Wireless technologies for industrial automation
Wireless networks for industrial automation enhance the ability to gather time-critical information, digest it, and react, key to continuously adapting to change with increasing reliability and profitability. No one type of wireless technology resolves all challenges.
Brent E. McAdams
The evolution in wireless technologies has opened the door to a new class of plant automation architecture that offers adopters a significant strategic advantage. Driven by substantial and measurable cost savings in engineering, installation, and logistics, as well as dramatic improvements in the frequency and reliability of field data collection, automation experts and IT professionals are presented with an opportunity to deliver a major, positive impact to the bottom line.
Cost benefits are the most intuitive among what’s driving adoption of wireless technologies. Other important considerations are safety and better management of legacy assets not previously on the network. Advantages include the following:
- Installation savings: Installation of wirelessly connected assets is up to 10 times cheaper than the wired alterative and offers much faster start-ups and accelerated profits. Engineering costs are dramatically reduced as extensive surveys and planning are no longer required to route wire back to junction boxes or control rooms. The reduced costs in wiring engineering, installation, and maintenance combined with the increased data gathering flexibility is the primary driver for wireless migration.
- Better information: Replacing manual readings with automated measurement results in information that is more accurate, timely, and consistent.
- Economy of scale: Deploying additional points in a network is incremental and may include integration onto legacy systems.
- Operational savings: Quickly diagnose and troubleshoot plant operations and support predictive maintenance programs by monitoring facility assets. Additionally, identify costly problems leading to excess use of energy or raw materials.
- Safer operations: Wireless technologies can reduce human exposure to hazardous environments [when the prior alternative was to send someone to take a reading or manually actuate a valve, for example]. Also, more frequent measurements and early detection of issues can help reduce or even prevent incidents or accidents.
Unfortunately, no one type of wireless technology solves all problems. Therefore, to maximize the return on industrial wireless networking investments, companies select the best technology for a given application.
By evaluating attributes of various wireless technologies, essential technology decisions can be made guaranteeing successful implementation of a wireless architecture solution. These attributes include the radio frequency (RF) technology, security, interference rejection, sensitivity, power management, and the ability to embed wireless into existing original equipment manufacturer (OEM) technologies. Furthermore, those working on the implementation need to determine if new systems should interface with existing systems to preserve [or extend functionality of] investments in existing infrastructure. Another consideration is to examine the radio provider’s commitment to backward compatibility, which extends the life of a wireless system and lowers lifetime implementation costs.
Licensed vs. unlicensed
In 1985 the U.S. Federal Communications Commission (FCC) issued rules permitting use in the Industrial, Scientific and Medical (ISM) bands (902-928 MHz, 2.4-2.4835 GHz, 5.725-5.85 GHz) at power levels of up to one Watt without end-user licenses. There are two very common spread spectrum modulation methods used in these bands: frequency hopping (FHSS) and direct sequence (DSSS).
Rather than transmitting over a static spectral segment, FHSS radios pseudo-randomly vary carrier frequency, quickly hopping through multiple channels while sending data. Interference is avoided by hopping over different frequencies, each of which has a different interference effect or characteristic. This provides FHSS with collision-free access by allocating a specific time slot and frequency for its transmission. A frequency-hopping scheme, combined with error detection and automatic repeat requests, ensures that the data is reliably delivered. Further, designers of FHSS systems anticipate competition for the airwaves. Therefore, interference avoidance and management are designed into the system. Other modulations are more susceptible to interference because they do not anticipate interference by design.
Direct sequence spreads a narrow-band source signal by multiplying it with a pseudo-random noise signal. The resulting signal is then spread over a large range of continuous frequencies. This introduces redundancy into the transmission, enabling a receiver to recover the original data even if parts of it are damaged during transmission.
In addition to the unlicensed ISM band, most licensed radios operate in the UHF and VHF bands, and as the name indicates, users must obtain a site license to operate radios in a specific geographic area. Consequently, these systems can be expensive to set up, and many offer significantly slower data rates (typically ≤ 19.2 kbps), which are not likely to support industrial data communication requirements in the future. However, UHF/VHF radios are allowed higher transmit power, which increases range, and because they operate at lower frequencies, they typically have better propagation characteristics. However, one of the drawbacks of a licensed system is that only one system can operate at that location. Therefore, overlapping networks and other communication capabilities using the same frequency band are not possible.
