I used to design agricultural electronics, and one of our products was a grain wagon scale. The axles of the grain cart formed the actual load cells for the scale system so that each axle had strain gauges glued to their surface. All four load cells were wired in series to produce one large Wheatstone bridge.
I began to get field complaints from a new customer who was having a very difficult time troubleshooting the problem. Since the load cells were wired in series, a shorted cell could appear anywhere in the Wheatstone bridge, and the symptoms would vary depending upon which socket of the scale the faulty cell was plugged into.
From the customer's point of view, he had a scale that wouldn't turn on, so he would swap the scale out of the system for a new one. Then, the second scale would turn on, but he wouldn't be able to balance the scale and show a reading.
He would try yet another scale and that one would work, but would be rather unstable. I tried to explain to the customer that the problem might be a faulty load cell, but the customer was convinced the problem was in the electronics. Finally, I convinced him to send me the complete systems for examination.
While examining the failed systems, I found that each of them had a short on one of the axle strain gauges. Using a microscope, I saw that each of the failed gauges had a burn mark on the polyimide backing, shorting the gauge foil to the axle's steel.
I called the customer to tell him of my findings. He was becoming quite agitated because he believed I was covering up a serious quality problem with our electronics. From his point of view, every time he swapped out the electronic scale, he saw a different problem, so clearly every one of the scales must be defective. “Besides,” he said, “why would we begin having these problems with the axles now that we've just improved the wagon by adding shock mounts?”
“You've recently changed your design?” I asked cautiously.
“Yes," he said, "now the whole wagon is mounted on huge shock mounts, so the axles aren't even getting bounced around like they used to.”
It was starting to make sense to me, but I had to be careful. I asked the customer to take an Ohm meter and check for continuity between the axle frame and the wagon body; sure enough, the two were isolated. I explained that the tires would build up a significant static charge while the wagon traveled down the road, and without a connection between the axle and the wagon frame, the high voltage would eventually punch through the strain gauge backing, onto the strain gauge, up to the scale, and then to ground.
The customer reluctantly added a ground strap between the wagon body and the axle frame. The field failures stopped with the addition of the ground strap, but within in a year we still lost the customer because he believed that the ground strap was simply a cover story for some sort of rampant quality problem.
This entry was submitted by Chip Curtis and edited by Jennifer Campbell.
Chip Curtis is a development engineer for Teko, designing small consumer appliances and medical devices.
Tell us your experience in solving a knotty engineering problem. Send to Jennifer Campbell for Sherlock Ohms.
The comment about small ground straps from the body to the engine block is a valid point. If you connect the negative cable to the engine block - not the frame - you will be fine. Making a good electrical contact with a jumper cable clamp and a rusted or painted part of a steel engine block may be difficult so you may have to choose the connection point carefully. Providing an adequate ground on moving equipment has proven to be a diffuclt challenge to meet. Imagine the static that used to build-up on cars in the 50's. Eventually methods were developed to allow the static charge to bleed-off, but before then, passing a bill to a toll attendant could cause some fairly dramatic shocks.
Instructions for jump-starting vehicles typically recommend that the last jumper cable connection be made to the chassis of the vehicle, rather than to the battery itself. This is supposed to prevent a spark near the battery vents where hydrogen gas could ignite. This is not bad advice to prevent a hydrogen explosion, but there is generally very little risk of an explosion (safety conscious people: bash me now).
If the dead/low battery is no longer capable of sourcing enough current to crank the engine after a few minutes of charging, then a different kind of fire hazard is created by following the "safe" instructions. The heavy-gauge negative cable from the battery typically connects to the engine block, because the starter usually draws more current than any other electrical device in most (production, non-electric, non-hybrid) cars and trucks. There is usually a lighter-gauge strap from the engine to body/chassis. If you connect the jumper cable to the chassis, and the cranking current must flow through the jumper cable, that is, if it is not sourced by the dead/low battery, then cranking current flows through the strap.
In this situation, the strap gets very hot, very quickly. I witnessed a not-so-helpful tow-truck driver try to jump start my truck and the procedure melted the ground strap and started a small fire.
One reason this doesn't happen more often is that the jumper cables most people own are of small wire gauge, so there is significant voltage drop across the cable during cranking.
Does anyone know of a safe, non-destructive procedure for jump starting???
I just love it when customers or end users make changes, without understanding the logic of what they have changed, and then blame the designer for producing a defective part. This problem runs across the full spectrum and is not confined to weak minded people. If two is good, why three must be better. I am sure this vehicle would perform better with a different tire size. Etc, etc.
Great job of trouble shooting and you did not lose a customer, you solved a problem yet to come.
Wit our sensors, which detected shock waves traveling in the metal, I found that every installation needed to either pick up the substrate that the sensor was mounted on, or else have a very thin insulkated shield bonded to the substrate, with the sensor then bonded to the shield. This avoided there ever being more than a few volts between the sensor and the conductive surface that it was mounted on. That might be a problem for strain gauges, I have not studied it. Possibly a rigid enoug epoxy would work.
Good point William, in practice the load cells were never attached to the shield because of problems with ground loops. Some of the load cells were trailer hitches or physical links between the wagon body and wagon frame. Tail lights, alarm horns and other accessories would wreak havoc with the signal if the shields were terminated.
This is an interesting example of one of the problems that will pop up when the efforts to keep low level signals in the low level realm are inadequate. I can understand that it was cheaper to simply glue the strain gauge to the axle, but the shiels of the cable should have been connected to that axle, or another conductor, if not the shield. MY reasoning is that aside from the voltage build up, the conductive axle is a good candidate to pick up a lot of noise and couple it to the strain guage. I learned all about the coupling of undesired signals while working with a non-strain-gage pickup as part of a product development project. Compensating cor the mount material potential is vital.
The two solution options would have been to either ground the axle to the input circuit, or to float the input circuit from all other potentials, but that would be difficult and expensive.
I remember that story, Jenn. I liked it for two reasons. First, it was a clever solution. Second, instead of coming up with some costly, convulted solution, they simply stopped testing their phones when a frieght train would pass.
On the subject of ground straps, I've seen missing ground straps on motor vehicles, and what often happens is the cranking current for the starter motor will go through the next best conductor, often the driveshaft U-Joints, fries them pretty well.
Hi Chip--great story. It is always challenging supporting customers in the field. You were lucky to get the parts back.
Once I was involved with design, production, and then support of an antenna module for commercial telematics. It consisted of an amplified GPS patch antenna integrated to a cellular omnidirectional antenna printed on an FR4 PCB. There was a long cable pigtail to allow the installer to locate the antenna in a good but hidden place in vehicles. Most often, it was placed under the dash on top of wire bundles or air ducts etc.
Two support challenges were memorable. I don't need to give all the details, just the solutions. Case 1: "We recommend you do not locate the antenna inside the glove box as the antenna won't perform well inside of a metal enclosure". Case 2: We added a label "This Side Up" to ensure the GPS patch was facing the sky, not the ground.
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