We’ve got a bunch of limit switches on our robot this year, and are seeing jitter between true and false when the switch is open. Our hypothesis is that the runs are getting RF noise. The issue is happening when the robot is standing still
The switches are the KOP Honeywell switches, and they’re connected to fairly lengthly (probably 3’) runs of 22ga unshielded wire. The switches, when closed, connect the signal and ground pins. Using a meter, they ohm out at 0.1 ohms when closed and infinity when open.
Talking to some folks at yesterday’s invitational, the suggestion was either to add lower valued pulldown resistors or go to shielded wire. We chose the latter approach, and built four external pulldown resistor units using 1000 ohm resistors we had in stock. These worked fine yesterday in eliminating the jitter, so we build a set of four more to cover the other four limit switches in our design.
Adding that additional load to the digital sidecar looks to have blown the 5V bus, as we no longer get a green 5V light on the sidecar.
Rewiring is a pain that we’ll probably start doing tomorrow morning (using 22ga shielded security system wire), but I wanted to check to see if there are other options – potentially going with higher valued resistors, for example – to overcome noise on long switch runs.
The added load of 8 1000 ohm pulldowns is negligible compared to the 3 amp 5v power supply.
However, a common problem is to not supply 12v to the digital sidecar, so the 5v supply doesn’t function. In this case, leakage through the 37 pin cable can let the digitial sidecar partially function, but not reliably. With everything disconnected except the power cable, are all three power LEDs brightly lit?
Another common problem is a small metal shaving shorting 5v to ground.
Are the BAT and 6V LEDs still lit?
Batt and 6v are green. Another sidecar connected to the same power yields green on all 3 LEDs.
Clarify one thing – where did you wire the 1000-ohm resistors to? There should be not static current flow through those, as for effective shielding you should only be grounding one side of the shield.
You know, a better option than an extra pullup resistor is to just connect your 5V wire to the NC contact on the microswitch. Does a much better job of pulling the signal pin up to 5V than a resistor.
Thanks everyone for their thoughts.
A couple more notes on this.
The sidecar is the one from 2009 and I’m informed that we did a lot of machining on that robot with the electronics installed. I opened it up (yes, the warranty is now voided blew it out a bit and now there’s a decent green, not quite as bright as the others, on the DSC.
We did look at running the 5V to the NC connector on the limit switches, but we are low on 3 terminal female terminals so are avoiding that one at this point.
We’re going to try another sidecar, with a couple of the switch runs converted to shielded cable grounded at the sidecar, and see what we get for results.
Another note on our setup – we are using the old style D connector cables between the cRIO and the digital sidecar, not the recent ribbon cable. Could that be the issue?