Strain Gauges in Truck Scales

If you have a piston on each corner, hydraulically connected together, the pressure of the fluid should be the total weight of the load divided by the total area of the pistons.

A practical problem that I can see is that if one corner is heaviest, it would tend to go to the bottom stop at the expense of the lightest corner. Once a piston reaches the bottom stop, part of the weight of the truck is not supported by the fluid, so the fluid pressure would not give a true reading.

In a non compressible fluid,very little movement is required to sense pressure,and this movement is caused by the type of sensor used:Microscopic in some cases.Existing electronics can be used for display,error reporting,zeroing,calibration,alarms,etc.

Information from the sensor(s) can be displayed in any engineering format desirable.

I am waiting to see if a company picks up on this and weighs the pros and cons of the system. (Pardon the pun).

I missed the part about the solenoids. I am assuming they are used to switch the single pressure sensor to read the pressure from the cylinder on each corner of the scale. Is this correct?

That sounds like it should work.

Yes,that is what I mean.

The multiple load cells perform an additional job. They make sure you have an accurate weight. If debris forms a fulcrum point anywhere but directly under the center of mass then the multiple strain gauges will each record a significantly different amount. Having the center of mass not precisely equidistant to the strain gauges will also vary the values slightly but not as much as an additional support point.

Any competent engineer will design piston movement far exceeding the max load rating of the scale.It does not displace much liquid top measure pressure. As long as all corners are free to move which could be accomplished with a knuckle or ball joint,but I don't believe that is necessary,readings should be accurate.I have supervised the installation and calibration of many load cells,and the mechanical linkage has many moving parts which are subject to wear or jamming. Liquid pressure transmitters are very cheap compared to load cells of the same accuracy,and they can be located a long distance from the actual scales.

Now of course,If I wanted to exclude all failure possibilities,I must allow for lightning strikes,earthquakes,meteorite strikes, floods, landslides,tornadoes,hurricanes,wind lift effect or downward pressure on the scales or truck,Solar Mass ejections fire ant infestations of the electronics and many more possibilities I cannot think of at the moment.

Pressure in a closed hydraulic system is equal at all points.A cylinder could be placed at each corner if required,or the center pivot could have a ball joint.

The total weight will be the same if the entire truck is on the scale,regardless of position.If it is desired to have individual weights for each corner,the cylinders can be isolated by solenoid valves,also cheap.

The sensor element in the pressure transmitter only has to move less than a mm or so to read the pressure,so not much fluid has to be displaced at the transmitter.

Most trucks are weighed to assure that the max axle weight for each axle is not overloaded, and also that the weight on the axle is balanced, then it is determined that the max weight of the load for the vehicle is not exceeded...

https://www.mylegalneeds.com/blog/what-are-florida-truck-weigh-stations-for.cfm

As I stated previously,wheel loads could be weighed separately.The drive wheels are separately weighed,as well as each axle.If the total weight exceeds the rated weight, heavy penalties can result.The reason they are weighed is to compensate for wear and tear on the highways,which is caused by weight on each wheel.

The max weight is determined by the manufacturer and/or the highway department,which ever is less.The cost of the annual license goes up as the weight goes up.Typically,thousands of dollars.

If it is <...simpler...>, then one might wonder why it isn't commonplace.

A lot of discoveries are accidental l like sulfur in rubber,why wet sand supports more weight than dry sand,when someone sees it they say"Why didn't I think of that.It is so simple!"

Why is everything always simple after the fact?

I myself wonder why it had not occurred to me sooner,when I could have profited from it. I have known hydraulics for many years,and didn't think of it till recently,when I observed another scale being installed.

Getting a patent is a long expensive project,and I do not have much sand left in the top of my hourglass.Besides,I have all I need and am satisfied.

But you are not measuring anything. You are just proportionally changing one unknown pressure value for another pressure value. Measuring pressure requires a sensor of one type or another. Critical to any measurement is the expected (or needed) dynamic range and resolution of the measurement. Requiring the mythic incompressible fluid and "known" piston areas adds more uncertainties instead of simplifying.

Hydraulic weigh scales exist but strain gauge types are more common because they are more accurate. In all my years in instrumentation, I've only seen one and it wasn't working. The owner wanted to replace it with strain gauge-type load cells.

Truck drivers seem to have been "educated" to get onto the scales and slam their brakes to jar the mechanism. They were told that, by doing that, the weight would read lower than what it really is. The only thing it does is it causes damage to the scale.

I spoke to the owner of that hydraulic weigh scale and they constantly had to have the thing repaired for fractures and leaks.

regards,

Vulcan

Sounds like a low bid scales.There are simple ways to prevent this,by installing overload absorbers and using higher strength connections and piping and penalizing those that deliberately try to damage the scale and make the penalties very painful.

The absorbers will require a couple of seconds seconds to settle down,but will not affect accuracy.

Consider all of the hydraulic systems in use all around the world, in mining,digging,earth moving etc,The sealing and strength technology is there,and as always, you get what you pay for.

Yes, the scale looked terribly constructed. No constrainers and no stoppers.

Strain gauge and hydraulic scales have the same inspection requirements except that the hydraulic ones need inspections for leaks included.

True,but hydraulic leaks are readily apparent,electrical problems not so easy to spot.

Maybe not as quick but easy enough for electrical guys who know their stuff.

A leak usually means the whole system goes down until the leak is fixed and the system refilled. A defective load cell usually means the defective load cell gets isolated from the circuit and the system can be brought back online. Accuracy may suffer some but the system will function.

It depends son the severity of the leak.The systems have lots of fluid capacity,so there is time to isolate the leak using the selector solenoids.There is very little almost no, fluid flow in the system,it senses pressure only,which displaces a very small amount.

A well designed system will give alarms when a leak is detected,as will an electric system when detecting a problem.

Electrical problems usually occur where there is flexing or abrasion due to rubbing,water or other infiltration into the system.As with any system,proper and timely preventive maintenance is the key to trouble free operation.

On a bridge scale it does not matter that much,but in a pit system,everything must be explosive proof,which is an expensive initial cost,and time consuming and expensive when seals are broken for repair.

Each system has it's weakness,and neither one is perfect for all applications.

They have engineers and bean counters on every job and things go well until the bean counter overrules the initial design to save money and cuts corners.

strain gages have the major advantage of little temperature sensitivity. both sealed gas and liquid systems need a temperature balancing system.

What is better, the simple system that works or the more complex system that works.

strain gages are quite reliable and have no seals to deal with.

Simply hit the (Zero;Tare) button before and after each truck, it only takes a few seconds,or it can be programmed to do it automatically.The time between two trucks can be used to zero the scales,so there is no accumulated error.The system will always be balanced when all solenoids are open.Zeroing the scales is also done with strain gauges,or should be.

Pneumatic and hydraulic systems were used to operate the gates in the Panama Canal for almost a century,so a well designed system can be designed.No effects from lightning,power surges,etc.

No system is perfect..If you find one,let me know.

Hmmm! ...Now that I think of it,the first Mars Rover (MER-1) explorer was very close,lasting over 14 years without any hands on for maintenance, many times over it's predicted and designed lifetime.

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I will never argue with you.Everyone is entitled to their opinion.