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Control Problem/ Solution: Controlling for Time/ Distance Lag

[#7] Last update 29 Jan 19
, Certified Vision Professional - Advanced
Machine Vision application with a two-camera time delay


Bottling lines operate at incredible speeds. This speed combined with the machine’s complexity increases the potential for catastrophic damage should an error occur. A miss-fill or cap mis-alignment can occur in the blink of an eye, making the use of automated inspection at strategic locations within the machine mandatory. One such location is the capper.

The capper, as its name implies, is where the cap is inserted onto the bottle top. The bottle passes beneath the capper traveler which moves with the bottle and spins at the same time to tighten the cap. A torque sensor senses when the cap is tightened to specification and the capper moves away from the bottle.

Although the caps may be torqued to within specification, flash or some other obstruction could cause a poor seal. A reliable way to verify that a cap is seated properly is to measure the distance between the shoulder of the bottle and the lower edge of the cap. Only one camera and a back light are required to perform the inspection.

The control challenge was to store the inspection results (pass/ fail) of the camera for triggering of a downstream reject mechanism.

Technical issues: machine access for sensor and camera; unequal part separation.


The picture above shows a simplified layout. The actual installation consisted of a sensor and a camera. The encoder and reject mechanism were part of the existing machine

A sensor detects the part at the beginning of the process. A sensor pulse and an encoder pulse are sent to the controller; the controller stores the relative position of the “current” part. The distance in encoder pulses between the sensor and the camera in pulses is known. When that part reaches camera location the controller sends a pulse to that camera and light. The inspection is performed and the results are stored. The distance between the camera and the reject mechanism in pulses is known. If the value of the inspection is “fail”, then the controller sends a pulse to the reject mechanism. The controller/ timer provided a simple method to sequence control without a PLC.