Feature Inspection & measurement Going the distance for quality
The quality assurance demands of manufacturing industry have changed out of all recognition in the last two decades. Parts are often manufactured to micron or sub-micron tolerances and must frequently be measured without physical contact. The simple photo-electric detection of whether a component is present is, in many cases, no longer enough and quality assurance specifications often require specific verification of the position of a component in an assembly. David Hannaby, product manager for imaging & measurement, Sick UK, explains
T
he requirement for specific varification, has resulted in the development of a wide variety of distance measurement sensors, usually optical, that provide analogue outputs defining the distance between the sensor and the target. Many dis- tance measurement systems use Class I or Class II lasers as their light source, rather than the infrared or LED sources used in previous years. The reliability, service life and spot size of lasers have all improved greatly in recent years and their low average wattage has made them safe in manufacturing and quality assurance applications. The most recent development in short-range distance measurement has been the optical displacement sensor, now routinely used in quality assessment procedures to measure dimensions, define positions and quantify characteristics such as curva- ture, deformation and vibration. Using optical displacement sensors in the race for ever-better quality is providing competitive advantage to manufactur- ers in a wide range of industries including automotive, packaging, machining, robotics and tooling. Optical displacement sensors are in many cases a better alternative to Linear Variable Distance Transducer probes (LVDT) and are particularly useful and cost-effective where movement of the probes would be required. LVDT probes have to be in contact with the sample being assessed, which has the potential either to damage the probes or the prod- uct. Optical displacement sensors need no contact, and as a result of receiver technologies, have been proved to be as accurate as LVDT measurement. Optical displacement sensors are available either as separate self-con- tained units, or as sensor systems, in which several individual sensors interface with a common controller. The sensor system offers many bene- fits because the easily-programmed controller makes it possible to apply filters, set output functions and
18
specify requirements at less cost than using a programmable logic controller. Some sensor system controllers can interface with three sensors to apply formulas to calculate results from the sensors as a group.
Applications
As an example, Sick has recently provided the optical displacement sensors needed for vital quality assur- ance routines in the manufacture of smartphone screens. Touch screens of smartphones and tablet computers consist of several layers of transparent materials. Assembly must be precise: unevenness or errors often cause breakage and to avoid this it is neces- sary to check glass thickness and the size of the gap between sheets of glass or similar materials.
The latest Sick optical displacement short range distance sensors measure glass thickness, or the gap between two transparent materials, with only one sensor head. This capability is used to measure the coplanarity of components, for example the displays and casings of smartphones and tablet computers or hard-disk drives. The laser triangulation principle, together with high-end CMOS (comple- mentary metal-oxide semiconductor) receiver technology, makes possible the measurement of glass thickness with only one sensor head. The Sick OD Precision short range distance sensor defines the positional coordinates of the front surface of the glass and the reverse side simultaneously and outputs any difference between the two. The same output rate, measuring frequency and accuracy are achieved as with normal distance measurement. It is even possible to measure the thickness of a gap between sheets of glass or other materials.
Defining your terms
When specifying the best distance measurement sensor for an applica- tion, it is important to consider several
Sick UK T: 01727 831121
www.sick.com/uk Enter 312
Above and top left: Applications using optical distance sensors to satisfy quality assurance specifications
issues in addition to the distance between sensor and target. The key ones are Resolution, Repeatability and Accuracy. It’s important to know what each means before talking to suppliers. Resolution: This is the smallest unit of measurement that the sensor is capa- ble of measuring. If a sensor has a specified resolution of 10mm, the next measurement in the scale will be 20mm, then 30mm, and so on. Repeatability: Repeatability refers to the deviation between several measurements of the same parameter carried out under identical conditions. If a sensor measures and transmits an analogue value every millisecond, each value would be very similar if repeatability is good. Current optical displacement sensors in the Sick short measuring ranges achieve repeatability of 0.6 micron. Accuracy: The maximum likely mea- suring error between the determined and the actual distance – often stated as a percentage of full-scale measuring range. If a sensor is stated to have accuracy of ±one per cent of full scale and the sensor has a full scale of 20 millimetres, then the accuracy would be ±0.2mm. It is also necessary to consider response time and output type. Analogue outputs of 4-20mA current or 0-10VDC voltage allow easy adjustments to process tolerances and variables.
For a successful specification it is best to work closely with your sensor supplier, who can help to ensure effec- tive installation and commissioning.
JULY/AUGUST 2013 Factory Equipment
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44