That is a very interesting story and great job about troubleshooting a field failure with an uncooperative customer...it is also a great reminder to inquire as to any modifications that a customer may have made that can introduce problems that they are blaming on the original design!
It was good of the vendor to offer a solution to the customer's modification. Good vendors will help as long as safety is not concerned. As a teen, I was working on an old Chevy loosening a bolt that was too close to the frame to reach with a standard box wrench, so I ground some off of the OD which promptly broke. I then took the Craftsman wrench back to Sears who said that it had been modified, but it should have been tough anyway and gave me a replacement.
The Sherlock Ohms stories never fail to amaze me. I never would have considered adding a ground strap between the wagon body and axle frame. Engineers -- especially those who submit our Sherlock Ohms stories -- are an amazingly clever bunch.
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.
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.
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.
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.
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.
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