Renishaw Probe

[popspipes]

I have too much free time on my hands again, I was wondering if someone has an input circuit from the probe to the machine to reduce the sensing current required? The measured current on mine is approximately 5 ma.
I dont use the interface, and I am thinking that if I could get the sensing current lower it would be a good thing?? I think KStrauss had a circuit for it to reduce it to about .05 ma but I havent located the post for it and I dont think I saw the schematic either. If anyone has something similar I would be very interested.

thanks for any input

[kstrauss]

For now I simply feed the contacts power via a resistor and monitor the voltage across the contacts using an analogue input on an Arduino clone. I'd have to take my probe apart to determine the resistor's value but ISR that it is perhaps 100K.

The attached patent shows in figure 3 what Renishaw does.

[popspipes]

Thank you sir, I didnt think about a resistor in series, that would work as well and be much simpler.
It works fine as it is but I could experiment with the resistor value and drop the current more as you stated.....

tnx

[popspipes]

Originally Posted by brianbonedoc
My boot was pretty much in tatters when I received my probe - so I just removed it all. Doesn't affect the probe as far as I can tell.

Pops - as far as backlash - you know PathPilot can do that automatically. Just go edit the tormach_1100-3.ini file in the tormach_mill directory.

Just add the BACKLASH line like I have below with your actual backlash. Do for ea axis. Axis 0 = X, Axis 1=y, and 2 = Z

---------------
[AXIS_0]
TYPE = LINEAR
HOME = 0.000

# Default=110 in/min 1.833/30.0 - just upped to 5/22
MAX_VELOCITY = 5
MAX_ACCELERATION = 22

# 20 % higher max-vel and 50% higher maxaccel for backlash comp 7.2/26
STEPGEN_MAX_VEL = 6
STEPGEN_MAXACCEL = 26

BACKLASH = 0.0008



The backlash figures can be entered into PP 2.02, set it up this evening, They do have to be entered into the same area in the .ini file though, I had it entered a few lines above and it wouldnt work.

Getting to the desktop does take a bit of doing though...........

- - - Updated - - -

Originally Posted by brianbonedoc
My boot was pretty much in tatters when I received my probe - so I just removed it all. Doesn't affect the probe as far as I can tell.

Pops - as far as backlash - you know PathPilot can do that automatically. Just go edit the tormach_1100-3.ini file in the tormach_mill directory.

Just add the BACKLASH line like I have below with your actual backlash. Do for ea axis. Axis 0 = X, Axis 1=y, and 2 = Z

---------------
[AXIS_0]
TYPE = LINEAR
HOME = 0.000

# Default=110 in/min 1.833/30.0 - just upped to 5/22
MAX_VELOCITY = 5
MAX_ACCELERATION = 22

# 20 % higher max-vel and 50% higher maxaccel for backlash comp 7.2/26
STEPGEN_MAX_VEL = 6
STEPGEN_MAXACCEL = 26

BACKLASH = 0.0008



The backlash figures can be entered into PP 2.02, set it up this evening, They do have to be entered into the same area in the .ini file though, I had it entered a few lines above and it wouldnt work.

Getting to the desktop does take a bit of doing though...........

[Zetopan]

Renishaw has many different probes and multiple methods of sensing any physical contact with their probes. Which probe do you have?

Also be aware that the Renishaw probes that use mechanical switch contacts internally do far more that what that specific patent states; that is only one of *many* Renishaw patents. My Renishaw probes actually detect minute resistance changes in the contacts and totally remove the contact current before the contacts fully open, preventing any arcing. In addition to that there is an oil gel on the contacts to prevent oxidation.

There are many ways to reduce the current down to at least nanoamp or even lower levels. Knowing which probe you have could assist with a simple design, powered by the DIN jack on the Tormach electrical cabinet (it obviously does not require using a microcontroller).

[kstrauss]

I suspect that most (all?) of us have a Renishaw MP3 as the basic "guts"; the MP7 that I purchased uses a slight variation of the MP3 together with additional electronics.

Obviously a microcontroller is not required since none of the older Renishaw units include a microcontroller. However, a microcontroller is often a less expensive/ simpler/ cheaper/ better way to provide the desired functionality. As an example, my homebrew IR attached MP3 uses a micro in the probe to measure contact resistance and to generate the IR pulses. Another micro connected to the DIN receptacle decodes the IR pulses, handles signal loss, provides a visual indication of probe status and IR signal strength on a tiny OLED display, etc.

In any case I am interested in your design.

[Zetopan]

I suspect that most (all?) of us have a Renishaw MP3 as the basic "guts"; the MP7 that I purchased uses a slight variation of the MP3 together with additional electronics.

