Help to desing the math behind a automatic weighing system for a wheel loader - Page 1
Forum section: electronics / projects

author: Velth
author_rank: Contributor
timestamp: September 08, 2022, 12:34:10 pm
content: Hi I'm trying to find how is the method behind some weighing systems that use a magnetic sensor on the bucket to measure the accelaration and then automatic calculate the wieght in that moment but I don't know how it does that.I find something abou tthe newton-euler method, but I'm not sure if this what I have to search, I need any suggest,help or idea about it.

author: ajb
author_rank: Super Contributor
timestamp: September 08, 2022, 04:01:29 pm
content: Using acceleration? Interesting. Do you have a link to someplace where this is described?Basically, force = mass × acceleration, so if mass is the unknown then you need to know the force as well as the acceleration. That applies whether you're using Newton-Euler methods or plain old algebraic methods.What makes things more complicated is how the force exerted by the wheel loader's hydraulic system is conveyed to the motion of the bucket. Whether you take the measurement by moving the boom or the bucket you don't have a simple linear acceleration or rotational acceleration about the center of gravity of the load--the load in fact swings around an external rotation point, and the path may not be particularly straight or circular. You'd need to construct a set of equations that account for all of that to translate the force applied at the cylinder to the force applied to the bucket as a function of bucket position. Calculus territory. The distance between the load's center gravity and the pivot point will also depend on how the load sits in the bucket--a lower density load like mulch will have its CG higher relative to the bucket than a higher density load like gravel. On top of all of that, I imagine that the speed the bucket can be moved is limited more by the volume flow from the hydraulic pump (which depends on engine RPM too!) than by the inertia of the load, so you might not see much change in acceleration anyway.Seems like it would be simpler to use a static measurement technique, possibly based on hydraulic pressure. I'm certainly not an expert on wheel loaders, but thinking about the kinematics I've seen on those machines, I would guess measuring the hydraulic pressure at the boom cylinders with the bucket in the upright position would be simplest. You'd need to know the position of the boom to be able to account for the mechanical advantage of the cylinders against the load changing through the motion of the boom--or fit something like a magnetic sensor to detect when the boom is in a defined position where the pressure/load relationship is known before taking the measurement.All that said, there are certainly systems that use acceleration to determine mass/inertia. We have a CNC lathe, for example, that needs to know the moment of inertia of the chucks on the spindles to adjust their synchronization control loops, and it does that by rapidly accelerating them for about a half rotation and back while measuring the force/acceleration. But that's a MUCH simpler problem, because it's done with high resolution encoders and big induction motors that are coaxial with the load, with a sophisticated vector drive measuring the power applied to the motors in real time. That all makes the math a lot more straightforward than something with a bunch of linkages that convert linear motion to angular in the way.

author: ajb
author_rank: Super Contributor
timestamp: September 08, 2022, 08:20:40 pm
content: I don't see any particular indication that that system accounts for acceleration directly--in fact I don't see how it could measure acceleration at all. The two position sensors would allow it to measure the transit time, but that's hardly the same as speed. I would guess that they're really only using the pressure sensors fitted to the hydraulics to do the actual weighing, with the position sensors used to determine how the measured hydraulic force is translated to a load weight, and possibly doing some correction based on transit time. They might even take a profile of the force over the range of motion between the two sensors with the bucket empty to establish a correction function, just guessing here. But I don't see how acceleration could be effectively used as part of weighing the load with that system.