Multi-sensor measurement system helps improve medical device quality

Specialized workpieces require specialized measurement techniques

Medical devices and their components have extremely strict requirements on form and function. For example, the prosthesis for middle ear repair used in ENT is very small. In addition to the size, the manufacturing of medical device components often allows only very small tolerances. The measurement system for these medical device components must have high-precision performance, often to the sub-micron extreme.

In the field of orthopedic implants, such as artificial total hip ball joints, tibia, total knee and ankle implants require highly accurate measurement methods. The surfaces of these implants are high-order curves composed of non-uniform rational splines (NURBS). Since the implant components must match the prosthetic parts or even the parts implanted in the human body, they are often irregularly shaped and curved. Complex 3D curves increase the difficulty of measuring all planes from one direction, making them difficult for some types of sensors to measure effectively.

Video measurement systems are suitable for measuring prismatic workpieces with intersecting planes. When planes intersect, edges appear, and video can easily measure edges. Orthopedic implant components are often made of continuous regular curves (hip implants) or complex contoured surfaces (knee implants) that mimic the contours of human organs. These surfaces have few or no flat or intersecting edges.

While video sensors excel at measuring edges and surface points, acquiring data from even a linear cross section of a contoured surface with a large number of data points is laborious and impractical. Trigger probes have the same limitations, as the probe needs to approach each point and return to its original position after triggering - although feasible, it is also not suitable for measuring large quantities of products.

Measuring knee implants

The best way to determine whether the contour of the artificial knee joint bionic curve meets the design requirements is to use laser measurement. How does the laser sensor work in the multi-sensor measurement system? The laser sensor projects light onto the surface of the workpiece, and the sensor obtains the reflected light and scattered light and automatically calculates the distance between the laser and the workpiece in 3D space.

The laser can measure a point, or when the workpiece moves under the laser or the laser scans the workpiece, it can also obtain and calculate a series of data points. The user can customize the point interval and sampling rate.

When the laser beam moves over the workpiece, the measurement software can continuously calculate the distance between the laser and the workpiece surface, and keep the laser sensor within the capture range through the closed loop positioning controlled by the Z-axis stage. In this way, the precise position of the data point can be quickly obtained. Laser focusing is faster and more accurate than video autofocus. Since the laser is a non-contact sensor, it avoids potential damage to the workpiece surface and potential contamination of sterile workpieces.

In most cases, it may be difficult for the operator to fix the knee prosthesis to ensure that the laser is directly on all critical surfaces. At this time, mounting the prosthesis on a rotary indexing table is a solution, while reducing the steps of manual loading and unloading of workpieces and fixtures, thereby speeding up the measurement.

Typically, a datum is established from the knee bend surface using a probe, and then the rotary indexing table rotates the knee prosthesis to present the ideal surface for the laser sensor to measure. Because the datum is set on the opposite side of the surface being measured, the measurement system must be equipped with full 3D measurement software that can rotate the coordinate system as the indexer rotates. This way, regardless of the position of the rotary indexer, each data point captured by the laser is traced in the 3D space controlled by the measurement software.

Another way to measure the complex contours of a knee prosthesis is to use a Renishaw SP25 contact scanning probe. Similar to the laser, the operator determines the start and end points of the scan, but the probe remains in contact with the surface of the prosthesis as the system moves over the surface and acquires data points.

Unlike a touch-trigger probe, the SP25 contact scanning probe remains in contact with the part. As with the laser, the data point density and scanning speed can be customized. Multi-sensor systems equipped with the SP25 must be equipped with matching 3D measurement software to track data points in three dimensions.

There are other options for measuring knee prostheses that are fixed to a rotary indexing table. The linear laser and touch probe scanning mentioned above can scan the top surface of the prosthesis on the indexing table. Because the linear scan represents a linear cross section of the 3D part, the cross section can be measured by the video sensor as an edge. The prosthesis is rotated 90 degrees, and when the light shines on the part from the back, the cross section becomes a distinct "edge". This technique requires a good measurement lens system with a long working distance and is less affected by the bevel of the knee prosthesis.

Because the "cross section" is larger than the optical window, functions such as "edge finders" can be used appropriately when the system automatically tracks the edge and acquires data points in multiple windows.

The knee prosthesis is mounted on a rotating indexing table and its entire surface can be measured. Slowly turning the indexer, only a few degrees at a time, multiple linear scans (or edge finders) are completed, generating data points. These data points can be imported into 3D fitting software, and after the center of rotation is determined, the software will show how the data of the part matches the CAD model of the part.

Some fitting software can even perform geometric dimensioning and tolerance analysis of the data points, meeting multiple requirements at the same time, and any deviations between the schematic and the design files. This analysis is not only used at the acceptance stage of each workpiece, but also during the production process, manufacturing engineers can improve the accuracy and efficiency of subsequent workpiece production.

Medical device manufacturers need to record and control the production process at all times, which also includes the application of testing equipment to control and monitor product quality. Multi-sensor measurement systems can quickly and accurately detect important dimensions of medical devices, minimizing the number of times the workpiece is loaded and unloaded. Ensuring that the workpiece is produced to meet the design specifications is the name of the game. The final result will affect the health of the medical device manufacturer's balance sheet - and ultimately the health of the patient.

Technical Tips

• Video optical measurement systems are suitable for measuring prismatic parts, which, as the name implies, have intersecting planes.

• Laser focus is faster and more accurate than video autofocus, and because laser sensing is non-contact, it avoids potential damage to the surface of the part and potential contamination of sterile parts.

• Multi-sensor measurement systems can quickly and accurately detect important dimensions of medical devices, minimizing the number of times the part is loaded and unloaded.