Rotary encoder break out board for scoping signals - Page 1
Forum section: electronics / projects

author: BillWojo
author_rank: Regular Contributor
timestamp: September 18, 2014, 01:25:19 am
content: Hi guys, one of the reasons I picked up a scope was to service CNC machines. Very common signals that I would like to check are the ones coming from the rotary encoders either mounted on the motors or at the end of the ballscrews. Common encoder signals are both the A and B channels and the marker pulse. Problem is getting to the signals easily. I need to check the signals both at the encoder and at the other end of the cable, at the control input.I have been hoarding old cables for years, I'm a bit of a pack rat. Same with old motors, drives, etc. So I have a lot of connectors that can be reused on the input and output side of a break out board. Some connectors I will have to source. The idea is to be able to unplug a cable at say the motor end and insert the break out board between the motor and cable, same idea at the control end. Board would have labeled test points or post to hook my scope up to. Really simple, no components on it at all. Old cable connectors would just be chopped off with about 2" of cable and soldered to the board.Does anything like this exist? I would need about a half dozen or so, maybe a few more.ThanksBillWojo

author: BillWojo
author_rank: Regular Contributor
timestamp: September 18, 2014, 09:16:38 pm
content: I guess nothing like this exist? Would have thought that someone, somewhere would have made a simple board for this purpose.BillWojo

author: kc9qvl
author_rank: Regular Contributor
timestamp: September 18, 2014, 10:10:46 pm
content: Put the tip of the scope probe in end the connector and done.

author: SL4P
author_rank: Super Contributor
timestamp: September 18, 2014, 10:46:29 pm
content: I like what you're thinking, and would make a straight-through breakout as you suggested, but would add a couple of non-retriggerable one-shots (or 555s) and LEDs to flash with each A/B transition. (A small PIC or AVR may offer more flexibility if you're up for it.)That way - without knowing the precision or reliability of the signals, you could at least see that A and B are firing before digging deeper with a scope or other diagnosis.

author: BillWojo
author_rank: Regular Contributor
timestamp: September 19, 2014, 02:25:07 am
content: SL4P, I like that idea but I'm such a noobie I wouldn't know how to do that but it makes sence.kc9qvl, I need to run the machine as I'm testing it. Yea, I could take the connector apart and try and probe from the back end but thats a royal pain, especialy with the mil spec connectors where a lot of the pins are in the middle.BillWojo

author: nowlan
author_rank: Frequent Contributor
timestamp: September 19, 2014, 03:41:19 am
content: I am used to see fly leads/crimped plugs on parts, not MIL connectors (although i dont work cnc/factory).Do the spare cables you have, have female & male ends that you can salvage for a break out board?What are the voltage levels on those signals? 24v?I was wondering if some 4000 series ripple counters might be able to drive a led bar etc.Would show the signal is being sent. Could count down for reverse.Unfortunately, 15v input. Would need to isolate with an opto-coupler perhaps.

author: BillWojo
author_rank: Regular Contributor
timestamp: September 19, 2014, 04:44:59 am
content: Hi nowlan, cable ends are usually diferent on each end. Typically there is a mil spec water tight connector at the encoder end and back at the control it could be a 9 pin sub D or propriatery connector or in the case of Fanuc they use Honda connectors. I have a lot of the connectors from old cables and trashed encoders, just missing a few from the control end. I'm sure I can round up what ever I might need.Most encoders use 5VDC to operate and the signals travel back on twisted pairs.The use of mil spec connectors out at the machine end is because of the hostile conditions found in these machines. Machine lubrication oils and water based coolants always getting to places you would never expect. Some of the cables run in flex tracks or tubes and they still disintergrate from the coolant degrading the PVC covering. When possible I source replacements using Igus chainflex cable, it has a urethane cover and doesn't care what kind of coolant it encounters. Really good stuff.BillWojo

author: ajb
author_rank: Super Contributor
timestamp: September 19, 2014, 04:59:34 am
content: Since you've already got a bunch of cables, the cheap solution would be to get somestripboard, chop a cable in half, and solder individual wires from each half of the cable to tracks on the strip board so you have a couple of extra holes in between. You can then just poke a scope probe at the bare tracks, or if you want to get fancy solder.in proper test points. You could even put a two row pin header in the middle of the board with shorting jumpers if want the ability to disconnect signals or inject your own test signals. Shorting jumpers also come with bonus test points you can clip a probe to. If you want something a bit more rugged, you could mount a bunch of banana jacks (or something) in a project box and mount that in the middle of a cable. Or panel mount connectors on the project box and make adapter cables from and to the other connectors you need.

author: SL4P
author_rank: Super Contributor
timestamp: September 19, 2014, 05:17:12 am
content: All good suggestions so far.One thing to keep in mind is that the tach pulses displayed directly on LEDs will probably be too fast to see a chain of pulses with the naked eye - hence the one-shots, or 555/micro I mentioned earlier - so at least you have a LED confidence check before lugging the scope out or replacing the encoder...Say for each A/B pulse, a fixed 50mS LED pulse, then off for a minimum 50mS before the next on event... which you can easily see in bright light / awkward access locations.I suspect the encoder phase outputs would be open-collector, terminated at the controller input - to minimise noise and voltage drop...??