Optical Reflectors: The Alternative to Rigid Measurement Setups

A lot has happened since computer-assisted metrology was invented. In its early stages, computer numerical controlled (CNC) machines were used to control manufactured parts, and it was not until the 1970s that computer-controlled coordinate measuring machines (CMMs) appeared on the market and brought increased precision and acquisition speed.

Later on came portable CMMs, which introduced freedom of movement to the concept, but failed to eliminate some of the drawbacks—such as setup rigidity, which is inevitable to measurement accuracy with this type of device. In the case of portable CMMs, both the device and the part require environmental stability because both entities are considered as one rigid body during the measurement process.

The HandyPROBE portable coordinate measuring machine (CMM) 

Optical CMMs entered the market at the turn of the millennium, and they were innovative in that optical reference models were used to reference the part to be measured. This provided accurate part control, even in an unstable environment, a low-quality setup, or with an inexperienced operator. A mechanical link between the part and the measuring device wasn’t needed because an optical CMM has two cameras that continuously track the position of optical components. All you have to do is attach optical reflectors onto the part, and the optical sensor analyzes the reflectors and automatically generates a local coordinate system, which ultimately eliminates errors resulting from manual measurement. Any instability in the environment will have absolutely no effect on the accuracy level because there are no physical links between the part and the measuring tool.

Creaform’s MetraSCAN Optical CMM Scanner

So how would equipment requiring a rigid setup perform under the same conditions? Well, it is a challenge to validate the results generated by this type of equipment in an unstable environment because the measurement error linked to the environment will depend on the degree of instability (which is extremely hard to quantify). Consequently, the validity of the measurement generated by this type of equipment must be questioned as soon as you have the slightest indication there might be instability.

Optical reflectors

It’s interesting to know that it’s now possible to measure simple geometrical entities and complex forms with one simple optical tool. An optical CMM can track a probe to measure geometrical entities like plans and cylinders, but it can also track a 3-D scanner for freeform elements analysis.

Let’s look at a fairly simple, repeatable product development project: designing and manufacturing a stamping tool. First, the tool is designed and the material spring-back estimated with the greatest precision possible. Once the first part has been printed, an inspection must be done to identify corrections that need to be made on the tool. The first step is to create an optical referential on the stamping tool by reading the optical reflectors’ coordinates with the optical CMM. The second step is the inspection itself, which will flag to the operator the areas requiring modifications.

Once the part has been modified accordingly, a second inspection is done using the same optical referential. This way, the measurements are aligned in the same coordinate system as the original measurements, which eliminates handling errors during alignment and leads to increased accuracy, as well as increased repeatability and reliability, because it eliminates variations that occur from having multiple operators. A second inspection using the same optical referential also lowers inspection costs and enhances productivity by reducing time to market, since these operations can be repeated an indefinite number of times without any negative impact on the results.

Most definitely, optical reflectors should be considered by companies that want to reduce their error rate during quality control. I know from personal experience that an unstable work environment is a great challenge when trying to conduct metrology processes in plants. Also, the optical systems’ ease of use is a great asset when it comes to reducing inspection time and costs.

The world of metrology is in constant movement. Optics is the most significant improvement to portable measurement that has occurred during the past few years, and I’m convinced that the migration toward optical CMMs for measuring and scanning of parts and production tools is far from over. This technology has the potential to help major manufacturers take the lead in high-quality production.

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