Improved method and appartus for adaptive deadtime process control

Abstract: An improved process-integral-derivative deadtime controller (PIDτ d ) utilizes an adaptive filter to compensate for mismatch between the controller deadtime and that of the process. The filter time constant is generated as a function of the manipulated variable signal noise level.

Description: This application is related to the following copending, commonly assigned applications, assigned to the assignee hereof and filed the same day herewith, and which claim priority from United States Patent Application No. 889,472 entitled METHOD AND APPARATUS FOR ANALYZING PROCESS CHARACTERISTICS (Attorney Docket No. 10227); and United States Patent Application No. 889,474 entitled METHOD AND APPARATUS FOR TUNING PROCESS CONTROL EQUIPMENT (Attorney Docket No. 10226). The teachings of the above-cited applications are incorporated herein by reference. Background of the Invention The invention relates to process control and, more particularly, to improved deadtime process control apparatus and methods. "Process control" refers to the control of the operational parameters of a process by monitoring one or more of its characteristics over time. It is used to ensure that the quality and efficiency of a process do not vary substantially during a single run or over the course of several runs. Process control has application in both the manufacturing and service sectors. A process control unit, or "controller," typically operates by monitoring and comparing a process characteristic, the controlled variable, with a desired "setpoint" level to determine whether the process is operating within acceptable bounds. As the controlled variable begins to deviate, the controller manipulates one of the process inputs, the manipulated variable, to bring the process back to the desired level of activity. For example, as shown in Figure 1, a process controller can oversee a process in which fluid flows at a constant rate from a continuously refilled tank. The controller monitors the liquid level in the tank and, when necessary to prevent the tank from running dry or overflowing, adjusts an inlet valve to increase or restrict the inflow. Among the controllers developed by the art are those in which the manipulated variable signal is generated as a predetermined mathematical function of the controlled variable. One such controller, illustrated in Figure 2a, is the proportional-integral-derivative (PID) controller. There, the manipulated variable is generated as a function of the "error," that is, the difference between the controlled variable and the setpoint: where, m is the manipulated variable; e is the error; P, I and D are constants SU-A-1,287,104 describes an adaptive regulator with regulated feedback comprising a PID regulator with switches connecting three linear filters to increase stability and improve the quality of regulation. In addition, the art has developed model-based controllers which model, mathematically, the operation of the process. One such controller is the deadtime controller, which relies on a model of the process deadtime and lag to determine values for the manipulated variable. See, for example, MEASUREMENT AND CONTROL vol. 23, no. 4, May 1990, LONDON GB pages 114-121; F. SHINSKEY 'HOW GOOD ARE OUR CONTROLLERS IN ABSOLUTE PERFORMANCE AND ROBUSTNESS' which describes a method and apparatus for controlling a process according to the preamble of claims 1 and 7, respectively. Deadtime is the time it takes a change in the manipulated variable to be reflected by a change in the controlled variable. For example, in a paper making process, deadtime is the time it takes for a change in the bleaching agent added to the initial slurry to be detected by a photo sensor that measures the whiteness of the final product web. Lag is the time, after the deadtime period, that it takes the controlled variable to move approximately 63% of its final value, following a step change in the manipulated variable. A deadtime controller of the type referred to above is shown in Figure 2b. It is constructed by adding a "deadtime element," i.e., a time delay, into the integral feedback loop of a PID controller. This controller is referred to by the mnemonic "PIDτ d ." Prior art PIDτ d controllers have not proven entirely satisfactory when utilized to control deadtime-dominant processes, that is, processes whose output characteristics are primarily determined by deadtime (as opposed to lag). It is, accordingly, an object of this invention to provide improved methods and apparatus for process control. More particularly, an object is to provide improved PIDτ d deadtime controllers. Still another object is to provide a deadtime controller capable of accurately and efficiently controlling deadtime dominant processes. These and other objects are evident in the description which follows. The method and controller according