Machine Design

Sensing without wires

Wireless sensing solves many problems, but introduces a few of its own.

Bob Peters
Robert Peters Consulting
Cleveland, Ohio

Reflected signal creates multipath interference

A factory building I-beam reflects transmission of wireless sensor data to a digital readout. The readout receives both the main and reflected signals that interfere with each other, disrupting the display.

The readout is moved out of the reflected signal path. Typically, movement of only a few inches is all that's required. If the readout can't be moved, then repositioning the antenna will have the same effect. With interference eliminated, the readout works normally.

You've decided that wireless machine sensors are in your future. But you still have many unanswered questions. Where does wireless technology do best? What are the strengths and weaknesses? What rules apply for fielding this kind of gear? Is it reliable? With over 3,000 manufacturers of wireless equipment it's easy to find the choices overwhelming.

Wireless technology today solves problems such as making it practical to put a sensor atop a crane by eliminating the need to string a cable. And similarly, consider the measurement of strain on a spinning flywheel. Slip rings introduce noise and offset errors disrupting meaningful readings. A wireless sensor mounted directly to the flywheel eliminates the problem, transmitting its result to the monitoring electronics.

Wireless sensors go in hard-to-reach locationsand eliminate the frustration of cable-failures caused by strain. On the manufacturing floor it reduces a sea of cables to a small pond at a central location monitoring all functions.

Wireless-enabled products can keep tabs on critical systems, report pending failures, and synchronize machine processes internally for smoother operation. Process control and manufacturing equipment from different manufacturers can share status, easing interoperability and increasing safety. Add wirelessbased inventory management, production control, quality monitoring, warehouse operations, and even salesfloor tracking, and what started as a simple shop monitor has turned into a fullfeatured wireless company network.

Many different technologies are involved in wireless sensing. Each sensor is a self-contained radio transmitter and receiver, or transceiver. Thus the operation of these devices falls under national regulations and international treaties. Two special frequency bands have been set aside for wireless systems: the ISM or Industrial, Scientific and Medical band and the Unlicensed Network Information Infrastructure, or U-NII, bands. In the U.S., the Federal Communications Commission licenses both companies that manufacture these devices and the devices themselves for proper operation, eliminating the need for a user license.

A wireless-sensor system is like any radio service. It will experience interference. There are other devices like Bluetooth blackberries and cellular phones that share operating frequencies with wireless sensors on both the ISM and UNII band. Microwave ovens, which operate at 2.45 GHz, may overwhelm many wireless technologies in the 2.4-GHz ISM band. Improperly filtered electric motors may generate enough electrical noise to make wireless transmissions unreliable. Even the physical placement of a transmitter can cause a significant loss of signal. Regardless of manufacturer claims, users must plan and test any wireless setup to ensure a reliable installation.

Care is in order when placing sensors to minimize interference. Keep wireless sensors away from other sources of radio-frequency interference (RFI) such as brush-type electrical motors, other radio transmitters or transceivers, or unshielded computer equipment. Sensors that must sit near such devices should connect to the transceiver via a short piece of shielded cable to stay as far away as possible from the source of the RFI. In an industrial environment, large iron and steel structures may create-a condition known as multipath propagation.

The best path for a radio signal to take is directly from antenna to antenna. Multipath propagation occurs when nearby metal reflects the radio signal the way a mirror reflects light. The receiver hears multiple signals, the original and the reflections, simultaneously and cannot decode any of them. Sometimes moving the receiving or transmitting antenna just a few inches is enough to fix this problem.

~10 to 100 m
Data throughput
~2 to 54 Mbits/sec
~1 Mbit/sec
~0.25 Mbits/sec
Power consumption
Battery life measured in:
Minutes to Hours
Hours to Days
Days to Years
Size relationship

Protocols determine the type of signal transmitted and how to encode the information it contains. There are many different wireless protocols. The most widely used are IEEE 802.15.1 (also known as Bluetooth); the IEEE 802.11a/b/g Series of wireless LANs; IEEE 802.15.4 which is also called Zigbee; and RFID tagging. Each standard possesses different benefits and limitations.

