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HIGH SPEED AUTO-TUNING LOOP
The present invention relates generally to distributed pro- 5 cess control networks and, more specifically, to a device and method for auto-tuning process elements communicatively connected within a distributed process control network.
Process control networks, such as those used in chemical, petroleum or other processes, generally include a centralized process controller communicatively coupled to one or more field devices which may be, for example, valve positioners, 15 switches, sensors (such as temperature, pressure and flow rate sensors), etc. These field devices may perform physical control functions within the process (such as opening or closing a valve), may take measurements within the process used in controlling the operation of the process or may perform any 20 other desired function within the process. Process controllers have historically been connected to field devices via one or more analog signal lines or buses which may carry, for example, 4-20 mA (milliamp) signals to and from the field devices. Generally, the process controller receives signals 25 indicative of measurements made by one or more field devices and/or other information pertaining to the field devices, uses this information to implement a typically complex control routine and then generates control signals which are sent via the analog signal buses to field devices to thereby 30 control the operation of the process.
More recently, there has been a move within the process control industry to implement field-based digital communication within the process control environment. For example, the process control industry has implemented a number of 35 standards including open digital or combined digital and analog communication protocols such as the HART®, PROFIBUS®, WORLDFIP®, Device-Net®, and CAN protocols. These digital communication protocols generally enable more field devices to be connected to a particular network, 40 support more and faster communications between the field devices and the controller and/or allow field devices to send more and different types of information, such as information pertaining to the status and configuration of the field device itself, to the process controller. Furthermore, the standard 45 digital protocols enable field devices made by different manufacturers to be used together within the same process control network.
Also, there is now a move within the process control industry to decentralize process control and, thereby, simplify the 50 individual process controllers. Decentralized control is obtained by having field mounted process control devices, such as valve positioners, transmitters, etc., perform one or more process control functions using what are typically referred to as function blocks or control blocks. The function 55 blocks may communicate data across a network structure for use by other process control devices (or function blocks) in performing other control functions. To implement these control functions, each process control device typically includes a microprocessor having the capability to implement one or 60 more function blocks as well as the ability to communicate with other process control devices using a standard and open communication protocol. In this manner, field devices can be interconnected within a process control network to communicate with one another and to perform one or more process 65 control functions to form a control loop without the intervention of a centralized process controller. The all-digital, two
wire network protocol now being promulgated by Fieldbus Foundation, known as the FOUNDATION® Fieldbus is one open Fieldbus communication protocol that allows devices made by different manufacturers to interoperate and to communicate with one another via a standard network to effect decentralized control within a process.
Tuning of any control block or control loop in a prior art system is fairly simple because the entire tuning routine can be stored in the centralized controller or field device. When tuning of a control loop of such a control routine is desired, the separate tuning block within the controller or field device forces the appropriate control block, such as a proportionalintegral (PI) or proportional-integral-derivative (PID) control block, through a tuning procedure like an induced oscillation procedure, to determine predefined characteristics of the process or the loop. During this dynamic data capture phase of the tuning procedure, the tuning block collects data generated by the loop, which is being delivered to the control routine per normal operation, and determines from this data one or more process characteristics, such as the ultimate gain, the time constant, etc. of the process. Once the desired process characteristics are calculated, the tuning block applies a set of rules or other algorithms using the calculated process characteristics to determine new tuning parameters for the control block or control loop. This step is, commonly referred to as the rule application phase of the tuning procedure. Thereafter, the tuning routine delivers the new tuning parameters to the control block (or control loop) and the tuning procedure is complete. Because, in a centralized process control system, all of the control functions are located within the controller and all of the data necessary for tuning is provided to the controller during normal operation of the process, the tuning block has direct access to the control blocks and to the data required to tune the individual control blocks.
Decentralized process control systems, in which control blocks or control elements, such as PI control elements, PID control elements, fuzzy logic control elements, etc., are located in a distributed manner throughout a process control network, are harder to tune because the control blocks are located away from the controller or field device where the tuning block is typically stored. Decentralized process control systems generally communicate in a scheduled or synchronous manner to implement specific control functions associated with the process control routine. During the periods in which synchronous communication is not occurring, other information, such as alarms, set point changes or other diagnostic signals (e.g., tuning signals), may be communicated in a non-scheduled or asynchronous manner. However, a tuning control block configured to communicate in an asynchronous manner is unable to send a deterministic tuning signal to a field device and to receive a deterministic response signal from a field device because the controller or field device must use asynchronous communications to implement the tuning functions. In particular, because the tuning signal is communicated in an asynchronous manner, the controller has no way to detect when the tuning signal is actually received by the field device or when the corresponding response signal is generated, thereby preventing strict control over the timing of the tuning procedure and increasing the likelihood of inaccurate tuning results.
In one known prior art system for implementing tuning in a distributed process control network, the entire network is reconfigured and taken off-line to perform the tuning procedure. In this configuration, the tuning procedure is performed using synchronous communications while the specific control functions are suspended. In another known prior art system used for implementing tuning, the entire tuning routine is
placed within the same device as the control block to be tuned (such as the PID function block) and, in fact, may actually be incorporated into the functionality of the control block. While this system is able to control the timing of the tuning procedure precisely and to collect data at any desired rate (up to and 5 including the speed at which the control block is executed), the tuning routine must be compiled along with and at the same time as the control block, which increases the overhead (e.g., the timing, processing, memory, etc. requirements) associated with the use of the control block during normal 10 operation of the process, even though the functionality of the auto-tuning routine is used relatively infrequently during normal operation of the control loop. Furthermore, a complete auto-tuning routine must be placed within each different device in which a control block is located in order to enable 15 auto-tuning of each control block, which adds unneeded redundancy to and increases the cost of the process control system.
