Touch Probe Part 2

 

Sometimes you buy (or make) a tool or accessory and when you’re done you never figured out how you got along without it.  This is one of those things.

The touch probe is actually a pretty basic thing that gets wired into the computer and simply lets you know when electrical contact is made.  From there the computer does all the magic.

The touch probe tip is broken down into a couple of sections:

  • Basic assembly of the probe

  • Connecting the probe and setting up Mach 2 (this section)

  • Using the probe (sounds scary doesn’t it?)

  • Making a probe for only a few bucks

 

 

 

Theory of operation:

Not Touching:

Positive Voltage (lets assume +5 VDC) comes from your PC to the top of R1, then to the top of the LED.  Current will not flow because the sensor wire from the PC is pulled up from internal resistors.  The signal wire to the PC will read “HI” (+5V).

Touching:

When the probe tip touches the material, a circuit will be formed from the cathode (negative side) of the LED through the casting of the machine to the ground (black wire).  This will cause the LED to light.  Because in essence the probe tip is connected to ground, the signal to the PC will be pulled “LOW” (0 V).  R2 is simply there to limit the current from the PC and protect it from damaging the PC.

Selecting the proper resistors:

Use the table below to determine the proper resistance values for your application:

Voltage

R1 Value

R2 Value

5

200

430

12

480

1200

Now of course these are nominal values, wiggling them around a pinch will not make any difference
 
Setting Up Mach 2
Start with the Ports and Pins configuration screen, go to the the Input Signals Selection Page and select the Digitize checkbox.  Click Apply

 

Go to the Printer Port Input Pins page and assign the proper port and pin to the Digitize Point, and select the Low Active checkbox.  Click Apply

 

You are now all set.
 
Testing the Probe
Go to the Diagnostics page in Mach 2 and look for the Digitize LED in the upper right quadrant.  With the probe connected and just sitting there the LED should be gray (like all the others) but when you make some connection between the top and the bottom of the probe the LED should turn green.

 

The easiest way to make the test connection is to use an hex wrench place the short leg of the L on the tip and with the long end touch the body.  The LED should turn green when you make connection.

It's a good idea to test the probe before each use.