Multi-part assemblies which are missing parts, including hidden internal parts, can be detected with noncontact sensors if the majority of the parts are nonconductive such as plastic. If the potentially missing part is metallic, either capacitive or eddy-current sensors could work. If the potentially missing part is not metallic, a capacitive sensor will be required.
Capacitive Sensor Version
Capacitive sensor measurements are affected by the dielectric constant of any material between the sensing surface and a grounded target surface. Passing an assembly through this gap will cause the capacitive sensor output to increase as the increased dielectric constant more effectively couples the sensor’s electric field to the grounded background surface. If a part is missing, the sensor’s output will not increase as much. This is especially true if the missing part is metallic, as capacitive sensors have much higher sensitivity to metallic parts.
Eddy-Current Sensor Version
Eddy-current sensors are unable to detect nonconductive materials. If an assembly which is primarily nonconductive (plastic) contains a metallic part, passing the assembly near the sensing surface will cause the sensor output to change when the metallic part is present; the output will not change if the metallic part is missing.