Renishaw TP-20 Probe Connection

I have a Renishaw TP-20 CMM probe that I want to use as a 3D CNC probe. I have measured the 2 contacts of the probe and without triggering, it measures about 500ohm and then open circuit when triggered.

Is that normal for the 500ohm resistance and how would I tie that to an input on the Mesa card? Pull-up resistor connected maybe?

Thanks for your help!

Hi Tommy,

Thanks for your reply, from what I have read of the Renishaw documentation it seams this probe just acts like a NC switch. With no extra circuitry in the body.

I am not sure why there is a resistance unless something is corroded but the unit is quite new.

I don't know what the Renishaw receiving end circuitry looks like, but I believe they use a different system than the typical NC switch.

As I understand it, the receiver senses the resistance drop. Once the resistance value starts to drop, the receiving circuit drops the voltage significantly so that there's no (or very little) spark across the contacts when they break. And the voltage is higher at the start to reduce false triggers from EMI (guessing here).

No spark = no contact pitting = longer contact life and more accurate triggering over time.

I assume that the circuitry would have to be really fast to accomplish this at reasonable probing speeds, but again I'm not familiar with the details of the the receiving end components.

Thanks for your replies, yeah I think you are right about the way they output the signal. This thread has a lot of info about the way Renishaw tackles the spark issue - www.cnczone.com/forums/tormach-personal-...25794-tormach-7.html

There are 2 circuits posted for inverting and non-inverting outputs which I think would work with the TP20 Probe, however this seams to use a 5v supply and the mesa inputs are 24v field power.

This is the non-inverting circuit diagram from the CNCZone post.

Of course my post was written pre-coffee. I meant increase in resistance as the probe triggers...

Duh.

Thanks for throwing up the diagram for future reference.

-R

So I have redrawn the circuit so I can make some PCBs for this, but I am having trouble with the output voltage signal. I am using the inverting signal version of the circuit to be able to trigger the opto during a probe, otherwise the LED in the opto would be always on.

I am running my Mesa card at 24v field power, but this circuit operates on 5v so I thought about using an optocoupler to close the signal on the Mesa card for the PROBE IN pin. So I would power this from a 5v pin on the Mesa, but the actual triggering circuit would be separate.

Would something like this work?

I think I could use the 5v Step/Dir for the supply and Enc Index in for the signal, as explained here - forum.linuxcnc.org/27-driver-boards/3668...stion?start=0#135209

Would then not need the optocoupler and simplify things and speed it up.