Since most packaging lines are a group of serial operations, sensor selection is best approached on an operation-by-operation basis. And when applying sensors to any operation—packaging notwithstanding—the key is to break that operation down into specific processes and identify the changing condition the sensors must detect.

The easiest way to fully understand these processes is to answer this question: If I were to manually package a product myself—vitamins, for instance—what would I have to do? First, bulk containers from the factory would have to be unloaded and prepared for filling. After filling the bottles with vitamins, the bottle must be closed and sealed. But prior to sealing the bottle, a wad of cotton would be added to prevent excessive movement and breakage of the vitamins within the bottle during shipping. A label would be applied. The individual bottles would then be shrink wrapped and packed into cases. Those cases, in turn, would be stacked on pallets for distribution.

In the automated equivalent on the plant floor, sensors initiate, inspect and confirm these processes. Sensors also monitor and control the movement of product between operations. In packaging industry vocabulary, those processes or events are depalletizing, filling (and closing), checkweighing, labeling, shrink-wrapping and palletizing of the final product. For the most part, these processes share similar sensor applications including container or product presence and position as well as void/jam/motion detection.

But the individual processes also have their own unique sensing applications.

Depalletizing
In the depalletizing stage, bulk containers—bottles, bags, cans, etc.—are unpacked from pallets and inserted into the process. Photoelectric sensors may be used to ensure the pallet is in position for unpacking, but limit switches and even capacitive sensors may be employed. Sensors with clear object detection capability such as photoelectric or ultrasonic sensors are ideal for confirming proper infeed of clear bottles, while opaque bottles are more easily sensed with standard photoelectric sensors. In either case, the sensor is wired into the PLC for motion detection, constantly checking for voids and jams; if a jam or gap in bottle infeed is detected, inbound conveyor speed and ultimately the unpacking of the bottles can be increased or decreased accordingly.

For canning applications, inductive proximity sensors may be used for the same operation. Special can sensors with built-in jam and motion detection have been designed specifically for these applications. Using a special sensing field, can sensors with underload and overload functions optimize line capacity by correcting for lags and surges in can flow.

Filling and Closing
Photoelectric sensors are typically used to confirm the arrival and position of a container to be filled. After filling, another sensor—stationed at the outfeed point—verifies that the filling operation has occurred. Depending on the type of material and the vessel in which it is stored, ultrasonic, capacitive and photoelectric sensors are used to determine if an adequate supply of the filling material exists. These level sensors will either signal the operator to manually feed or a control system to automatically gate more product to the filling apparatus, whether it is a tank, tube or scale.

After the container is filled with product, the space between the level of the product and the top of the bottle is often stuffed with a filling material to absorb vibrations that could cause damage to the product during shipping. An example of this would be the wad of cotton in an aspirin bottle. To verify the presence of the cotton filler, the most common solution is a background suppression photoelectric sensor. By actively sensing for both the cotton target and the pills which represent the background, a background suppression photoelectric sensor can effectively determine whether the cotton is present without false triggering due to reflections from the vitamins themselves.

In today's security conscious world, most packaged products have either an internal seal or external overwrap (and sometimes both) to guarantee that the goods inside are as they left the factory. Environmental conditions, such as humidity, may also merit the use of sealing methods so that product can be warehoused in facilities with uncontrolled climates. Sometimes the seal is made of foil, in which case its presence can be most easily confirmed with an inductive proximity sensor looking in from above. (Although inductive proximity sensors do not sense aluminum well, foils are the exception—their crystalline structure is closer to that of ferrous metals.) In other cases, fixed-focus photoelectric sensors can be used to verify seal presence.

Closing of the container, of course, varies depending on the container type—cans get lids, bottles get caps or corks, boxes get folded and glued. As in the rest of the processes, sensors are used to see containers before they arrive at the closing station, either to stop infeed in the event of a jam or when commanded to do so by the control system. Another sensor is used to ensure the container is positioned properly under the closing apparatus. Additional sensors, usually photoelectric sensors, continually check the cap/lid supply and subsequently confirm proper cap placement on the bottle before sending it off to the next station.

In some packaging systems, a checkweigher is part of the filling and closure process. The checkweigher determines if the proper amount of product has been placed in the container. Too much or too little product weight will cause the offending bottle to be rejected. There are many consumer protection regulations that drive packaging companies to include checkweighing procedures in the process. This helps protect the consumer by ensuring a.) that the quantity/weight printed on the container is provided; b.) proper dosages or serving sizes; and c.) quality. Generally, a photoelectric sensor will be used to verify the package has entered the checkweigher and will start or stop product flow depending on the results of the weighing process. Additional photoelectric sensors are used to divert rejects to a discard path when the weighscale (or other condition sensor) provides feedback not within the producer's specified parameters.

In some cases—dry cereal packaging being the most common—internal packages such as prizes and coupons are automatically inserted prior to closure. These items may be hermetically sealed in plastic or foil envelopes. Usually a photoelectric sensor is employed to detect plastic envelopes, but capacitive proximity sensors have also been known to work in these applications. For foil envelopes, as mentioned in the sealing phase, inductive proximity sensors are often the easiest and most effective detection method.

Labeling
Although the product itself is ultimately the most important aspect of the packaging process, the appearance of the container and labeling is the most crucial commercial aspect of a product.

Up to this point, sensors have been used to determine whether we have a container to fill; whether the container is in place to be filled; the status/level of the product supply; confirmation of container content. Now, in typical packaging application fashion, sensors will ensure that the bottles are in the proper position for label application and that labels are available to start the process. During the actual labeling, however, an encoder could be used to mark where the label is touched to the container, allowing the container to be rotated through 360 degrees for complete application of the label. After rotating the container through a full revolution, the encoder signals stop and the controller releases the bottle to the next stage where label presence and position is verified. Label verification may be achieved with a color sensor, a clear object sensor or fiber optic sensor looking for the edge of the label.

But in many applications, the package is the container, sealing method and label all rolled up in one. The best example of this might be the foil-embedded plasticized bag that holds your favorite brand of potato chip or cheese curl. Typically, the high speed filling, sealing and cutting of these bags is controlled through the sensing of color registration marks. A photoelectric color registration control signals when a bag is at the proper point along the web for the machine to expand the tape (bag stock), crimp the bottom, toss in the chips, seal the top and cut the bag to length. Gating photoelectric sensors are used to signal to the registration control to start looking for a color mark that will initiate the process.