Process control systems, such as distributed or scalable process control systems like those used in chemical, petroleum or other processes, typically include one or more process controllers communicatively coupled to each other, to at least one host or operator workstation and to one or more field devices via analog, digital or combined analog/digital buses. The field devices, which may be, for example, valves, valve positioners, switches and transmitters (e.g., temperature, pressure and flow rate sensors), perform functions within the process such as opening or closing valves and measuring process parameters. The process controller receives signals indicative of process measurements made by the field devices and/or other information pertaining to the field devices, and uses this information to implement a control routine to generate control signals which are sent over the buses to the field devices to control the operation of the process. Information from the field devices and the controller is typically made available to one or more applications executed by the operator workstation to enable an operator to perform any desired function regarding the process, such as viewing the current state of the process, modifying the operation of the process, etc.
Some process control systems, such as the DeltaV™ system sold by Emerson Process Management, use function blocks or groups of function blocks referred to as modules located in the controller or in different field devices to perform control and/or monitoring operations. In these cases, the controller or other device is capable of including and executing one or more function blocks or modules, each of which receives inputs from and/or provides outputs to other function blocks (either within the same device or within different devices), and performs some process operation, such as measuring or detecting a process parameter, monitoring a device, controlling a device, or performing a control operation, such as the implementation of a proportional-integral-derivative (PID) control routine. The different function blocks and modules within a process control system are generally configured to communicate with each other (e.g., over a bus) to form one or more process control loops.
Process controllers are typically programmed to execute a different algorithm, sub-routine or control loop (which are all control routines) for each of a number of different loops defined for, or contained within a process, such as flow control loops, temperature control loops, pressure control loops, etc. Generally speaking, each such control loop includes one or more input blocks, such as an analog input (AI) function block, a single-output control block, such as a proportional-integral-derivative (PID) or a fuzzy logic control function block, and an output block, such as an analog output (AO) function block. Control routines, and the function blocks that implement such routines, have been configured in accordance with a number of control techniques, including PID control, fuzzy logic control, and model-based techniques such as a Smith predictor or Model Predictive Control (MPC).
To support the execution of the routines, a typical industrial or process plant has a centralized controller or room communicatively connected with one or more process controllers and process I/O subsystems, which, in turn, are connected to one or more field devices. Traditionally, analog field devices have been connected to the controller by two-wire or four-wire current loops for both signal transmission and the supply of power. An analog field device that transmits a signal to the control room (e.g., a sensor or transmitter) modulates the current running through the current loop, such that the current is proportional to the sensed process variable. On the other hand, analog field devices that perform an action under control of the control room is controlled by the magnitude of the current through the loop.
More recently, field devices have been designed operate to superimpose digital data on the current loop used to transmit the analog signals. For example, the Highway Addressable Remote Transducer (HART) protocol uses the loop current magnitude to send and receive analog signals, but also superimposes a digital carrier signal on the current loop signal to enable two-way field communication with smart field instruments. Another protocol generally referred to as the FOUNDATION® Fieldbus protocol is an all digital protocol, that actually defines two sub-protocols, one supporting data transfers at a rate up to 31.25 kilobits per second while powering field devices coupled to the network, and the other supporting data transfers at a rate up to 2.5 megabits per second without providing any power to field devices. With these types of communication protocols, smart field devices, which may be all digital in nature, support a number of maintenance modes and enhanced functions not provided by older control systems.
With the increased amount of data transfer, one particularly important aspect of process control system design involves the manner in which field devices are communicatively coupled to each other, to controllers and to other systems or devices within a process control system or a process plant. In general, the various communication channels, links and paths that enable the field devices to function within the process control system are commonly collectively referred to as an input/output (I/O) communication network.
The communication network topology and physical connections or paths used to implement an I/O communication network can have a substantial impact on the robustness or integrity of field device communications, particularly when the network is subjected to adverse environmental factors or harsh conditions. These factors and conditions can compromise the integrity of communications between one or more field devices, controllers, etc. The communications between the controllers and the field devices are especially sensitive to any such disruptions, inasmuch as the monitoring applications or control routines typically require periodic updates of the process variables for each iteration of the control routine or loop. Compromised control communications could therefore result in reduced process control system efficiency and/or profitability, and excessive wear or damage to equipment, as well as any number of potentially harmful failures.
