Wireless discrete monitoring and controls standards emerging
By Bill Lydon
Industrial wireless is proving valuable for automation professionals in many areas of industrial automation with products and standards emerging. People are comfortable with wireless since they use it in their daily lives with cell phones, personal computers, security monitoring, and other devices. Wireless standards to date have focused on analog sensors, but there is growing interest and adoption of wireless for discrete monitoring and for controlling digital output points. Discrete monitoring and control points significantly outnumber analog input and outputs in automation systems and are the largest installation cost on most projects. Discrete points monitor contact closures from a wide range of sensors and use contact outputs to control a wide range of devices, including motors, two position valves, and solenoids. If wireless cost and reliability improve to compete with hardwiring, this would be a real improvement in automation systems. Today, wireless sensors are being applied to select applications that have a high return on investment as a low-cost means for monitoring hard-to-reach locations and deploying new innovative applications. Examples include connecting far distant sensors that are too expensive to wire, such as tank monitoring/control, and as an alternate to electromechanical slip rings on rotating machines connecting electrical signals from a stationary to rotating structure.
One of the challenges for wireless is the majority of discrete points (contacts in/out; digital in/out) in factory automation require high-speed response since they are typically part of interlocking control logic that synchronizes machine and assembly line operations. In addition, the point density is much higher than analog points in process applications. There are a wide range of devices, including limit switches, proximity sensors, relays, push buttons, stack lights, machine stops, and motor starters. Applications that do not require high response speed are the first candidates for wireless. Interestingly, there are approved safety applications in operation using wireless 802.11, but it is important to remember that wireless becomes part of the safety loop. If there is a communications breakdown, this causes a safety fault.
Early adopter users have been ahead of wireless standards using various proprietary wireless devices primarily to monitor and control hard-to-reach discrete points. There are a number of proprietary industrial wireless offerings in the U.S. and Canada and other countries, and many operate in the 900-MHz band or lower frequency, providing a strong signal that allows communication through walls and other structures. Legal frequencies for these applications vary by geography. Common ISM (industrial, scientific, and medical) bands for industrial and commercial applications are:
- 220-MHz band in China
- 433-MHz band in Europe and some other countries
- 869-MHz band in Europe
- 900-MHz band in North America and some other countries
- 2.4-GHz and 5.7-GHz bands, allowed in most parts of the world
As radio waves travel, the radio signals gradually lose energy. The higher the frequency of transmission, the quicker the radio wave will lose energy down to a point where it cannot be detected by a receiver. Higher frequency waves also lose energy more quickly when trying to penetrate walls, trees, or other obstructions. If both a 900-MHz radio and a 2.4-GHz radio had the same output power and receiver sensitivity and were compared side by side, the 900-MHz radio would get almost twice the range of the 2.4-GHz radio.
Building automation industry
The building automation industry is significantly less demanding in communications response requirements than industrial automation with more rapid adoption of wireless. The BACnet building automation protocol standard for building automation hardwired networks defined by ASHRAE (American Society of Heating, Refrigerating, and Air Conditioning Engineers) has agreements with ZigBee and EnOcean wireless standards organizations that have defined interfaces and protocol mapping to BACnet.
ZigBee is an 802.14.4 mesh-based standard that has been implemented in the 2.4-GHz, 915-MHz (Americas), and 868-MHz (Europe) frequency bands.
EnOcean technology is based on ultra-low power electronics and radio technology that allows it to be powered using energy harvesting to transmit wireless signals over a distance of up to 300 meters using 868- or 315-MHz frequencies with 125-kbps data rate. EnOcean invented and patented energy harvesting wireless sensors in the 1990s. A good EnOcean example is a light switch that, when pushed, generates power for an EnOcean radio to transmit to a receiver to turn on a light. EnOcean contributed key technology to the ISO/IEC 14543-3-10:2012 standard, titled Information technology—Home Electronic Systems (HES)—Part 3-10: Wireless Short-Packet (WSP) protocol optimized for energy harvesting—Architecture and lower layer protocols.
