JRK12V12 Feedback Pot value extremes?

Support Motor controllers/drivers and motors
1

I have a servo system using a JRK12V12 and when I use a 10 K feedback pot everything works just fine. Pot connected to AUX, FB & GND. Input range set to 0 and 4095. Powered only by USB with no motor connected. If I change the pot value to 30K I get a pot disconnect error at about 3000 counts of feedback. The plot shows the pot position accurately but I get the error flag. Disable the error flag and everything works fine. A DVM connected between FB and GND tracks the pot position monotonically. Obviously the source impedance of the 30K pot is 3X the 10 K pot source impedance. It acts like the FB input on the JRK outputs a glitch into the higher source impedance and trips the error flag.
It seems like the FB pin should only go to the PIC A to D input and shouldn’t put out anything. Several different JRK units behave this way.

What’s happening here and what is the official maximum pot resistance?

The 3T 30K I am using are mechanically unique and expensive to duplicate in a lower value.

Eric

2

Hello, Eric.

I would generally recommend a pot in the 1-10k range. For higher impedances, the 100k pull-up resistor on the jrk’s FB line starts to have a larger effect, and it can cause trouble with the disconnect detection feature. If you disable that feature by unchecking “Detect disconnect with AUX” in the Jrk Configuration Utility then everything will probably work fine with your 30K pot.

The FB pin is always an input, but the AUX pin as an output that goes low sometimes to detect disconnection of the pot. If the voltage on the FB pin is close to 0 but not too close and the impedance of the potentiometer is high, then it is hard for the jrk to see a noticeable difference when it drives the AUX pin low, so it could interpret that to mistakenly interpret that the potentiometer is disconnected.

–David