In the interest of assuring robust communications, I/O communication networks used in process control systems have historically been hardwired. Unfortunately, hardwired networks introduce a number of complexities, challenges and limitations. For example, the quality of hardwired networks may degrade over time. Moreover, hardwired I/O communication networks are typically expensive to install, particularly in cases where the I/O communication network is associated with a large industrial plant or facility distributed over a large area, for example, an oil refinery or a chemical plant consuming several acres of land. The requisite long wiring runs typically involve substantial amounts of labor, material and expense, and may introduce signal degradation arising from wiring impedances and electromagnetic interference. For these and other reasons, hardwired I/O communication networks are generally difficult to reconfigure, modify or update.
More recently, wireless I/O communication networks have been introduced into the process control environment to alleviate some of the difficulties associated with hardwired I/O networks. For example, U.S. Pat. No. 7,519,012, entitled “Distributed Control System for Controlling Material Flow Having Wireless Transceiver Connected to Industrial Process Control Field Device to Provide Redundant Wireless Access,” the entire disclosure of which is hereby expressly incorporated by reference herein, discloses a system utilizing wireless communications between controllers and field devices to, for example, augment or supplement the use of hardwired communications. The use of wireless communications between devices within a process control network, such as between controllers and field devices, has quickly gained momentum. In response to this trend, various wireless communication protocols have been established to support wireless communications within process plant environments, including the WirelessHART® protocol.
Complete reliance on wireless communications for control-related transmissions has been limited however due to, among other things, reliability concerns. As described above, modern process control monitoring and control applications assume reliable data communications between the controller and the field devices to achieve optimum control levels. Moreover, typical process controllers execute control algorithms at fast rates to quickly correct unwanted deviations in the process and these control algorithms rely on the availability of new process measurement data during each controller execution cycle. Undesirable environmental factors or other adverse conditions may create intermittent interferences that impede or prevent the fast communications necessary to support such execution of monitoring and control algorithms.
Moreover, power consumption is sometimes a complicating factor for the implementation of wireless communications in process control environments. When disconnected from the hardwired I/O network, the field devices may need to provide their own power source. Accordingly, wireless field devices are typically battery powered, draw solar power, or pilfer ambient energy such as vibration, heat, pressure, etc. For these types of devices, however, the energy consumed when performing data transmission via a wireless network may constitute a significant portion of total energy consumption. In fact, more power may be consumed during the effort to establish and maintain a wireless communication connection than during other important operations performed by the field device, such sensing or detecting the process variable being measured.
Thus, the relatively recent introduction of wireless transmitters in the process control industry has presented many challenges when a wirelessly transmitted measurement is to be used in closed loop control, because the process variable measurements provided in such systems are often reported on a much slower basis (e.g. 15 second update rate) than is typically provided by a wired transmitter. Also, the measurement value provided by a wireless device may be communicated on a non-periodic basis. For example, windowed communications supported by some WirelessHART® devices may transmit new measurement values on a non-periodic basis. Still further, it is important that the loss of communications in any wireless implementation be automatically treated by the controller in a manner that does not introduce a process disruption.
In response to these problems, methodologies have been developed to enable some process controllers, such as proportional-integral-derivative (PID) process controllers, to work effectively with slow measurement updates, non-periodic measurement updates and intermittent loss of communications, i.e., situations more frequently associated with wireless communication networks. In general, these control methodologies receive and process unreliable or non-periodically received feedback signals (e.g., intermittently received process variable measurements) while still controlling a process loop adequately. These systems thus enable, for example, a PID controller to operate properly without receiving new process variable measurements for each execution cycle of the process control routine. In particular, U.S. Pat. Nos. 7,620,460; 7,587,252; and 7,945,339 and U.S. Patent Application Publication No. 2008/0082180, each of which is expressly incorporated by reference herein, describe how, for example, a PID control routine may be modified to perform closed loop control using a wireless transmitter that performs intermittent communications, thereby enabling process variable measurements to be communicated to the controller via a wireless communications link only when the process variable changes a particular amount.
While effective, these new control techniques generally modify the manner in which a PID control routine or a PID control block handles the intermittently received process variable measurements at the input of the controller or the control routine. However, there are many types of control techniques, including some PID based control schemes, that use estimates of process variables as inputs to the controller instead of using measured process variable values and control signals as inputs to the controller. For example, process control techniques that use Kalman filters typically operate on the measured process variable value to produce an estimate of the process variable value, which is then provided to the control routine for generating a control signal. In these cases, the intermittently received process variable measurements signals are not provided directly to the control routine. Still further, traditional Kalman filters assume the presence of zero-mean process or measurement noise within the process when producing an estimate of the process variable value. In processes in which this assumption is not correct, the Kalman filter based control system may operate poorly.