Automated Defect Detection in Ceramic Roof Tiles

The Challenge

In many roof tile factories, operators still tap each tile and judge its ring by ear to catch internal fissures and cracks. The method tires them, varies from one inspector to the next, and lets defects slip through as false passes or false rejects. On lines running close to 3000 tiles per hour, and for cracks too faint to hear, automated non-destructive testing is the obvious answer, yet the complex geometry of modern tiles, with its valleys, lugs and grooves, makes a robust automated decision hard to engineer.

The Solution

On a French tile plant, GrindoSonic tested one tile model, with and without engobe, across sound parts and parts with light fissures or severe cracks. Impulse excitation applies a light impact and reads the tile’s natural resonance frequencies; because a tile rings in flexural, torsional and longitudinal-compression modes, where you support, strike and listen matters as much as the algorithm. The team built a three-stage learning protocol: learn the tile “free” on foam to map its modes, learn it “captive” on the line’s transport bars to set tolerance windows, then run it live with the automatic hammer. Frequency-sensitivity calculations confirmed the shifts come from cracks, not from engobe, mass, thickness or temperature, leaving three peaks as the OK/NOK criteria: flexural near 615 Hz, longitudinal-compression near 845 Hz and torsional near 1260 Hz.

Results

Light fissures dropped the characteristic frequencies by 3.8 to 6.6 percent and severe cracks by 6.7 to 10.7 percent, far beyond any drift from mass or temperature. At roughly 3000 tiles per hour, the MK7’s OK/NOK verdicts tracked the human sonador and a reference inspector, and caught cracks the ear missed. The one catch surfaced when an unloading robot left a tile out of position: an off-centre hammer strike raised a peak near 520 to 560 Hz that mimicked a defect, so hammer repeatability is the point to watch in any line automation.

Key takeaway: Cracks shift roof tile resonance frequencies by up to 10.7 percent, well beyond the drift from mass, engobe or temperature. That margin lets automated impulse excitation match or beat human tap-testing at 3000 tiles per hour, as long as the hammer keeps hitting the tile centre.