The eyes and ears of the system: sensors grow in importance

Heiko Luckhaupt, Category Marketing Manager - Automation & Control, RS Components

 

When assessing technological progress, it is very common to focus on the massive increases in processing power that have made the machines around us progressively more ‘intelligent’.

Against this backdrop, it’s easy to neglect the role of the sensors that provide the opportunity for today’s central processing units to do their work. How ‘smart’ would a smart phone seem, for example, without capacitive or resistive sensing technology? How much less fun would we have with home video games consoles, without advanced accelerometer technology to control them?

The same is true in the industrial control arena. Today, there is virtually nothing that an engineer cannot measure or sense, and then use as a system input. Sensors and transducers are built to suit a wide range of environments: and just as important as acquiring the signal is the conditioning and transmission system that ensures that it is safely conveyed to the central processing unit.

Perhaps the most prominent trend in this arena is the dramatic increase in the use of visual information as a system input. Industry analysts IMS Research estimate that the machine vision market grew 10% in 2011 to reach nearly $2.9 billion. The company says that despite the on-going difficulties in the macroeconomic climate, growth is likely to continue over the next five years, albeit at a more modest single-digit rate. Today, manufacturing accounts for more than 80% of machine vision revenues; but IMS also identifies a trend towards a broader use of the technology in areas such as traffic monitoring, security and surveillance, control and medical applications.

Products such as Omron’s ZFV series smart vision sensors allow engineers to implement vision systems that would have been impossible just a few years ago. The devices combine a sensing head equipped with a high-speed CCD camera and intelligent, adjustable LED lighting source that communicates digitally with an amplifier/controller module. Sensing heads are available with a range of field of vision specifications, from 5 to 50mm, while amplifiers can be either single-function or programmable/multifunction, depending on the application and customer requirement.

The amplifier module is equipped with an LCD screen and ‘teach and go’ software that allows users to define their inspection application with a few keystrokes. Pre-programmed basic algorithms include area, brightness, width, position, character, count and pattern. The screen is also used during the inspection process itself to display pass/fail information, and to show an image of the product being inspected.

Figure 1:  the ZFV series from Omron is designed specifically for visual inspection

Available in both colour and monochrome versions, the ZFV series can be programmed to perform inspections in 12ms, 6ms or 4ms (that is, at up to 250 inspections per second). As many as five sensors can be connected to a single controller bus, allowing five different inspections in one pass to greatly increase productivity. Up to eight different inspection targets can be stored within each system, for added flexibility.

While vision systems have undoubtedly seen much progress in recent years, more traditional sensor types have also evolved rapidly. One of the most important functions that needs to be implemented is position and motion sensing, and a vast range of linear and rotary position encoders is now available. While some offer absolute encoding, that is, they retain memory of the last position of the system after power-down, the vast majority of applications can be served by incremental encoders.

Typical of the new generation of such devices is the DFS60 line from SICK Stegmann. These devices offer high resolution of up to 65,536 pulses per revolution (PPR), but are based on a freely programmable platform that allows the user to set the output voltage, zero set position and PPR count via a software interface. The motor shaft and encoder are electrically isolated, increasing interference immunity and allowing the devices to be used in electrically demanding conditions. This electrical robustness is supplemented by the devices’ physical design, which employs an innovative bearing design and nickel code disc that enables an operating temperature range of -20°C to +100°C.

Figure 2:  The DFS60 range from SICK Stegmann is one of the new generation of incremental encoders

With the choice of axial or radial mounting, and TTL/RS422 and HTL/push-pull output signal options, the DFS60 series is suitable for a wide range of applications including printing, textile and packaging machinery.

Encoder outputs need to be chosen carefully according to the electrical environment, application and external signal conditioning apparatus. Manufacturers such as Baumer therefore offer a range of configurations specifically designed to cope with these different requirements.

Complementary line driver outputs, for example, can be useful in situations where cable runs are long, and noise due to EMI (electromagnetic interference) is possible. Push-pull outputs with short-circuit protection are more useful with short cable lengths, while a sine output may be required if the signal needs to go through an external interpolation process.

