Wouldn't it be better to measure the weight in the box it's self? One way would bet to use a pin attached to the box, going through a spring rated to deflect under the load limit desired, the pin would then when the limit was reached push on a simple limit/snap action switch. Springs are rated in load per inch if you get them from a spring company, or you could use a urethane spring if the steel ones might get rusty.

You could even use the limit switch as a scale, have the NC contacts power a light, and when the weight limit is reached the light would go out, then you remove weight until it just goes back on. Doing this would make loading go faster than waiting until you try to move the truck. Anything done with the suspension its self will be a problem when going over bumps.

https://www.google.com/search?client=firefox-b-1-d&q=urethane+springs

 

Anything done with the suspension its self will be a problem when going over bumps.

The leaf spring should provide a good balance between damping and dynamic response. Locating a strain gauge closer to, or farther away from the axle should provide for broader parameters and tuning. The real advantage is cost and ease of application. Shifting loads on cornering, speed regulation on uneven surfaces. Throw in an accelerometer, just for fun.

 

I would go with a micro switch on the axel and a pin mounted to the frame that will disable the battery during startup use an SRC from the battery when the thing is turned on so it will not turn off if the box bounces on a bump while driving.

 

Nice images. In the interest of keeping things simple I would run with the Micro Switch approach. Try a Google of "Pin Plunger Micro Switch" to get an idea. Frame mount center of leaf springs. Use 1 or 2 switches (one each side). Load the bed to max to see where the distance between frame and axel is and work from there. I suggest this only because it id quick and simple. Anyway my vote runs with those who made this suggestion. Good pictures too!

Ron

 

This idea is also much easier.
But dang these things are expensive

Certainly the "string-pot" sensors are expensive, but they do work well and they do not require expensive amplifiers and calibrations. A limit switch scheme could serve to warn of an overload in advance of attempting to start moving, and it would not involve any changes to the drive power wiring.
Using a limit switch that is operated by a string with a series spring when the box is not loaded too much will allow a lot of bounce without damaging anything. The switch can change from powering a green light until it is loaded too much, then a red light. And since the switches do not cost that much, a second switch to illuminate a yellow light when the load is at the recommended maximum amount. And calibration will only need adjusting when the vehicle leaf springs are replaced.
No load cells or strain gage amplifiers, no display, and no structural or electrical modifications. Total materials would be two limit switches and three lights, plus some brackets and some wire. And a simple enough circuit so that a plant electrician can figure it out.

 

What ever the sensor method we use my disable method would be to prevent it from accelerating from rest. Once it takes off the disable circuit would not stop the vehicle but would indicate an alarm of overload during bumps.
I can disable the throttle or activate hand brake easily.
I would need to read the speed from the RPM wire that comes from the BLDC controller. Apart from the sensor, this RPM sensing would be a bit of circuitry But I believe a comparator and a small PIC would do to inhibit startup from overload

 

Time for a vote.
Less hassle and long term is the target. As for price I will talk to the customer.
1. String pot.
2. Load cell.
3. Limit switches.

 

3. Limit switches.

There I voted.

Ron

 

3. limit switches BUT not on the suspension, too much movement on the wheel/frame suspension. Your load box looks like it lifts to dump so use that to it's fullest, you're measuring load after all, not suspension movement. most people don't appreciate the movement in a vehicles suspension when moving even on a paved surface.

 

The requirement was, and still appears to be, avoiding travel with the vehicle overloaded. This tells us that there is no need for the load sensing system to operate while the vehicle is in motion.
A tension operated limit switch set to trigger as the distance between a frame member and the axle, and with a spring arrangement to allow full suspension travel without damaging the limit switch, can provide either one or two switch points so that fully loaded and overloaded conditions can be known easily.
I am guessing that accuract within 5% should be adequate for this application, and easily within the capability of this scheme.

 

I’ll vote limit switch, only because I wouldn’t want you to waste your time with a string pot (sorry Bill). The ‘limit switch’ Could be a simple magnetic reed switch. You surrender dynamic sensation, but that was your criteria

 

The string pot and a digital meter with a switching function would work very well. Of course the cost will be a lot more, probably about $1500 for materials. But it would be simple to set and simple to calibrate and give a believable number, which would probably be close to correct. maybe. But the limit switch can perform the task required and so why do any more? And it will probably be the most durable and easy to use as well.

 

I’ve only seen a couple of string pot installations, and for the application they worked very well. Pretty sure they have a max acceleration rating that the string/spool assembly can withstand. Pretty sure a bumpy road would exceed that.

 

Hmmm...So limit it is for initial setup.
lemme find some waterproof ones
Thanks all.

Some Year later .
How's these ?