Draw Wire Sensor Selection Guide: Range, Output and IP Rating

What a Draw Wire Sensor Does

A draw wire sensor — also called a cable extension transducer, string encoder or yo-yo pot — converts linear motion into an electrical signal using a stainless steel cable wound on a spring-loaded drum. Pull the cable out, the drum rotates, and a sensing element inside (potentiometer, encoder or digital absolute encoder) reports how far it moved.

The appeal is geometric: a 1.5-metre measuring range in a body smaller than your fist. No other linear sensor technology packs that range into that volume. That is why draw wire sensors dominate long-stroke position feedback on cranes, lifts, presses, valve actuators and telescopic equipment.

Step 1: Fix the Measuring Range

Specify the maximum travel plus a safety margin of 10–25 %. Never use the full rated range as your working stroke — the cable needs pre-tension and the drum needs headroom.

Watch the units. Celesco families are quoted in inches, KMBT own-brand DWS models in millimetres. A "400 inch" sensor is just over 10 metres — a common and expensive ordering mistake.

Step 2: Choose the Output — This Decides Your Electronics

The output is where most projects go wrong, because it dictates which PLC card you need, cable length limits and whether you can put several sensors on one bus.

Rule of thumb: if the cable from sensor to cabinet is longer than about 10 metres, or the machine has VFDs and servo drives nearby, go 4–20 mA or digital. A 0–10 V signal over 30 m next to a drive is a noise antenna.

If you are unsure which current or voltage standard fits your control system, our output signal comparison walks through the trade-offs.

Step 3: Match the IP Rating to the Environment

This is where cheap sensors fail first. Ratings in our catalogue:

• IP50 / IP65 — clean indoor, dust only, occasional splash. Adequate for OEM builds inside an enclosure.
• IP67 — temporary immersion, washdown, outdoor with rain. The industrial default; the largest single group in our range (68 models).
• IP68 — continuous immersion. Required for outdoor gates, marine, tanks and wet processes.
• NEMA 4 / 4X / 6 — North American equivalents; 4X adds corrosion resistance, 6 adds temporary submersion. Frequently quoted alongside IP for US customers.

The PT9420EX is rated IP67 and IP68 (plus NEMA 4, 4X, 6) across a 600–1700 inch range with 4–20 mA output — the specification you want when a machine is hosed down daily or sits outdoors year-round.

Do not overlook the cable exit. A sensor rated IP68 with a standard cable gland installed upside-down collects water. Specify the gland orientation, or a connector version, and route the cable with a drip loop.

Step 4: Check the Mechanical Details Nobody Quotes

Acceleration. The cable must stay tensioned. If your axis accelerates faster than the spring can take up slack, the cable goes slack, then snaps taut — producing a spurious reading and eventually a broken cable. Ask for the maximum acceleration figure and compare it with your machine's actual dynamics.

Cable direction and exit angle. Draw wire sensors are usually specified with the cable exiting at a fixed angle. Mounting the body so the cable pulls at more than a few degrees off-axis wears the guide and shortens life. Some models allow a rotating head; use it.

Mounting repeatability. The sensor measures cable extension, not absolute machine position. If the body shifts, every reading shifts. Bolt it to a rigid, machined surface, not to sheet metal.

Return spring force. Light spring = gentle on delicate targets, but slower take-up. Heavy spring = fast response, but it loads your mechanism. For long strokes the spring force at full extension can be several newtons.

Step 5: Cost Expectations

Draw wire pricing scales with range, output and sealing, roughly like this:

• OEM compact, voltage divider (SG1, SM1): lowest — suitable for high-volume builds
• Industrial 4–20 mA, IP67 (PT9420, SKD): mid — the workhorse tier
• Hazardous-area / long range, IP68 (PT9420EX): highest

Resist the temptation to save on the output stage. The cost difference between a voltage-divider and a 4–20 mA version is small compared with the cost of an unreliable signal on a production machine.

Common Specification Mistakes

1. Sizing to the rated range with no margin — the cable bottoms out and the drum takes the shock load.
2. Ignoring cable length limits on voltage outputs, then blaming the sensor for noise.
3. IP rating on the body only — the cable entry and connector are the real weak points.
4. Mounting on a vibrating panel — vibration couples straight into the drum and appears as signal jitter.
5. No strain relief on the cable where it attaches to the moving target. Use a swivel clip and a free loop.
6. Mixing inch and metric when cross-referencing families.

Frequently Asked Questions

How accurate is a draw wire sensor? Typical industrial accuracy is ±0.1 % to ±0.5 % of full stroke. Our SKD quotes ±0.35 % FS; the PT9301EX ±0.10 % full stroke; the DWS1S ±0.1 % FS. Long-range units are relatively less accurate in absolute terms but the percentage figure stays similar.

How long does the cable last? It depends almost entirely on routing and contamination. Clean, on-axis, moderate acceleration: years of service. Abrasive dust and side loading: months. Cable and drum kits are replaceable on most industrial families.

Can a draw wire sensor measure rotary motion? Indirectly, yes — pull the cable around a drum or attach it to a rotating arm of known radius. For continuous rotation, use a rotary sensor instead (RT8420 covers 0.125–200 turns with 4–20 mA).

Can it work outdoors? Yes, with IP67 or IP68 and UV-stable cable. Specify a stainless cable and confirm the housing finish; powder-painted aluminium and 303 stainless are both offered on the PT9420EX.

What is the difference between a draw wire sensor and a string potentiometer? Functionally the same principle. "String potentiometer" usually implies a resistive voltage-divider output; "draw wire sensor" or "cable extension transducer" implies an industrial package that may use a potentiometer, encoder or absolute element inside. See what is a string potentiometer.

Get a Model Recommendation

Tell us: stroke, output, IP requirement, cable run length, acceleration and mounting orientation. We will narrow 149 models down to the two or three that fit, with datasheets.

Draw wire position sensors → · Compact OEM draw wire →