# What type of sensor could I use to determine the mass of a small object (under 1 kg)?

I'm building a robot that needs to pick up a piece of material and then determine its mass. My robot will retrieve the material using a scoop at the ended of a mechanized arm. Once the material is in the scoop, the arm will tilt upwards until the material slides down the arm and into the chassis. I hope to weigh the material while its in the chassis, but I'm not sure how to do this. I've found that load sensors are useful for taking masses, but I'm not clear on how they're implemented. For example, I've found load beam cells, but how would I implement one in my design? Does it require a metal plate of some sort (like a weighing pan) on top of it like a laboratory scale? The sensor must be able to communicate with an Arduino.

My robot needs to be under 6 lbs and have a footprint of 35 x 35 cm. Thanks!

• The problem is that there are cheap kitchen scales with obscure electrical output decoded and compensated in specialized circuitry you would need to re-engineer from scratch and then there are expensive load cells and sensors with documented output. An intermediate might be a USB postage scale, at some price premium and probably suggesting something more than an Arduino to host it. – Chris Stratton Oct 13 '15 at 1:00

Just a little brainstorming:

If you have a spring, then you can easily calculate the physical force (mass) needed to stretch the spring to a certain extent. The law is called: Hookes's Law - you could then just measure the increase in distance between the initial position and the position with load. Use a servo or any distance measure.

• I wouldn't really use a servo as a distance measure? You could use any distance sensor below it or a potentiometer that will turn based on the position of your weighing pan. – Paul Oct 13 '15 at 14:02
• Servo for the angle though. I was thinking faster, than I typed. But yes a potentiometer does the trick as well. – AnyOneElse Oct 13 '15 at 14:38
• I like the distance sensor and potentiometer ideas. Could you please elaborate on the pontentiometer idea? How would I rig one up to turn based on the position of the weighing pan? – user14000 Oct 15 '15 at 18:21
• it depends on how your weighting pan is linked to the robot. if you have a schematic drawing, I could wrap my mind round it. – AnyOneElse Oct 19 '15 at 7:20

A load beam cell would be good. Put it as one leg of your robot, and calibrate it with the arm in a particular location dangling over the cell. You'd need the beam load cell, and an instrumentation op amp, or HX711 (https://learn.sparkfun.com/tutorials/load-cell-amplifier-hx711-breakout-hookup-guide) to do the interfacing.

Also good would be the cells from a bathroom scale as the four feet -- you could hook them up like https://electronics.stackexchange.com/a/199907/30711 or https://electronics.stackexchange.com/a/199470/30711

One thing cool about using the robot feet for measurement is that you could calibrate for more sensitive measurements while reaching far from the base, and rougher measurements closer. You could also calculate how far you could reach without toppling over.