Spread spectrum advantages
Spread spectrum has two significant advantages over fixed frequency licensed radio transmissions. The first is that no FCC license is required by the user. Even though licensed spectrum is available, the user must go through the process of obtaining the license. Once obtained, the license is good for a single site and has a defined term.
The second advantage spread spectrum, specifically FHSS, has over fixed frequency transmission is that spread spectrum radio transmissions are far less susceptible to interference. In an industrial plant environment, machinery and other equipment generate interference over a very broad spectrum of frequencies. Therefore, if one frequency is affected in a FHSS system, for example, the data is quickly transmitted over another, clear channel. This gives the technique greater coverage, channel utilization, and resistance to noise than comparable direct sequence systems. A licensed solution doesn’t have that capability.
FHSS characteristics, security
FHSS technology has immediate advantages in security, immunity to interference, robustness, and network reliability.
FHSS systems were originally designed for military applications during World War II to provide data security and interference avoidance that existing fixed frequency systems could not reliably provide. Concerns about the integrity of signal transmission and reception are prevalent today among adopters worried about leaving control and business networks vulnerable to hacking or denial of service attacks. In fact, security is widely seen as the most significant barrier to industrial wireless adoption. FHSS technology has inherent advantages for security, immunity to interference, robustness, and network reliability.
When FHSS radios communicate, communication frequency changes rapidly (as much as 1,000 times per second), providing an additional layer of security by making transmissions difficult to detect. To outside listeners, transmissions simply look like noise spread over the spectrum, with only a small signal at any one given frequency.
This technique assures the integrity of the data, because without the hopping sequence, no outside sources can listen in. This technique also allows communications to continue even if some frequencies in the band are blocked. The devices simply hop to another frequency.
Additional data security is gained through 128-bit and 256-bit Advanced Encryption Standard (AES). The AES algorithm uses an encryption key (password). Each encryption key size causes the algorithm to behave slightly differently: Increasing key sizes offer a larger number of bits for data scrambling and increase the cipher-algorithm complexity.
As with existing data transmission over wire, packet protocol acknowledgment is supported by error checking. Error checking is designed to ensure that spread spectrum radio data are not forwarded from the buffer until it is acknowledged as a correct transmission to guarantee that what is received is identical to what is sent. To accomplish this, a cyclic redundancy check (CRC) is generated, giving the packet a unique digital signature.
Typical packet structure
The probability of detecting any random error increases as the width of the checksum increases. Specifically, a 16-bit checksum will detect 99.9985% of all errors, far better than the 99.6094% detection rate of an eight-bit checksum, but not as good as the 99.9999% detection rate of a 32-bit checksum. More than 4 billion possible CRC values exist with 32-bit CRC—4,294,967,296, to be exact. By comparison, a 16-bit CRC offers a chance for data error with every 65,536 transmissions, a relatively small number of transmissions in a work cycle, since many radios transmit packets as often as 50 to 100 times per second.
Sensitivity can be counterintuitive
Receiver sensitivity is an important specification. More sensitive receivers can receive weaker transmitted signals, allowing greater distance and more obstructions between a transmitter and receiver.
Receive sensitivity can be confusing because it is expressed in a unit of measure commonly used for audible sound, the decibel (dB), which is a ratio expressed on a logarithmic (exponential) scale. A 10:1 ratio is 10 dB and a 2:1 ratio is 3 dB. A 1:1 ratio is 0 dB, while ratios of less than 1:1 are expressed as negative numbers. For example, a 1:2 ratio equals -3 dB.
Because receive sensitivity indicates how faint a signal the radio can successfully receive, the lower power level, the better. This means that the larger the absolute value of the negative number, the better the receive sensitivity. For example, a receive sensitivity of -110 dBm is better than a receive sensitivity of -107 dBm by 3 dB, or a factor of two. In other words, at a specified data rate, a receiver with a -110 dBm sensitivity can hear signals that are half as strong as a receiver with a -107 dBm receive sensitivity.
