What Is a String Potentiometer and How Does It Work?
Definition
A string potentiometer — also sold as a string pot, yo-yo pot, cable-position sensor or draw wire potentiometer — measures linear displacement using a flexible cable wound on a spring-return drum. Pulling the cable rotates the drum, the drum turns a potentiometer shaft, and the potentiometer produces a voltage proportional to how far the cable has been extended.
Think of it as a tape measure with an electrical output.
What makes it useful is packaging: because the cable coils up, a 1.5-metre measuring range fits in a housing roughly the size of a coffee cup. Nothing else in linear measurement offers that range-to-size ratio at that price.
How It Works, Stage by Stage
1. Cable and drum. A stainless steel or nylon-coated cable is wound in a single layer on a grooved drum. A constant-force spring keeps tension so the cable retracts when the target moves back. Single-layer winding matters: overlapping turns change the effective drum diameter and therefore the reading.
2. Shaft coupling. The drum drives a potentiometer shaft, an encoder shaft, or a non-contact sensing element, depending on the model.
3. Resistive element. In a true string potentiometer, a resistive track with a wiper converts shaft angle into a voltage divider ratio. Three wires: supply, ground, and wiper output. The wiper voltage is a fraction of the supply proportional to extension.
4. Signal conditioning (optional). Industrial versions add electronics to output 4–20 mA, 0–10 V, SSI or a bus signal instead of a raw divider.
5. Spring return. The return spring sets the retraction force. Heavier springs retract faster but load the target; long-stroke units can pull with several newtons at full extension.
How to Wire a String Potentiometer
The three-wire version is straightforward:
| Wire | Connection | Note |
|---|---|---|
| Supply (V+) | Sensor excitation | Match the potentiometer's rated voltage |
| Ground (0 V) | Common | Tie to the ADC ground reference |
| Wiper (signal) | Analogue input | Voltage = V+ × (position / full scale) |
Two practical points:
- Ratiometric means supply-independent — if your ADC references the same supply, a sagging supply does not shift the reading.
- The wiper draws current. Keep the input impedance high (≥ 10 kΩ). A heavy load pulls the divider and introduces a nonlinear error near the ends of travel.
For longer cable runs or noisy plants, use a version with a 4–20 mA output (such as the DWS1MA) rather than running a divider signal 30 m through a drive cabinet.
Range, Output and Sealing Across a Real Product Line
Our own-brand DWS family covers 0–1500 mm in three output variants, which illustrates how the technology scales:
| Model | Range | Output | Sealing |
|---|---|---|---|
| DWS1A | 0–1500 mm | Voltage divider | NEMA 4 / IP65 |
| DWS1MA | 0–1500 mm | 4–20 mA, 0–20 mA | NEMA 4 / IP65 |
| DWS1S | 0–1500 mm | SSI, BiSS, ±0.1 % FS | NEMA 4 / IP65 |
Same mechanics, three integration paths. Above that, the Celesco OEM families split by duty: SG1 (0–5 inch), SM1/SM2 (0–25 inch), SP1–SP3 (4.75–50 inch), SKD (0–400 inch, 4–20 mA / 0–10 V), and the long-range PT9420 family (75–550 inch, and 600–1700 inch on the PT9420EX).
Accuracy: What to Expect
Typical figures across the string pot class:
- Compact OEM units: ±0.25 % to ±1 % FS
- Industrial draw wire: ±0.35 % to ±0.5 % FS
- Precision digital versions: ±0.1 % FS (DWS1S, SR1E, PT9301EX)
On a 1.5 m sensor, ±0.1 % FS is ±1.5 mm. That is fine for a lift or a valve position, and hopeless for a servo axis. Scale the specification to the job — buying ±0.1 % for a gate position costs money for accuracy nobody uses.
String Pot vs LVDT: The One-Paragraph Version
A string pot wins on range and cost; an LVDT wins on life and stability. The wiper touches the track every millimetre of every stroke, so a string pot has a finite cycle life and its accuracy degrades as the track wears. An LVDT has no contact and no wear. If your machine cycles millions of times or runs in abrasive dust, pay for the LVDT. If it moves a few times an hour over a long distance, the string pot is the sensible engineering choice.
Full comparison: LVDT vs potentiometer.
Where String Potentiometers Are Used
- Lifting and access equipment: platform height, mast extension, outrigger position.
- Cranes and hoists: hook height, trolley travel, boom extension.
- Material handling: stacker height, fork position, telescopic fork extension.
- Valves and dampers: long-stroke actuator position where an LVDT would be oversized.
- Test rigs: crosshead displacement, actuator stroke, structural deflection.
- Agricultural and construction: implement depth, auger position, header height.
Installation Rules That Extend Life
- Pull straight. Cable misalignment over 2–3° wears the guide and eventually chafes the cable. Use a rotating head or a swivel clip.
- Never let the cable snap back. Retraction under no load at full speed can whip the cable off the drum. Keep a small residual tension.
- Mount rigidly. The sensor measures cable extension; if the body moves, every reading is wrong.
- Respect the acceleration limit. If the target accelerates faster than the spring can take up slack, the cable goes slack and then shocks tight.
- Keep the cable exit dry. Mount the gland downward, or specify IP67/IP68 versions for outdoor duty.
- Treat the cable as a wear item. Keep a spare; they are inexpensive compared with downtime.
Frequently Asked Questions
How accurate is a string potentiometer? Between ±0.1 % and ±1 % of full scale depending on model. On a 1500 mm unit that is 1.5 mm to 15 mm — always convert the percentage to millimetres before deciding.
What is the maximum range? Our range extends to 1700 inch (about 43 m) on the PT9420EX long-range family. Practical limits come from cable sag, drum size and spring force rather than electronics.
Can it measure fast motion? Up to a point. The limiting factor is cable take-up during retraction. For high-speed or high-acceleration axes, an LVDT, magnetostrictive or encoder-based solution is more appropriate.
Does it need a signal conditioner? Only if you want 4–20 mA, 0–10 V or a digital output. The three-wire voltage divider version connects directly to an ADC.
What's the difference between a string potentiometer and a draw wire sensor? Same principle, different packaging and output. "String potentiometer" usually means a resistive divider output; "draw wire sensor" or "cable extension transducer" implies an industrial housing that may contain a potentiometer, encoder or absolute element.
Which output should I order? Short cable, in-cabinet electronics: voltage divider. Long cable or noisy plant: 4–20 mA. Absolute position across power cycles: SSI. See the output comparison.
Get the Right Model
Tell us the stroke, output, environment and mounting orientation. We will match it against the DWS own-brand range or a Celesco OEM family, and send the datasheet.
Need help specifying a sensor?
Tell us your stroke, output signal, environment and mounting constraints — we reply within one business day with a model recommendation and datasheet.