Renishaw touch probe MP1 with inductive coupling

Yep. Some time ago I acquired a proper Renishaw touch probe from ebay. It's one of the poncy inductive coupled versions. The guy wanted £265 but I got it for £150, pointing out that it's not much use by itself. A new one would cost significantly more than that of course. However, if you get one of these without the matching interface box, it won't be much use to you if you aren't prepared to bugger about with it. Of the 3 versions shown here, mine is the one on the right ("MP1/S" with side electrical connection).

What I got:

Here's what I got. There's a BT40 shank in the box too, which I'll convert to ISO40 by welding one of my M16 threaded collars onto it:

The blue face is the surface of the inductive pickup:

When you remove the arm containing that inductive pickup, you find a 5 pin DIN connector:

Pins 2 and 4 connect to the physical contacts, so I will make up a lead that plugs in there and brings the contact connections out to the controller probe inputs. A suitable connector only costs about £1.49 (plus £3 carriage and vat) from CPC, so it's a simple enough modification.

The black cylindrical sponge thing contains the sacrificial probe (ceramic arm with sapphire tip). Don't expect that to last long. For now, I've stuck a brass M4 screw in the hole for finger testing:

How it works:

It's not rocket science and of course, there's a patent that explains it all for you. There are several basic formats for patent applications and the one that Renishaw used (they were part of Rolls Royce at the time it seems) pretty much gives the actual schematic diagram of the internal circuit.

The circuit is pretty simple. There's a resonant circuit / oscillator that uses the capacitor that is mounted in the probe. The probe circuit is coupled by the transformer action of the inductive transducers. When the probe contacts are opened during probing (as a result of contact with the workpiece) the frequency of oscillation changes. So the detector circuit looks for a change in frequency. 

There are various ways of detecting if a frequency is above or below a given threshold but the way Renishaw did it was with a couple of monostables. From memory, this looks like the 74123 (dual retriggerable monostable multivibrator). Given the vintage of this thing (the datasheet is dated 1983), that seems about right.

Interfacing with the Centroid Acorn:

There's a section on touch probes on the Centroid website. Once enabled (by buying the upgrade license key), there is a range of handy probing functions, including digitising if you like that sort of thing.

The connection to the Acorn is fairly simple, although it uses up 2 of the precious inputs. This is for the DP-4 touch probe but in fact all of the various touch and tool setting probes seem to have the same connections. One is for the actual touch probe signal and the other "detect" line tells the controller that a probe is connected, to which point the spindle signal is disabled to prevent it turning with the probe in place(!):

So I'll need the obvious 2 wires for the probe contacts, plus a loop for the "detect" connection. Sounds simple enough......