Probe Interface Board Design (resistance switching)

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21 Jan 2024 18:32 - 22 Jan 2024 11:15 #291267 by fletch
Today's tech never ceases to amaze me. 24 hours ago development of the board stalled because my probe broke yet now I have a 3D printed one! Shame the chrome rods are not the best conductors but nothing a quick spin on the lathe tomorrow back to the brass won't solve (tungsten rods on order though).

 

Thanks again Andy - I'll be definitely have a go at designing/making a more precise one - I like the idea of being able to repair/replace it when (not if) I crash it.

Edit: Now I've removed the chrome this now shows similar electrical characteristics as the Renishaw TP6.
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Last edit: 22 Jan 2024 11:15 by fletch. Reason: It works! Yes, you heard it - IT WORKS!
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21 Jan 2024 22:46 #291289 by cakeslob
the peizo stuff im pretty sure, was the counter measure to the 3 lobe issue. No idea how to make the newer TP20 probes to work with it though.

Try playing with the tension thing, and back it off all the way. when i was playing with mine, i thought the tension rod had something to do with my issue, but it turned out to not work well when just using a volt meter, i needed to use 24v

The tp6 is bigger than the tp2, so it might be easier to disassemble, but for the tp2 it looks like i need to make some custom tools. i couldnt get it apart easy at all, so i stopped trying because i didnt want to break it. 
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22 Jan 2024 11:12 - 22 Jan 2024 18:30 #291334 by fletch
Thanks Cake - now I want to know more about how the piezo compensates for the 3 lobes but I suspect I'll be disappointed.

The TP6 looks like the top cap should unscrew from the main body and/but there are three 'pin holes' that could either be for a specialised pin spanner or need punching out before disassembly. I have called Renishaw but while hopeful, I don't expect an answer soon. The tension screw on mine has never worked - fully out it would stop and there was no fully in. The pressure required to move the stylus never changed so I suspect there's a bit on the loose inside which is now stopping the contacts making.

Up until it broke, the contact resistance was a bit variable (probably because of the lack of spring tension) but operated fine at 5v with the 'check if still open loop' able to detect closed at 0.5v. Now I've removed the chrome, my '3D printed special' shows similar characteristics.

Philip
Last edit: 22 Jan 2024 18:30 by fletch.

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22 Jan 2024 19:10 #291358 by fletch
The Reishaw TP6 probe is fixed! Hopefully. I modelled a couple of 'tools' to use with 5 x  1.5mm drill bits to use as pin spanners and then bravely took it apart.



Apart from an interference fit that caused a fair bit of head scratching, the process went smoothly. Some of the smallest springs I have ever seen but the culprit was the central spring knocked sideways - a complete pig to reassemble (three times) but it is done. Electrical resistance is quite high, so I suspect I'll have to have it apart again to do a proper clean up.

If anyone is interested in seeing the internals of a Renishaw, then I have a couple of photos, otherwise normal service on the interface board will be resumed.

 
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22 Jan 2024 23:52 #291377 by vre
I have a renishaw omp60 but i don't have the wireless ir-receiver so it is useless.
Knows soneone the comunication ptotocol to impement a receiver or
if it is posible mod it and use it with wired connection?

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23 Jan 2024 08:19 #291404 by dm17ry

Up until it broke, the contact resistance was a bit variable (probably because of the lack of spring tension) but operated fine at 5v with the 'check if still open loop' able to detect closed at 0.5v. Now I've removed the chrome, my '3D printed special' shows similar characteristics.

i played with a few renishaw probes a while back - namely TP6, TP1s. yes, the resistance can be quite high and variable. but PI4 and MI8 interface units i had could reliably detected it. AFAIR, they supply 10 VDC at 0.5 mA to the probe. resistance less than about 3 kOhm considered as a seated probe.
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23 Jan 2024 08:51 #291410 by vre
The probe must triggered not by resistance absolute value but by increase in resistance.
So must have mcu with a voltage comparator?

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23 Jan 2024 10:03 #291414 by fletch

I have a renishaw omp60 but i don't have the wireless ir-receiver so it is useless.
Knows soneone the comunication ptotocol to impement a receiver or
if it is posible mod it and use it with wired connection?
 

That is a very nice probe. There's a lot of information on the Renishaw site here but predicatably no information at all the protocol. Reading the IR outputs from the probe would be fairly(!) trivial with an IR reader/oscilloscope/Arduino but without the OMM receiver,  the IR inputs to the probe would be challenging to say the least.

I suspect it would not be prudent to convert the probe to wired - there's a whole lot of electronics packed inside!

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23 Jan 2024 10:14 #291415 by fletch

i played with a few renishaw probes a while back - namely TP6, TP1s. yes, the resistance can be quite high and variable. but PI4 and MI8 interface units i had could reliably detected it. AFAIR, they supply 10 VDC at 0.5 mA to the probe. resistance less than about 3 kOhm considered as a seated probe.

 


Thank you Dmitry, that is very useful information. It validates my decision to implement a Wheatstone bridge to be able to 'tune out' the probe resistance but only up to 2K atm!

I'm using 5VDC with 10K resistance (2x 20K in parallel in the bridge) which fortuitously turns out to be 0.5mA! Happy camper.

Philip

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23 Jan 2024 10:20 #291416 by dm17ry
i've found some old notes when was playing with an MP700 optical interface:

The probe turns on after receiving 500Hz for 1/4 sec.. it has internal a switch for selecting auto power of or power of with the same signal that turn it on.

Once turned on, the probe sends out a 19.2K baud byte every 16 ms, and when there is a change of state of the probe tip. When not triggered, the byte is :
01010100 (with 1 meaning LED lit)
on the first transmission after the probe is deflected it sends:
11010100
after the first transmission of a trigger, it sends
10010100 as long as the probe is still deflected
The last bit goes to a 1 when the battery is low.
The start bit of the data is not shown above, it is with the
LED lit.

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