Basic info needed on reflective photointerrupters

[jgreene6]

I am brand new to this forum and also very new to electronics. So if I don't explain things well, let me know and I will try to make any points as clear as possible.

I am looking at photointerrupters for a project I am working on. I haven't been able to find a traditional transmission photointerrupter that that the correct geometry to meet my needs, space is very small in all but one direction. However a reflective photointerrupter has the geometry I would like to have, but I am not sure of their exact operation. I am wanting to detect a fairly slow moving object (~1mm/s) move past the detector. I want it to reproducibly switch states within a fairly narrow range (1mm). From what I can tell, the data sheets for reflective photointerrupters don't include data for this type of information like transmission photointerrupters do. But In don't know what all the terms mean in the data sheet such as L(HLS) and L(HLL). Will a reflective photointerrupter meet my needs?

 

[ke5frf]

I believe I can help a little, at least on reading the datasheets.

The first L in your specifications refers to distance (length) of detection.
in the parenthesis, such as (HLS) the data is indicating an output voltage level change. HLS indicates a transition from High to Low. LHL would indicate a transition from Low to High. At 5 volt Vcc this would be 5 volts to 0 volts and vice versa. Or, perhaps more correctly, the transition "threshold" voltages where the logic level is considered to be high or low.

The S and L at the end would (I think) indicate short range detection versus long range detection.

LHL would indicate the distance at which the detector no longer picks up the reflected signal because it is too far away. LHS would be the closest point at which a detection output transition can occur.

Now, it would appear after reviewing one of these datasheets that the output switching characteristics are quanitified by both the reflective object moving toward the detector and the reflective object moving away from the detector. In other words, the distance at which the output switches would not be the same for both cases. You might say there is a bit of hysterisis in the response. In the datasheet I looked at, the output goes low when the reflective surface is detected, and is high when the surface is not detected. There is a distance that is too close where the focal performance of the source will not be adequate to be picked up when reflected. This would be the "S" indication at the end. Then there is a distance that is too far, which would be the "L" indication at the end. The distance inbetween is the "detecting distance" where the reflective surface will properly reflect the optical source to the detector. This would be your operating distance, and the LHS, HLS, HLL, and LHL distances can be understood and accounted for as the reflective object moves in and out of the operating zone of the "detecting distance".

So, as the object moves away from a point too close to the detector, the output will be high. At a certain distance, the reflective surface will be in the "detection region" and the output will transition L(HLS) to a low state. In the detection region, the low state will be maintained. When the end of the detection region is approached, the output will transition back to the high state L(LHL) and will maintain that state as long as the reflective surface is beyond this distance. As the reflective object changes direction and approaches the detector, the L(HLL) transition to low will occur, and will maintain low as long as the reflective surface is in the detection region. As it moves closer, to the point of being too close again, it will go high again L(LHS) and stay high as long as it is too close. Once it reaches the point once again of the detection region, L (HLS) will occur and the output will once again be low.

Again, the H to L and L to H distances are not the same for an approach or a wayward movement. There is a shift so to speak.

I hope this is clear.

 

[ke5frf]

One question about your detection needs, are you looking for an in-out method of detection or a side to side method? You say "a slow moving object move past the detector", which seems to indicate a lateral movement.

If this is the case, my interpretation of the datasheet is that the reflective photointerrupter would be optimal within the detection region, meaning the performance would be most consistent within this zone. This would be ideally between distance L(HLS) and L(HLL). My disclaimer would be that I have seen reflective photointerrupters in operation on devices such as opticouplers for motor movements. This is generally a lateral movement. But I'm not certain that THESE particular photointerrupters are specifically designed for lateral detection, though I see no reason why they shouldn't.

For the particular datasheet I have been referencing, as an example...
https://www.electro-tech-online.com/custompdfs/2009/12/gp2a200l_e.pdf

I interpret the detection region that would be ideal for lateral movement as between 2 and 22 mm for Kodak reflective paper as their reference example. Depending on the size of the optical focal point or beam and the size of the reflective surface, when the reflective surface passes in front of the beam it should at some time reflect the beam into the path of the detector and output LOW. Once it moves past and the beam is no longer detected, it should transition back to HIGH. I would imagine that a similar lag or hysterisis applies in lateral motion, meaning that a surface moving INTO the optical path will switch at a similar point as L(HLL) and a surface moving out of the path should switch at a similar point as L(LHL). BUT I MAY BE WRONG. It might, in fact, be that within the detection region a lateral interruption will have a more immediate response and be very consistent.

Perhaps someone with more knowledge and experience can clear up these details.

 

[jgreene6]

Just to be perfectly clear, I do mean a lateral movement. Thank you for your definition of the terms within the datasheet. I just wish the data sheets would tell information about lateral distances. But as you said, this was probably not what they were designed for.

 

[ke5frf]

Just to be perfectly clear, I do mean a lateral movement. Thank you for your definition of the terms within the datasheet. I just wish the data sheets would tell information about lateral distances. But as you said, this was probably not what they were designed for.

I don't know. Like I said, I know reflective opticouplers are used in lateral rotations around a motor shaft axis, very commonly. Perhaps the datasheets do not specify information about lateral interruptions because within the detection distance (2 to 22 mm in my reference datasheet) there is no need for specification, i.e. function is implied.

I might suggest purchasing one for experimentation to see if it works as you desire.