Using 2 distance sensors for speed calculation

Looking to use 2 @ #4069 or #4067 lidar sensors relatively near each other in a beam break concept with an Arduino. Block first sensor to start timer, block second one to stop timer, then calculate speed. Measuring speed of a ball dropping through a channel. Will the sensors interfere with each other? How close can the sensors be to each other? Accuracy is secondary to repeatability and reliability.

Hello.

There is potential for the Pololu Digital Distance Sensors to interfere with each other if they are too close, so I recommend designing your setup so that each sensor’s field of view (FOV) does not interfere with the FOV of another. The effective FOV can vary with object reflectance and ambient conditions, but for these sensors it is typically around 15°.

- Patrick

Thank you, Patrick. To confirm what I understood from your post: if I’m considering an 18" channel, then a 15º isosceles triangle centered on 1 sensor mount gives a base of ~4.75". As long as my sensors are mounted at least 5" or 6" apart, then I should be OK. That’s great - I can afford 10" to 12" of separation.

It sounds like you understood my post correctly, and your proposal sounds like it should be okay.

- Patrick

I would like to refine my question about using the #4069/#4067 sensors as a beam break. It is a vertical beam break setup - dropping a ball, for example. I understand that the sensors should be mounted at least 6" apart vertically to get accurate start time and stop time info. But if I wanted to use 3 or 4 sensors as a beam curtain for the start (and stop), it seems like I can mount them closer together and there would be no problem with interference since it doesn’t matter which sensor emits the light and which sensor picks up the reflection. I just “OR” all together to trigger the start/stop. Very rough testing makes me this is the case. Thoughts?

The Pololu Digital Distance Sensors operate by measuring the amount of time it takes IR light emitted from the sensor to return to the sensor, and they try to correlate the light they receive with their own emitted signals, so they will generally ignore signals from other sensors or see it as noise. That means they are not really suited for use as break-beam sensors where the signal is emitted and received by different units. However, you might still get adequate performance when they are mounted close together as long as each sensor is not overwhelmed by the noise from the others.

Given your particular application, the approach you proposed might be more appropriate for with sensors where the output depends on the intensity of reflected light instead, like our 38 kHz IR Proximity Sensors or QTR reflectance sensors.

- Patrick

First, thanks for the help. Second, I was able to stack a series of sensors ~3" apart and it seems to work very well for my “dropping ball” activity. My question is about the Arduino coding. In general, I use INPUT_PULLUP for digital switching. The $4067/#4069 sensors are HIGH by default (un-triggered) and I’m powering them externally. Is INPUT_PULLUP necessary/desired or is it possibly making the sensor fractionally less sensitive or slower?

Hello.

As described on the product page, the output on those digital distance sensors is driven both high and low depending on whether or not an object is being detected, so it is not necessary to use a pull-up, but whether or not you use one shouldn’t affect the sensor’s response time.

Brandon