Photoelectric Sensor Detection Errors and Chattering: Causes and Fixes, Starting from the Symptoms

Detection errors occur when the variation in received light reaches the threshold

A detection sensor decides its output by comparing a value such as the amount of received light with a threshold (the boundary value at which the output switches between ON and OFF). This value is not exactly the same every time. It differs slightly with the color of the workpiece and the position at which it passes, and it also shifts from day to day with dirt and temperature.

In other words, both the value with a workpiece and the value without one vary within a certain range. When the edge of that range reaches the threshold, detection goes wrong.

• Missed detection: The value with a workpiece falls below the threshold
• False detection: The value without a workpiece exceeds the threshold
• Chattering: The value goes back and forth around the threshold, and the output rapidly repeats ON and OFF

How far the value is from the threshold is called the excess gain. It is expressed as how many times the threshold the amount of received light (or other value) is, and the larger it is, the less a little variation affects the judgment. Finding the cause of a detection error starts with finding why the excess gain has dropped.

First, check which value has moved toward the threshold

When detection errors occur, it is tempting to start by replacing the sensor or adjusting the sensitivity, but that often turns out to be a detour. First, read the amount of received light with and without a workpiece on the amplifier’s display, and compare it with the records from installation. Depending on which value has moved toward the threshold, the likely causes fall into three groups.

• The intended signal has weakened (for photoelectric sensors, “the light-received level drops”): dirt on the lens, optical axis misalignment, loose mounting, bending of the fiber, and changes in workpiece color or gloss
• An unwanted signal has been added (“the light-blocked level rises”): reflection from the background, light going around or passing through the workpiece, ambient light, and mutual interference between sensors installed side by side
• Neither value has changed: Errors occur even though both values are far from the threshold. Suspect the response time relative to the workpiece speed, workpiece wobble, and the wiring or the input on the receiving device

Without records to compare against, you cannot even tell which value has shifted. Recording the amount of received light at installation is the shortcut to resolving trouble quickly.

Finding likely causes and fixes from the symptom

Common symptoms, with their likely causes and fixes, are summarized below.

Be careful with the idea that “raising the sensitivity will fix it.” If the cause is an unwanted signal such as reflection from the background, raising the sensitivity actually increases false detection. If the cause is a dirty lens, the dirt keeps building up, and the sensor eventually misses workpieces again. Changing the sensitivity without checking the cause may only change the form of the symptom.

Hysteresis and timers against chattering and miscounting

Chattering occurs when a workpiece wobbles near the boundary. If you widen the hysteresis (the difference between the value at which the output turns ON and the value at which it returns to OFF), wobbling within that width no longer switches the output. Conversely, the narrower the hysteresis, the smaller the differences the sensor can tell apart, but the more prone it is to chattering.

With glossy, uneven workpieces, there are moments as the workpiece moves along when the reflected light misses the receiver. If the amount of received light drops below the threshold for just an instant, the output turns ON twice for one workpiece, and the PLC counts it as two. You cannot see this symptom by checking the amount of received light with the workpiece stopped.

In this case, use an “off-delay timer,” which delays the return to OFF by a set time, so that the short OFF is ignored. If you lower the threshold instead, the sensor picks up even reflections from the background more easily, and this time it turns ON on an empty conveyor.

Timers also involve trade-offs. When the workpiece is moving, the delay time translates directly into a position error. If the delay is too long, two workpieces that arrive one after the other are counted as one. The same goes for response time: the faster you make it, the lower the excess gain, and the fewer sensors the interference prevention function can handle.

Check “easy things” and “things that changed” first

When the cause is not clear right away, follow a set order of checks.

1. Write down the symptom (since when, how often, which workpiece, at what time)
2. List what has changed (replacements, modifications, product types, settings, materials, wiring)
3. Check the easy things first (display, dirt on the window, connectors, set values)
4. Change one condition at a time to narrow it down
5. Confirm the cause before fixing it, and keep a record

How the symptom appears is also a clue. If it started suddenly at some point, suspect whatever was changed just before. For example, after a safety cover on the equipment is replaced with a glossy metal plate, its reflection turns the output ON even when no workpiece is present. If the problem gets worse gradually, look for dirt or loose mounting; if it occurs only now and then, look for something that moves at the same time.

If the malfunction occurs only when other equipment is running, also suspect noise entering the wiring. Compare with that equipment stopped, and take measures such as routing signal lines away from power lines and grounding the shield.

Swapping in a spare sensor is a way to check whether the cause lies in the sensor itself. If the symptom remains after the swap, the cause lies in the mounting, the wiring, the workpiece, or the surrounding environment. Fixing the problem is not the same as finding its cause.