Hi Guys,
@pontiac428
You can send me any magnetic DRO you want to get rid of! They are fine.
All of the sensor technologies have limitations. I worked in the field of sensors for years and even longer in the field of magnetism and magnetic recording. The optical sensors (DRO) do not do well in dirty environments. Neither do the capacitive. Magnetic sensor systems are far more tolerant. I have a magnetic DRO on my lathe. I have never gotten around to putting a DRO on my Mill as it is CNC. On big motors moving large paper webs (news print) where the position must be measured accuratly so that the speed can be carefully controlled (do not break the web) they got rid of the optical sensors as they fail from the paper dust. They were replaced with magnetic sensors (magneto-resistive).
I have never tried this experiment of waving a magnet around my DRO sensor. Of course it could have bad effects if the sensor and stripe are not built correctly or well.
Just as you could scratch the optical scale and it would start to fail, you could demagnetize the magnetic stripe, but is is pretty strong, high coercivity (resistance to change in the magnetic state), and you would essentially have to bring a magnet into contact with the stripe to mess it up (you would have to replace the stripe, which is not hard to to if you can get it). The stripe is usually made from a polymer containing ferrite particles with magnetized patterns written into in. (A digital recording pattern.) It should have a coercivity of several hundred oersted or larger. For reference, the earth's magnetic field is about 1-2 Oe. The magnetic stripe on a credit card can be as much as a couple of thousand Oe, but is usually about 700-800. Most better digital tape (video/audio recording tape) is about 800 Oe. MP, metal particle, tapes have even higher coercivity.
The magnetic sensors themselves are not very subject to EMI, but the cables to the readout might feed a time sensitive noise to the readout electronics. If EMI at the cables is the problem it probably does not matter which type of DRO you have. Ideally the sensor head has enough electronics in it to convert the sensor signal into a digital signal before sending it on to the readout electronics ... so has to minimize EMI/noise effects. There is lots of EMI coming from the motor etc of a lathe, especially if you have a VFD. There is a huge amount from a motor with brushes.
The DRO magnetic sensors, Hall Effect or Magnetoresistive (there are several kinds of MR sensors) measure the DC and slowly varying AC magnetic fields from the pattern that is on the stripe. Since the lathe motion is so slow, this is essentially DC. Waving a magnet around would indicate to me that you are changing the DC field slowly (quasi-static field) at the sensor and this could produce a change in the sensor output that is interpreted as steps along the magnetic stripe. One would not normally do this during operation. Your magnetic base is designed to clamp to the steel somewhere (lathe bed). As you rotate the knob or lever of the base the magnetic is turned so that the poles cause the internal magnetic field to point through the base surface and into the steel. The magnetic field (magnetic flux) comes out of one surface of the base, through the steel lathe bed, and then returns into the other side of the base. This way the flux is essentially coming from the north pole and back to the south pole of the magnet that is inside the base. In this magnetic circuit there is virtually no external fields outside of the base and the lathe bed. When it does this the field/flux is contained in the lathe bed and so does not /should not be much of any where else. That is, there are virtually no magnetic fields outside of the device and the steel it is clamped to. Hence, the sensor and DRO should work perfectly fine as it would see no field from the magnetic base.
The magnetic sensor "housing" should be made from a soft magnetic material to deflect/re-route any external fields away from the sensor itself, but by necessity the sensor must be able to sense the stripe. Ideally the back of the stripe would be of soft magnetic material to also prevent any external field from reaching the sensor. In both cases the soft magnetic material would then essentially short out most of any external field. Even simple steels tend to be pretty good soft magnetic materials for this purpuse. Someone mentioned mu-metal which is an extremely soft (near zero coercivity) magnetic material. This would be good to shield the sensor head as well as to put on the back of the magnetic stripe spaced a short distance away from the magnetic particle materials. (think a layered structure: magnetic particles/non-magnetic materials/soft magnetic base)
Some stainless steels are not magnetic, but some are. Aluminum, brass, copper and many other metals are not magnetic and so would NOT work as shields from external fields.
I had a project a couple of years ago where I needed to observe the magnetization patterns of a material changing with applied field. These are small enough that I needed to do the measurements via a special microscope (polarized light is changed a little bit when it reflects from a magnetic material surface --- Kerr effect). However, the applied field was time dependent and the steel parts of the microscope would be pulled as the field varied. Hence, parts of the microscope wiggled with the varying magnetic field screwing up the optical signal. I had to take the microscope and all of the associated mechanical stages apart and replace any parts made from ferromagnetic materials with parts made from non-magnetic materials. In many cases I used brass or non-magnetic SS for the bolts, nuts, rods, etc. However, in one of the objective housings there were bearings that ran on steel runners. I replace the balls with ceramic balls and the runners (~0.25 wide by 2.5" diameter washer shapes) which were very thin ~0.015" steel with Titanium. It is always hard to clamp and cut sheet material, but it is really hard to machine Ti this thin!
So either shield you sensor head or stop waving magnets around! If you are going to clamp a magnetic base near your magnetic DRO I suggest that you reset your origin/reference after you clamp the magnetic base down. After all the magnetic base should have no effect on the DRO once it is stationary.
On the other hand, just send me the DRO's you are taking off line!
Good luck.
Dave L.