The HART Foundation reports that many end users have requested that HART technology support discrete applications, and they now have a discrete applications specification. Wally Pratt, HART Communication Foundation chief engineer commented, “Where WirelessHART adopted orphaned process instruments and applications, this new specification adopts orphaned discrete applications.” HART has defined a discrete variable for on/off or state-related values that may be inputs from, or outputs to, plant equipment. The discrete variable may also contain a copy of the register values from a connected programmable logic controller (PLC). Inputs receive a plant signal and status and convert those signals into a digital value. The conversion process may include signal conditioning, termination, isolation, and/or indication for that signal’s state. The input may be a simple Boolean value, push-button inputs, or binary coded data (BCD). If the input is an on/off or open/close type, such as with a push button or limit switch, the signal can be represented in a single bit. If, on the other hand, the state of the input varies, such as with a blocking valve, where the valve is open, closed, opening, or closing, the state requires a full word. Output modules transmit single bit or state signals to activate various devices, such as actuators, blocking valves, on-off valves, solenoids, and motor starters. The output maintains a target value and may include the actual value as well for a discrete output. Often the output module maintains the status of the output, too (i.e., whether the output is functioning correctly). A host application modifies discrete outputs by writing the target value of the output, and then monitors the transitions to intermediate and/or final states by reading the actual value. All HART discrete products must include core mandatory capabilities that allow equivalent device types to be exchanged without compromising system operation. HART discrete features are backward compatible to HART core technology, such as the device description language.
The ISA 100.11a Working Group 16 has been considering discrete factory automation and published a technical report, ISA-TR100.00.03-2011 Wireless User Requirements for Factory Automation. ISA100 WG16 is chartered to investigate applications for wireless technology in the factory automation and discrete manufacturing industries, such as automotive manufacturing, packaging machinery, machining, and robotics. The technical report presents descriptive user and market-related requirements of wireless communication in factory automation applications and explores use cases, factory automation topologies, and recommendations for attributes and values for existing, emerging, and conceptual solutions for wireless communications as applied to factory automation applications. The report highlights factory automation functional and technical requirements that place unique demands on wireless, including the high transaction response speeds. For example, one of the most demanding applications cited is very high-speed processing requirements for sensor feedback of a motion control loop to actuate a servo drive to achieve precise positioning in microseconds.
The report noted that data from a single node should be represented as a register, or set of registers, in a way that is similar to, and modeled after, data read from a remote I/O unit of a PLC. If all the data are discrete, then all inputs and outputs are to be represented as binary bits of a single data element. For example, one 16-bit word = 16 inputs or outputs. It recommends that a single data model similar to that of the PLC be defined. Model use cases cited include discrete inputs, intelligent limit switch tuning parameters, proximity switch sensitivity value, discrete outputs, opening or closing time delay, and pulse output parameters.
This technical report presents classes of needs defined as use cases:
Simple substitution of wires on stationary equipment (e.g., automation controller to I/O, controller to controller, controller to enterprise system) describes a general case for wireless.
Robot end effector
A robot end effector is the working end of a robot that interacts with tooling to perform specific functions.
Track-mounted equipment includes overhead cranes, hoists, gantries, and rail cars that are used to move material and personnel.
Rotary equipment (e.g., packaging fillers) typically spins around a single fixed axis.
Torque and gauge tools
Torque tools are used on automated assembly lines to tighten fasteners (e.g., bolts) to a prescribed tightness. Gauge tools are used in manufacturing operations to measure specific attributes of a unit of work against a prescribed tolerance.
Mobile material containers
Mobile material containers (also called intermediate bulk containers or IBCs) are used to transport raw materials, work-in-process (WIP), and finished goods to various locations within a manufacturing operation. Examples include totes, super sacks, barrels, and similar vessels.
Mobile high-value assets
Mobile high-value assets include removable or replaceable tooling (e.g., molds, dies), storage (e.g., movable racks), maintenance tools (e.g., powered hand tools), and other transportable assets that are used in the manufacture of product or maintenance of machines.
