Series 18 External Explained: Matching Follower-Rod and Sliding-Carriage

Series 18 is Germanjet’s external magnetostrictive displacement sensor. It is mounted outside the machine and transfers the linear motion of the measured mechanism to the magnet ring through a follower-rod or sliding-carriage structure. It does not enter the cylinder, does not take pressure and does not require any change to the mechanical body, so it is the first choice for adding displacement sensing to existing plant. Measurement is still non-contact absolute position. The Series 18 analogue resolution is listed as infinite (depends on the controller A/D and supply ripple), repeatability < ±0.005% FS, non-linearity < ±0.02% FS (min. ±90 μm), update time 0.5 / 1.0 / 2.0 / 5.0 ms by stroke. The real selection difficulty is not the figures, but matching the mechanical follower structure — the great majority of external-type field problems originate there.

Structural differences between follower-rod and profile-mounted types

An external type needs a mechanical link that “passes motion to the magnet ring”. Two kinds are common:

Follower-rod type (rod follower). The magnet ring is fixed on a follower rod, one end of which is connected to the measured mechanism. The structure is simple and low-cost, and suits mechanisms with a clear motion path and a single stroke direction. The drawback is limited stiffness of the follower rod: on a long stroke it readily sags under its own weight or yaws under side load, causing magnet-ring eccentricity.

Profile-mounted type (sliding-carriage follower). The magnet ring is fitted on a sliding carriage that runs along a rail on the sensor body; the carriage is linked to the measured mechanism by a connecting piece. The carriage is constrained by the rail, so coaxiality of magnet ring and sensing rod is guaranteed by the product itself. Resistance to lateral offset is strong, suiting long strokes and slight misalignment. The cost is a slightly larger installation envelope and a slightly higher price.

Comparison	Follower-rod	Profile-mounted
Coaxiality guarantee	Depends on external installation accuracy	Guaranteed by the product’s own rail
Resistance to lateral offset	Weak; needs a coupling / floating joint to compensate	Strong
Long-stroke suitability	Fair; the follower rod tends to sag	Good
Installation space	Compact	Needs a continuous mounting face along the stroke
Cost	Lower	Higher
Typical application	Short to medium stroke, well-aligned mechanisms	Long stroke, mechanisms with assembly offset

The biggest pitfall of the external type: misalignment

The motion axis of the measured mechanism and the sensing-rod axis cannot be perfectly parallel; there is always some offset in practice. The consequences come in two grades:

Mild misalignment makes the magnet ring eccentric, so the magnetic working zone is asymmetric. Readings show a progressive deviation over certain sections of stroke — the appearance is very like non-linearity out of specification, but changing the sensor has no effect.

Severe misalignment puts a side load on the follower, which over long running wears the rail and fatigues the connecting pieces, ending in seizure or fracture.

The engineering remedy is a floating link at the connection: a ball joint, universal joint or flexible connector, so that the follower transmits only axial displacement, not side force or angular offset. This is a general principle of external mounting; practical methods are in External mounting: how to fix the bracket and sliding carriage so they stay stable.

Bracket stiffness and resonance

The external type introduces an extra mechanical chain: machine body → bracket → sensor. Insufficient bracket stiffness produces two kinds of problem:

• Static deflection: the bracket bends under self-weight or installation stress, so the sensing rod is not parallel to the motion path; deviation is most obvious at the far end of a long stroke;
• Dynamic resonance: the bracket natural frequency falls near a machine excitation frequency, and readings show regular jitter in operation. If readings are normal when stopped and jitter in operation, and the jitter frequency tracks machine speed, bracket resonance can essentially be diagnosed.

The treatment principle is “short, straight and stiff”: keep the bracket as short as possible, fix it directly to the rigid main structure of the machine, and avoid thin-plate cantilevers. Mounting points for vibrating environments are also in Installing in vibration: locking and vibration resistance.

Where Series 18 sits against other options

Comparison	Series 18 external	Series 13 mobile hydraulic	Series 16 / 17 cylinder-integrated
Mounting position	Outside the machine; follower-rod or sliding-carriage follower	In-cylinder / head-space integration; vibration-reinforced	Inside the cylinder
Environmental reinforcement	Standard	IP69K, 25 g vibration, 100 g shock	Protected by the cylinder body
Mechanical modification	No; only a bracket is added	No	Yes; piston-rod bore ≥12.7 mm
Pressure-containing	No	No	350 / 530 bar (Series 16)
Replacement downtime	Short	Short	Long; cylinder must be stripped and drained
Main risk points	Misalignment, bracket resonance	As left, but the structure is already reinforced	Bore machining, pressure-rating calculation
Typical use	Retrofit of existing plant, non-hydraulic linear mechanisms	Construction machinery, outdoor mobile equipment	Hydraulic cylinders on new-machine designs

Simple rule: if the cylinder cannot be touched, choose external. The full decision path for retrofit projects is in Selecting for retrofit of existing plant; comparison with cylinder-integrated options is in Series 16 explained.

Series 18 four-pin voltage wiring

The 180 analogue voltage type commonly uses a four-pin connector. Facing the catalogue pin assignment:

Pin	Function
P1	+24 V supply
P2	10-0 V output
P3	0-10 V output
P4	0 V

• The controller analogue-input negative (AI−) must connect only to sensor 0 V (P4), not to the machine internal earth;
• Swapping supply and signal wires will burn the sensor immediately and is outside warranty;
• Use four-core shielded twisted pair of at least 0.2 mm²; ground the shield on the controller side.

Series 19 analogue uses a different D60 six-pin assignment — see 191 analogue wiring in practice. Do not map Series 18 four-pin onto Series 19 six-pin.

Suitability beyond hydraulics

Series 18 is not limited to hydraulic equipment. Any linear mechanism that needs continuous position feedback can be covered by an external type: feed positioning on woodworking machines, sealing mechanisms on packaging machines, roller tension displacement on textile machines, positioning of laser-cutting tables, and so on. These applications are usually clean, with a medium stroke; the selection focus shifts from “pressure rating / protection” to “whether resolution and repeatability match the controller”. The criteria are in The three accuracy figures.

Choose the output interface from the downstream system: for a PLC with analog inputs, the matching configuration of the Series 191 analog; for multi-axis synchronisation or diagnostic information, use fieldbus — the framework is in Analog or fieldbus. See the Series 18 external product page. The 19P in Series 19 is also an external type; the trade-off is in Series 19P external explained. Horizontal selection across all external options is in Comparing the whole external range.