to the present invention is characterized by the features of the characterizing portions of claims 1 and 7, respectively. These and other objects are attained by the invention which provides, in one aspect, a deadtime controller, based on a PIDτ d design, which incorporates an adaptive noise filter in the output path, upstream from the feedback to the integration section. More particularly, the invention provides an adaptive PIDTd controller having a proportional derivative section that generates an error signal representing an error in the control signal, particularly, as a function of (i) a time rate of change of the controlled variable signal and (ii) a difference between the controlled variable signal and a predetermined setpoint. A summation element sums that error signal with a time-delayed integral signal to generate a summation signal. That summation signal, in turn, is filtered by the aforementioned adaptive noise filter to generate a manipulated variable signal, which is selectively applied to the process by an output section. A feedback element processes the manipulated variable signal to generate the time-delayed integral signal, referred to above. That processing includes time-delaying the manipulated variable signal an amount substantially equal to the deadtime of the process, τ d , and passing that time-delayed manipulated variable signal through a first-order lag of time constant I to generate the time-delayed integral signal. According to a further aspect of the invention, the adaptive noise filter varies the filter time constant to maintain the level of noise in the manipulated variable signal at a predetermined level. Particularly, that time constant can be generated in accord with the mathematical relation: τ f = τ d /20 * σ m /σ set where, τ f is the filter time constant; τ d is the process deadtime; σ m is the noise level in the controlled variable signal; and σ set is predetermined noise level. According to still another aspect of the invention, the filter time constant can be limited to a pre-selected range between 0 and 0.1 * τ d . Still other aspects of the invention include methods for process control in accord with the operations described above. A better understanding of the invention may be attained from the following non-limiting description with reference to the drawings, in which: Figure 1 depicts a fluid-flow process under control of a process controller; Figures 2a and 2b depict conventional PID and PIDτ d controllers; Figure 3 depicts an effect of mismatch between the deadtime of a PIDτ d controller and that of the process it controls; Figure 4 depicts an adaptive PIDτ d controller constructed in accord with the invention; Figure 5 depicts a preferred sequence for filter adaption in a PIDτ d controller constructed in accord with the invention; and Figure 6 depicts the control of a deadtime dominant process by an adaptive PIDτ d controller in accord with the invention. Detailed Description of the Illustrated Embodiment It has been discovered that prior art PIDτ d controllers, when not perfectly tuned to the processes they control, generate a manipulated variable signal with a pattern of high-frequency pulses. Figure 3 illustrates this phenomenon, as particularly reflected when the deadtime of the controller is greater than that of a first-order deadtime dominant process that it controls. Referring to the drawing, the controlled variable, c, experiences an initial rapid increase of value in response to an applied step load (not shown) on the process. As the controller compensates for the disturbance, the controlled variable converges toward the setpoint, s, which is represented by the horizontal axis. The controller output, represented by manipulated variable signal, m, responds to the initial change in the controlled variable by rapidly decreasing in value. As the controlled variable begins returning toward the setpoint, the manipulated variable, too, returns to its normal value, n. Because of the mismatch between the deadtime constants of the process and controller, however, the manipulated variable signal begins to spike at each transition. These spikes, which increase in magnitude, effect changes in the process itself, as reflected in spiking of the controlled variable signal. Figure 4 depicts an adaptive PIDτ d controller according to the invention. Aspects of the controller 10 are constructed and operated in accord with a conventional PIDτ d controller, as particularly modified in accord with the discussion which follows. The controller 10 includes a proportional-derivative section 12 that accepts as inputs the controlled variable signal, c, and a desired setpoint. The controlled variable signal can be generated in a manner conventional to the art, for example, by measuring equipment coupled with the process itself. The setpoint, too, can be generated in a conventional manner, e.g., via the operator at a console (not shown). The proportional-derivative section 12 operates in a manner like that of the corresponding section of a conventional PIDτ d controller. The section 12 generates a PD signal representative of an error in the controlled variable signal, c, particularly, as a function of a time rate of change of the controlled variable and a difference between it and the setpoint. The gain of proportional-derivative section 12 can be set in a conventional manner. Preferably, however, the gain of that section 12 (and, accordingly, that of the illustrated controller 10) is set in accord with the teachings of related application United States Patent Application No. 889,474, for METHOD AND APPARATUS FOR TUNING PROCESS CONTROL EQUIPMENT (Attorney Docket No. 10226). Illustrated integral control element 14 and deadtime element 16, together, generate a signal, ID, representing a time-wise integral of a time delayed form of the manipulated variable signal, m. These elements, too, operate in a manner like that of the corresponding elements of a conventional PIDτ d controller. More particularly, the deadtime section 16 time-delays the manipulated variable signal, m, by the controller deadtime τ d . The integral control section, 14, integrates that time-delayed signal with a time constant I. A summation section 18 sums the PD signal generated by the PD section 12 with the ID signal generated by the integral control 14. The output of element 18 is passed through filter 20, having a time constant τ f , which attenuates high frequency components of the summed signal. The filtered signal can be passed through high and low limits 22, which limits the filtered signal to limit integral windup. The signal output by filter 20, and optionally limited by limiter unit 22, is applied to the process as manipulated variable signal m. That signal is fed back to the integral control and deadtime elements 14, 16, as illustrated, for production of the signal ID. The controller 10 automatically adjusts the filter time constant τ f to maintain the level of noise, σ m , in the manipulated variable signal at a predetermined level, σ set . A procedure for adjusting τ f is shown in Figure 5. There, in step 30, the controller 10 samples values of the manipulated variable signal, m. In step 31, the controller sums values of m, as well as squares of those values. In step 33, the controller determines whether to sample more output values, preferably, so as to obtain a full set of samples over the period of a complete controller deadtime τ d . Following sampling and summing steps 30 - 33, the controller determines the noise level of the manipulated variable signal; see step 34. Preferably, this is determined in accord with the mathematical relation: σ m = n·Σ(m 2 )-(Σm) 2 n·(n -1) where, n is the number of sampled manipulated variable values; (Σm) 2 is the square of the sum of the sampled values; and Σ(m 2 ) is the sum of squared sampled values. In step 35, the controller determines the filter time constant in accord with the mathematical relation: τ f = τ d /20 * σ m /σ set where, σ set is predetermined noise level. In step 36, the controller limits the value of the filter time constant, τ f , between the values 0 and 0.1 * τ d . Those skilled in the art will appreciate that the aforementioned sequence of steps for determining the filter time constant, τ f , can be implemented in special purpose hardware. Preferably, those steps are implemented in software for execution, e.g., on a general purpose microprocessor. In this regard, it will be appreciated that such implementation can be achieved using conventional programming techniques as particularly adapted in accord with the teachings herein to provide the disclosed structure, signaling and functionality. Figure 6 illustrates the operation of an adaptive PIDτ d controller according to the invention in controlling a first-order deadtime dominant process. As in Figure 3, the controller deadtime is initally greater than that of the process. The initial response of the controller of Figure 6 mirrors that shown in Figure 3. However, because the controller adapts the filter time constant τ f in the manner described above, spiking in the manipulated variable is prevented. Consequently, spiking in the process output, i.e., the controlled variable signal, is also prevented. The foregoing describes improved deadtime process control apparatus and methods. These apparatus and methods improve on the techniques provided in the art by permitting better control of deadtime dominant processes. Those skilled in the art will appreciate that the illustrated embodiment of the invention are exemplary, and that other embodiments incorporating additions and modifications to that described above fall within the scope of the invention. Thus, for example, the level of noise in the manipulated variable signal can be determined by processes other than that described above.

