Yesterday I was troubleshooting an issue with a Durant counter and a rotary encoder. The encoder was coupled to an acme screw and the counter was used to indicate linear position of the apparatus that moves along the ball screw. The machine hadn't been ran in over a year, and it was unknown whether it had ever worked with the Durant counter that was installed. The counter looked newer than the machine.
The issue was that it was very inconsistent. You tell it to go 10", and it might go 10", or it might go 8", or 16." I scoped the encoder signal, and this is what I saw:
Encoder channel A, bite taken out of one side:
Encoder channel B, bite taken out of the other side:
Both encoder channels:
So with the bites taken out of the channels, the counter has very little margin of error to resolve direction, and I suspect that while travelling in one direction, it may have been counting up sometimes and down sometimes.
I disconnected the encoder channels from the counter and scoped them again, and I had nice, clean square waves. So I deduced that the problem was with the counter. I theorized that for whatever reason, the counter's input impedance was too low, and was pulling down the edges of the square waves. I added 2KΩ pull-up resistors at the counter terminals (in parallel with the existing 2KΩ pullup resistors internal to the encoder) and then the waves looked better, but only slightly better, and that was "enough" to make it work.
Does my theory sound right? Could too low of input impedance cause this? My theory doesn't explain why only the corners of the waves are bitten off, and not the whole top of the wave chopped off.
Supporting information: It is believed that the Durant counter is a refurbished unit, and it is unknown when/by whom it was installed or refurbished. It is possible that it may have input circuitry bandaids that we don't know about.
The issue was that it was very inconsistent. You tell it to go 10", and it might go 10", or it might go 8", or 16." I scoped the encoder signal, and this is what I saw:
Encoder channel A, bite taken out of one side:
Encoder channel B, bite taken out of the other side:
Both encoder channels:
So with the bites taken out of the channels, the counter has very little margin of error to resolve direction, and I suspect that while travelling in one direction, it may have been counting up sometimes and down sometimes.
I disconnected the encoder channels from the counter and scoped them again, and I had nice, clean square waves. So I deduced that the problem was with the counter. I theorized that for whatever reason, the counter's input impedance was too low, and was pulling down the edges of the square waves. I added 2KΩ pull-up resistors at the counter terminals (in parallel with the existing 2KΩ pullup resistors internal to the encoder) and then the waves looked better, but only slightly better, and that was "enough" to make it work.
Does my theory sound right? Could too low of input impedance cause this? My theory doesn't explain why only the corners of the waves are bitten off, and not the whole top of the wave chopped off.
Supporting information: It is believed that the Durant counter is a refurbished unit, and it is unknown when/by whom it was installed or refurbished. It is possible that it may have input circuitry bandaids that we don't know about.
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