Speed and length: The endless quest for precision
17 February 2021
Darren Pratt, SICK’s UK Product Manager for motion control, considers the challenges for achieving precise measurement of continuous and cut-to-Length materials. He introduces the new SPEETEC non-contact laser measurement technology from SICK that can make processes more efficient and reduce the need for product give-away.
Measuring speed and length is a common requirement in many industrial processes and one in which the quest for precision is never-ending, especially where more challenging materials are being produced and processed.
While the measurement principles have traditionally been similar, the materials and processes are markedly varied – everything from packaging, to plasterboard, to plastic pipes. What they have in common is that they are produced in a continuous or ‘endless’ process, whether it be extrusion, casting or blow-moulding. Speed and length must be accurately synchronised during the creation of these materials to optimise process efficiency and ensure a high-quality end product.
Avoiding product give-away
Further down the line, when the material is cut to length, often on a conveyor of some kind, ensuring an accurate measurement of the piece goods is closely aligned to profitability. In reality, manufacturers will often allow a little extra length to avoid the consequences, and even potential financial penalties, of under-fulfilment. But allowing give-away eats into the bottom line and investment in accurate instrumentation to ensure a more accurate result could rapidly pay back, protecting all-important margins especially when trading conditions are tough.
So, let’s consider some examples. During the manufacturing of building materials like plasterboard, chipboard, or extrusions of uPVC window profiles, the amount of material produced needs to be carefully synchronised with the supply of raw materials into the process to ensure consistent quality. Accurate data about the speed and length of material coming out of the process is therefore critical.
Additional processing steps are likely to follow and the speed, say of a conveyor, needs to be synchronised with the exact position of the product. That might be to control a machine that adds a laminate, prints onto paper or textiles, or adds a label, or it might be to conduct a quality inspection.
The rotary encoder has generally been the technology of choice to provide a speed and length measurement for these purposes. Many engineers will be familiar with the need to fit a rotary encoder to the drives of the conveyor, and they will know that slippage between the conveyor and the material can be the enemy of accuracy.
For some processes and materials, measuring wheel encoders can solve the problem of slippage. The set-up, usually comprising an encoder, a measuring wheel and a spring mechanism, can either be ‘home-made’ in the workshop or supplied as a ready-made all-in-one device by manufacturers. SICK offers several tailor-made measuring wheel solutions and our latest product, the SICK MWS120, offers the advantage of an easy manual and repeatable adjustment of the force applied by the wheel to the product.
However, the measuring wheel encoder has its limitations. While the benefit of contact with the material can provide good measurement results, wear of the measuring wheel on an abrasive product will threaten to degrade accuracy over time and therefore require closely-monitored maintenance and replacement. Some endless materials, such as rubber or extruded plastic products, can be delicate and easily damaged by a wheel in contact with the material.
Measuring piece goods
When measuring piece goods, direct detection of the material is normally not possible with a measuring wheel. Instead, encoders are typically used to monitor the conveyor together with a photoelectric trigger sensor to detect when the piece good is present. But then the problem of slippage returns.
To solve these challenges, an accurate non-contact measurement principle is needed. However, current non-contact options on the market have tended to be at the higher end of the investment scale, and therefore limited to specialist applications. So, for the past few years at SICK, we have had our eyes on developing an affordable motion control sensor that can use eye-safe infra-red laser measurement as the basis for reliable machine integration.
Introducing SICK SPEETEC
The result is the SICK SPEETEC non-contact sensor for speed and length measurement, which uses Laser Doppler measurement directly on the material surface. The Doppler effect is familiar as the pitch change in acoustic waves when an emergency vehicle moves towards or away from us. This pitch change is a frequency shift caused by the relative movement of wave source and observer. In the same way, the SICK SPEETEC detects the frequency shift of the light caused by the movement of the material. Just a little larger than a mobile phone, it can be mounted above, below or at the side of the material. With intelligence onboard, the SPEETEC can then automatically convert the measurement into TTL/HTL signals identical to those of an incremental encoder, so that they can be easily integrated into the control system.
The SPEETEC works at speeds between 0.1 and 10m/s to measure directly on the material surface with a resolution of 4 µm, an accuracy of 0.1%, and a repeatability of 0.05%. As a result of this high performance combined with unprecedented affordability, new automation opportunities are opening up which would not have been feasible previously. For existing processes, adding a SICK SPEETEC could reduce reject rates or increase throughput speeds.
The SPEETEC’s non-contact measurement principle means it can be used where a measuring wheel in contact with the substrate would never have produced completely reliable results. There’s no danger of damage to the material – important, for example for delicate, smooth or soft materials such as extrusions or textiles. There’s also no configuration or calibration needed, simply mount the sensor and start measuring.
Initial feedback from the first SICK SPEETEC customers, as well as that gained through extensive beta trials, has been hugely positive. We have found the laser measurement principle works well with a wide range of the challenging materials, from plasterboard and insulation, to tyres, from wooden parts to plastic extrusions and paper hygiene products. We have also seen how well the sensor works on shiny metals and near-transparent materials.
What’s best for your application?
However, the arrival of the SICK SPEETEC does not mean the end of the road for rotary encoders and measuring wheels. It’s more a question of ‘horses for courses’, and the correct measuring principle will be guided by the application and the material.
If you have a material or process that could benefit from non-contact measurement, then why not speak to a member of the SICK team? SICK UK is currently offering a 30-minute one-to-one presentation and online demonstration to anyone who would like to discuss whether the SPEETEC is right for their application. Please contact firstname.lastname@example.org for more details.
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