We really need to weigh our MX-5e before and after conversion. We also want to better understand the weight distribution and this means using vehicle scales to measure the weight at each corner. The issue we have is that vehicle scales are really expensive!
So we have decided to simply build our own using readily available components and some open-source hardware and software. The basic parts required are some load cells, some signal conditioning boards and an Arduino processor to do the measurements and display the results on a laptop.
Weight measuring of heavy loads such as a vehicle is done with a load cell. These come in various versions that weigh from a few kilograms to up to two tons. For this project we purchased eight YZC-516C S type load cells on eBay that can each weigh up to 300kg.
These load cells have M12 fixing holes and a long screened cable (over 3m long), with 4 wires to connect them to a load cell amplifier.
Each load cell needs to be connected to an amplifier to perform the signal conditioning. It is these amplifiers that are interfaced to our Arduino processor.
Each load cell needs to be connected to an amplifier to perform the signal conditioning. These are also cheap and readily available on eBay and other sites.
To simplify the wiring of the 8 load cells to the Arduino processor, we developed this custom PCB. It uses a DIL header to plug straight into the Arduino Mega 2560 and has pins aligned to enable the amplifiers to be plugged in using SIL headers.
I used the code sample from this tutorial to calibrate my test amplifier and load cell, using a lead-acid battery as a known test weight.
We initially got a single load cell working and were able to accurately measure some quite heavy weights on it by balancing them carefully.
With the PCB delivered, we could then test all eight channels. This allows us to put two (300kg) sensors on each corner of a car to corner weight it, with up to 600kg per corner.
With calibration completed, we were able to repeatedly measure the weight of an 11.3kg car battery to within 20g.
All four weighing plates completed.
On the 27th September we weighed the MX-5e for the first time. Having weighed all the bits we had already taken off and added, we estimate that the Eunos Roadster originally weighed 1021.5kg when we bought it. The weight was measured at 987.2kg.
We have since updated our code to also display weight distribution as a percentage left/right and front/rear.
We are really pleased with how this project has turned out. For less than £200, we have some accurate vehicle scales that provide good repeatable results. We can weigh objects up to 2400kg in theory but each of the four individual corner 'weigh plates' is limited to 600kg.