366: 2363: 2348: 2344: 2321: 2317: 2286: 2282: 2278: 2273: 2270: 2267: 2252: 2249: 2218: 2215: 2198: 2195: 2192: 2187: 2182: 2179: 2176: 2173: 2170: 2167: 2164: 2161: 2157: 2151: 2147: 2144: 2138: 2130: 2126: 2122: 2119: 2116: 2113: 2110: 2107: 2104: 2098: 2094: 2088: 2085: 2081: 2075: 2071: 2067: 2063: 2059: 2056: 2053: 2050: 2047: 2042: 2039: 2036: 2032: 2004: 2000: 1997: 1991: 1986: 1983: 1979: 1975: 1972: 1969: 1966: 1963: 1935: 1931: 1927: 1924: 1921: 1918: 1915: 1912: 1909: 1903: 1899: 1893: 1888: 1885: 1881: 1857: 1854: 1851: 1848: 1845: 1842: 1838: 1833: 1828: 1824: 1811: 1808: 1791: 1788: 1785: 1780: 1775: 1772: 1769: 1766: 1763: 1760: 1757: 1754: 1749: 1746: 1742: 1738: 1734: 1728: 1724: 1721: 1715: 1709: 1706: 1703: 1698: 1695: 1691: 1684: 1681: 1677: 1671: 1668: 1664: 1658: 1654: 1650: 1646: 1642: 1639: 1636: 1633: 1630: 1625: 1622: 1619: 1615: 1587: 1583: 1580: 1574: 1569: 1566: 1562: 1558: 1555: 1552: 1549: 1546: 1523: 1520: 1515: 1511: 1505: 1501: 1480: 1477: 1474: 1469: 1465: 1459: 1455: 1432: 1428: 1407: 1404: 1399: 1396: 1392: 1366: 1362: 1356: 1352: 1348: 1345: 1339: 1335: 1329: 1324: 1321: 1317: 1291: 1287: 1281: 1277: 1273: 1270: 1263: 1259: 1253: 1249: 1242: 1237: 1234: 1230: 1206: 1203: 1200: 1195: 1192: 1188: 1181: 1178: 1174: 1168: 1163: 1160: 1156: 1129: 1126: 1123: 1118: 1114: 1106: 1102: 1096: 1092: 1085: 1082: 1076: 1073: 1070: 1065: 1061: 1053: 1049: 1043: 1039: 1031: 1026: 1023: 1019: 995: 992: 989: 984: 980: 973: 969: 963: 958: 954: 941: 938: 925: 918: 915: 911: 888: 880: 876: 870: 866: 862: 859: 852: 848: 842: 838: 831: 824: 821: 817: 783: 776: 772: 733: 729: 725: 720: 716: 692: 691: 653: 651: 644: 638: 635: 622: 619: 616: 613: 591: 587: 560: 557: 554: 549: 525: 522: 519: 514: 502: 501: 489: 486: 476: 464: 461: 451: 440: 437: 434: 431: 428: 425: 422: 419: 416: 413: 389: 386: 383: 380: 370: 356: 352: 341: 326: 323: 320: 315: 304: 292: 289: 275: 274: 262: 259: 256: 253: 250: 247: 244: 237: 233: 229: 223: 220: 217: 212: 192: 189: 169:cruise control 157:on–off control 134:control system 110: 109: 45:. Please help 31: 29: 22: 15: 9: 6: 4: 3: 2: 2472: 2461: 2458: 2456: 2453: 2451: 2448: 2447: 2445: 2436: 2433: 2432: 2420: 2414: 2410: 2409: 2401: 2399: 2394: 2387: 2383: 2381: 2377: 2373: 2362: 2346: 2342: 2319: 2315: 2305: 2284: 2280: 2276: 2271: 2268: 2265: 2257: 2248: 2245: 2240: 2223: 2214: 2211: 2193: 2190: 2177: 2171: 2168: 2165: 2159: 2155: 2149: 2145: 2136: 2128: 2124: 2120: 2114: 2111: 2108: 2102: 2096: 2092: 2086: 2083: 2079: 2073: 2065: 2057: 2051: 2045: 2034: 2021: 