looking for suitable force sensor - Page 1
Forum section: electronics / beginners

author: honeycomb0
author_rank: Newbie
timestamp: January 16, 2020, 03:41:00 pm
content: Hi all,I want to make continuous measurements of relative force applied by the tip of the pen to the surface during writing/sketching. I want to locate a sensor on the pen, hence I am looking for relatively small and light type of sensor. The most important characteristic would be sensitivity - it would be amazing to capture load variantion as low as 0.01N. Additionally, it is desirable for sensor to be insensitive to accelerations and changes in orientation.Can you point me to the right sensor type. that would suit such application?Does anyone has experience with working with such type of sensors?Thank you!

author: RoGeorge
author_rank: Super Contributor
timestamp: January 16, 2020, 04:05:07 pm
content: Either with a strain gauge, or maybe with a custom sensor based on a strong spring that compress very little, so it won't alter the feeling of a solid pen, yet it moves very little, so the distance can be measured with capacitive or inductive sensing.

author: ComradeXavier
author_rank: Contributor
timestamp: January 16, 2020, 04:08:28 pm
content: I think you're probably looking for a very small strain gauge, of the sort that is made of traces on a flexible film which you can glue to your specimen.Strain gauges of this sort can detect very small deflections, and are often affixed to specially-engineered metal structures to make force sensors (sometimes called load cells). The strain gauge changes resistance based on how much it is deflected; calibration determines how to convert between measured resistance and force applied to the structure.I work on the software side, so while I've used strain gauges of this sort, I don't know where to source them or how you'd design your pen to differentiate between, for example, the force of the pen tip on the paper vs. the flexing force applied by the hand holding the pen tightly (which will certainly show up if you're trying to measure to the tens of millinewtons).

author: andycsmith
author_rank: Contributor
timestamp: January 17, 2020, 03:28:56 pm
content: I used to work in mechanical testing and strain gauges were used to measure movement and load.I would imagine this would be the best solution as they would be tolerant of g forces to an extent and orientation.You would have to put several around the circumference because the load will not be down the axis of the pen and would involve some maths to calculate the movement and convert this to force knowing the properties of the material the pen is made of.I would very much doubt you will be able to use a load sensor to directly measure the force with the required resolution and be also able to withstand the forces without considerable expense.The other way would be as RoGeorge suggests and add a flexibly part and measure the movement, the sensors required are what are used performing Tensile Tests to determine properties of materials. This method would be more sensitive to g forces and orientation.

author: honeycomb0
author_rank: Newbie
timestamp: January 17, 2020, 11:08:58 pm
content: Thank you guys for replying. After exploring the options, I stumbled on inductive sensing solutions, like LDC1312. I wonder if it is possible to measure the displacement of the conductor inside the coil (i.e. coil is being wrapped around the conductor) - constructively this would be ideal!

author: Lesolee
author_rank: Regular Contributor
timestamp: January 18, 2020, 02:51:58 pm
content: Quote from: honeycomb0 on January 16, 2020, 03:41:00 pmI think these are mutually exclusive ideas. A force is the same as an acceleration of a mass (F=mA, Newton's second law)There is no reason not to have a conducting cylinder within a coil, moved by the pen. But the cylinder would need to move a significant distance (say 1mm). That is adisplacementas well as a force.The cylinder has a mass, so if you shake the pen (axially) that will give force measurements. The best you can hope is thattransverseaccelerations do not give force measurements.Load cells don't deflect much, so they approximate measuring a force. SparkFun do some really nice and cheap load cells. Trouble is the force axis is at right angles to the load cell long axis, so the load cell would stick out the side of the pen body.

author: Prehistoricman
author_rank: Regular Contributor
timestamp: January 18, 2020, 03:30:52 pm
content: Quote from: honeycomb0 on January 17, 2020, 11:08:58 pmYou can do this in a few ways. If you have two coils, you can excite one and load the other. Look for how much voltage is being induced in the sensing coil, and you can infer where your conductor is (magnetic conductor in this case).Or, with a single coil, you can measure its inductance. As you move the core out of the coil, the inductance will decrease. You can make an LC or RL filter out of this and therefore sense the change in inductance.Regarding strain gauges, aren't they too insensitive for this task? 0.01N resolution would be difficult to achieve according to my intuition. You'd need lots of gain and good wiring, power supply, etc. They aren't that easy to apply to a surface either.The benefit of stain gauges is that by using a 4-way wheatstone bridge, you can null out bending forces and amplify axial forces.I think what I would try is having a long-ish spring on the pen so you can vary the force by varying the height of the pen.