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• Force Gauge

  Matt Dombroski • 03/26/2019 at 00:25 • 0 comments

  The centerpiece of this project and the component that took the most time to develop is the force gauge. The basic idea of most force measurement is knowing the relationship between the displacement of an object to the force that caused that displacement - i.e. a spring. For measuring the force on a control surface of a plane however the 'slip' from a spring is undesired. The servo can only travel so far and in addition any slip will cause a bit of a time delay to moving the control surface.
  

  An obvious answer would be strain gauges. However, strain gauges are extremely sensitive, need quiet electronics (hard to do on an RC plane), and typically need bulky mechanical structures to be attached to. Instead, I happened to read another HaD article featuring [Wold Tronix] linked here, which shows a project with almost an identical goal as this one, and showed a great idea for force measurement. Here an analog hall effect sensor placed over a very small magnet. The hall effect sensor part number is the Allegro A1304 and a 1/32 diameter neodymium magnet are used, as per the recommendation from the article. 
  

  The hall effect sensor is sensitive to both the magnitude and direction of the measured magnetic field. Paired with the small diameter neodymium magnet, the sensor is extremely sensitive small movements, probably in micrometer range up through about 1 or 2 mm. So very little displacement is needed and a stiff, strong sensor body can be designed. This particular hall effect sensor is also very easy to interface, it is completely analog and only needs an analog port on an Arduino or any other embedded platform. One note is that it runs off 3.3V, so watch out before you plug it into the 5V rail of an Arduino!

  Now, while [Wolf Tronix] builds in the hall effect senor into the servo arm and into a jig frame for the servo, I wanted to make a more traditional discrete force sensor that was physically separate from the servo. Part of this was because I did not want to modify the servo arms and possibly weaken them, nor did I want to design a frame for the servo that had to be design or at least built into the airframe. I also decided to do this in case I came up with a project down the road which didn't use servo (say stepper motors or linear actuators); I didn't want a design that was bound to a specific servo size and style.
  

  A good force gauge will be linear over its intended measurement range and have low hysteresis so that forces going one direction and then the other read the same values. For my design, I wanted to 3D print the gauge because its faster and more accurate than me hacking at something manually, and I have no access to lathes or CNC's. I went through a number of design ideas:

  And each had they're own set of drawbacks that took them out of the running. (That big round one at the end would have probably worked well through, just far too heavy for use on an RC plane, and it needed springs more precise assembly...). The design I found to work the best was predictably, one of the simplest:

  Why this design? A few things I have observed from testing:

  1. 'Short' gauges work better overall. Short here means the bending area is small and the arms transferring the force are close together. This reduces the amount of flexing possible, but the hall effect sensor is sensitive enough to need only fractions of a mm of travel. Why keeping the bending area short is useful is due to minimizing undesired flexing directions. For example look at the green clover leaf prototype in the pictures above. As a force gauge it works very well, but because the arms are so spaced there are really two major directions of bending - compression and tension and bending up and down (think bending the plane of the gauge into a curve). Due to the geometry of the sensor, both the compression/tension flexing (of interest) and up/down flexing (of no interest) are measured with roughly equal sensitivity. So the result is looking at my sensor measurements...

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