2018: 2002: 1998: 1989: 1984: 1981: 1977: 1973: 1967: 1961: 1952: 1933: 1929: 1925: 1919: 1916: 1913: 1907: 1901: 1897: 1891: 1886: 1883: 1879: 1852: 1849: 1846: 1840: 1836: 1831: 1826: 1822: 1807: 1804: 1786: 1783: 1770: 1764: 1761: 1758: 1752: 1747: 1744: 1740: 1736: 1732: 1726: 1722: 1713: 1707: 1704: 1701: 1696: 1693: 1689: 1682: 1679: 1675: 1669: 1666: 1662: 1656: 1648: 1640: 1634: 1628: 1617: 1604: 1601: 1585: 1581: 1572: 1567: 1564: 1560: 1556: 1550: 1544: 1535: 1521: 1518: 1513: 1509: 1503: 1499: 1478: 1475: 1472: 1467: 1463: 1457: 1453: 1430: 1426: 1418:; therefore, 1405: 1402: 1397: 1394: 1390: 1364: 1360: 1354: 1350: 1346: 1343: 1337: 1333: 1327: 1322: 1319: 1315: 1289: 1285: 1279: 1275: 1271: 1268: 1261: 1257: 1251: 1247: 1240: 1235: 1232: 1228: 1204: 1201: 1198: 1193: 1190: 1186: 1179: 1176: 1172: 1166: 1161: 1158: 1154: 1127: 1124: 1121: 1116: 1112: 1104: 1100: 1094: 1090: 1083: 1080: 1074: 1071: 1068: 1063: 1059: 1051: 1047: 1041: 1037: 1029: 1024: 1021: 1017: 993: 990: 987: 982: 978: 971: 967: 961: 956: 952: 937: 923: 886: 878: 874: 868: 864: 860: 857: 829: 804: 800: 795: 781: 759: 755: 751: 698: 688: 685: 677: 667: 663: 657: 654:This section 652: 643: 642: 634: 617: 611: 589: 585: 575: 547: 512: 487: 484: 477: 462: 459: 452: 438: 435: 432: 429: 426: 423: 417: 411: 403: 384: 378: 371: 354: 350: 342: 313: 305: 290: 287: 280: 279: 278: 260: 257: 254: 248: 242: 235: 231: 227: 210: 202: 201: 200: 197: 188: 186: 182: 181:PI controller 176: 174: 173:control valve 170: 166: 162: 158: 153: 151: 147: 143: 139: 135: 132: 128: 121: 116: 106: 95: 92: 88: 85: 81: 78: 74: 71: 67: 64: –  63: 59: 58:Find sources: 52: 48: 44: 38: 37: 36:single source 32:This article 30: 26: 21: 20: 2407: 2384: 2368: 2306: 2258: 2254: 2241: 2228: 2217:Offset error 2212: 2022: 2019: 1953: 1813: 1805: 1605: 1602: 1536: 943: 802: 798: 796: 757: 753: 749: 703: 680: 671: 655: 576: 503: 401: 276: 198: 194: 177: 154: 126: 125: 100: 90: 83: 76: 69: 57: 33: 2244:Hooke's law 475:: Set point 2444:Categories 2390:References 2376:hysteresis 2372:duty cycle 2365:Advantages 674:March 2024 161:thermostat 103:March 2021 73:newspapers 2197:∞ 2163:Δ 2143:Δ 2137:× 2087:× 2070:↘ 2041:∞ 2038:→ 1996:Δ 1990:× 1790:∞ 1756:Δ 1753:× 1720:Δ 1714:× 1690:τ 1670:× 1653:↘ 1624:∞ 1621:→ 1579:Δ 1573:× 1476:− 1427:τ 1391:τ 1334:τ 1316:τ 1187:τ 1113:τ 1060:τ 979:τ 433:− 43:talk page 185:integral 148:and the 138:setpoint 131:feedback 2247:error. 2231:SP − PV 660:Please 87:scholar 2415:  2307:So if 2292:  1220:where 277:where 191:Theory 89:  82:  75:  68:  60:  758:error 94:JSTOR 80:books 2413:ISBN 1534:). 