Linear Position Sensors for Hydraulic Cylinders: In-Cylinder vs External

Three Ways to Measure a Cylinder's Stroke

Every hydraulic cylinder position project reduces to one of three architectures:

1. In-cylinder (embedded) sensor — the measuring element lives inside the piston rod or the cylinder bore.
2. External draw wire sensor — a cable attached to the rod end, body mounted on the cylinder barrel.
3. External LVDT or linear probe — a contactless probe on a parallel measuring axis or a guided rod extension.

They are not interchangeable. Each solves a different combination of stroke, pressure, environment, retrofit feasibility and cost.

Option 1: In-Cylinder Magnetostrictive

A waveguide tube runs down the centre of the piston rod; a permanent magnet in the piston interacts with a torsional pulse that travels back to the electronics head. Time-of-flight gives absolute position.

Strengths

• Nothing protrudes — the machine envelope stays clean and the sensor is mechanically protected.
• Absolute output (SSI, analogue or bus), so position is known at power-up.
• High resolution and repeatability; unaffected by oil, pressure or contamination.
• Ideal for mobile hydraulics and cylinders that must survive impact and dirt.

Constraints

• The cylinder must be designed or modified to accept it — a drilled rod, a specific bore and a pressure-tight head. Retrofitting an existing cylinder is usually a machining job, not a bolt-on.
• Higher unit cost and longer lead time.
• Rod drilling reduces rod strength; check buckling margins on long strokes.

Choose this when the machine is being designed now, or when the cylinder is being rebuilt anyway.

Option 2: External Draw Wire Sensor

A stainless cable from the sensor drum attaches to the rod end (or to the moving load). Cable extension maps directly to stroke.

Strengths

• Bolt-on retrofit — no cylinder modification, no machining, installs in under an hour.
• Long strokes at low cost: SKD covers 0–400 inch (about 10 m) with 4–20 mA or 0–10 V; the PT9420EX reaches 600–1700 inch for very long axes.
• Wide output choice: voltage divider (DWS1A), 4–20 mA (DWS1MA, SKD, PT9420), SSI (DWS1S) — see the output comparison.
• IP67/IP68 versions handle washdown and outdoor duty.

Constraints

• The cable is exposed. Side loading, abrasive dust, weld spatter and someone stepping on it are all real failure modes.
• Cable slack during rapid retraction can cause a snap-back. Check the maximum acceleration rating against your cylinder speed.
• Mounting must keep the cable parallel to the rod axis. Even 5° of misalignment over a long stroke wears the cable guide.
• Not suitable where the rod end is inaccessible or the area must stay clear.

Choose this for retrofits, long strokes, and low-to-medium duty cycles. It is the fastest path from "we need position feedback" to a working machine.

Option 3: External LVDT or Linear Probe

A contactless LVDT probe mounted parallel to the cylinder, actuated by an arm on the rod end.

Strengths

• Contactless — no wear, essentially unlimited life, and stable accuracy that does not degrade.
• Very high repeatability, ±0.25 % FS or better (e.g. PTL-ARTSD, IP68, 0–50 mm; PTL-TSD, 0–40 mm).
• Digital bus variants exist: PTL-TSK-T080 (0–15 mm, Modbus RS-485), DLBB (1/2/5 mm, RS-485, ±0.1 % of reading).
• Survives oil mist, coolant and vibration.

Constraints

• Needs a parallel mounting surface and a mechanical link to the rod — extra bracketry that must stay aligned over the full stroke.
• Practical stroke limit around 250 mm; beyond that the LVDT body becomes long and costly (PTL-TSD-HR reaches ±254 mm using an AC output).
• AC versions require a signal conditioner in the cabinet.

Choose this for short-stroke, high-cycle cylinders — clamping, pressing, injection, valve actuation — where wear life decides maintenance cost.

Selection Matrix

● ● ● strong fit · ● ● ○ workable · ● ○ ○ compromise · ○ ○ ○ avoid

Pressure, Temperature and Fluid Considerations

• Pressure only matters for in-cylinder sensors; external units never see system pressure, which is a large part of why retrofits default to draw wire.
• Oil temperature on mobile equipment can exceed 80 °C. Check the sensor's rated ambient, not just the fluid rating.
• Contamination is the main killer of external cable systems: metal particles, grit and ice. Specify IP67 minimum and a protective guide if the cable runs near the ground.
• Rod speed drives cable take-up requirements. Fast retraction is the classic failure point of a draw wire on a cylinder.

Wiring and Commissioning Checklist

1. Confirm the controller input: 4–20 mA card, 0–10 V, SSI channel, or Modbus. Match it — do not convert later if you can avoid it.
2. Set the measuring range with 10–25 % margin beyond the physical stroke.
3. Mount the sensor body on a rigid, machined surface. Sheet metal flexes and shows up as drift.
4. Keep the cable parallel to the rod; use a swivel clip at the rod end and a slack loop for strain relief.
5. Zero the system at a known mechanical reference, at operating temperature.
6. Log the first week of data. Any slow drift usually means a mounting fastener is moving, not that the sensor failed.

Frequently Asked Questions

Can I add a position sensor to an existing cylinder without machining it? Yes — an external draw wire sensor is the standard answer. It bolts to the barrel and clips to the rod end, and no hydraulic work is involved.

What is the most accurate option? In-cylinder magnetostrictive and LVDT-based probes are both high accuracy. For short strokes, an LVDT reaches ±0.25 % FS or better; for long strokes, magnetostrictive holds accuracy that a cable system cannot match because the cable adds mechanical compliance.

How long do draw wire cables last on a cylinder? On a clean, well-aligned axis with moderate speed: years. With side loading, fast retraction or abrasive dust: months. Treat the cable as a wear item and keep a spare.

Do I need an intrinsically safe sensor? Only if the area is classified. If so, choose a 4–20 mA loop-powered unit with the appropriate certification — the PT9420EX is specified for hazardous areas and rated IP67/IP68.

Which output for a mobile machine? CAN Bus or 4–20 mA. Both tolerate long runs and electrical noise; CAN additionally lets several sensors share a harness.

Ask Us to Size It

Send the cylinder bore, stroke, rod speed, pressure, mounting space and controller input. We will tell you whether the in-cylinder route or an external draw wire is the better engineering trade — and which of the 149 models to quote.

Draw wire sensors for cylinder retrofit → · LVDT probes →