Sensor data transmission has come a long way since the days of simple digital (on/off) machine control, hardwired analogue signals, and RS-232, RS-422 and RS-485 serial protocols. With so many alternative networking technologies from which to choose, engineers seeking the optimum for an application must consider what their requirements are - and whether the sensors they need are available with the necessary networking capability.
The actuator/sensor interface (ASI), developed by a consortium of European automation companies which saw a need for networking the simplest devices at the lowest level, offers the dual advantages of simple configuration and low cost. ASI is designed for small systems employing discrete I/O (input and output) devices; it caters for a maximum of 31 slaves, which can provide for up to four inputs and four outputs each for a total of 248 I/O (Fig. 1).
ASI is typically used to connect proximity sensors, vision sensors, limit switches, valves and indicators in equipment such as packaging machines and material handling systems.
For more complex networking, engineers today can select from a broad range of fieldbus protocols. In some cases an engineer's choice of sensor manufacturer will determine the protocol, and there are currently two consortia trying to encourage manufacturers to use their technologies: FDT Group and IO-Link.
Historically, Field Device Tool (FDT) technology was associated with factory automation users and vendors of discrete manufacturing equipment such as ABB, Endress+Hauser, Invensys, Metso and Siemens. Following the publication of an international standard, IEC 62453, Field device tool (FDT) interface specification, FDT is claimed to be platform-independent and supports a multitude of protocols including Hart, Profibus, Foundation Fieldbus, Devicenet, Interbus, AS-Interface and Profinet.
FDT standardises the communication and configuration interface between field devices and host systems. Moreover, it provides a common environment for accessing the devices' most sophisticated features; any device can be configured, operated, monitored and maintained through the standardised user interface, regardless of the supplier, type or communication protocol.
IO-Link is an intelligent point-to-point connection system at the field level, which is compatible with existing I/O standards and is supported by a consortium of automation product suppliers. These companies have come together to support what has been described as a new concept in all areas of sensor, actuator and control technology.
According to the consortium, IO-Link promises simplified installation by replacing parallel wiring and various types of analogue signals with IO-Link. Also included are function modules for automated parameter setting and tool-assisted parameter setting, as well as remote diagnostics down to the field device level, cable break detection and device-specific diagnostics. Additional sensor information can be accessed with minimal effort in terms of hardware and software.
To see the benefits of IO-Link, consider a packaging system equipped with a colour sensor. A sensor is normally connected using an industry-standard M12 eight-pin connector; termination is expensive and time-consuming, and connecting cables are rarely available pre-assembled - meaning that in-house cable assembly is required. It is therefore inevitable that mistakes are occasionally made, which have to be identified and remedied. In contrast, with IO-Link a sensor is installed and electrically connected using a standard, readily available M12 three-pin connector. This eliminates the need for in-house cable assembly and the associated time and cost associated with wiring errors. Overall, sensor wiring is therefore quicker, easier and cheaper.
To give an example from a real handling and assembly automation installation that has been simplified using IO-Link, a valve assembly line uses a fully automatic, 20-stage process to assemble various components such as plastic parts, O-ring seals and compression springs to create tested and ready-to-install valves. An industrial RFID (radio-frequency identification) system with 17 reading stations provides information exchange between material flow and the controller. A large number of standard sensors such as diffuse photoelectric, inductive proximity switches and cylinder switches are also used, whose signals are carried to the controller in as simple a manner as possible. On the actuator side, a larger number of parallel wired pneumatic valve terminals are connected.
In contrast to the previous installation, now only a simple standard sensor cable is used. This connects each RFID device to the closest IO-Link Master over the shortest distance. The use of IO-Link valve terminal plugs, together with IO-Link sensor hubs for collecting standard sensor signals on the sensor and actuator side, means that parallel wiring is largely replaced by the serial IO-Link system. Customer benefits include simplified connection of RFID read heads, valve terminals and standard sensors. In addition, electrical connections can be made at the time of installation by mechanical personnel.
Instead of complying to open standards or those agreed by consortia, some sensor manufacturers have opted to develop their own network systems. Solartron Metrology, which makes precision dimensional measurement and position measurement transducers, has developed the Orbit and, more recently, the Orbit3 network system that combines RS-485 communications and 'plug and go' functionality (Fig. 2). Any Orbit3-compatible transducer, probe, encoder, I/O module or analogue input module can be used in any location and each is automatically given a unique address.
One of Solartron's customers, oven manufacturer Neff, recently wanted to reduce the thickness of the steel sheet used in oven cases. To do this - without sacrificing product quality or manufacturing yield - some of the manufacturing processes had to be more tightly monitored and controlled. To this end, a metrology system was conceived to gauge the exterior and interior case dimensions, flatness and perpendicularity, as well as the location and orientation of mounting surfaces and points. In all, 36 measurement points were required. Furthermore, to prevent the thinner steel from deforming, the measuring system had to exert a contact force of less than 0.7 N on the sheet metal.
Given the high volume of parts and measurement points, co-ordinate-measuring machines (CMMs) were deemed too slow for this application. Data handling and computation were key elements to the control of manufactured parts. Solartron's transducers and Orbit network were compared against those from an alternative supplier, but Neff found that the quality and reliability of Solartron's Feathertouch probes was superior.
Devices equipped with USB (Universal Serial Bus) communications have proved to be remarkably popular for consumer electronics, and USB has also been successful in industrial applications. One manufacturer that offers USB-compatible sensors is Futek, which recently claimed to offer the first miniature digital load cell with a USB output and microgram part-per-million (PPM) resolution. Targeted at the test and measurement industry, nanotechnology, pharmaceuticals, medical device manufacturers, process control of fine powders or packaging, and aeronautical experiments, the sensor's associated software displays the peak/valley and tare/gross values with numerous filtering options.
With such a USB-based system, the analogue amplifier, power supply and display are eliminated, enabling sensors with analogue outputs to interface seamlessly with digital devices. As the diagram indicates, there are several components to consider in order to build a traditional sensor system.
An ideal platform will include an amplifier/signal conditioner with an active analogue filter, which will require an external power supply. In addition, a multimeter display and data storage are necessary.
Futek argues that traditional sensor systems' shortcomings include power consumption, unwanted noise and voltage drop-outs that occur if the distance between the amplifier and display instrument is too long. The advantages of USB sensors, besides reduced cost, are high-resolution digital output, an integrated digital filter to reduce mains 50-60 Hz and high-frequency noise, an increased sampling rate for high-speed applications, and the ability to store calibration values in the on-board non-volatile memory.
Other sensor manufacturers have been quick to grasp the importance of USB connectivity, as, for example, Ellison Sensors International has done with its GS4200-USB digital pressure transducer. Designed to measure, analyse and record pressure directly to a PC without the need for costly I/O interface boards, the GS4200-USB is powered by the computer's USB port.