1519:> 1473:> 1403:> 1307:and 118:The 66:news 2277:100 2062:lim 2031:lim 1951:. 1645:lim 1614:lim 1600:. 756:is 664:to 404:. 49:by 2446:: 2397:^ 2236:p0 2017:. 805:. 152:. 2421:. 2347:p 2343:K 2320:p 2316:K 2285:p 2281:K 2272:= 2269:B 2266:P 2194:= 2191:t 2186:| 2181:) 2178:t 2175:( 2172:y 2169:= 2166:R 2160:= 2156:) 2150:s 2146:R 2129:c 2125:k 2121:+ 2118:) 2115:1 2112:+ 2109:s 2106:( 2103:s 2097:c 2093:k 2084:s 2080:( 2074:0 2066:s 2058:= 2055:) 2052:t 2049:( 2046:y 2035:t 2003:s 1999:R 1985:L 1982:C 1978:g 1974:= 1971:) 1968:s 1965:( 1962:y 1934:c 1930:k 1926:+ 1923:) 1920:1 1917:+ 1914:s 1911:( 1908:s 1902:c 1898:k 1892:= 1887:L 1884:C 1880:g 1856:) 1853:1 1850:+ 1847:s 1844:( 1841:s 1837:1 1832:= 1827:p 1823:g 1787:= 1784:t 1779:| 1774:) 1771:t 1768:( 1765:y 1762:= 1759:R 1748:L 1745:C 1741:k 1737:= 1733:) 1727:s 1723:R 1708:1 1705:+ 1702:s 1697:L 1694:C 1683:L 1680:C 1676:k 1667:s 1663:( 1657:0 1649:s 1641:= 1638:) 1635:t 1632:( 1629:y 1618:t 1586:s 1582:R 1568:L 1565:C 1561:g 1557:= 1554:) 1551:s 1548:( 1545:y 1522:0 1514:c 1510:k 1504:p 1500:k 1479:1 1468:c 1464:k 1458:p 1454:k 1431:p 1406:0 1398:L 1395:C 1365:c 1361:k 1355:p 1351:k 1347:+ 1344:1 1338:p 1328:= 1323:L 1320:C 1290:c 1286:k 1280:p 1276:k 1272:+ 1269:1 1262:c 1258:k 1252:p 1248:k 1241:= 1236:L 1233:C 1229:k 1205:1 1202:+ 1199:s 1194:L 1191:C 1180:L 1177:C 1173:k 1167:= 1162:L 1159:C 1155:g 1128:1 1125:+ 1122:s 1117:p 1105:c 1101:k 1095:p 1091:k 1084:+ 1081:1 1075:1 1072:+ 1069:s 1064:p 1052:c 1048:k 1042:p 1038:k 1030:= 1025:L 1022:C 1018:g 994:1 991:+ 988:s 983:p 972:p 968:k 962:= 957:p 953:g 924:, 917:l 914:c 910:g 887:, 879:c 875:g 869:p 865:g 861:+ 858:1 851:c 847:g 841:p 837:g 830:= 823:l 820:c 816:g 799:r 782:, 775:p 771:g 754:e 750:r 732:c 728:k 724:= 719:c 715:g 687:) 681:( 676:) 672:( 658:. 621:) 618:t 615:( 612:e 590:p 586:K 559:t 556:u 553:o 548:P 524:t 521:u 518:o 513:P 488:V 485:P 463:P 460:S 439:V 436:P 430:P 427:S 424:= 421:) 418:t 415:( 412:e 402:t 388:) 385:t 382:( 379:e 355:p 351:K 325:t 322:u 319:o 314:P 291:0 288:p 261:0 258:p 255:+ 252:) 249:t 246:( 243:e 236:p 232:K 228:= 222:t 219:u 216:o 211:P 105:) 101:( 91:· 84:· 77:· 70:· 53:. 39:.

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fly-ball governor
feedback
control system
setpoint
process variable
float proportioning valve
fly-ball governor
on–off control
thermostat
proportional–integral–derivative (PID)
cruise control
control valve
PI controller
integral
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