Like encoding, non-contact proximity sensing is a long-established technology that is used in a vast variety of positioning applications, ranging from hoisting in the factory environment to mobile lifting, automatic livestock feeding systems and automation in the food and beverage industry. The trend here is to provide improved performance using a traditional underlying technology, with advances in specifications such as sensor speed, robustness and smaller size. Progress has also been made in cost reduction, and offering increased flexibility, such as the ability to run from both AC and DC power supplies.

A product family such as the Schneider OsiSense range therefore includes both general-purpose devices and sensors designed for specific applications – for instance, IP69K rating for components that need to withstand high-pressure wash-down or steam cleaning, as often encountered in the food industry and in specialist areas such as car wash centres.

Figure 3:  The OsiSense XS range of inductive sensors can be used not only for position sensing but also for ferrous / non-ferrous discrimination

The OsiSense family includes the XS inductive technology for sensing of metal objects up to 60mm, and the XT capacitive technology, that can sense any material up to 20mm.

When position and presence detection is required on a much smaller scale, fibre optic sensors are becoming increasingly popular. Aside from their small size and their ability to detect small object and movements, they have a number of other benefits; they are resistant to electrical noise, which is an advantage in many environments; conversely, they do not themselves create electrical noise, so they are safe to use in situations where fire is a risk; and, via the use of glass, rather than plastic, fibre, they can function at elevated temperatures.

The principle of operation is very similar to that used in any other light-based sensing scheme. Light from an LED is coupled into an optical fibre, which conveys it to the site where sensing needs to take place. At the sensing head, the light is dispersed. Objects nearby will cause a certain amount of it to be reflected back along the fibre.

The fibre optic sensing system therefore consists of two main components: an amplifier that generates light and senses reflections, and the fibre itself. Amplifiers are available in digital and manual types. As the name suggests the digital variety provides a numerical display of the sensed light value, with the switching point set digitally. Manual units provide a bar graph display, and allow the switch point to be set via a potentiometer.

A number of different types of fibre can be used. A single-strand plastic fibre is most common. For more accuracy, users can choose a coaxial configuration, in which the central core is used to transmit incident light and the smaller surrounding fibres are used to convey reflections back to the sensor. This type can also be more effective in sensing objects that may enter the area of interest from any direction. A third type, multi-core, is constructed of a large bundle of small fibres. Because multi-core fibre is extremely flexible, it is often used in robotic applications in which the sensing head may have to move large distances.

E3X Series Programmable Fibre Optic Amplifiers and E32 Sensing Heads from Omron are good examples of the kinds of capabilities available. The standard E32 cylindrical sensing heads provide reliable object detection, easy installation and long sensor lifetime for all general applications, and include models with a hexagonal back for simplified mounting. For quick and easy installation on flat surfaces, the E32 family also includes square heads that are 3mm or 4mm thick with a choice of sensing in the X, Y or Z axis. A selection of miniature heads is available for object sizes between 500µmm and 3mm, and sensing in confined spaces. A diffuse coaxial sensor and additional lens extends the sensing range down to 100µmm. Finally, longer distance heads with a built-in focal lens allow sensing at distances up to 20m.

The E3X series of amplifiers includes the E3X-HD, which combines a dual digital display for clear reading and setpoint, with fast and simple one-button teaching dynamic power control and EtherCAT and ComponNet communications units.

Figure 4: The E3X-HD from Omron provides fast and simple teaching with a dual digital display

Designers can also choose from E3X models with enhanced water resistance, fast 20µs response time for high-speed detection, and a dual version that allows the detection of two objects simultaneously.

 

Conclusion

In this short article, we have barely been able to scratch the surface of what is possible with modern sensor technology. As well as vision sensors, proximity switching and fibre optics, today’s automation systems can include a vast array of bar code readers, light intensity, colour and contrast sensors. In the plastics industry, and in many types of machine tool, fluid level and pressure monitoring, temperature and flow rate measurement are all important functions.

The importance of these many and varied devices is often underestimated: but make no mistake, any automation system would be lost without these vital ‘eyes and ears’.