The three local Renishaw probes that I am aware of around this area are all TP1S. They can be used without an interface box but both the sensitivity and life are vastly improved by using an appropriate interface box (these interfaces are available for $75 to about $250 used). The sensitivity and repeatability are both well under 0.1 mils.

In any case I am interested in your design.

Since the goal for preserving the internal contacts of this type of kinematic probe is to have a low current and low voltage it is necessary to use a high impedance sensor. The contacts will arc no matter how low the voltage so Renishaw goes farther by turning the current off before the contacts open. The probe then trial reestablishes the current after a time interval and if the contacts are still open the current turns off again.

I have included a simple reference design that only puts maximums of about 20mV and 7uA on and through the touch probe contacts. The resistor values can easily be varied to change this. The design also features hysteresis to ensure swift switching without oscillation. In contrast, the Tormach direct connection places 5V and several mA (resistance values not shown in their documentation) on and through the touch probe contacts. I have also attached a snapshot of the Tormach DIN connector pinout. This design uses a low cost CMOS opamp as a Schmitt trigger level detector driving an NPN transistor to drive the Tormach DIN input LED and a circuit description theory of operation follows.

R2 defines a 5uA current reference for R3 and the latter resistor provides for a 10mV reference voltage for the inverting input of opamp U1. When the touch switch is closed the non-inverting input of opamp U1 is at ground, hence the output voltage of the opamp will be at ground since the inverting input is at a greater positive voltage than the non-inverting input.

When the touch switch opens R4 defines a 5uA current reference for R5 and the latter resistor provides for a 15mV reference for the non-inverting input of opamp U1. This is 5mV greater than the inverting input, but since the input offset voltage specs of this opamp over the industrial temperature range is +/-3 mV the actual sensed voltage difference between the two opamp inputs could be as small as 2mV or as large as 8mV. Since the minimum open loop gain at DC is 95dB (about 56,234) the opamp output voltage will easily switch from 0V to +5V. When this switching occurs the positive feedback supplied by R6 will force the non-inverting input to 20mV. Resistor R1 then drives about 1mA of base current for Q1, which has a high Beta (>100X current gain) to drive the Tormach DIN connector optoisolator input.

Please note that this design assumes that a touch probe contact turns on the Tormach optoisolator. The opposite polarity (non-inverting, relative to connecting the probe directly to the Tormach DIN input) requires a slightly different circuit. If anyone wants to see that version I can easily generate that version as well.

[MFchief]

I'd be very interested in seeing the other circuit whenever it would be convenient for you.

Thanks
Terry

[popspipes]

I took the easier alternative, Ebay for 115 dollars, brand new MI-8.

At my age I will get some use out of it, and the brain power isnt all that good anymore either........... when it comes to reinventing the wheel.



The main problem I have now is the Backlash, entered in the axes, the probe seems to be right on. The problem is an artifact that it leaves when the transition from X to Y occurs that I have noticed, the parts cut without the backlash entries do not have it.

I think my solution will be to delete the backlash entries in X and Y, then probe the part, then add back in half the backlash (+ for both) for the X and Y axes, hit zero for X and Y and it should be right on probe wise. Then it will cut without the artifact........ who knows maybe I have that screwed up too ha!!

On the other hand, if it werent for the problems and changes the Tormach wouldnt be quite as much fun..................

[Zetopan]

Here is the non-inverting version. In this case the reference threshold voltage at the opamp inverting input is 20mV (also the maximum voltage across the touch probe switch contacts) and the maximum current through the touch probe switch contacts is under 7uA. R2 defines the maximum current and R3 defines the reference voltage. Since the difference in resistance values is so large the opposite resistor can be largely ignored when defining the current and the voltage.

This reduces the voltage across the touch probe contacts by a factor of about 100X (assuming about 3V across the Tormach LEDs) which will reduce the arcing distance by that same factor, and the current through the contacts by a factor of at least 250X (assumes at least 5mA through the Tormach LEDs), both relative to a kinematic touch probe direct connection to the Tormach DIN input. The switching power reduction in the arc is obviously reduced by a factor of at least 25,000.

On the opamp non-inverting input R4 defines the current (5uA) and R5 defines the voltage (9mV when the opamp output is at 0V). Positive feedback resistor R6 provides for 3mV of Schmitt trigger hysteresis when the opamp output is at 5V, making the upper switching threshold 12mV. As you can see, the average opamp non-inverting input voltage is about midway between the opamp inverting reference voltage and ground.

