Process control for industrial automation processes or industrial automation devices is often supervised and regulated by a process control system as a number of control loops including one or more closed loop control processes. A traditional approach in the use of closed loop control is to measure a value of a process output and compare the measured value with a reference value. There are also other objectives of control loop control, including set-point regulation, tracking (time-varying reference path), path following (varying reference independent of time), disturbance attenuation etc. The most common form of control is a Proportional, Integral, Derivative (PID) control for feedback control. In PID control a sensor measurement is used as an input for a feedback control loop, and any difference between the measured sensor value and a reference (setpoint) value or signal is determined by a controller. The controller then in turn sends signals to an actuator connected to the control loop in question, making changes to the process, so that the sensed value approaches the reference value over time.
The traditional closed loop feedback system comprises hard-wired communication links. A disadvantage with hard-wired communication links is that changes in position of any component or field device in the closed loop, such as a sensor or actuator, usually requires a stop in production or an extensive shutdown, especially in the case of analogue wired connections, and/or digital wired connections. Alternatively, such changes have to be delayed until a process shutdown may be programmed. In addition, hard wiring may be both expensive to replace and sometimes also technically challenging to replace.
Wireless technologies give several advantages to industrial automation in terms of gain in productivity and flexibility. Industrial sites are often harsh environments with stringent requirements on the type and quality of cabling. Moreover large sites often require many thousands of cables and it could be difficult to install or engineer additional wires in an already congested site. Thus wireless communication can save costs and time during an installation phase. At the same time wireless communication can improve reliability with respect to wired solutions by offering several mechanisms of diversity, such as space diversity, frequency diversity and time diversity. Furthermore the ad-hoc nature of wireless networks allows for easy setup and re-configuration when the network grows in size. However, when the field devices such as sensors and/or actuators are part of a closed-loop control system, an industrial application will require hard limits on the maximum delay allowed during the communication, so strict timing requirements have to be applied and consistently achieved.
Another requirement is the coexistence of the network with other equipment and competing wireless systems. The WirelessHART standard has been developed to fulfill these demands. WirelessHART is a wireless mesh network communication protocol for process automation applications, including process measurement, control, and asset management applications. It is based on the HART protocol, but it adds wireless capabilities to it enabling users to gain the benefits of wireless technology while maintaining compatibility with existing HART devices, tools and commands. A WirelessHART network may be connected to the plant automation network through a gateway. The plant automation network could be a TCP-based network, a remote I/O system, or a bus such as PROFIBUS. All network devices such as field devices and access points transmit and receive WirelessHART packets and perform the basic functions necessary to support network formation and maintenance.
Devices can be deployed in a star topology, that is where all devices are one hop to the gateway, to support a high performance application, or in a multi-hop mesh topology for a less demanding application, or any topology in between. These possibilities give flexibility to WirelessHART technology enabling various applications (both high and low performance) to operate in the same network. WirelessHART specifies the use of IEEE STD 802.15.4-2006 compatible transceivers operating in the 2.4 GHz ISM (Industrial, Scientific, and Medical) radio band. Communications among network devices are arbitrated using TDMA (Time Division Multiple Access) that allows scheduling of the communication link activity.
WirelessHART uses TDMA and channel hopping to control access to the network and to coordinate communications between network devices. The basic unit of measure is a time slot which is a unit of fixed time duration commonly shared by all network devices in a network. The duration of a time slot is sufficient to send or receive one packet per channel and an accompanying acknowledgement, including guard-band times for network wide-synchronization. The WirelessHART standard specifies that the duration of the time slot is 10 ms. The TDMA Data Link Layer establishes links specifying the time slot and frequency where 1.3. WirelessHART Standard communication between devices occurs. These links are organized into superframes that periodically repeat to support cyclic and acyclic communication traffic.
WirelessHART standard does not specify a particular scheduling algorithm to be used for scheduling communication in a WirelessHART network. However, for all network devices accessed through a WirelessHART gateway, the user has to configure how often each measurement value is to be communicated to the gateway. In order to support multiple superframes for the transfer of process measurements at different rates, the size of superframes should follow a harmonic chain in the sense that all periods should divide into each other, in particular, scan rates should be configured as integer multiples of the fastest update time that will be supported by network devices. The correctness of the process control system behavior depends not only on the logical results of the computations performed in each controller, but also on the physical instant at which these results are produced, in other words, it is a system with explicit deterministic (or probabilistic) timing requirements.
Thus the task of making up a schedule for communication links in a control loop may be subject to many conditions and constraints. In addition, in a typical process installation, communication scheduling and formation of superframes may be required for many hundreds or even thousands of control loops. This is a time consuming process for an installer or engineer, and one that also presents opportunities for errors to be made when an installer manually constructs a time schedule or configures a superframe for a control loop based on information for a control diagram or P&I diagram (Piping and Instrumentation). U.S. Pat. No. 7,460,865 entitled Self-configuring communication networks for use with process control systems, assigned to Fisher-Rosemount Systems, Inc., discloses methods for automatically assigning a first and a second wireless link to a first field device, wherein the first wireless link wirelessly couples the first field device to a second field device and wherein the second wireless link wirelessly couples the second wireless field device to a controller; the assignment being dependent on at least one first predetermined signal criterion.