Wireless solutions are making headway in factory control and monitoring applications, driven by the obvious advantages of reducing infrastructure requirements such as cabling, conduit complexities, and installation requirements. The newest entry in this space is what Banner Engineering is calling the world's first completely self-contained wireless photoelectric sensor. The SureCross Q45 features a proprietary, self-contained battery, radio, and sensor in an all-in-one package. Using a its own network protocol to optimize bandwidth and power usage, the unit offers communication capabilities of up to 3,000 ft/1 km line-of-sight, and a battery life of up to five years.
Banner's proprietary wireless technology used in the new photoelectric sensors is based on the 2.4 GHz ISM band, and creates 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 this application requires. Specifically, they are not designed for small amounts of data, exceptionally low power, deterministic data transfer, and very long range communications.
"The wireless photoelectric solution is only sending a few bits of data over the air with each update," Bob Gardner, senior product manager at Banner Engineering, told Design News. "The standard infrastructures are designed to handle a lot more data. For example, Bluetooth (which is very inexpensive) is designed to be very short range and typically opens a voice channel which is far more data transfer than the photoelectric system requires. After investigating these other potential solutions, it ends up we can have an order of magnitude better performance if we built a solution from the ground up. Our DX80 wireless system has been on the market for more than five years, and it's the technology we have used in the new Q45 sensors."
A key to the design is that the wireless photoelectric sensor requires 2,000 times less power than a cellphone. Compared to a standard photoelectric sensor, the cost premium for the wireless solution is less than the cost to hard wire 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. One technique used to reduce power consumption is to quickly turn the sensor on and off.
"We made the sensor very fast so it could take a reading in a fraction of a millisecond. Then, we can turn the sensor off for, say, 100 milliseconds," Gardner told us. Many times a second, the sensor is turned on and operated for a very short period, but the majority of time it is essentially off. Sophisticated algorithms within the sensor control when it is using power.
Application advantages and limitations
"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 what you want," said Gardner. "A wired system is still faster for those types of applications with more brute force power available."
But if the application has long cable runs and more real-time performance requirements, versus high-speed operation, Gardner said the wireless sensors typically 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.
Another 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.