Using wireless technology for photoelectric sensors created a need for a solution that offered low power operation, deterministic data transfer and the ability to communicate over long distances in the factory. Opting for a proprietary network protocol adapted for the specific needs of automation and control, new systems provide line-of-sight communications up to one kilometre and battery life up to five years.

Wireless photoelectric sensor technology is targeting factory monitoring and control applications, driven by the obvious advantages that wireless brings to any application by reducing infrastructure requirements such as cabling, conduit complexities and installation.

New wireless technology from Banner Engineering implements a self-contained battery, radio and sensor in an all-in-one package. Using its own proprietary network protocol to optimize bandwidth and power usage, the technology offers communication capabilities of up to 3,000 ft/1 km line-ofsight, and a battery life of up to five years.

Communications is based on the 2.4 GHz ISM band, which is used to create a frequency- hopping, spread spectrum solution. The system doesn't use Zigbee, WiFi or other standard radio communication architectures because those solutions can't provide all the characteristics that these types of automation application requires. Specifically, more standardized solutions are not designed to accommodate small amounts of data transfer, exceptionally low power, deterministic data transfer and very long range communications.

Networking Technology

The key to making the technology work in a photoelectric sensor is that it only needs to send a few bits of data over the air with each update cycle. In most wireless implementations, the standard infrastructures are designed to transmit and receive a lot more data,. Bluetooth, for example, is designed to provide effective communications over a short range, and typically opens a voice channel which is far more data transfer-intensive.

So after investigating potential solutions, Banner decided to build the technology from the ground up, and only implement the specific functions required to achieve an order of magnitude better performance.

One key is that the wireless photoelectric sensor requires 2,000 times less power than a cell phone. Compared to a standard photoelectric sensor, the cost premium for wireless is less than the cost to hardwire the system.To achieve the five-year battery life, the design also needed to reduce the power required to operate the sensor over a thousand fold, on what was already a low power sensor.

The technique used to reduce power consumption is to quickly turn the sensor on and off. By making the sensors fast enough to take a reading in a fraction of a millisecond, the objective is to turn the sensor off for very short periods (measured in microseconds).

The technology then is turned on and off many times a second. The sensor is turned on and operated for a very brief period, so the majority of cycle it is essentially off. Sophisticated algorithms within the sensor control when it is using power.

Application Advantages & Limits

If a customer is trying to implement a system with millisecond or sub-millisecond operation which could be categorized as high speed, wireless is not an appropriate solution. Wired systems are still faster for those types of applications with more brute force power available.

But if an application has long cable runs and more real-time performance requirements, versus high speed operation, the wireless sensors can typically only respond in 125 milliseconds. Wireless is also suited for applications where there is motion between the PLC and end device such as a transport mechanism or conveyor. A wireless system design is much less expensive than implementing slip rings using cable trays and flexible cables.

An additional application advantage is if the sensor is located a long distance from the control. On a small machine where the distance is only five feet, it may not make sense but if the distance is 500 feet installing the wireless sensor costs less than installing the cable.

Another potential application class is where there is a requirement for system-wide monitoring such as a call-for-parts system, for example, that delivers a tote of parts to a location. Wireless is often used on forklifts, so that the driver can have an indicator and real-time feedback which identifies which areas are out-of-parts. Since the forklift is mobile, wireless offers a clear advantage.

Many manufacturing facilities are looking to create flexible layouts where production can be laid out for specific needs depending on their product mix. To collect information from a machine used in the process that might be moved on a periodic basis, wireless works well since you can move the machine without a need to re-wire it.

PLC Network Features

One potential application concern is overall communications reliability. But the signal used provides 10 to 100X the range of a WiFi-based system, and also has built-in all of the characteristics of a PLC-based level network. This includes time-out mechanisms, CRC checks and a pre-defined state for the output in case a link is lost. For years, these techniques have been used in automation and PLC level networks, so they are also implemented and built into the wireless photoelectric sensor protocol as well. With its built-in site survey which reports signal strength, if everything is setup properly and within range to guarantee good signal strength, Banner confirmed that the system should as reliable as a wired solution. It also provides immediate indication if communications is lost to a part of the system, and has been designed with the reliability and determinism issues firmly in mind.

The system can also implement a system heartbeat where wireless devices check on a regular basis, every five seconds for example. If the devices don't check in, those devices can be reset to a pre-defined state to fail in a predictable manner. For setup, all the user needs to do to establish communications is bind it to a gateway. Since there can be multiple gateways and devices in a single area, communications only occur with devices that they have been paired with.

One design consideration was operating the sensors in noisy environments such as variable frequency drives (VFDs) operating in the area. Noise reduces the range of the signal but not significantly because the wireless system operates within a very narrow band. Frequency-hopping, spread spectrum technology creates very narrow, well-defined frequencies, and these narrow frequencies are noise immune because the system filters out any frequencies above and below that level.