Photoelectric Sensing
Photelectric sensing, as with many other sensors of different technology, can be applied in different modes of operation.
A common mode is the proximity mode, where the light source from the sensor is reflected off the target and returned to the sensor’s receiver. This return of light to the sensor confirms the presence of the target. The proximity mode allows sensing from one side of the target, but the strength of the return light depends on distance to the target, color of the target, and incident angle to the target. Where those factors can be controlled, the proximity mode is easy to install, and can provide reliable sensing. Depending on physical spacing between sender and receiver in the proximity mode, will impact this mode’s sensitivity to accumulation of dust and dirt, and its survival in a that challenging environment.
Another popular mode for photoelectric sensors is the through-beam (or opposed) mode. In this mode, one way or another, the sensor’s sender and receiver are displaced, and the target passes in-between them. When the target passes between the sender and receiver, the light from the sender is blocked from the receiver, and this absence of light at the receiver confirms target presence.
The disadvantage of the through-beam mode is that one-half the sensor needs to be physically on the other side of the target, and where a fiber-optic through-beam is not used, both components of the through-beam system need to be powered, requiring wiring to each component. Transparent and some translucent targets may not provide sufficient attenuation of the light beam for the sensor to reliably detect their presence.
The third mode, Retro-Reflective, uses a reflector on one side of the target, and the sensor with its sender/receiver on the other side. When a target passes between the reflector and the sensor, the light beam path is blocked, and this absence of light at the receiver confirms the target presence. The advantage of this mode is independence of target color, incident angle to the target, and distance to the target.
The disadvantage of the Retro-Reflective mode is that the reflector may experience gradual accumulation of dust, dirt, residue, or moisture with a corresponding attenuation of light returned to the sensor, reduced sensing range, and subsequently, loss of effectiveness. Depending on the installation, the reflector may easily become misaligned or damaged. Some applications, because of the specifics, may not permit installation of a reflector. Transparent and some translucent targets may not provide sufficient attenuation of the light beam for the sensor to reliably detect their presence.
The terms light-operate or dark-operate merely describe whether the sensor’s output is in the ON state when light is returned to the sensor, or ON when light is not being returned to the sensor, respectively. Most sensors have this function as a selectable feature, so the user can switch to whatever logic state is needed for the application.
Some applications are space constrained, or simply need better control of the light beam. Applications of this nature are often best served by using fiber-optic light guides with the photoelectric sensor. The fiber-optics are a conduit for the light beam and are available in different apertures and geometric shapes. Fiber-optics, when chosen appropriately, may change the way the sensor responds to the target and performs. The geometric shape of the fiber can be chosen to favor the sensing task. Fiber-optics can be selected to survive in either higher temperatures than the sensors, or in wash-down applications, where the sensor is not rated for that exposure.
In general, most applications for photoelectric sensing should be relatively clean environments. If the application is dusty or dirty, special considerations need to be addressed for the sensor to remain a reliable means of sensing, such as periodic sensor lens and/or reflector cleaning.
Compared to other sensing technologies, photoelectric sensing has some of the fastest response times, due in part to the fast speed that light. Ultrasonics, traveling at the slower speed of sound, have inherently slower response times.
The choice of LED light source for the photoelectric sensor is important. IR (infrared) is non-visible, and can pass through some target materials, and can more easily pass through a reasonable accumulation of dust and dirt. Both properties could be used as an advantage for some applications. IR LED sources usually offer the longest sensing range, compared to other visible LED sources in the same product family from the same manufacturer. IR is the optimal LED source when attempting to sense darker colored targets in the proximity mode.
Since IR will typically pass-through transparent targets, and some translucent targets with minimal attenuation, it is not a recommended source for sensing such targets in either the retro-reflective or through-beam modes. IR is an excellent source for sensing these targets in the proximity mode.
Visible LED light sources have the advantage of being able to see where the light beam strikes the target in the proximity mode, but this mode and amount of light returned to the receiver are very dependent on the color of the target. Darker colors absorb more of a visible light source, so that the amount of reflected light and its subsequent sensing range is significantly reduced. The lesser light returned from darker targets could be an advantage, if trying to distinguish between lighter targets and darker targets.