Providing the slowest data rate is the IEEE 802.15.4 or Zigbee standard. Its maximum rate of data transmission is only 250 kbits/sec, with speeds down to 20 kbits/sec, slower than most telephone modems. It has the lowest power requirement of the group. Zigbee devices are projected to run several years on a single set of batteries, making them candidates for unattended or difficultto-reach locations.

Bluetooth is a short-range (about 100 m or 328 ft) communication protocol widely used in cellular-type phones and Blackberries. It runs in the 2.4-GHz ISM band and is reliable, but only has a bandwidth of approximately 1 Mbit/sec.

IEEE 802.11a/b/g is actually a collection of related technologies that operate in the 2.4-GHz ISM band, the 5-GHz ISM band, and the 5-GHz U-NII bands. It provides the highest power and longest range of the common unlicensed wireless technologies. Transmission data rates can reach 54 Mbits/sec. At least one of the 802.11 protocols now comes preinstalled on most new laptops and is available for PDAs and cellular phones.

Radio Frequency IDentification, or RFID, is the one form of wireless sensing that requires no power at all in the tag. It is a passive technology used for labeling and tracking. The RFID tag is the sensor, responding when power is beamed to it through the reading device. Current RFID tags can hold only 96 bits of information, but 128-bit and 256-bit tags are on the horizon.

The choice of protocol depends on how the sensor will be used. If slow speeds are okay and long battery life is a necessity, then look into Zigbee. The 802.11 families are a good choice if the system must respond to changes quickly and power isn't a problem. Theoretically, it is possible to mix and match the different sensor protocols, but that might create more problems than it would solve at this time. An example would be differing response speeds can give rise to signal race conditions triggering false actions in the machine.

Even wireless sensors need power to operate. The type of power source depends on the demands the sensor sees. In general, higher data rates demand more power and shorten battery life. The high-rate 802.11 protocols have a typical battery life measured in only a few hours in the absence of other power-saving actions.

Another factor affecting power needs is how often the sensors transmit data and the format the data takes. Continuous measurement consumes more energy than operations taking readings and sending out data every minute or only when there is an exceptional event. Periodic transmittal can lower power consumption dramatically.

Sources other than batteries or wired power may be used with wireless sensors. Outdoors, solar power can keep batteries at full charge while providing operating power during the day. Piezoelectric power supplies can produce power from vibrations in machinery. Inductive power supplies pick up stray magnetic fields surrounding electrical services and converts their energy to electrical power. Coupled with energy-saving transmission modes, such as event-triggered readings and low data rate transmissions, it is possible to create a wireless sensor with no batteries at all.

Topology is the name given to the methods used by different sensors in an installation to connect to one another. Connection schemes are important. For example, they can determine how the loss of a transmitter or sensor would affect the overall system. In some topologies a malfunctioning sensor would block data from other sensors creating a cascade failure. The number of sensor nodes that a base station can process may affect the speed of data acquisition. Some sensor nodes collect data and transmit it at the same time while others handle such tasks according to a specific sequence. High-speed connections favor the first method while the latter offers longer battery life. Point-to-point or star networks frequently have limitations on their ability to handle multiple nodes simultaneously.

A new technology, mesh networks, promises to resolve some of these problem by turning each node into a storeand-forward site for neighboring nodes. The shorter transmission distance between nodes reduces power requirements because transmitters need not be as strong. The data transfers from node to node the same way buckets of water were passed along old fire brigade lines. Software supporting these networks should function with wired sensors to cover both legacy systems and situations where a wireless installation is not appropriate.

Wireless sensors are unlicensed radio transmitters. This means anyone can listen in if the data is not encrypted and the connections lack reliable authentication. This may not be a problem inside a large metal-frame building, but security is a major issue for wireless components used in an open environment. Security cannot be taken lightly. The implications of stolen data can extend far beyond the value of the information itself, particularly if privacy issues are involved.

All in all, wireless sensing is a technology with many pitfalls and rewards. There are still many questions about its use. Wireless solves problems with an ease no other technology can match, but it is not a silver bullet. It can put manufacturers that use it ahead of the game for years to come.

Bluetooth SIG,
IEEE 802 Wireless World,
ZigBee Alliance,

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