Quality control in packaging operations demands reliable, automated inspection systems capable of keeping pace with high-speed production lines. One of the most common packaging defects — and one with significant implications for product quality, customer satisfaction, and regulatory compliance — is a foil wrapper that has been sealed without its intended contents. Manual inspection by human workers is slow, inconsistent, and impractical at modern line speeds. An automated detection system using industrial-grade infrared photoelectric sensors provides a highly reliable, non-contact solution to this problem.
The Application
Many industries use foil wrappers to package individual portions of products — food items such as chocolates, cheeses, and single-serving condiments; pharmaceutical tablets and capsules; medical devices; and consumer goods of all kinds. In each case, the production process involves placing a product into a foil wrapper and sealing it. When the placement step fails — due to a missed pick, a jam in the feed mechanism, or an empty supply hopper — the line continues to produce sealed but empty wrappers.
Empty wrappers that make it through packaging and reach end customers represent a quality failure, a potential regulatory violation (particularly in food and pharmaceutical production), and a source of customer complaints. Detecting and rejecting them automatically, before they leave the production area, is the goal of an infrared-based inspection system.
Why Infrared Photoelectric Sensors?
Infrared photoelectric sensors offer several properties that make them ideal for this application:
High-speed response — industrial photoelectric sensors can switch at rates that keep pace with production lines running hundreds of units per minute, making them suitable for inline integration without creating a bottleneck.
Non-contact operation — sensors detect the wrapper and its contents without touching either, preserving package integrity and eliminating mechanical wear on the detection system.
Adjustable sensitivity — the gain or sensitivity setting on an industrial photoelectric amplifier can be tuned to discriminate between filled and unfilled wrappers based on the difference in infrared transmission or optical density. This tunability is the foundation of the dual-sensor detection approach.
Reliability — industrial photoelectric sensors are designed for continuous, 24-hour operation with long service lives and minimal maintenance requirements.
The Dual-Sensor Detection Method
The system uses two sets of through-beam infrared photoelectric sensors, positioned at the same point along the production line so that each foil wrapper passes through both sensing zones simultaneously. The two sets serve distinct functions.
Set 1: Presence Detection (High Sensitivity)
The first set of sensors is adjusted to its maximum sensitivity setting. At maximum sensitivity, the infrared beam is strong enough to detect the foil wrapper itself, whether it is full or empty. The output from this sensor set is connected to the PLC and serves as a presence signal — telling the control system that a wrapper is currently in the inspection zone.
The purpose of this sensor is simply to tell the PLC when a package is present. It does not differentiate between full and empty.
Set 2: Fill Detection (Reduced Sensitivity)
The second set of sensors is adjusted to its minimum sensitivity setting — or more precisely, the sensitivity is reduced to the point where the infrared beam can pass through an empty foil wrapper but cannot pass through a foil wrapper containing product.
This works because an empty foil wrapper is a very thin material with relatively high infrared transmissibility. When the sensitivity is turned down sufficiently, an empty wrapper presents a thin enough obstruction that the beam still reaches the receiver. A wrapper containing a solid or dense product blocks enough of the infrared energy that the beam does not reach the receiver, and the sensor output indicates that the beam is broken.
Important note: The two sensor sets must operate on different modulation frequencies to prevent cross-talk. Industrial photoelectric sensors transmit their infrared beam as a modulated signal at a specific frequency, and the receiver only responds to that frequency. Using sensors from the same product family with different frequency settings, or sensors from different manufacturers with incompatible frequencies, ensures that Set 1 cannot interfere with Set 2 and vice versa.
Logic at the PLC
With both sensor sets reporting to the PLC, the control logic is straightforward:
- Set 1 output ON + Set 2 output ON — a wrapper is present and the beam passes through it; the wrapper is empty. Trigger a reject mechanism.
- Set 1 output ON + Set 2 output OFF — a wrapper is present and the beam is blocked; the wrapper is filled with product. Allow the package to continue.
- Set 1 output OFF — no wrapper is in the inspection zone; no action required.
The PLC uses this logic to activate a reject mechanism — typically an air jet, a pusher arm, or a diverter gate — to remove empty wrappers from the production stream and direct them to a reject collection bin.
System Configuration and Calibration
Calibrating the sensitivity of Set 2 is the most critical step in deploying this system. The target is a sensitivity setting that is low enough to pass the beam through an empty wrapper but high enough to have the beam blocked by a filled one. This window depends on the wrapper material's infrared transmission properties and the density and composition of the product inside.
Calibration procedure:
- Set Set 2 sensitivity to its minimum (lowest gain) setting.
- Pass an empty wrapper through the sensing zone. Confirm the output indicates the beam is passing through (beam received = sensor in light state).
- Gradually increase the sensitivity while passing filled wrappers through. Find the point at which a filled wrapper reliably blocks the beam.
- Set the sensitivity at a point between these two conditions where empty wrappers reliably pass the beam and filled wrappers reliably block it. Run a statistically significant sample through to confirm reliability.
The ideal sensitivity setting will have margin in both directions — meaning it will not be at the edge of sensitivity where small variations in product density or wrapper material could cause inconsistent results.
Sensor Frequency Separation
When two sets of photoelectric sensors are mounted in close proximity — as they are in this application — the risk of optical crosstalk must be addressed. If Set 1's transmitter emits at the same frequency as Set 2's receiver is tuned to accept, the signal from one transmitter may be received by the wrong receiver, causing unpredictable behavior.
Select sensor sets that operate at different, non-interfering frequencies. Many industrial photoelectric sensor manufacturers offer models in multiple frequency variants specifically to allow multiple sensor pairs to operate in close proximity without interference. Consult the manufacturer's selection guide or application support team to confirm frequency compatibility before finalizing the sensor selection.
Benefits of Automated Inspection
Implementing an automated foil wrapper inspection system delivers quantifiable benefits:
- Consistent detection — the system performs the same test on every package at every line speed, without the fatigue and attention lapses inherent in manual inspection
- High throughput compatibility — sensors respond in milliseconds, keeping pace with the fastest production lines without becoming a limiting factor
- Data collection — PLC integration allows reject counts to be logged, enabling trend analysis that can identify root causes of empty wrapper production and support continuous improvement efforts
- Regulatory compliance support — in food and pharmaceutical applications, documented automated inspection contributes to compliance with product quality and traceability requirements
- Reduced downstream costs — catching defects at the point of production eliminates the far more expensive process of sorting, repackaging, or recalling products after they have been distributed
Stainless-steel sensor bodies rated for washdown environments are available for food-grade applications where the production area is subject to regular cleaning with water or chemical agents, ensuring the inspection system remains operational through cleaning cycles without corrosion or moisture damage.