Claims: A method for controlling a process that has a characteristic that can be measured by a controlled variable signal and that responds to a manipulated variable signal for varying that characteristic, said method comprising: A. generating an PD signal (PD) indicative of an error in said controlled variable signal (C), said generating step including determining said PD signal (PD) as a function of a time rate of change of said controlled variable signal (C) and a difference between said controlled variable signal (C) and a predetermined setpoint, B. summing said PD signal (PD) and a time-delayed integral signal to generate a summation signal, and c. selectively applying said manipulated variable signal (M) to said process, the method being characterized by: D. filtering said summation signal, with a filter time constant τ f to generate a manipulated variable signal (M), and E. feeding-back said manipulated variable signal (M) to generate said time-delayed integral signal, said feed-back step including i) time-delaying said manipulated variable signal (M) an amount substantially equal to a deadtime, τ d , and ii) passing that time-delayed manipulated variable signal through a first-order lag of time constant I to generate said time-delayed integral signal. A method according to claim 1, wherein said filtering step comprises A. sampling noise in said manipulated variable signal (M) to generate a signal, σ m , representative of a level thereof, and B. varying said filter time constant τ f to maintain said noise at a selected level, σ set . A method according to claim 2, wherein said filtering step includes generating said noise level signal, σ m , as a function of noise value sampled over a time period substantially equal to said deadtime constant τ d . A method according to claim 2, wherein said filtering step comprises an adaptation step for generating said filter time constant τ f as a function of said deadtime constant τ f and a ratio of said noise level σ m to said setpoint σ set . A method according to claim 4, wherein said filtering step comprises generating said noise level signal, σ m , as a function of (i) a sum of sampled values of said noise and (ii) a sum of squares of those sampled values of said noise. A method according to claim 4 or claim 5, wherein said adaptation step includes generating said filter time constant τ f as a function of a mathematical relation τ f = τ d /20 * σ m /σ set , and/or includes limiting said filter time constant τ f to a selected range of values, and/or includes limiting said filter time constant τ f to values substantially between 0 and 0.1*τ d . A proportional-integral-derivative deadtime process controller for measuring a controlled variable signal (C) representative of a first characteristic of a process, and for applying a manipulated variable signal (M) to that process to vary that first characteristic, said controller comprising A. proportional derivative means (12) for generating an PD signal (PD) indicative of an error in said controlled variable signal (C), said proportional derivative means including means for generating said PD signal (PD) as a function of a time rate of change of said controlled variable signal (C) and a difference between said controlled variable signal (C) and a predetermined setpoint, B. summation means (18), coupled with said proportional derivative means (12), for summing said PD signal (PD) and a time-delayed integral signal to generate a summation signal, and C. controlled variable signal output means for selectively applying said manipulated variable signal (M) to said process, characterized by: D. noise filter means (20), coupled with said summation means (18), for filtering said summation signal to generate a manipulated variable signal, (M), said noise filter means (20) having a filter time constant τ f , and said filter means (20) for instance being a second-order Butterworth filter or a first-order low-pass filter, and E. feedback means, coupled to said controlled variable signal output means, for processing said manipulated variable signal (M) to generate said time-delayed integral signal, said processing including i) time-delaying said manipulated variable signal (M) an amount substantially equal to a deadtime, τ d , and ii) passing that time-delayed manipulated variable signal through a first-order lag of time constant I to generate said time-delayed integral signal. A proportional-integral-derivative, deadtime process controller according to claim 7, wherein said noise filter means (20) comprises A. means for sampling noise in said manipulated variable signal to generate a signal, σ m , representative of a level thereof, and B. means for varying said filter time constant τ f to maintain said noise at a selected level, σ set . A proportional-integral-derivative deadtime process controller according to claim 8, wherein said noise filter means (20) includes means for generating said noise level signal, σ m , for a time period substantially equal to said deadtime constant τ d . A proportional-integral-derivative deadtime process controller according to claim 8, wherein said noise filter means (20) comprises adaptation means for generating said filter time constant τ f as a function of said deadtime constant τ d and a ratio of said noise level σ m to said setpoint σ set . A proportional-integral-derivative deadtime process controller according to claim 10, wherein said noise filter means (20) comprises means for generating said noise level signal, σ m , as a function of (i) a sum of sampled values of said controller output and (ii) a sum of squares of those sampled values of said controller output. A proportional-integral-derivative deadtime process controller according to claim 11, wherein said adaptation means includes means for generating said filter time constant τ f as a function of a mathematical relation τ f = τ d /20 * σ m /σ set , and for example said adaptation means includes means for limiting said filter time constant τ f to a selected range of values, and for example said adaptation means includes means for limiting said filter time constant τ f to values substantially between 0 and 0.1*τd.

Inventors: SHINSKEY FRANCIS G, SHINSKEY, FRANCIS G.
Applicants: FOXBORO CO, THE FOXBORO COMPANY
Application year: 1993
Main classification: G05B13
IPC codes: G05B13, G05B11