Five Magnet Installation Pitfalls: Coaxiality, Gap, Loosening and Magnetic Interference Taken Apart

The position magnet is the only moving part of a magnetostrictive displacement sensor, and the site of the most concentrated field problems. Typical symptoms of poor magnet installation are: jumping readings on a local stretch of stroke, repeatability failing to reach the stated ±0.002 mm, and an overall shift in the reading after a period of running. This article gathers five classes of frequent installation defect — coaxiality out of tolerance, improper radial clearance, reversed magnet polarity/orientation, loose fasteners, and nearby ferromagnetic parts and magnetic interference — and gives checks that can be executed on site. For the installation overview see Installing Magnetostrictive Displacement Sensors in Practice; for selecting the magnet itself see How to Choose the Permanent Magnet (Position Magnet).

Pitfall one: magnet and rod not coaxial

Magnetostrictive measurement depends on the orthogonal superposition of the annular field produced by the magnet and the pulsed-current field in the waveguide. When the magnet axis is at an angle to the rod axis, or is eccentric, the region of magnetic action is stretched and the peak is weakened; the leading edge of the echo waveform becomes slower, the timestamp taken by the detection circuit jitters, and the macroscopic symptom is an unstable reading on a particular stretch of stroke.

Site judgement: move the magnet slowly by hand over the full stroke and watch whether the reading shows clear jitter or a step in a particular section. If the jitter location is fixed, coaxiality or clearance in that section is essentially out of tolerance, rather than an electrical problem. The remedy is to slacken the magnet bracket, realign on the rod as the datum and then tighten — not to adjust controller parameters.

Pitfall two: radial clearance too small, even rubbing the rod

A non-contact clearance must be left between the magnet bore and the rod. If the clearance is too small, mechanical run-out in operation will cause periodic friction between the magnet bore and the rod outer wall; at light severity this leaves wear marks on the rod surface and damages protection, at heavy severity it displaces or even jams the magnet. It must be emphasised: the premise of magnetostriction being maintenance-free and long-lived is precisely that the magnet and the rod do not touch; once the rod is rubbed, that premise no longer holds.

Clearance should not be too large either. Field strength falls with distance; an oversized clearance lowers echo amplitude and with it the noise-immunity margin, so jumps are more easily triggered in a strong electromagnetic environment. Actual values should follow the installation drawing of the magnet type chosen, and after assembly a feeler gauge should be used at several points around the circumference to confirm that the clearance is even.

Pitfall three: magnet reversed or the wrong type used

Common ring magnets have an axial magnetisation direction, and the mounting face is clearly marked. A reversed direction does not necessarily mean no output at all; it may appear as a weaker echo and a greater tendency to lose signal at the two ends of the stroke, which is harder to troubleshoot than a complete failure. Another class of error is substituting an ordinary ferrite washer or a home-made magnet for the OEM magnet — remanence and temperature coefficient both mismatch; it may work after a fashion at room temperature, then remanence falls as temperature rises and the reading starts to drift. For differences of remanence, temperature coefficient and mounting form see How to Choose the Permanent Magnet (Position Magnet).

Pitfall four: fastening and anti-loosening not done

A loose magnet bracket is the number-one cause of "normal at installation, offset after a few weeks of running". Reversing shock and vibration on hydraulic equipment will gradually back off ordinary threaded joints; the magnet then displaces by millimetres relative to the piston rod, and what the controller reads is a slowly changing zero error. Vibration duty needs particular attention: Series 13 mobile hydraulics has 25 g vibration and 100 g shock resistance in the body, but anti-loosening of the external magnet bracket still has to be guaranteed by the installer.

Anti-loosening should use mechanical means such as spring washers, thread-locking compound or lock washers, with preload applied to the torque marked by the fastener manufacturer. Do not "tighten by feel until it will go no further"; over-preload will crack a non-metallic magnet carrier. For the complete practice on vibration applications see Installing in Vibrating Environments.

Pitfall five: nearby ferromagnetic parts and external magnetic interference

Large ferromagnetic materials near the magnet (steel pressure plates, flanges, welded brackets) will distort the magnet's field distribution, equivalent to worse coaxiality. More hidden is an electromagnet, high-power motor, inverter power cable or permanent-magnet chuck nearby, whose alternating field after superposition may cause low-frequency swing of the reading. The judgement method is to stop the machine, de-energise external magnetic sources and move the magnet by hand alone; if the reading recovers stability, the problem is from the external field rather than the installation itself. For the related mechanism see EMC: Why Strong Magnetic Fields Disturb Measurement.

Symptoms and remedies of the five classes of defect

The minimum verification after installation

Do not go straight into automatic running after the magnet is fitted. First complete one full-stroke round trip in manual mode, dwell at 10%, 50% and 90% of measuring range and record the readings; the difference between forward and return readings should be stable at the stated order of repeatability (around ±0.002 mm, which will degrade under site vibration). This step can also expose bore concentricity problems — machining error of a cylinder-integrated hole likewise presents as "magnet eccentricity"; see Cylinder Bore Machining Requirements. Only after verification passes should zero calibration be executed; do not reverse the order. For the steps see Zero Calibration Step by Step. If there is still no reading at this point, go to No Output in Three Steps.

Product matching: cylinder-integrated types such as Series 16 cylinder-integrated and Series 17 hydraulic cylinder integrated usually have the magnet in the piston moving with it; external types such as Series 18 external rely on an external carriage to drive the magnet, and the latter's bracket stiffness requirement is higher.