Infrared Distance Sensor question

[cosmonavt]

I have to choose between the following two distance sensors:

https://www.pololu.com/catalog/product/1136
https://www.pololu.com/catalog/product/1137

One has range 4-30 cm but greater accuracy and the other has range 20-150 but less accuracy.

I want to make a robot that moves towards a wall and stops when 25cm from it.

If I am using the 20-150 range one, I get less accuracy.

If I am using the 4-30 range one, the initial position of the robot will be out of range for the sensor:

**broken link removed**

When out of range, if the sensor gives an output which is less than 0.3V we can still compare the distance of the robot from the wall with a reference voltage.

BUT

If it actually goes above 0.3V (breaks the trend of the graph beyond its range), then it can cause trouble. Does this happen or can I confidently use this sensor beyond its range?

 

[jpanhalt]

It measures intensity of the reflected light. It is hard to think of a likely mechanism that would cause an increase at greater than 40 cm. I'll bet $13.95 USD it won't increase at distances greater than 40 cm. Some pulsed IR at exactly the right frequency might interfere, but it would also interfere at shorter distances too.

John

 

[cosmonavt]

Exactly what I was thinking but thanks a lot man! And that $13.95 joke was a good one!

 

[Mr RB]

It measures intensity of the reflected light. It is hard to think of a likely mechanism that would cause an increase at greater than 40 cm. I'll bet $13.95 USD it won't increase at distances greater than 40 cm. Some pulsed IR at exactly the right frequency might interfere, but it would also interfere at shorter distances too.
...

Sorry to nitpick, but the Sharp sensors don't measure light intensity they measure the distance using triangulation. They are quite 'safe" when the object is beyon the sensing distance, they are designed to be safe that way (as they are designed to be used for things like hand sensing in restroom hot air dryers etc).

For the OP, you could use both sensors, the 150cm one to detect the walls and the 30cm one for fine positioning, or you could just use the 30cm sensor and drive forward until a wall is detected (when the wall is closer than 30cm).

With a typical ADC of 1023 resolution you should sense the wall to better than a cm even with the 150cm sensor. There is also a 80cm range sensor too.

 

[jpanhalt]

@RB Sorry for my error. These sensors have been discussed here before, and I should have remembered. The fact that the reported distance varies little with reflectivity was a clue, it made me pause, but I guess it was just too early for me to wake up. I confused them with some analog Vishay sensors (**broken link removed** ). To help make amends, here is a link that describes the Sharp sensor mechanism: **broken link removed**

John