Strain Gauge Question.

Thread Starter

R!f@@

Joined Apr 2, 2009
9,784
I haven't done this so I am here for ideas.
I need to disable a battery pickup type vehicle to disable if the cargo is too heavy.
My idea for a gauge is this or this ....I dunno what to chose. Max weight say about 200kg.

Where should this mounted around the chassis ?
Rear axle or the box itself where the cargo is kept ?
Should be bolted really tight ?
The box is rather thin I believe.

I can deal about how to disable the vehicle once I can measure the weight.

Any ideas ?
 

ericgibbs

Joined Jan 29, 2010
15,516
hi R!,
It would be usual to mount a load cell at each corner of the platform, which would form the Base of the Load box.
What are the dimensions of the load box and the expected overweight.?
E
 

MisterBill2

Joined Jan 23, 2018
11,877
It will undoubtedly be quite a task to add strain gage load cells to measure the weight of a vehicle. And if the load cells are not arranged to carry the load through on their intended axis they will need to be individually calibrated.
A simpler scheme would be to use a limit switch to detect when the vehicle suspension had compressed excessively because of the weight. That switch will certainly need to accept quite a bit of overtravel to avoid being damaged in use.
A better description of the project will certainly lead to getting more useful answers.
 

SamR

Joined Mar 19, 2019
4,194
I'd go for a simple limit switch. Call it a sag switch that is engaged when the axle gets too close to the bed of the truck. Only problem to overcome would be hitting bumps in the road. Used to be you could tell when the forklift was overloaded when the rear wheels came off the ground. I've driven my old pickup with so many old crossties in the back it felt like the front tires were coming off the roads every time I hit a bump.
 
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Reloadron

Joined Jan 15, 2015
6,584
Both of your links are to strain gauge load cells. Each load cell will need an amplifier and signal conditioning. Can it be done? Yes but rigging a trailer would not be easy. Road bumps and pot holes will, as mentioned, produce problems. Using 4 sensors you would also want a level / centered load for it to be practical. Simple solution would be a limit switch configuration and even then you need to account for bumps false triggering but I see a limit switch as a simpler solution. Everything really depends on your chassis design and suspension.

Ron
 

MisterBill2

Joined Jan 23, 2018
11,877
Based on an old application note from the HBM company, which used to be a supplier of strain gages and equipment, if the four load cells are all the same, they can be connected in parallel and only require one amplifier system. At the time they included the math to show that it would work.
 

MrSalts

Joined Apr 2, 2020
1,762
You could also measure the mass by measuring inertia - that is, how much motor current it takes to start moving the vehicle. An overweight vehicle will cause the motor to draw "too much" cutting to get moving. You'll need to create a weight vs start current chart to see if this idea is sensitive enough.
any slope, expansion joint near the loading area could impact the accuracy. Change in tire pressure could also impact the result.
 
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MisterBill2

Joined Jan 23, 2018
11,877
I imagine that it is some sort of material moving vehicle in some sort of manufacturing environment. And measuring the drive current after starting up makes reducing the load much more complex. So a limit switch in series with the start command would be the choice, because once the start is given it is not needed, since it is latched.
Imagine the sand-delivery cart in a foundry. It leaves the sand pile and then drives up a ramp to dump sand into the mold filling station. But if it is too heavy it can't get up the ramp, which is not near the sand pile. (This is just my guess at a possible application that would need to inhibit an overload.)
 

Thread Starter

R!f@@

Joined Apr 2, 2009
9,784
So many good ideas.
I never thought of uneven distribution.
And also the limit switch idea.
I can tackle the electronics later. The weight measuring device is the key issue.

One idea @MrSalts said about measuring inertia is not bad but there is a slight hiccup. The controller is those cheap Chinese ones.
These are not bad but hacking into it is a bit tricky. It could be done if I can figure out a way to measure the 3 phase current of the BLDC.

I will get some pictures of the under carriage.

Thanks to all.
Regards
Rifaa
 

MisterBill2

Joined Jan 23, 2018
11,877
Consider that the majority of the load weight is carried by the rear suspension to the rear axle.And then note that the rear suspension consists of two leaf springs. A logical method of determining the load weight will be to measure the change in distance between the rear frame and the rear axle, where the springs are attached. This would be the equivalent of a spring scale. The distance between the frame and the axle will be directly affected by the added load, and should be stable when the vehicle is at rest. So either a limit switch arrangement or a linear position sensor measuring the distance between the two will be able to provide a reasonable indication of the added load weight while the vehicle is not in motion.
The difficult part will be assuring that whatever is used will not be damaged by the suspension moving through it's entire range. The very good part is that such a sensor can provide a much greater voltage output that does not require additional amplification.
One such type of device is often called a "string pot", although they tend to use a flexible steel cable.
 

MrSalts

Joined Apr 2, 2020
1,762
If you like the idea of measuring current draw to infer the load, you can measure directly at the battery - especially since you want to cut supply to the motor control if current draw is too high.
 

Thread Starter

R!f@@

Joined Apr 2, 2009
9,784
I am leaving all options open.
I believe I can measure current from battery.
The issue here is tht I need to consider the initial load when the vehicle is accelerated from rest
 

strantor

Joined Oct 3, 2010
5,967
Vehicle mounted load cell will experience quite a lot of shock load during operation which is not something they seem to tolerate well in my experience. Also while cornering, braking, side-loading that they aren't designed for. If you must use load cells my choice would be a pin-type, and one rated many many times higher than the maximum weight of anything you could possibly fit in the cargo area.
But, if I could measure suspension sag with a limit switch as suggested, that would be much preferred. I think I would make it a prerequisite to take off, but not to remain in motion.
 
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