When the touch probe switch is closed the opamp non-inverting input voltage is greater than the opamp inverting input voltage by 12mV and when the touch probe switch is open the opamp non-inverting input voltage is less than the opamp inverting input by 11mV. The Schmitt trigger positive switching point is at 9mV +/- 3mV and the negative switching point is at 12mV +/-3mV over the industrial temperature range. Those represent the opamp inverting input voltage levels where the Schmitt trigger action will take place. Since the opamp inverting input switches between 0mV and 20mV, there is plenty of input signal for the Schmitt trigger to always operate reliably.

The total parts cost for this design should be about $5 or less and some electronic assembly expertise is assumed. If a sufficiently high number of people want something like this it could be worth fabricating an EC board, although the cost would also increase.

Even much lower threshold voltages and contact currents are possible but the cost would increase by a lot since higher precision opamps would be required. Instead of using opamps with input offsets of +/-3mV, there are opamps available with offsets in the microvolt range. Considerable shielding and a clean electrical construction would be required for this or to get into the nA or less touch probe switching current range.

[MFchief]

Thank you for the circuit, I think this would be worth trying out.

Terry

[Zetopan]

Thank you for the circuit, I think this would be worth trying out.

Let me know if you encounter any problems with this design (I do not expect any, this is all very simple). The opamp is static sensitive so be careful when handling it. I would recommend using a piece of vector board with plated through holes. If you don't have any and cannot locate any just ping me and I can mail a small piece to you. If you cannot sneak this circuit into your touch probe (some touch probes are huge) the circuit should be placed in a small aluminum box which is grounded.

[jwatte]

For what it's worth, I find that it's easier to build a small PCB using cheap tools like Eagle or free tools like KiCad, rather than use protoboards.
OSH Park will make a very nice PCB and mail it to you for $5 per square inch in about 14 days. While I'm no electrical engineer, having an actual PCB for the actual circuit eliminates many problems, and lets me put mounting holes where I need them, have cut-outs in the outline of the board, and so forth.

[MFchief]

I'll certainly let you know if I run into difficulty. It might be a while before I try it, just had cataract surgery yesterday, with the other eye coming up in a couple of weeks.

Terry

[popspipes]

I received my renishaw MI-8 interface yesterday, installed it, and the results I have noticed so far are that when probing a hole, it comes up in the X direction with all Zeros 90% of the time, before it would read usually -.0001 or + .0001, all Zeros were rare. So making an assumption with very little data to back it up, it seems to be more accurate........

So far I am well pleased with it and the install was fairly painless even for a guy in my age bracket!!

This is used with a MP 3 probe with the leads connected for manual operation.

[popspipes]

I'll certainly let you know if I run into difficulty. It might be a while before I try it, just had cataract surgery yesterday, with the other eye coming up in a couple of weeks.

Terry

The difference will be amazing Terry, I had both eyes done a few years back and it made a big difference in clarity and it made everything bright white again.

[brianbonedoc]

I received my renishaw MI-8 interface yesterday, installed it, and the results I have noticed so far are that when probing a hole, it comes up in the X direction with all Zeros 90% of the time, before it would read usually -.0001 or + .0001, all Zeros were rare. So making an assumption with very little data to back it up, it seems to be more accurate........

So far I am well pleased with it and the install was fairly painless even for a guy in my age bracket!!

This is used with a MP 3 probe with the leads connected for manual operation.

Pops - I have my mi-8 arriving today tomorrow. Can you give a quick rundown on the connections as I have the MP3 as well - saves me from having to think!

Thx!

[kstrauss]

Agreed. Cataract surgery is a great tuneup for us old folks!

[MFchief]

Having just had only the first eye done, I'm already pretty happy! If nothing else the increase in brightness is amazing, I actually had to turn down the brightness on the monitor I'm typing on. LoL

Terry

[popspipes]

Pops - I have my mi-8 arriving today tomorrow. Can you give a quick rundown on the connections as I have the MP3 as well - saves me from having to think!

Thx!

I will go look.
The input supply I fed off the 12 volt lead that goes to the probe connector socket. A5 Red lead from pin 3
12 Volt zero volts isnt a machine ground, this needs to be connected to the Green lead A6 Green lead from pin 4

Probe inputs A1and A2 (Pin 4 and 5 off the DIN socket on the machine) where the probe leads connect, (where blue and green connected)

Outputs are B6 and B7 (Green and Blue)

Hopefully this is clearer than mud!!

I just clipped the White 5 volt lead, taped it as it isnt used.

I didnt use sheilded cables as they call out in the instructions.

Velcro'ed to the cabinet door.

I am going to change out those bogus looking connectors to RC 3 lead connectors, I think they would look nicer.................and be a more appropriate size ha!!