Mobile test and calibration fixtures
Mobile test and calibration fixtures refer to combinations of sensors and recorders that capture data related to units under test.
Wireless Ethernet (802.11)
Standard 802.11 wireless Ethernet has become commonplace throughout industry and provides a transparent transport for many industrial Ethernet protocols, including Modbus TCP, EtherNet/IP, and PROFINET. Users are simply connecting Ethernet PLCs or Ethernet remote I/O devices to a wireless Ethernet (802.11) adapter to communicate. Wireless Ethernet modules are also available for many controllers and remote I/O products that plug directly into them in place of hardwired network interface modules. In addition to controllers, there are many devices using industrial Ethernet, including sensors, motor drives, and robots that can easily be connected to an 802.11 wireless Ethernet network. Key considerations in these applications include total number of 802.11 access points required to adequately cover desired area, existing wireless Ethernet networks operating in the same area, wireless range, installation environment (indoor or outdoor), and security. Since these applications are running over standard wireless Ethernet, there can be a lot of other communications traffic with the potential to create problems. Some companies are communicating with safety devices over 802.11, and these are approved safety applications. These safety applications are interesting but if there is a communications fault, the machine process must go to the specified safe state stopping production.
The PI organization responsible for PROFIBUS and PROFINET has a wireless working group focused on wireless. Since PROFINET is Ethernet-based, it is already being applied running over 802.11 and Bluetooth. In addition, the PROFISAFE safety protocol is being applied over 802.11 wireless using PROFINET as the transport mechanism. The wireless working group is also developing the FA WSAN (factory automation wireless sensor actuator network) specification. The technology is based on the ABB WISA technology and uses the IO-Link standard as the protocol. The IO-Link standard defines a point-to-point connection for discrete I/O that provides information from smart discrete devices. In a hardwired application, this is accomplished with a three-conductor sensor/actuator cable and an IO-Link Master. Only one IO-Link device can be connected to a single port. A unique characteristic of the WISA/WSAN technology is that the radio and sensors can be powered from the radio waves.
Users are learning that industrial plant wireless requires a systems approach. For example, wireless has been growing at a fast rate, which has the potential to create performance issues at a plant site without proper system level management. If you have ever been in an Internet café and frustrated because you cannot get e-mail with too many people using the Wi-Fi, you have experienced what can happen in an unmanaged wireless network. While this is frustrating, an unmanaged network with the potential for the loss of data communications in a wireless industrial plant application could become catastrophic. I have spoken with users who experienced wireless problems caused by several sources, including microwave ovens, walkie-talkies, and IT adding wireless devices without notifying plant personnel.
It is easy to think of wireless communication as limitless, but it has limitations similar to wired industrial networks. Wired industrial networks have a finite bandwidth available for communications, and overloading the network will create performance problems. Wired industrial networks are in a closed system, making them inherently more deterministic than unmanaged wireless communications. Wireless networks also have limitations on communications bandwidth and response issues if not managed.
Sorting it all out
The WirelessHART group has the first and only specification out for discrete wireless, and some members have products with other organizations developing standards. New wireless points will continue to be added to systems to access difficult-to-reach sensor/actuator locations for new functions. What percentage of total points on a project will be wireless in the near future is anyone’s guess.
The number of wireless points deployed today is a small fraction relative to the number of hardwired points. In the future, the number of points that will be connected wirelessly is likely to increase, assuming wireless for discrete applications becomes more responsive, reliable, and cost-effective. It is hard to compete with the reliability and availability of hardwiring to an I/O card for discrete points.
ABOUT THE AUTHOR
Bill Lydon (firstname.lastname@example.org) has a wide range of control and automation experience, including design engineering, application engineering, and business. He is currently the editor of InTech magazine and Automation.com. Bill is an automation industry consultant as well as a North American representative of PLCopen and a member of industry committees including OMAC.