AVR Freaks

Firstly, you listed only 7 bit combinations, while in reality 8 are possible. One is missing. Who can help muke12 to find the missing combination?

Secondly, there's really no reason to implement it through enumeration of bit combinations, although for just 3 sensors this might be a viable idea too.

The whole logic boils down to:

S1==S2==S3==1 - "you are too long, go away!!!" (not sure whether you need to handle this)

otherwise:

S1==1 - "please move forward"

S3==1 - "please move back"
otherwise:

S2==1 - "OK, that's the spot"

otherwise:

"yawn... empty"

This all assumes that commands are active only when a truck is entering the scales (not when it is leaving). If the system is constantly active, it will also issue commands to leaving trucks forcing them to oscillate back and forth indefinitely. It also assumes that a truck can't enter while the preceding truck is still there. So, strictly one truck at a time. Your table is built under the same assumptions, as I see.

RSCA Series Pit-Type Truck/Track Combo Scale | Cardinal Scale Manufacturing Co.

[8 load cells]

A guess on the local light truck -to- semi-tractor trailer weigh scale :

• load beam that pivots
• limit switches to indicate the beam is off the pads
• one torque arm weigh scale (pivot -to- scale operator who balances the scale beam then reads the weight)

edit : strikethru

I would also suggest that "exact position" (as written in the title) is not an issue, here. Suppose that you have a truck that is smaller than the distance between the light beams. Then, technically, the truck could be anywhere inside the beams.

How do you know that the system does not use ultrasonic sensors aimed at each side and each end of the truck? That would give you a "more exact" position!

Just how close does the center of the truck (or, its center of mass) have to be to the precise center of the weighbridge?

Jim

agreed.

the OP, is also making life somewhat hard on themselves too.  
 

to almost quote a famous Freak:

”Stop.  Go make a cup of tea….”

I think the OP should do this to clear their head, and the community as well as they are repeating themselves, and starting to get a bit abrasive.

the basic concept and logic seems to have already been solved by the OP, yet it seems they don’t understand how they did it.

if indeed this is a homework assignment, then the OP should consult with their teacher as well as ask questions here.

as for the rest of us, to me, the solution, and suggestions have been presented.  Time for the OP to go make that cup of tea and review what’s been presented, and we should as well go make a cup of tea and stop trying to make the horse drink..

cheers,

jim

OK, then, YOU need to define how good is good enough. Factors to consider include:

1. Are all trucks exactly the same size?

2. How important is it that the truck be centered over the weighbridge? The other vendor should be able to tell you. Related to this is the issue of unevenly distributed loads in the truck. Does this cause errors? If so, how much?

3. If the trucks are all the same, why not use an "index mechanism" that hangs down even, lets say, with some hood or windshield detail. Then the driver drives up until that detail just touches the index mechanism. This mechanism might not be anything more than a string with a soft weight on the end. When done, the truck just proceeds. It would have to be suspended from something taller than the truck but that might be a LOT less expensive and more reliable. 

4. If trucks are not all the same length, then what sort of error occurs if a short truck is all the way forward compared to all the way back? Is such an error significant? The weighbridge vendor should be able to tell you.

5. If good centering is important for different truck lengths, then light gates are not sufficient, I would suggest ultrasonic ranging sensors at each end and a light bar visible to the driver. Then, you change the position of the lighted section on the bar according to whether too far forward vs too far back. Here, the important measure is that the front and back sensors read the same distance to the truck. If the front sensor reads more than the back, it is too far back, and the truck needs to move forward.

Jim

My guess is that the IR sensors are good enough.

The exact positions of the sensors likely matter,

as does the mechanics of the scale and

the positions of the trucks' wheels.

BTW I'm pretty sure all 8 combinations of sensor readings are possible,

even without broken sensors.

101 would not necessarily require two trucks,

just a negative hump.

What do you need help understanding?

Basically everything, including how to ask the question...at least a diagram is at hand, but not much beyond.

How do you get the weight ?
Is the weight-information a continuous stream from the weighbridge, or do you need to 'ask' for the current weght ?
Is there a 'weight is stable' indicator ?

What?!?

You drive the truck onto the scale. There are three sensors. One says the truck is somewhere on the scale, two of them tell you if the truck is hanging off one end or the other and should be centered.

When the combinations of sensors is correct, read the weight from the scale.

There are no torques involved.

There is no work involved, not by the physics definition or the colloquial one.

There is amazingly little logic required.

What's the fuss? Reheating a cup of coffee in a microwave oven can be more involved.

As I mentioned earlier, the requirement for the truck to be centred, as opposed to merely having both wheels on the platform, is an overcomplication. Whether the scale uses one load sensor or several it doesn't matter. The weight of the truck is the sum of the weights on each of the sensors, irrespective of the position of the truck or of its centre of gravity.

If this is not the case, then this is not a software issue relating to position: the scales are broken. And given that people have been using weighbridges for approximately as long as they have been using trucks, that seems unlikely.

It can be solved visually by an external operator: are the truck wheels all on the platform.

It can be solved using two side viewing optical beam breaking detectors: weigh after the rear beam has broken but if the front beam is broken before the rear beam clears, the truck is too long.

It can be solved with *no* beam detectors: merely weigh continuously and log the highest weight observed - though this doesn't allow for a truck longer than the platform, and requires an operator (or an external sensor) to reset the weight.

It can be solved with a fore-and-aft sonar sensor: weigh when they both report the same distance, but not infinity. And hope the truck doesn't squash them on the way in or out.

It can be solved with...

Neil

Driver is told his 5 tonnes of coal is too much so  he leaves 1 tonne in the driveway. Oh hum...

Well for me everything in the world looks like a vision processing problem these days so I'd use a camera and OpenCV doing something like Structure for Motion to determine exactly where in the 3D space the vehicle is. 

However, if you're keeping it simple then who dictated 3 optical sensors? A bit like nyquist and sampling a wave with ADC the more sample points you have the more accurate idea you get of exactly what is positioned where.

As this is presumably a one off rather than something you are building 50,000 examples of then whether you pay $3 for three $1 sensors or $10 for 10 of them hardly really matters does it?