The NI Signal Conditioning Chassis (SCC) system accepts modular signal conditioning modules that allow customers to mix and match input and output signal levels. Our versatile DC I/O module design actually proved quite versatile, and was used in several other applications for power control, and in at least one production test system for marine controls for dual high power CAT diesel marine engines.
I used this Feedthrough module to develop my prototype.
http://sine.ni.com/psp/app/doc/p/id/psp-473/lang/en
The client was very happy with the resulting modules, and specified them as standard equipment for all of their product development test stands in both North Carolina and Michigan.
I used this Feedthrough module to develop my prototype.
http://sine.ni.com/psp/app/doc/p/id/psp-473/lang/en
The client was very happy with the resulting modules, and specified them as standard equipment for all of their product development test stands in both North Carolina and Michigan.
Speed Glitches?
During one follow-up call the client casually mention being frustrated by occasionally erratic readings with my module, and being an engineer who wants their designs to ALLWAYS work, caused me some anxiety. A few quick questions clarified that the same problem predated my module design. It turns out they had been trying to address the erratic tachometer readings with software filters for almost two years.
With a little prodding, the client put scope on the module and verified that tachometer input into my module, and the conditioned signal output of my module were identical other than scaling and voltage shifting, so my circuit design was in the clear. The client still had a problem to solve but could provide no additional information.
Our local field engineer visited the customer asking if they have any new test or automation projects. The engineer called me after the visit and told me the client was very pleased with the modules I designed, and the client demonstrated the tachometer speed reading glitch for him. He said he could see the incoming signal trace moving up the oscilloscope screen on my input signal, while doing the same thing on the output signal from my module.
With a little prodding, the client put scope on the module and verified that tachometer input into my module, and the conditioned signal output of my module were identical other than scaling and voltage shifting, so my circuit design was in the clear. The client still had a problem to solve but could provide no additional information.
Our local field engineer visited the customer asking if they have any new test or automation projects. The engineer called me after the visit and told me the client was very pleased with the modules I designed, and the client demonstrated the tachometer speed reading glitch for him. He said he could see the incoming signal trace moving up the oscilloscope screen on my input signal, while doing the same thing on the output signal from my module.
Something sounded off in his description, so I asked him "Do you mean you saw the rising edge of the square-wave? His reply was "No, it was not a square-wave, we could watch the voltage rise smoothly as the voltage from the inductive pickup increase". I felt like we had cornered the dragon and found the detail that would let me solve the issue!
Slowly changing signals can wreak havoc on a digital input, with any noise causing multiple transitions for what should be a single edge, and the slower the signal, the worse the problem. But this is well-trodden ground with known solutions. Adding just a little positive feedback can add hysteresis to the system by creating separate thresholds for low-to-high and high-to-low transitions. Having two different thresholds can prevent multiple-transitions.
Slowly changing signals can wreak havoc on a digital input, with any noise causing multiple transitions for what should be a single edge, and the slower the signal, the worse the problem. But this is well-trodden ground with known solutions. Adding just a little positive feedback can add hysteresis to the system by creating separate thresholds for low-to-high and high-to-low transitions. Having two different thresholds can prevent multiple-transitions.
I suspected that the quickest solution would be changing to an optocoupler with a Schmitt-trigger input. A quick search through DigiKey turned up a chip that was signal level compatible with my design but with a different pinout. I ordered a handful of chips for overnight delivery, and showed up early the next day to watch for the FedEx van to roll through the parking lot!
I quickly modified a module dead-bug style and verified the Schmitt-trigger input device worked on my bench. Next I called the client and told him I have an idea that might solve his signal glitch, and that I can hand-modify a handful of modules for verification testing, and I should be able to send them out by the end of the week.
I quickly modified a module dead-bug style and verified the Schmitt-trigger input device worked on my bench. Next I called the client and told him I have an idea that might solve his signal glitch, and that I can hand-modify a handful of modules for verification testing, and I should be able to send them out by the end of the week.
A week later the engineer called, thrilled to tell me the signal glitches were completely gone! Not better, gone! For the first time in two years there were no glitches in their speed data, and his test stand was the only one in the world that could do it properly! The client immediately ordered replacements for every IO module (24 units) to upgrade all of their test stands in North Carolina and Michigan.
One stumbling block we had was that the client didn't want to return the hand-prototypes modules from his system because he now had the only properly working test bench in the world for that product line. Out compromise was that I would no-charge him for the last three replacement modules, but wouldn't ship them out until I received my returned hand-wired prototypes.
My client was a company hero for fixing the long lingering problem with the tach sensor's slow-speed reading jitter. I love it when something clicks from one of those early EE lectures, or those hours of reading application notes, and hands you exactly the tool you need to slay a dragon!
