Wireless photoelectric sensors are finding more use in industry, thanks to recent developments that have boosted their range and battery life. You can thank better low-power microcontrollers for making such improvements possible.

Generally, wireless systems fit into two classes, powered and unpowered. Powered systems essentially have their own power supply, often a battery. When the power requirements get higher, solar systems become more common. Only the control signals are wireless. Unpowered signifies no power wires at all. The device harvests all its power from its environment.

Advanced sensing nodes can extend battery life by switching themselves off when they are not specifically taking measurements. Thus, the sensor spends most of its time not consuming power. This technique can boost battery life by a factor of 100 or more compared to that of nodes powered continuously. However, the trade-off is response time. Understandably, the sensor cannot sense when it is off.

The sensing technology determines how long the battery can last as well. For example, a dry contact sensor requires almost no power to operate because it applies no power to the sensing contacts. A dry contact sensor can be sampled at 10 Hz or more and still last five years on two AA batteries. This type of wireless sensor typically dissipates about 100 to 200 μW.

In contrast, powered sensor systems can be always on and generally respond more quickly. They can also run at higher power levels to provide a longer wireless range. And, of course, there are no batteries to replace in a powered system.

Reliable communication

Industrial wireless sensors use frequency-hopping spread-spectrum (FHSS) techniques that minimize the chance of interference from electrical noise. FHSS basically transmits RF signals by rapidly switching a carrier among many possible frequencies, using a pseudorandom sequence. Data packets get transmitted using these frequency channels randomly in a pattern known only to bound or paired devices that communicate with each other.

The bandwidth necessary for frequency hopping greatly exceeds that required for transmitting the same information on just one carrier frequency. A point to note on frequency-hopping systems, though, is that transmission takes place only on a small portion of this bandwidth at any given time. Consequently, the effective bandwidth of any interfering signals is really the same as that for a narrow carrier. Thus, though it provides no extra protection against wide-band thermal noise, frequency hopping does greatly diminish interference from narrowband sources.

Wireless sensors also use a time-division multiple-access (TDMA) technique which ensures all sensors in a network get adequate time to transmit their data and receive instructions. TDMA basically divides communication time into specific time slots for each node. Sensors transmit each using its own time slot. This lets sensors share the same RF channel while using only a part of its channel capacity. TDMA effectively eliminates the possibility of multiple sensors trying to communicate simultaneously. Using TDMA to avoid such data collisions also eliminates the possibility of wasting power to resend data packets lost in collisions.

It is customary for operators to conduct a site survey before installing wireless sensors. The survey basically determines whether RF interference is strong enough to be a problem. And the radio master device (gateway) will poll its sensor nodes at specific intervals to verify radio communications are still operating. If one of the sensors doesn’t respond, these new systems can react deterministically; this means the system goes into a state that maintains control in a fail-safe way.

Scalability

Multiple sensors can connect to a single master node, and dozens of wireless sensor nodes can work within a single radio network. Large installations may aggregate hundreds of sensor readings into a single gateway device before forwarding the sensor readings to a host system for analysis. For example, a single wireless network can handle up to 47 sensors per gateway.

Further extending this wireless I/O network are serial-data radios. In telecommunications parlance, these are backhaul devices that receive serial data from other serial-data radios or from a gateway. They forward the data to another serial device miles away. Chaining data radios can expand a network indefinitely.

Wireless sensors are compact, self-contained, and inexpensive. They can easily be relocated as the application or environment changes in what is known as a “peel-and-stick” installation process — basically, positioning the sensor in place and leaving it to run with no further attention from a human operator.

The latest wireless-sensing technology typically house a battery, radio, and sensor in a single enclosure. These devices generally can generate signals good for ranges of up to 3,000-ft/1-km line of sight to a gateway. Among the applications for this technology is notification or call-for-service tasks. For example, a wireless sensor connected to a tower light can boost production-line efficiency by immediately notifying technicians when there is a problem with a particular assembly operation.

In this scenario, each production area might have a switch box and a tower light connected to the input on a wireless sensor. If the production cell goes down, an employee flips a switch. A wireless sensor notes the switch closure and broadcasts a signal that is picked up by a gateway installed near a manager. It can actuate a tower light indicating which production line needs attention. This kind of notification system can reduce the need for supervisors to monitor production lines.

Wireless sensing is also sometimes used to call for parts. Production operators need an easy way to call forklift drivers to deliver parts or remove completed assemblies from a workstation. Of course, a hardwired system could handle the task, but production areas frequently get reconfigured when requirements change. There would be rewiring involved with every reconfiguration.

Using wireless sensors and indicator lights eliminates complex cable installation and provides easy rearrangement capabilities if the plant layout changes. Each production operator is equipped with a toggle switch and a wireless sensor with a dry contact input. The gateway is connected to a large lane-status display board with multiple indicator lights. When a production operator needs supplies, the operator toggles the switch connected to the wireless sensor, which causes the corresponding indicator light on the display board to light up — indicating an operator needs parts. After the parts are delivered, the operator toggles his switch again, turning the light off. The result is higher productivity with real-time part status of the production line.

Edited by Leland Teschler, leland.teschler@penton.com

Resources: Banner Engineering

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