Wireless proximity switches eliminate failure source

Sensors and actuators are found in large numbers on every production line in every industry. And each and every one of them requires data and power cabling. Not only are these cables costly to install, they are also a frequent source of failure. Now, ABB is introducing a novel wireless proximity switch that incorporates a communication module for the power supply, signal transmission and man-machine communication, and so has no need for cables

Wireless devices are gradually being introduced into industrial environments. Their use, however, has tended to be limited. Today, you may be able to use a wireless link to configure a field device or check a motor's condition, but no-one so far has been able to solve the complex task of closed-loop control with wireless and battery-free sensors.

The last few years have seen a revolution in wireless communication. High-volume production in the consumer and office automation markets has made advanced communication solutions available at an astonishingly low cost. The telecommunication industry has facilitated matters by creating worldwide standards for wireless links, like 802.11, Bluetooth, GSM and Zigbee, thus removing the need for region-specific solutions.

But what is the special attraction of wireless technology for industrial applications, and what does industry expect to achieve by utilising this technology?

Wireless technology has three main advantages: it reduces cost thanks to its easy installation, simpler engineering and the reduction in materials needed; it increases productivity by introducing mobility, flexibility and fast network access; and it allows new value-adding applications and services like portable clients and operator terminals, faceplates, remote diagnostics, etc. These inherent advantages of wireless communication are fully exploited in the new ABB wireless proximity switch.

Inductive proximity switches are the most widely used position sensors in machine control. With high reliability and without actual physical contact, they inform the controller about the progress of a machine's movement. Since inductive proximity switches are designed to detect metal targets, they are generally insensitive to dirt.

However, there is a problem: the connections between the sensors and the control system. These are fixed multicore cables, fitted with either a plug or with terminals. Although integrating inductive proximity switches in the machine design has become relatively simple, cable engineering and installation is still relatively laborious, especially when cables move with each machine movement.

Today, the machine designer has various technical solutions at hand for increasing the reliability of electrical connections between moving machine components, but most of these, like slip-ring transmitters, are only suitable for special applications. Using flexible cable ducts and highly flexible cables is a more standard way to increase the reliability of such connections. Nevertheless, cables remain the main source of sensor faults and machine downtime. Getting rid of the cables, then, represents a major step forward.

Assured reliability

This is exactly what ABB's new wireless proximity switches were designed to do. Researchers based in Norway, Germany and Switzerland divided the problem into three areas: the wireless communication system, the wireless power supply, and the low-power sensor. The first of these, the wireless communication system, has to be just as reliable as wired sensors. As these sensors are part of closed-loop control systems, strict timing constraints also apply. The technology should be cheap and also have a low power budget. To add to the difficulty, the sensors should be able to co-exist with interfering systems, such as Bluetooth and Wireless Local Area Networks (WLANs), as well as with any self-interference arising from up to a few thousand wireless sensors on the factory floor. Clearly, no existing radio standard comes even close to fulfilling these requirements.

The wireless power supply posed a similar, if more fundamental challenge. Whereas battery technology may have improved over the last decade, no battery could provide the minimum of ten years maintenance-free operation required by the system. A number of technologies were investigated, including thermocouples, photovoltaic cells and fuel cells. The only solution that proved viable was inductive coupling, whereby a small magnetic field set up throughout the volume of a machine is converted into usable electrical power by the sensor units.

The third task was to develop the basic sensing technology, at power levels two orders of magnitude below traditional technology.

System architecture

Central to the solutions for all these problem areas is the overall system architecture. The system has four primary loops installed around the manufacturing cell. These are fed by two power supplies that set up an alternating current in the loops, producing a magnetic field throughout the cell. Wireless proximity switches within the cell have small coils that pick up the energy from the magnetic field and convert it to electric power. The sensors also have small radio transceivers and low-power electronics that handle the wireless communication link. The sensors communicate with an input module via antennae mounted in the cell. This module behaves rather like a traditional, wired, input module. It can handle up to 60 wireless proximity switches simultaneously and is connected to the control system via an ABB FieldBusPlug. Five input modules can co-exist within the same area, allowing up to 300 sensors in one manufacturing cell to be served.

The wireless communication subsystem transmits the sensor signals to the input module, which can be compared to a base station in a cellular system. It must satisfy the rigorous demands of an industrial environment - have very fast response times (generally much less than (10-15ms), serve a large number of sensors located in a cell of several metres radius, and guarantee high data transmission integrity, even where radio propagation may be affected by obstacles and interference.

It was therefore decided to design a new system tailored to the needs of the wireless sensor, while re-using as many of the available standard low-cost components as possible. The new system operates in the 2.4GHz radio band allocated to ISM (Industrial, Scientific and Medical) users. A sophisticated input module designed by ABB ensures that the complexity resides in the input module rather than in the sensor. One such module wirelessly can handle up to 60 sensors. Although similar to a WLAN base station in many respects, the ABB design has several features that clearly set it apart: simultaneous transmission and reception of radio signals (full-duplex operation is not possible with Bluetooth and WLANs); simultaneous reception of the strongest and weakest signals; interference suppression - reception of a very weak sensor message or signal is possible even though a large interfering signal may exist at some adjacent frequency; transmit and receive antennas at the input modules may be periodically switched to provide a diversity of radio propagation paths as a protection against fading and shadowing effects.

The sensor communication hardware is based on a standard Bluetooth transceiver (radio) in order to benefit from economies of scale, component integration (small size) and low power consumption. In particular, the communication antenna on the sensor transceiver module must be carefully chosen. Its radiation characteristics should be omni-directional, in order to achieve uniform transmission performance irrespective of the sensor's orientation with respect to the input module antennas.

ABB
q122@industrialnetworking.co.uk