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Tools – inductive probes

DTI’s usually resolve 0.01mm or some can resolve 0.001mm. This is the limit. A device that can detect even smaller distances is the inductive probe.

These depend upon electronics to work. The probe is rather like a DTI but without the display. In a way that mirrors DTI’s probes can be plunger types or lever types

fig plunger type probe

4323 lever type inductive probe

4323 lever type inductive probe

fig lever type probe

The probe is connected via a cable to a box that contains a display. On older systems the display is a meter. Often the meter would contain two pointers to set maximum and minimum values. On this particular display these are set using the screwdriver adjustable pots above the meter.

4324 inductive probe display

4324 inductive probe display

fig the display


Details of the probe

The probe consists of a very small transformer which has three windings. It is arranged so that a high frequency sine wave is fed into one winding. This voltage can be picked up on either of the other two. The key to its working is that the core of the transformer can move between these windings so that the signal induced in second and third windings depends upon the position of the core. The core is connected to the tip of the probe. These are known as linear variable differential transformers – LVDT.


fig the circuit inside the probe

Like dti’s inductive sensors can be similar to the plunger type dti or the lever type dti.

4326 inside of lever type probe

4326 inside of lever type probe

fig the inside of a lever type probe

The transformer is wound on a plastic bobbin and is covered with some black material. The side of this bobbin can be seen in the middle of the probe.


The output of the inductive type of probe is simply a cable. This has to be plugged into a box containing a display. On older types this display uses a meter.

The inductive probe is different from a dti in several very different ways.

1   The smallest distance that can be resolved is much less down to 100nm or less.

2 The display always goes from minus to plus with zero in the middle as can be seen in the photo above.

3 At the same time  it will have set of ranges. But the movement of the core in the probe is very limited.

4 It will have two inputs so that two probes, A and B, can be fitted together. If one probe is fitted it will work just like a DTI But if two probes are fitted then it is possible to display other permutations.

There are three ways this can be used. This can be seen from the diagram on the back of the display. There are two sockets for probe A and for probe B. These signals can be used directly or inverted using the switches above the connectors. This gives the three ways they can be used

4327 detail of display

4327 detail of display

fig back of the display box

There are two scales  going 3-0-3 and 10-0-10 either can be used either for metric or imperial.The full scale, metric, readings are:

+ – 3micrometers     one division on the scale is only 0.1 micrometers

+- 10 micrometers ie +_ 0.001mm

+-30 micrometers

+-100 micrometers ie +- 0.1mm

+-300 micrometers

+-1000 micrometers ie +-1mm

Notice that though there are six scales the maximum range is +- 1.0mm

Stands for inductive probes

Since inductive probes would generally be used because of their high sensitivity it is essential that the stand being used to hold the probe is sufficiently rigid.






  1. Great article. Do you have any information on the pin inputs for the Metem 520 that you might like to share. There is so little information available online for probe and meter wiring.

    • johnf said:

      The pins on the metem 520 are
      1, 3 and 5 these are separate for a and b probe
      then the center pin on a and b are joined as is pin4.
      I also have one of these made by pretec in Switzerland
      On this pins 1,3 and 5 are separate and go to either probe and pin 4 is earth.
      I don’t have a power cable here to try these out. I’ll try and get one and sent you result.

      john f

  2. Thanks very much John. I will wind up a small test inductor to check my unit using those pins. Next month when I have a little more time I am going to do a reverse engineer of my unit and create some schematics and service data on calibrating it. I will pass them on to you when done. Am hoping to interface a Mitutoyo 519-521 lever probe to mine. Yours seems to work ok with a single probe. What is the make of leaver probe in your picture? (Sorry about the double post in the model engineering section. Don’t know what went wrong there).

    Cheers and thanks you are a legend.
    Pete from down under.

    • johnf said:

      I think the photos are probes made by Mahr.

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