Robicon 454GT series drive Analog input question

I have a Robicon 454GT series drive, and am noticing some weird behavior. It was configured to use analog input 1 (4-20ma corresponding to 0-100%). LOS was set to shut off the drive.

What I'm finding is that the analog input signal is currently at 9.88mA with the wires disconnected. I set up a 4-20mA generator, and as I increased the signal the drive would get closer. 20mA matched perfectly, and the 16mA was read as 80%. Which made me think it was initially scaled 0-20mA.
But when I disconnected the wires the drive still saw an input.

I switched control over to analog input two, and while the signal is better than an1, it's still 9.24mA. There are no wires landed at tb18-19 (an1) yet it still reads 9.88mA.

I'd like to trim it, if at all possible, but I suspect there's something more going on.

Any help would be greatly appreciated.

I suppose it's possible that there are parameters that got messed with or corrupted to throw it off, but it might be a damaged A/D converter in the drive. I saw this once with a low end drive many years ago and it affected multiple channels. My symptoms were almost exactly like yours.
The VFD shows a 50% run command with the wires pulled out and would scale up to 100%. We needed to run it at 30% at times so I had the replace the whole drive to solve the problem. For a while, I cheated and hacked the min and max scale values, but it continued to drift higher and higher and we never trusted it after that.

I think many analog input modules with multiple channels internally multiplex them through a single A/D converter. I know nothing about that model of drive so I can't be of more help.

Thank you. That makes sense. The drive is 20 years old.

It still runs, but the scaling of both analog inputs are off in the low range.
PLC Output
0mA (wires disconnected)
4mA
8mA
12mA
16mA
20mA


VFD Analog Input
Wires disconnected they read 8.28mA and 8.98mA
Using Analog input two, I got the following readings
4mA 38%
8mA 50%
12mA 60%
16mA 80%
20mA 103% (I believe this is due to the card giving slightly more than 20mA. When I tested it with my signal generator the 20mA output read as 100% at the drive)
The drive is configured for a 4-20mA input:
4mA - 0%
20mA - 100%
I have a minimum speed setting (25%) but even when I changed it to 0% (test) the analog input values didn't change.

Fortunately it rarely operates at low speeds. I can live with a 33% (roughly 21Hz). It's not much higher than my ideal offline speed (15Hz) and doesn't affect the burner operation. The previous company working on the drive had it opersti ar 50% offline, which was crazy, but not surprising considering who they are.

Robicon used to use signal conditioners to convert the 4-20 mA into (I think?) 0-5 VDC to go into the analog input card. Back in the day I had lots of trouble with those being off calibration, as well as the analog input itself being poorly calibrated. You might want to check that out.

At 20 years old it would definitely be wish to start budgeting for a replacement drive and even moreso considering it's Robicon. It's on short time for sure.

DickDV said:

At 20 years old it would definitely be wish to start budgeting for a replacement drive and even moreso considering it's Robicon. It's on short time for sure.

Click to expand...

I get requests to swap out older 454GTs at a rate of about 2 per week now, because Siemens is not interested in supporting the old Robicon NIH (Not Invented Here) drives; they are trying to drive people to change to German made Siemens drives and quoting lead times of 30-50 WEEKS for replacement Robicon control boards! So yes, start making new plans. And don't think you can just buy replacement boards on Fleabay, I know one large user here in California that buys every single one that comes up for sale the instant they see them, even if they don't need them yet. They don't even know (or care) if they work, because they just buy them and immediately send them to re-man companies to be repaired whether they need it or not. It's crazy...