Need help/opinion on scales or load cells - Page 1
Forum section: electronics / projects

author: KTP
author_rank: Frequent Contributor
timestamp: February 12, 2014, 01:28:06 pm
content: I am working on our truck camper (flatbed camper for a flatbed truck) and would like to be able to measure the current weight instead of just guessing. Obviously the easy thing would be to put a bathroom scale under the front two jacks, take readings, then move them to the back two jacks, take readings, then add them all up. This would probably get pretty close if I made blocks for the non measured legs the same height as the scale.The problem is the camper is probably around 2500 to 3000 pounds already. Truck scales are hugely expensive.So I went on ebay and looked up load cells. There seem to be a huge variety of these that go from 5kg small ones to huge 25000kg models (actually the cells are not that big in size). There are beam type, s-type, puck type and others. They all seem to take 10 to 15V excitation and have a signal output.There you have about as much as I know about load cells. Any recommendation? Have any of you played with these or interfaced to them?As a tease, here is what the camper looks like so far and the jacks where I would measure force:

author: SeanB
author_rank: Super Contributor
timestamp: February 12, 2014, 07:09:36 pm
content: Buy 4 puck types rated for around 5-10 tons and buy a remote display for each as well. That way you have 4 simple to use additions to get all up mass. otherwise you will have to just get 4 identical loadcells and a single display and accept the reduced accuracy if you use the 3 non calibrated load cells with it.

author: Rigby
author_rank: Super Contributor
timestamp: February 12, 2014, 07:59:00 pm
content: Something is telling me that you can't just add up the weights and get the total. That would only work when measuring a body whose center of gravity is exactly equidistant from the four measuring points, and even then it might not work out right. I can't remember why, but I have a very strong feeling that this won't work.Measuring the front half then the back half DEFINITELY will not give you the right answer if you add them up.Easiest way to measure it is probably to use a hoist from the ceiling with a good old mechanical analog strain gauge.

author: SeanB
author_rank: Super Contributor
timestamp: February 12, 2014, 08:09:30 pm
content: Adding the mass works if you do not change position during the moving of the meter from one location to the other. On a large tank with liquid or something that levels out you can get by using a single load cell on the one leg and then calibrating in place assuming the load is carried out equally by the legs.I weighed aircraft using this method using multiple load cells on the landing legs. We did a lot of checking when the one was 50kg heavier than when it was last done, the added mass being found by going through the paperwork and seeing just how many times the flooring had been repaired with epoxy from mounting points that had been torn loose from vibration and shock, and in each case the repair had been to use more epoxy to attach back to the honeycomb floor. Made sense when you saw the fitters using gallon drums of epoxy mix per mount. As well there was an electric fuel transfer pump added for refuelling from drums instead of using a bowser or a hand pump. That was 30kg.

author: kolbep
author_rank: Frequent Contributor
timestamp: February 12, 2014, 08:12:10 pm
content: Just take it to the tip/rubbish dump. They can weigh it as you go in. Normally th3y weigh you on the way in and out, and charge you for the difference you dumped.

author: SeanB
author_rank: Super Contributor
timestamp: February 13, 2014, 04:23:10 pm
content: If you want to avoid charges ( they charge for weighing the vehicles) you just go in with under 30kg of garden refuse, which is free.

author: bronson
author_rank: Regular Contributor
timestamp: February 14, 2014, 02:38:52 am
content: See if you can buy a used set of race scales. Race car drivers are super anal about their corner weights. Be warned, they're pretty expensive.It's easy to find "race scales" on ebay for 1/4 the price: just four bathroom scales and some levered platforms to reduce total weight. Stay away. They're terrible.As far as load cells go, buying them is easy but reading them is difficult (unless you have a 6 digit multimeter). Full scale readings are normally just a few mV. You'll need an instrumentation amp to pull the reading off the cell's wheatstone bridge, then a second and maybe third stage to boost the signal to a few V, where it's suitable for an ADC input. A fun weekend project but, unless you're we'll versed in op amps, probably not something that you'll get right on the first try.Like the project, I say go for it.

author: calexanian
author_rank: Super Contributor
timestamp: February 14, 2014, 05:58:48 am
content: Just look up Public Scales. they are everywhere and very inexpensive. Like the other poster said as well. Garbage dumps have scales. Jut pull in, throw the guy a tip, and have him tell you what the scale says. Also if you are in the U.S. most highway patrol offices have truck scales you can use if you ask nicely. Public scales usually just have a kiosk and are open 24/7 you just drive on, feed money into it, and it blurps out the weight. don't forget about your own weight!

