730 Payload System

[Chrisso]

730, 3F3 prefix w/ payload fitted from factory.

So the problem was an active 6863- 9 Payload Weight #1 : Abnormal Update Rate code.
Played looksies and swapsies, identified the #1 payload sensor has failed internally.
It shouldn't have, because all literature for this code alludes to there being a communication error between ECMs and or a data link error, but anyway...

I would like to know how these systems calculate weight. There are 2 accelerometers (front & rear) and 8 x 550-0975 Strain Sensors (4 on each side). What the heck is a strain sensor?

 

[Nige]

I would like to know how these systems calculate weight.

When you find out let everyone know. The perceived wisdom was always that an accurate payload system for an ADT was not possible because of the rubber suspension on the rear.
Need to get hold of a copy of the STMG for the truck, or maybe there is a standalone STMG for the payload system because I imagine it will be the same across the complete product range of ADTs.

 

[mg2361]

Look at the 2:30 time stamp.

 

[Nige]

I refer my learned friend to the word "accurate" that I used in my original comment. Strain gauge technology is not new but I'm still to be convinced of its use in this application.

 

[Chrisso]

Good video but falls just short of explaining exactly how they measure the strain.

When you find out let everyone know.

I will. These sensors are mounted each with two bolts, about 1" apart. So for now I'm guessing they're just measuring movement (via changes in electrical resistance) between those two mounting bolts, either that or each sensor must have a way of communicating their precise position with each other, which I doubt.

According to SIS the sensors have an "integrated temperature sensor" which makes sense if you're measuring how much steel moves.

 

[Nige]

According to SIS the sensors have an "integrated temperature sensor" which makes sense if you're measuring how much steel moves.

They'd have to have some sort of temperature compensation because of the expansion/contraction vs temperature of the steel on which the sensors are mounted.

 

[John C.]

Sounds like another load of BS so that Cat warranty can claim their units were being overloaded when the center hitches go bad. It used to be that the size of the box ensured that the truck couldn't be overloaded.

 

[icestationzebra]

I do a lot of work with strain gages. They are a small metallic grid that undergoes minute changes in resistance that directly correlate to strain (i.e. deformation) in a part. The problem is that strain can be caused by mechanical loading AND thermal growth/contraction. Temperature compensation is used to cancel out the thermal strain so you are only measuring mechanical strain.

From an application standpoint there are three main options:
A. Load cell: A device or custom made part that has strain gages bonded internally and is calibrated before install. It may have built in amplification. Usually fairly expensive. Drive on scales would be a good example.
B. Bolt-on sensor/strain gage: A bolt on device consisting of a strip of metal with bonded strain gage(s). May have built in amplification. Can only be fully calibrated once installed. Usually easy to replace but have to recalibrate the zero point. As used on the ADT. Similar systems are sold to retrofit bulk storage bins, just weld studs to the bin legs. Also used for "load moment indicators" on off-road forklifts.
C. Strain gages bonded directly to the structure: usually reserved for engineering studies only. Difficult to install unless specifically trained and must use with precision instrumentation to accurately measure data.

In this application they are trying to measure the deflection in the walking beams. They have them on the inside and outside to counteract the unequal loading caused by having the tires on only one side of the beam. Whether or not they have suspension really shouldn't matter as the force still has to go thru the walking beam - but it would affect how much the measured value jumps around and what damping/filtering would be appropriate.

BTW in that video at 2:43 he says "you can't squeezer metal." FALSE - you can certainly have a compressive failure. Every punch with a mushroomed head is a compressive failure.

 

[Chrisso]

Thank you, that was the detailed answer I was hoping for.

 

[Nige]

Sounds like another load of BS so that Cat warranty can claim their units were being overloaded when the center hitches go bad. It used to be that the size of the box ensured that the truck couldn't be overloaded.

It generally was, until customers started installing greedy boards. "They're to prevent spillage, not to allow us to get more material in the dump body" - yeah, right.

According to the Specalog this system is only an attachment, so customers have to pay to get it installed. You don't have to have it........

One thing that tends to get forgotten is that systems such as brakes and steering are certified "safe" under various standards BUT this only applies to a machine weighing its rated Max GVW plus a small percentage above that. Extracts below from the 730 & 795 Specalogs. So if you are a customer and you are running your machines up & down hills and/or want to go round corners with them wouldn't you want to know if you were doing it outside the certified design criteria of the machine.? In case of something going awry the finger of blame tends to be pointed.

730

795

 

[John C.]

All the artics I had in fleets could lock up all the wheels fully loaded going down hill. Problem was never the brakes, it was learning to control the machine when the wheels locked up.

Steering issue on an artic is another story. All those problems were design issues. Cats were the worst. Let the engine slow down at speed and steering got heavy and sluggish. Tying the steering to the box cylinders was another issue in that damaged cylinders dropped trash into the shuttle valves and then all steering pressure went away and the front turned suddenly to right or left. We had an early 50 ton truck go over a bank at full speed. In that accident the good thing was that the brakes worked fine. All six tires locked up when the driver nearly stuffed his foot through the floor while looking at the ground coming up at him.