Sensors: The eyes of automation

If you compare a typical machine with the human body, while the PLC would be the brain, and the servo the arms, the sensors would be the eyes. Essentially, sensors give information and are a major input to automation systems.

Sensors are used extensively in production lines that are manufacturing semiconductors, consumer electronics, food & beverages, etc. Innovations in these industries are always on a fast track, with new products constantly coming out. Product life cycles are shorter than before, and product variety is getting wider.

Hence, there is no time for manufacturing engineers to develop a fully automated production line and ensure 100 percent error-free process. The trend is changing from mass production of few items to small lot, high variety production, with more and more semi-automated production lines where humans work with machines. This new manufacturing environment is creating new challenges for production lines to maintain productivity, yield, quality and safety.

Sensors can help. A typical example is a flat panel display (FPD) manufacturing plant, where a robotic hand is used to transfer glass panels. In order to ensure secure lifting, a sensor is required to confirm positioning of the glass panel on the robot hand. The sensor has to be very small, flat, and able to detect 99 percent transparent glass. An optical sensor is used, with light conveyed by fibre optic cable to the tip of the robot hand. The advantage: safe and secure handling of the glass panels.

Illustrating the variety of applications for sensors in manufacturing: parallel beam laser sensors ensure precise diameter and warp of a photocopier roller (below), and safety sensors isolating workers from a dangerous zone (above).

Various sensing techniques

On a production line, sensors are typically used for three purposes: to detect the presence of an object or a part; to inspect the quality of a product or a process; and to protect workers from accidents.

The question then arises, if there are just three main application areas, why then the need for so many different kinds of sensors? The reason is that no single sensor model can do everything and there are many different environments in which they are required to operate.

Photoelectric sensors  The sensor which uses light is called a photoelectric sensor. Depending on the sensing task, either red, blue, green or infrared light is used. Infrared sensors are normally used in dusty environments as the IR beam is able to pass through dust and smoke easily. Depending on the contrast of the object and background, the choice of whether to use red, green or blue light is made. There are colour sensors which use RGB technology and can detect subtle colour differences.

Fibre sensors  Fibre sensors use optical fibre to transmit light from the sensor amplifier to the object. Advantages are that the amplifier is isolated from the object, and the sensor heads can be made in many different shapes and sizes to suit the manufacturing environment. Thus, fibre heads can sense objects in very narrow and tight spaces.

Laser sensors  Spot beam or parallel beam type laser sensors are commonly used for high precision displacement measurement. Combined with CMOS or CCD technology, they can achieve high resolution of up to sub-microns.

Vision sensors  Probably the most advanced sensors of all. Using CCD for image capture, they are particularly suited for 2D inspection. Vision sensors are commonly used to detect quality attributes of the product such as appearance, scratches, dents, colours, orientation, shape and size, as well as for character recognition.

Inductive sensors  Used to detect metallic objects in harsh environments such as high temperature, dust, oil, grease, corrosive gases, welding spatter, etc. Most come in a metal body and are cylindrical in shape, with some teflon-coated.

Contact sensors  The best choice for high precision measurement of objects where surface conditions can vary from shiny, matt or colour. Usually, the measurement range is very small and accuracy is in microns. It is often used in multi-point measurement of precision metallic parts.

Ultrasonic sensors  Used for detecting various surfaces, with a common application being overhead measurement of liquid level in a tank.

As well as selecting the right sensor for the application, engineers need to work with devices that are easy to use and that can be rapidly configured. Hence, apart from the core sensing element, sensors increasingly come with built-in functions and features such as counters, timers, logic, data logging, and network compatibility, so allowing the user to use additional control devices.

In addition, having the sensor capture data with real-time date/time in a built-in storage card or sending to the PC/PLC, can go a long way in tracing the root cause of a quality problem, and potentially saving a great deal of money for the manufacturer.

Smart sensing

After recognising that quality issues are a major consideration in modern production plants, especially in sectors like displays, semiconductors, digital appliances, and even automobiles and food and beverage, Omron decided some years ago that sensors could go beyond simple on/off detection and aid in solving pressing quality issues of todays manufacturers.

But the company also realised that the many different types of quality applications and manufacturing conditions would require many kinds of sensing methods and many kinds of sensor heads to suit different manufacturing condition. Users would face cost issues and also need a high level of technical support too.

So Omron developed Smart Sensors, which are based on a platform concept whereby one sensor amplifier can support several sensing heads. The result: affordable to produce and users only need to get familiar with a single user interface.    

Now, Omrons ZX, ZS and ZFV range of Smart Sensors are using laser, inductive, vision, contact, microwave, etc, techniques to perform a variety of advanced, high accuracy sensing applications, eg: height, shape, eccentricity, wobble, inclination, scratch, dents, etc.

Bringing the plug & play concept to sensors, a single amplifier can be used with many different sensor heads, increasing usability and affordability. 

Many companies are trying to survive against global, low-cost competition, so it is essential to have an error-free production line. Sophisticated but affordable devices such as Smart Sensors can help manufacturing companies to solve quality issues at various levels, from process variation to product packaging, and in doing so, allow them to stay competitive.

Ketan Mistry is assistant manager, Marketing Management Division, Omron Asia Pacific.