Micrologix 1200 PTO relay debouncing

Hello!
First time poster here. I've been in and out of PLC programming for around 10 years now. My current project has me way deeper into RSlogix than I have ever been (and I have learnt a lot).
I am basically building an indexing axis for a CNC router. The CNC controller writes to the PLC via Modbus, the PLC outputs step & direction for the driver, etc. fun stuff.
My issue lies in the high speed relay output. My first test run was quite disheartening, as motion was terribly rough. I did some experimenting (wired the PTO to the high speed counter) and found my issue was noise from the mechanical relays. I played with the filter on the HSC and found a filter of 250usec would clean it up nicely. I should still have 4khz to play with (4khz x 250usec = 1sec). Should be more than enough for my application.

As a side note, what is the point of having a high speed output capable of 20khz, when there is so much noise in the output that anything above 4khz would be a complete mess? I guess I assumed when picking this plc that if it says it can do it, it should be fine.

Anyway...I built a low pass filter, and it almost works great. I attached a pic of the circuit below, works great below about 400hz. Above that and the driver doesn't see any steps. Not sure why it is limited so low, I followed calculations from here: http://sim.okawa-denshi.jp/en/CRlowkeisan.htm
Does anyone have experience with this? Any recommendations for moving forward? I need more speed now! If I could get up to 1khz I would probably be happy!
One more question, does anyone know what the part number is for the high speed FET that AB uses?
Thank you for your time.
Joe

A little more experimenting with the HSC shows that above 500 hz, nothing happens. Output is just one steady on with no filter. Lame, apparently high speed output is not possible with relay outputs.
So I guess my biggest question would be how can I find the part number for the AB output FET's? I should be able to change it out myself.
Thanks,
Joe

Look into Optocouplers



http://www.clrwtr.com/PDF/Weidmuller/Weidmuller-TERMSERIES.pdf

ahh, that sounds much better. I was trying to figure out how I was going to make a transistor work. Optocoupler would be much simpler, I may even have some.
I don't look forward to hacking into this thing.
Thanks!
Joe

The relay outputs of a MicroLogix are never used for PTO or PWM. The whole point of having a hardware subsystem is to use the fast solid-state output.

Are you running an interposing relay ? I can't figure out how you got the PTO instruction to run an onboard relay output in the first place.

Eddie Willers said:

Are you running an interposing relay ? I can't figure out how you got the PTO instruction to run an onboard relay output in the first place.

Click to expand...

I don't know what an interposing relay is, so probably not!
Not sure how, but it does.

3.5-15 V DC 5 kHZ

It's never occurred to me that you might try to use a relay output with the high-speed subsystem, so I've never tried it.

The PTO feature is only intended to be used with Output 2 and Output 3 on a MicroLogix 1200 controller, which are the special high-speed outputs that are wired directly to the PTO/PWM hardware subsystem.

In the MicroLogix 1200 Instruction Set Reference, Chapter 6 is "Using High-Speed Outputs". The second "IMPORTANT" block on the first page says:

"The PTO instruction should only be used with MicroLogix 1200 and 1500 BXB units. Relay outputs are not capable of performing very high-speed operations."

Maybe I'm still confused by your description of using a mechanical relay output with the PTO instruction. Which actual output terminal on the MicroLogix do you have wired to your indexing driver pin ?

Did you try to use a relay because the indexing driver voltage is lower than the usual 24V DC used by the MicroLogix ?

If so, then a fast optocoupler is almost certainly the solution you're looking for.

Don't know what his pulse input voltage is but i'm sure FET output volgage from the 1200BXB is 24vdc.

I am using output pin 2. The reason I am using relays is because the PLC I have has relays, lol. Didn't know at the time to look for one with FETs. Makes sense now that I know the manual says that.
In any case, I got it working. I disassembled the PLC, soldered wiring from the relay coil and brought it out to the same output pins (disconnected relay output lines). I ran these lines out to power a 4N35 optocoupler, then 5v through the output to the stepper driver. Crazy, I know, but it works with no need for a filter. I am up to 500hz now, haven't had a chance to test faster yet.
The stepper driver already has an optocoupler, so I may bypass the one I added and wire directly to the driver (with resistor inline).

Now running 20khz! I ended up leaving the opto as an added bit of separation between the two.

Which model of 1200 are you using

It is a series c, not sure of much else. Not in front of me anymore.

you can try a micrologix 1500 with the
176428BXB, it has digital solid outputs so you PTO will work great

I will probably leave it as is for now, then for my next project I will get the proper one and switch them out.
I have access to a 1500 now, but it is a series a and I need modbus!
Joe