An auto-tuner is adapted to be used in a distributed process control network having a communications network that communicatively couples a process controller, which executes a process control routine, and one or more process devices used 25 in a process control loop. The auto-tuner includes a first tuning element configured to cause a control entity to force the process loop to under go an auto-tuning procedure and a tuning data stack operating within one of the process devices to receive and store a tuning signal associated with the control 30 entity along with a time stamp indicating the time the tuning signal was acted on by the device. A measurement data stack is disposed in the same or a different process device and operates to receive and store a response or measurement signal generated by the process device along with a time 35 stamp indicating when the response signal was generated or detected. A second tuning element which may be, for example, in a controller or a workstation, periodically receives data from the tuning data stack and the measurement data stack and determines a tuning parameter to be used in 40 tuning the process loop.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic block diagram of a distributed process control network including an auto-tuning system; 45
FIG. 2 is schematic block diagram illustrating the information flow in one embodiment of the auto-tuning system of FIG. 1;
FIG. 2A is function block diagram illustrating a control routine in one embodiment of the auto-tuning system of FIG. 50 1;
FIGS. 3A-3C are graphs representing signals that may be used and stored in one embodiment of the auto-tuning system of FIG. 1;
FIG. 4 is schematic block diagram illustrating information 55 flow into data registers associated with an auto-tuning system;
FIG. 5 is a schematic block diagram illustrating a data collection procedure that collects and uses data stored in a pair of data registers of an auto-tuning system; and 60
FIG. 6 is schematic block diagram illustrating information flow in another embodiment of an auto-tuning system.
FIG. 1 illustrates a distributed control network (DCN) 2 including one or more user interface devices, generally indi
cated by the numeral 4, connected via a communications network 6. The network 6 may be an Ethernet local area network (LAN) compliant to the IEEE 802.3 standard or any other suitable communication network. The user interface devices 4 may be any variety of networkable terminals such as a touchpanel 8, a personal computer 10, a laptop computer 12 having wireless network capabilities and/or a wireless personal digital assistant 14 (PDA) connected via a wireless router 16. The wireless router 16 may be compliant to the IEEE 802.1 lx (where the x indicates a specific wireless protocol, such as a, b or g, for example) and allows for seamless communications between the LAN and the wireless devices 12 and 14.
The DCN 2 further includes controllers 18 and 20, which may be connected via a hub 22 operating on the network 6, and which are capable of storing a process control routine in a memory thereof and implementing the process control routine on a processor (not shown within the controllers 18, 20). The controllers 18 and 20 are further capable of communicating with function blocks located in a plurality of field devices distributed throughout the physical process, generally indicated by the numeral 24. The controllers 18 and 20 may be, by way of example only, the Delta VTM controller sold by Fisher-Rosemont Systems, Inc. and may be configured to use any proprietary or open source communications protocol, such as the HART®, PROFIBUS®, and the Fieldbus protocols. In this configuration, the wireless PDA 14, laptop 12, touchpanel 8 and personal computer 10 may be used to communicate with the controllers 18 and 20 to obtain information about the individual elements of the physical process 24. If the controllers 18 and 20 are DeltaVTM controllers, they may be configured to provide graphic depictions of the process control routine implemented within the controllers 18 and 20. Furthermore, if desired, a user may initiate an auto-tuning routine via any one of the user interfaces 4 connected to the network 6.
The controllers 18 and 20 are connected to numerous field devices located throughout the physical process 24 through any standard input-output (I/O) devices 26, 28, and 30. The I/O device 26 is shown communicating to field devices 32-36 in a point-to-point topography required by the HART® protocol. Alternatively, the I/O device 28 is shown communicatively coupled with the field devices 38-46 in a ring configuration required by the PROFIBUS® protocol, while the I/O device 30 is shown connected to field devices 48-54, which may be Fieldbus devices, using a bus 56 configured to conform to a fieldbus protocol, such as the FOUNDATION® Fieldbus protocol. The I/O devices 26, 28, and 30 may be any standard I/O devices capable of connecting to analog devices using 4-20 mA signals, digital devices using digital protocol signals, or any combination thereof. Furthermore, the field devices 32-54 may be any type of field devices including, but not limited to, optical sensors, thermocouples, valve positioners, servo positioners, valve controllers, etc.
FIG. 2 illustrates a schematic block diagram of a set of routines, some of which may be function blocks, connected to form an exemplary auto-tuning loop in the DCN 2 controlling the physical process 24 represented in FIG. 1. In the diagram of FIG. 2, the user interface (UI) 4 includes an active graphical control display 58, which may be generated by a DeltaVTM application, representative of the control routine 60 implemented by the controller 20. It should be noted that, if the UI 4 is a wireless device such as the laptop 12 or the PDA 14, the control display 58 is likely to be an HTML (hypertext markup language) or XML (extensible markup language) representa