Contact vs. Non-Contact Measurement
Understanding which dimensional inspection sensor type, contact, or non-contact, best meets your requirements can be a bit complicated. With this article, we aim to help answer some basic questions and give readers an idea of how to go about choosing measurement systems and methods. For a simplified introduction, contact measurement devices must navigate the object/part point-to-point, where non-contact systems measure with absolutely no contact or direct touch probing whatsoever.
The types of objects/parts and the accessibility to the features to be measured will most likely be the deciding factor when choosing an inspection method. The most common methods include both contact and non-contact devices, such as Coordinate Measuring Machines (CMM), Optical Comparators, Vision Systems, or a combination of the two, such as, Multi-Sensor Vision Systems which include both contact and non-contact probing.
When selecting equipment, it is best to reduce all aspects of uncertainty. That is why it is also important to select devices and systems with robust software, reliability, longevity, and with the right level of accuracy and repeatability. Other factors to consider may include your manufacturing process, frequency of inspection, speed and performance, traceability, and ease of documentation.
To Contact, or Not to Contact? That is the Question
Which inspection system, contact or non-contact, best meets your requirements? Although the question is a relatively simple one, the answer, however, can be fairly complex. The growing need for exact and interchangeable components that require tighter and tighter tolerances means that understanding your manufacturing process is vital, as is having the ability to back it up with precise, meaningful measurement data from the right piece of inspection equipment.
As parts reach the micro scale, they become more fragile, can be more easily contaminated, damaged, and deformed with even the slightest contact from inspection probing. Or the part may have tiny features that can be near impossible to measure with such methods. In these cases, non-contact inspection methods found in Vision Systems, Structured Light, and CT Scanning systems may solve the problem.
Additionally, the part may be difficult for vision and structured light methods due to line-of-sight issues or have inaccessible features (e.g., deep bores and undercuts). In these cases, contact or even CT scanning may be the appropriate method. On the other hand, maybe the material is too dense to penetrate the available CT Scanners on the market. There are a lot of variables that need to be taken into consideration when selecting the right method for inspection.
Inspection System Types
Non-Contact Systems: 3D Vision Systems, CT Scanner, Laser Scanner, Articulating Laser Scanning Arms, Photogrammetry, Structured Light Scanners, LiDAR Range Scanners, 2D & 3D Microscopes, and CFS Sensor Systems
Contact Systems: CMMs, Hard Probe Articulating Arms, Gage Pin sets, Calipers, Gage Blocks, Height Gages, Surface Plates, Micrometers, Contracers, and Roundness Testers.
Understanding Non-Contact Measurement
Non-contact or touchless dimensional measurement devices are the modern alternative to the assortment of contact inspections that have traditionally been used for inspection. Examples of non-contact devices include Vision Systems, CT Scanners, Laser Scanners, Photogrammetry, Articulating Laser Scanning Arms, Structured Light Scanners, and CFS Sensors (Confocal White Light Sensor) devices. One exception is the Optical Comparator, which has been around in some form or another for almost a century. Classic Optical Comparators themselves do not inherently produce digital data, although variations with enhanced digital systems can have this ability.
The main benefit of non-contact devices or systems the large amount of data they can collect quickly while not having to make contact with the object/part when gathering measurement data. Almost all non-contact systems produce large amounts of raw point cloud or voxel data when scanning. It is important to note that non-contact measurement devices (excluding those designed for nano-level measurements) are typically less accurate than contact devices. Still, that variance is getting smaller and smaller every day.
Non-contact devices gather more of the objects/parts surface. They usually are better at quickly capturing very complex shapes commonly found in industries like aerospace, automotive, and medical. Non-contact devices are especially good at complex characteristics such as multiple axes, repeating patterns, or a multitude of features. Non-contact inspection systems are also ideal for sensitive work such as inspecting surgical devices, objects related to legal investigations, and damaged or contaminated parts.
Another aspect of non-contact we should discuss is Range Scanning, also known as LiDAR Scanning. LiDAR scanners (laser scanners) can capture detailed measurements for large areas like entire rooms, buildings, or even cites by compiling multiple scans. Range scanners measure the time it takes for a laser pulse to travel to an object and then reflect light back to the scanner sensor. Then it uses the time it takes to calculate the distance. Each scan is made up of millions of separate scan point measurements. But each scan is line-of-sight, so many scans are potentially needed to provide a complete 3D digital representation of an object or large area.
Understanding Contact Measurement
Contact measurement devices represent the majority of what is routinely used for dimensional metrology. The most advanced contact measurement devices are CMMs. CMMs tend to be more accurate than many non-contact devices and come in a variety of sizes. They generally consist of 3 independent axes (XYZ) that use scales to indicate each of their positions relative to each other and the CMM. They are usually more expensive, require a greater footprint, and are not easily moved. CMMs also tend to be more time consuming because they navigate the part from point-to-point during inspection.
Modern CNC CMMs usually have the advantage of being programmable and are typically used in most general inspections. Manually operated CMMs (not CNC controlled) exist but are not as common today. All the major CMM manufacturers have relatively similar levels of accuracy (.0001-.0002″ for entry-level systems). The most advanced CMMs on the market are currently capable of achieving is .3 microns (intrinsic). So, if you require highly accurate inspections? Most likely, you will require a device such as a CMM.
Other methods for contact-based measurement are fairly standard in the quality industry and include a classic array of manual and open setup equipment such as Gage Pin sets, Calipers, Gage Blocks, Height Gages, Surface Plates, and Micrometers, but these are usually only capable of 1 dimension of measurement. Modern height gages can measure 2-dimensional circles and 3-dimensional cylinders in some situations. The accuracy for each of these different types of measurement tools differs but typically range from millionths to low thousandths of an inch to millionths on an inch (such as super-micrometer).
Should You Make Contact? Ask the Experts
Hopefully, you now have a better understanding of contact and non-contact inspection systems. But don’t worry if you’re still not sure. We are glad to help. At IIA, we are experts in all aspects of dimensional measurement because it is what we do and have done so now for 33 years. We work to combine the best equipment and methods to properly address your scope of work. Our goal is to provide you with the best information for your project in the most efficient and cost-effective ways. We offer a wide range of measurement services as well as products for purchase. Have a look at our offerings to learn more about what we can do for you and your business. We offer a no-cost needs assessment to help you make the right decision, or simply contact us today.
