Patent GB-1361211-A (GB)
Title: Control method and apparatus for a machine or process having dead time
IPC: D21G9/00
Filed: 1971-04-19
Abstract: 1361211 Retro-active control of papermaking INDUSTRIAL NUCLEONICS CORP 19 April 1971 [3 Feb 1970] 20092/71 Heading G3R In the control of a plant 11, for example a paper-making machine, steel rolling system, or plastics extrusion system, having the property that there exists a dead time or transportation lag T 2  between the time of reception of a control input change, and the consequent change in the plant output, the error signal is fed both to the plant, and to a model 17, 20 of the plant, unit 17 representing the steady state response of the plant, unit 20 the transient response. The modelled transient response signal is delayed by the time T 2  and combined in opposed polarity with the steady state response, the composite signal being fed back to the input of controller 16, via summation unit 15 receiving also a set print signal R(s), and the actual plant output signal C (s) such that the steady state model signal is negatively fed back; the delayed modelled transient response positively. In such a system, although the set point, actual and model feedback signals are sampled at intervals T 1  less than the time T 2 , the average of the latter two signals in any sample period T 1  being derived by units 12, 21, it is possible to retain full control during any one sampling interval. Units 8 and 9 accumulate their respective input signals, changing state upon synchronous operation of switches 13, 14, 22. The system of Fig. 2 enables adaptation of the system to allow for gain changes in the plant,  since unit 20 represents only the dead time T 2  and other transient characteristics of the plant, but otherwise the configuration of Fig. 1, (not shown), with units (17) and delaying and transient response unit (18) may each feed an input to summation unit (19). For weakened feedback, the response to changed control input signal may be spread over several sampling intervals, T 1 . Application to simultaneous control of moisture content, M, and bore drag basis weight, BDBW, of paper, Fig.3. Capacity type moisture gauge 67, and nucleic type basis weight, BW gauge 68 scan transversely to the paper, the sampling interval T 1  corresponding to the traverse time between limit switches 71, 72. Thick stock valve 32 and the steam supply pressure to dryer 57 are controlled. Signals from both gauges are combined in unit 73, in the form BW (1-M) = BDBW, the latter signal being combined in summing unit 83 with a corresponding output from the BDBW model 101, detailed in Fig. 4, (not shown), the output of unit 83 being fed to the controller 105. The actual moisture signal M is fed to summing unit 82 also receiving an output from the moisture model 102, detailed in Fig. 5, (not shown), and providing an input to controller 105 whose outputs provide an open loop control signal Qstk S E T  defining the thick stock valve setting, and a steam pressure set print signal P SET  controlling the steam pressure in dryer 57 via retro-active system 62, 63, 64, 59. Unit 107 receiving the set pressure signal provides, for the model 102, data simulating the change in drying capacity of dryer 57 consequent upon steam pressure change, unit 108 simulating the required change in drying brought about by change in the thick stock valve setting. The models 101, 102 are coupled since change in BDBW SET  85, requires a change in drying capacity, and a signal # v/v representing any change in the velocity of Fourdrinier wire 48, controlled by retro-active speed loop 52, 53, 54, 56, is fed to model 102. The system is digital, with a/d and d/a converters at machine/control inter-faces. The averaging units normally average the signals over a transverse scan of gauges 67, 68, but, for changing the grade of paper, switches 94 and 89 are open, allowing progressive change through a scan. Operation of the model 101, simulating the transport, or dead time T 2  for fibre movement from valve 32 to gauges 67, 68 is disclosed in conjunction with Fig.4, (not shown), and model 102 in Fig. 5, (not shown), the latter dead time being equivalent to the mean transit time for paper movement between heater 57 and the gauges.