Fresnel zone and antennas
For the shorter-range installations in industrial facilities, a common question is, "Is line of sight required for radio links?” “No” often is the answer. Radio waves can travel through a variety of objects with different levels of attenuation. The area over which the radio waves propagate from the antenna is known as the Fresnel zone. Like the waves created by throwing a rock into a pool of water, radio waves are affected by the presence of obstructions and may be reflected, refracted, diffracted, or scattered, depending on obstruction properties and their interaction with radio waves. This is often how the signal gets to the receiver when there is no line of sight. However, this effect attenuates the signal and affects how a radio will operate without line of sight.
Proper antenna use and ability to adjust output power assist in overcoming these issues and getting messages through. Industrial quality directional and high-gain omni-directional antennas allow radio communications at long distances through a crowded industrial facility. Low-gain antennas can be used to keep radio signals from straying unwanted distances or directions.
With many existing wired networks, the user is locked into using one protocol at a time. Alternatively, by using wireless architecture, several protocols can operate over the same communications layer, giving the user greater flexibility.
Any wireless device needs to connect to existing control systems, not a small task given the myriad of existing control systems. The 4-20 mA signal and switch closures are universally translatable. Digital input allows more data flow at significantly lower cost, but generally adds a level of complexity to any system. Servers using Modbus communication protocol or OPC Foundation communications offer degrees of acceptance where large data flows are required.
Products intended for industrial applications should use industrial-rated components and operate reliably over industrial temperature ranges, typically -40 C to +75 C (-40 F to 167 F). Temperature extremes are commonplace in many applications. In addition, these products are generally better constructed than consumer devices and continue reliable operation under shock and vibration conditions.
Industrial wireless modems typically carry some form of UL certification. Most commonly this UL certification is for Class 1, Divisions 1 and 2 environments, which permits radio operation in the presence of flammable or explosive gases, fluids, or vapors. Having this certification is also beneficial because a company can standardize on one type of device, and use it for many applications, regardless of environment.
Where to use wireless
- Wireless I/O (input/output) connections— Asset information is available from applied and embedded sensory points enabling sophisticated diagnostics, remote monitoring and control, and plant optimization. The form factor of wireless devices allows for easy integration into existing OEM technologies and housings.
- Safety — Environmental alarms and personnel management allow for greater safety and compliance with OSHA regulations, especially in dangerous environments and in locations where the plant is close to residential areas. Also, completely automated first response systems are available to limit human exposure in the event of a release or catastrophic incident.
- Security — With wireless, one can detect intrusions, control access, report smoke or fire, or perform video surveillance within the facility.
- Workforce mobility — Wireless connectivity allows mobile workers to access applications on the go, which can increase efficiency. Whether it is logging data or managing operations, worker mobility impacts productivity.
- Mobile asset and material tracking — Tracking asset location allows for better use of assets, as well as regulatory compliance for the use, storage, and transport of hazardous chemicals.
- Integrating wireless sensor networks, or mesh networks — Sensor integration represents an emerging technology that has great potential for widespread applications. These networks consist of a large number of simple nodes with limited power sources and functionality, but they offer greater utility than the sum of those individual nodes. Greater flexibility and connectivity may be achieved by integrating these networks with other longer range wireless technologies.
More efficient info use
Information is power. As such, the ability to gather time-critical information, digest it, and react upon it is the key to continuously adapting to change with increasing reliability and profitability. As stated, no one type of wireless technology solves all problems. Therefore, it becomes very important that the necessary monitoring, management, and security capabilities be evaluated to ensure the wireless architecture selected maximizes limited resources, while allowing disparate applications to share spectrum within context of importance, time sensitivity, and mission criticality.
- Brent E. McAdams is director, customer advocacy, FreeWave Technologies Inc.; Edited by Mark T. Hoske, content manager CFE Media, Control Engineering, Plant Engineering, and Consulting-Specifying Engineer. He can be reached at mhoske(at)cfemedia.com.
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