Selecting a Photoelectric Sensor for Detecting Part Ejection

Because of their large detecting areas and long sensing distances, photoelectric sensors are a common choice for detecting part ejection. The three most popular types for this application are the diffuse reflective photosensor, the mini-light curtain, and the through-beam photosensor. The first two types are best suited for smaller air-ejected parts, while the through beam is better at detecting larger parts that follow a repeatable path.

Success or failure often hinges on sensor selection. This month's tip provides selection guidelines for photoelectric sensors used for part ejection.

The most important application problem faced by photoelectric sensors in a stamping environment is the presence of oil. When lubricant coats the lenses of the sensors, the amount of light both coming from the transmitter and getting into the receiver is diminished, which can result in intermittent sensor actuation and frequent nuisance stops. Also, in some cases a mist of oil being blown through the sensing field can 'trick' a sensor, causing it to actuate as if the part came out - even if the part is still in the die. Both of these scenarios can be avoided with proper sensor selection.

A diffuse reflective sensor is a small, self-contained reflective photosensor. The emitter sends out a low-intensity cone-shaped field of light. The sensor will actuate when the receiver detects a small amount of this emitted light reflected off the part as it passes. These devices are quite sensitive, allowing them to easily detect small, fast moving (even non-reflective) parts quite easily. The downside of this sensitivity is that the diffuse reflective sensor is prone to false actuations caused by background objects such as the ram, upper die, barrier guards, etc. Care must be taken during installation to ensure that background items will not cause false actuations.

Diffuse Reflective Photosensor Detecting a Part

When sensing air-ejected parts, it is important that you only use infrared diffuse reflective sensors. The reason for this is that the air blast used to eject the part will often pick up some of the lubricant on the die, and send it through the sensing field along with the part. Infrared light is better able to 'shine through' lubricant, whereas visible light reflects off of the lubricant. In other words, visible light sensors are more easily 'fooled' by oil in the sensing field. This will result in a die crash if the part sticks in the die and lubricant is ejected (and detected) by the sensor. Infrared sensors minimize the possibility of this occurring.

Mini-light curtains consist of a separate emitter and receiver. The emitter contains a line of photoelectric transmitters that together emit a low-intensity curtain of light which is detected by the receiver. The sensor will actuate when a part breaks a portion of this curtain. Like the diffuse reflective sensors, it is important that you use only infrared mini-light curtains to minimize the chances of a false actuation when oil is ejected through the curtain.

For larger parts that are pushed out of the die (rather than being air-ejected), a through-beam photosensor is a good choice for detecting part ejection. A through-beam sensor features a single beam of light from the transmitter that is aimed directly at the receiver, and actuates when the target object breaks the beam. The single small beam makes the sensor unsuitable for detecting small flying objects (like air ejected parts).

The through-beam sensor - like any other photosensor - is susceptible to problems with oil in the sensing field, but a different selection strategy is available for users of these sensors: Power. Compact through-beam sensors are commonly available with sensing ranges over over 20 feet. For in-die use, the emitter and receiver are seldom separated by more than a foot. The extra power of the sensor is then available to 'burn through' any oil on the lenses or in the sensing field. The abundance of extra power enables even visible light through-beam sensors to be used in oily environments. In fact, the visible light versions of these sensors are actually preferred, because the visible spot of light from the emitter makes it easier to align the sensor at installation.