# Measuring distance to a non perpendicular surface

I am a Mechanical engineering student and i am completely new to arduino and need some advice. i have a project to measure the maximum deflection in a sailplane wing (up to 9 meters)

1. How do i measure distance using ultrasonic or light sensors to a non perpendicular surface

2. what hardware would measure such a large distance as ultrasonic have a max range of 4 meters

ANY HELP would be greatly appreciated

the attached

• I would use a Time-of-flight laser sensor
– chrisl
Commented Mar 14, 2021 at 17:27
• I suspect this is better suited to the Engineering SE, as it is only moderately associated with an Arduino, while selecting methodology and sensors are certainly engineering. Commented Mar 14, 2021 at 17:40
• Your question reads as though the wing will deflect up to 9 m which is unlikely. Can you edit to clearly state the wing length and maximum deflection to be measured and the required accuracy. Are we allowed to attach a vertical scale to the wingtip? Commented Mar 15, 2021 at 13:12
• you just need a steady camera. then you can see what row/column of pixels the wings extend to, and that tell you the position. I used to have a golf rangfinder that was simply a bar scale overlay that expected a 6 foot hole pin; by confining the top and bottom of the pin to the visible slope, you could tell how far away that 6' pole was... Commented Mar 15, 2021 at 23:06

It sounds to me like you're planning on putting a distance sensor on the top of a sailplane's canopy, aimed out to the sides and slightly downwards to point directly at the wing tips. You then correlate the distance to the wingtip deflection. Under higher Gs the wing tips will flex up more.

Since this will likely need to take measurements during flight, even if ultrasonic sensors had sufficient range, the wind noise would likely render them useless, in addition to the fact that you're literally flying away from the sound waves as they occur, causing them to not return as well, and drastically affecting the return time if the sound waves travel with or against the wind at all. If that were the case, you'd have to take airspeed into account to adjust the ultrasonic return time. Plus, the near zero angle is far from the ideal 90 deg angle to the wing tips, making the ultrasonic return very very weak, if it returns at all.

I suggest you buy a laser rangefinder which you can hook up to an Arduino to log measurements. If it doesn't work well due to the shallow reflection angle, then a better plan may be to mount strain gauges to the bottom (and maybe top too) of the wing at a fixed point along the wing (ex: 2/3 of the way from the root to the tip). Then, you can put the sailplane in a test setup on the ground to experimentally flex the wings and correlate strain gauge readings to wing tip deflections.

Another option is to place a large fiducial sticker on the top of the wing tip. Have a high resolution camera on the canopy aimed at the wing tip. Use computer vision and photogrammetry to correlate the angled view of the fiducial to wing tip deflection. The greater the upwards deflection, the more perpendicular the fiducial will be to the camera.

Note: if using the strain gauge sensor approach, calibrating strain gauge readings to wing tip deflection in a ground bench test setup would be best done with a laser range finder aiming straight up at the wing tip during the test, close to perpendicular, from the ground, during forced wing tip deflection. It would be the source of truth for wing tip deflection, allowing you to determine wing tip deflection as a function of strain gauge readings.