IF you compare a typical machine with the human body, if the PLC is the brain and the servo the arms, the sensors would be the eyes.
Sensors provide a major input to automation systems at a time when the trend is changing from the mass production of a few items to small lot, high variety production, with more and more semi-automated production lines.
This new manufacturing environment creates fresh challenges for production lines to maintain productivity, yield, quality and safety. Here, sophisticated devices such as Omron’s Smart Sensors can help.
A typical example would be a flat panel display (FPD) manufacturing plant, where a robotic hand is used to transfer glass panels. 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 and flat, and be 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.
It’s all a matter of technique
ON production lines, sensors typically have three purposes: to detect the presence of an object, to inspect the quality of a product or process, and to protect workers from accidents.
The question then arises: if there are only three main application areas, why is there the need for so many kinds of sensors?
Sensor catalogues from any reputable manufacturer contain many models of various shapes and sizes, simply because no single sensor can do everything and there are many environments in which they are required to operate.
Let’s look at different ways to sense object presence, position and surface characteristics:
These sensors use light. Either red, blue, green or infrared light is used - depending on the sensing task. Infrared sensors are normally used in dusty environments as the IR beam is able to pass through dust and smoke easily. The choice of whether to go red, green or blue depends on the contrast of the object and background. Colour sensors that use RGB technology can detect subtle colour differences.
Fibre sensors use optical fibre to transmit light from their amplifiers to an object. Here, amplifiers are isolated from objects. Another benefit is that sensor heads are available in different shapes and sizes to suit the manufacturing environment. Thus, fibre heads can sense objects in very narrow and tight spaces.
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 are 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 the quality attributes of a product such as appearance, scratches, dents, colours, orientation, shape and size. Vision sensors are also used for character recognition.
Inductive type sensors are 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. Some are Teflon-coated.
Contact type sensors are the best choice for the 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. Such sensors are often used in the multi-point measurement of precision metallic parts.
Ultrasonic sensors are used for detecting surfaces. Here, a common application would be the overhead measurement of liquid level in a tank.
DETECTING defects or performing measurements however are not enough, especially if a sensor is being used for product or process quality applications.
Unless the root cause of a defect is found, problems will recur.
Devices - such as Omron’s Smart Sensor - not only undertake high precision measurements but also high speed data logging, storing data in real-time in a built-in storage card or sending it to a PC or PLC. The data can then be analysed to trace the cause of the defect.
Thus faults can be caught well in advance, helping to eliminate any factor that has led to the defect.
At the same time, manufacturers are able to reduce the amount of scrap, tighten up on product rejection and minimise customer claims.
As well as selecting the right sensor, engineers need to work with devices that are easy to use and that can be rapidly configured.
Apart from the core sensing element, sensors thus increasingly come with built-in functions such as counters, timers, logic and network compatibility, as well as data logging.
Omron’s Smart Sensors are based on a platform concept where one sensor amplifier can support several sensing heads. As a result, users only need to get familiar with a single user interface.
The ZX, ZS and ZFV range uses laser, inductive, vision, contact, microwave, etc., techniques to perform a variety of advanced, high accuracy sensing applications e.g. height, shape, eccentricity, wobble, inclination, scratches, dents, etc.
* Commentary by Ketan Mistry, assistant manager, marketing management, Omron Asia Pacific