author: mjrandle
author_rank: Contributor
timestamp: February 14, 2014, 07:09:46 pm
content: Quote from: Rigby on February 12, 2014, 07:59:00 pmReally? Ignore this response. Yes, you can weigh front then rear of camper and total so long as chassis is horizontal as OP has indicated. A weighbridge is best bet if camper is in a condition where it can be transported otherwise you can hire wheel scales for half a day. The problem with buying a load display or making an instrumentation amp circuit is load cell requires field calibration, depending on the style of load cell you choose the factory calibration sheet may not correlate with how you it is loaded in this application. Field calibration would require known weights.I also wouldn't recommend suspending a camper from the ceiling if you are fond of your house!Regards,Mike

author: woodchips
author_rank: Frequent Contributor
timestamp: February 14, 2014, 07:48:42 pm
content: Or use four old hydraulic rams, or cheap trolley jacks, each with a pressure gauge?

author: G7PSK
author_rank: Super Contributor
timestamp: February 14, 2014, 09:59:46 pm
content: There was someone recently on the amp hour that was talking about load cells, as I remember it he said that the strain gauges used in all the different size load cells were all the same. Unfortunately I cant find the relevant one, but whoever it was said that all that was altered was the beam size and I think the voltage on the cells.The load cells are fairly cheap.http://www.ebay.co.uk/itm/500g-Capacity-Load-Cell-Aluminium-Alloy-Weighing-Sensor-/190847275423?pt=UK_BOI_Electrical_Test_Measurement_Equipment_ET&hash=item2c6f62519fhttp://www.ebay.co.uk/itm/4pcs-Body-Load-Cell-Weigh-Sensor-Resistance-strain-Half-bridge-sensors-50kg-/181284138618?pt=UK_BOI_Electrical_Test_Measurement_Equipment_ET&hash=item2a35606e7a

author: bronson
author_rank: Regular Contributor
timestamp: February 14, 2014, 11:10:16 pm
content: That's partly true for part of the market.The strain gauges on hundred or thousand dollar load cells are awesome: made out of different materials so tempcos cancel out, exotic layered glues so the adhesive is consistent across temp/press/humid, and so on. Mounting locations are important too: fractions of a mm make a big difference.Expensive load cells first mount the gauges then mill the beam to meet spec. Cheap ones just mass-produce the beams, splat some adhesive over the gauges, and ship it. Cheap doesn't get you much accuracy. If you want consistent accuracy, your product will probably have to go through a cal step, which will probably cost more than just using the expensive cells.That said, the cheap cells have been getting pretty darned good recently. I haven't kept up with the technology but I was impressed with the performance of some junk Chinese scales I saw recently.The voltage on the cell is determined by the resistance of the gauges, the heat dissipation of the beam and adhesives, and your power budget. Obviously you should use the highest voltage reasonable.

author: KTP
author_rank: Frequent Contributor
timestamp: February 15, 2014, 01:15:42 am
content: Thanks for the suggestions. It doesn't sound like there is a easy way to do this without taking the camper somewhere (impossible right now) or renting scales.I did have the thought that I could use some large diameter pneumatic actuators with a pressure gauge connected to the extend stroke air input. I happen to have some 4 inch diameter piston short stroke actuators rated to at least 100 psi. My quick calculation says the piston area would be 12.56 sq inches, which would mean 1256 pounds would generate 100psi on the gauge.Not really accurate, but if you could read +/- 2psi you could get within a few tens of pounds of the actual weight.Crazy.

author: bronson
author_rank: Regular Contributor
timestamp: February 15, 2014, 04:30:10 pm
content: Funny, I just bought two of these:http://www.ebay.com/itm/261260635816?ssPageName=STRK:MEWNX:IT&_trksid=p3984.m1439.l2649They arrived fast but I haven't had a chance to hook them up yet. Looks like the seller has a full range from 15 to 1600 psi.

author: mjrandle
author_rank: Contributor
timestamp: February 15, 2014, 10:27:55 pm
content: Quote from: KTP on February 15, 2014, 01:15:42 amSo long as you use a hand pump to pressurise the pneumatic cylinders with hydraulic oil rather than air and don't exceed their pressure rating. Air being compressible will be problematic.