Technology Update: Wireless and photoelectric sensing technologies have merged to create sensors without wires.

Bob Gardner

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Wireless and photoelectric sensing technology have each made their mark in industrial automation applications, from using a photoelectric sensor for production line product sensing to employing wireless for easy reconfiguration. While each technology brings its own set of performance features and benefits, combining them into a wireless photoelectric sensor includes design constraints. Operators expect the sensor to behave the same as a traditional wired sensor. The idea is simple; the task was difficult.

Basic functions, parameters

Ideally, a wireless photoelectric sensor component would perform identically to a wired sensor, meaning that optical parameters, such as lensing and LED color, remain unchanged. Mechanically, one piece with rugged, industrial design makes sense.

A wireless transmitter needs to transmit sensor output immediately when the sensor changes state. The network must be deterministic, highly reliable, and secure. The wireless transmission must be noise immune, even in the presence of variable frequency drives (VFDs) and other noisy industrial equipment. Communications based on PLC-level networks provide enough wireless range to easily cover large plants, exceeding the range of off-the-shelf wireless or WiFi technologies.

A wireless sensor should economical, if installed as one sensor, and also be able to operate as one of hundreds of sensors in the same building, without interfering with other sensors or existing WiFi systems. Easy setup and maintenance is required to offset work required to install a wired sensor.

Careful power management is required to have batteries last years rather than hours.

Based on these difficult and competing requirements, attention to each of three areas was required: photoelectric sensing, battery power management, and wireless networking.

Photoelectric sensors have been used in industrial controls since the late 1940s. Since then, major advances in circuit technology and manufacturing engineering allow manufacturers to produce smaller, faster sensors, offering more features at a lower cost. For wireless operations, a sensor capable of operating on exceptionally low power levels was needed.

Battery power management

Battery power management included consideration of power harvesting technologies, solar panels, rechargeable lithium ion batteries, and alkaline batteries. Tests showed that a Thionyl Chloride Lithium battery has three to five times the energy of a comparably sized alkaline or rechargeable lithium ion battery. Cost, size, and power density were favorable. Efficient switching power supplies and sensing systems ensured that the battery slept 99.9% of the time. A typical 24 V photoelectric sensor consumes around 20 ma of current. This is 0.48 watts or about ½ watt. To reach five years of battery life on 2 AA batteries, the sensor would need to operate on 0.1 ma of average current at 3.6 V or 0.00036 watts. This is a reduction in power of 1300 times.

Wireless networking

To operate within the desired parameters, the wireless networking infrastructure required:

1. Extremely low power
2. Bidirectional design, so every end point could send and receive data. One-way communications offer limited control and cannot be acknowledged, which has a significant impact for industrial controls.
3. Real time: If the target is a five-year battery life and real-time response, the system should respond typically within 125 ms. Higher speed requires more power and decreases battery life.
4. Extended range
5. Site survey to accurately test wireless signal strength
6. Multiple sensor support so each wireless gateway could support many sensors

Wireless I/O system

Before designing the photoelectric wireless sensor, a wireless I/O system was developed, available with standard digital, analog, and temperature inputs and outputs, replacing PLC I/O cards and the PLC I/O network.

In the PLC world, sensors are always connected to power, but often an I/O node requires battery power. To ensure reduced power and extended battery life, a concept called “switched power supply” is used. Each battery-powered I/O node has one or more power supply terminals providing 5 V-24 V for a short time—this is done using a voltage pump and battery power. The power can be switched on to power a sensor. Once the sensor is on and operating, the output is read. Then, the sensor is turned off. By switching the sensor on and off, the sensor spends most of the time off, increasing battery life by 100 times or more. Trade-off is response time, since the sensor cannot sense when off.

Using the switched power supply method, the wireless photoelectric sensor can operate on two replaceable AA Lithium batteries for up to five years, depending on the application. This type of wireless sensor uses an average power of 100 to 200 microwatts, producing 1,000 times less power than a 24 V wired sensor with similar performance.

In addition to extended battery life, the wireless I/O system made it quick and easy to wirelessly connect many I/O types to a PLC. Range exceeds WiFi with a real time response rate at 125 ms. Data traffic is fully acknowledged and is highly reliable and deterministic. It also is extremely noise immune and does not interfere with existing wireless systems.

Self-contained, peel and stick

Integrating these technologies resulted in a wireless photoelectric sensor. It combines photoelectric sensing and wireless technology in one housing, operating on battery power.

No configuration software required for setup. For installation, simply bind (or pair) a wireless sensor with a wireless gateway. The sensor’s output appears at the gateway output terminal, providing up to 3,000 ft/1 km line-of-sight sensing distance. By eliminating the need to wire sensors, operators can add a photoelectric sensor in minutes, creating a scalable, wireless sensor network infrastructure. Users can now apply sensing technology in situations where it was not previously possible and reduce installation and maintenance costs.

Multiple photoelectric sensing modes include retroreflective, convergent, and fiber. Dry contacts (push button or inductive proximity sensors) also are possible. Gateway functions include two out and six out with Modbus RTU. The two-out function serves entry level single wireless links; one output indicates the sensor state, and the second provides a fault signal if there is a problem. The six out model with RTU can connect up to 47 sensors. Sensor status reports to the PLC using a Modbus interface. If an application requires more than 47 sensors, multiple gateways can co-exist without interference. The system can support analog and digital data, which will remove limits to sensor types that can be made wireless.

Wireless sensing is practical with key advancements in low power sensing, battery management, and industrial wireless technologies. Wireless sensing can support photoelectric, inductive, and dry contacts, resolving challenging applications including moving, rotating, or reconfiguration applications. With enhanced performance features, new wireless sensors provide an easy-to-deploy way to increase productivity.

- Bob Gardner is senior product manager, Banner Engineering. www.bannerengineering.com. Edited by Mark T. Hoske, content manager, CFE Media, Control Engineering, Plant Engineering, and Consulting-Specifying Engineer, mhoske(at)cfemedia.com.

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