Help - my magnetic scale DRO is not reading correctly!
You may go to work one day and find that your magnetic DRO scale is no longer reading correctly. This
document examines what may be causing that to happen, why, and how to fix it.
In general, there are three different scale failures. They are:
Flickering
. The last 2 digits on the display window are flickering. As the machine is moved, the
leading digits change and appear to be correct, but no matter where the machine is moved, the
last two trailing digits flicker. Cause: Improper readhead/scale orientation. Go to Step 1.
Digits Frozen / Not Moving
. All digits on the display window are "frozen" and do not change
value regardless of whether the machine is moved or not. Cause: Malfunctioning display or
readhead. Go to Step 2.
Value Not Correct
. All digits on the display appear to work fine, but the value shown in the
display window does not match the actual travel. Cause: Improper readhead gap or scale not
level. Go to Step 3.
1. The hash marks on the readhead, and the hash marks on the scale need to be on the same 'side' for
the scale to work properly. If the last two digits are flickering, this is a classic sign of a mis-match of hash
marks between the readhead and the scale. Disconnect the readhead brackets and run the readhead
back and forth along the scale track by hand (much like a "Hot Wheels" car). Make sure to run the
readhead with the cable pointing in both directions (ie cable to the left, and then cable to the right) to see
which orientation is correct. Most likely, orienting the readhead in the opposite direction will instantly 'cure'
the issue. At times, this may present a problem as the cable may now be pointing in an inconvenient
direction (like towards the front of the machine). The solution is to swap the scale end for end, and mount
the readhead in the same orientation as it originally was mounted.
2. There are three items that work together for a scale to 'work' properly. They are the scale, the
readhead, and the display. The first step is to identify which piece is malfunctioning. Let's assume the
digits on the X axis display window are "frozen". The first step is to turn off the display, and then swap the
X and Y leads on the back of the display. Turn the display back on, and let it boot up. Now move your
machine and see if the issue remains on the X axis window, or has 'moved' to the Y axis window. If the X
axis window digits remain frozen, the issue is with the display. If the Y axis window digits are frozen, then
the issue is with the scale and / or readhead. Continue to Step 4.
3. The first question to ask is this - "How do you know for
certain
the scale is misreading?" Technically,
the only proper methodology to validate scale movement is with a laser. This statement bears repeating:
The only proper methodology to validate scale movement is with a laser
We do realize that most of us do not have the resources to buy a laser interferometer. As such, on the
DRO PROS Library page, we've published a guide for "Validating DRO movement" which, while not an
'approved' method for validating movement, may prove extremely useful. This article highlights several
very important variables to isolate
before
you conclude your DRO is mis-reading.
The next question to ask is "Has anyone ever attempted calibrating the display using either Linear Error
Compensation (LEC) or Non-Linear Error Compensation (NLEC)? If so, or if you're unsure, go to the DRO
PROS library page, download / print the appropriate "LEC and SLEC Error Removal" procedure for your
display. Run the procedure and re-check your results.
If you're still certain something is amiss, proceed to Step 4.
