Draw-Wire vs Magnetic Linear Sensors: Choosing Linear Position Measurement
When a machine needs to know where something is along a straight line, such as a press ram, a hydraulic cylinder, a saw carriage or a lift platform, two technologies cover most industrial jobs: draw-wire sensors and magnetic tape (magnetic linear) sensors. Both are rugged and both are widely used in India. They suit quite different situations.
How a draw-wire sensor works
A draw-wire sensor (also called a wire-actuated encoder or cable-extension transducer) has a thin steel wire wound on a spring-loaded drum. The end of the wire is clipped to the moving part. As the part moves, the wire pulls out or rewinds, turning the drum, and a potentiometer, analog converter or rotary encoder inside converts that rotation into a position signal.
- Fast to retrofit: one fixing point for the housing and one for the wire, with no long guide or scale to install.
- Long ranges in a small housing: SIKO models range from the miniature SG5 (up to 600 mm) through the SG10, SG20 and SG30 (up to 3000 mm) to the SGP/1 (up to 6000 mm).
- Flexible outputs: potentiometer, voltage or current outputs, or a mounting for your own rotary encoder, as on the SG21 and SG31.
- Inside hydraulic cylinders: the SGH10, SGH25 and SGH50 are absolute, pressure-resistant designs (up to 350 bar) that sit inside the cylinder, with CANopen or analog output.
Watch out for: the wire must leave the housing in line with the direction of travel; side-pull wears the wire and drum. Very high accelerations and heavy dirt or chips around the wire exit also need attention.
How a magnetic linear sensor works
A magnetic tape with alternating north and south poles is fixed along the travel path. A sensor head passes over it at a small air gap without touching it and reads the pole pattern. Because nothing touches, there is no wear, and the tape can be cut to almost any length.
- High resolution: from 0.25 mm on the MSK2000 down to 1 µm on the MSK5000 and 0.2 µm on the MSK1000.
- Incremental or absolute: incremental sensors such as the MSK500/1 feed a counter, display or PLC; absolute sensors such as the MSA510 and MSA511 output position over SSI or RS485 and need no homing.
- Long and curved paths: suited to long axes, gantries and rotary applications with ring magnets.
Watch out for: the reading distance is small (for example up to 0.4 mm on the MSK1000 and up to 2 mm on the MSA511), so the sensor head must be guided precisely along the tape. Strong external magnetic fields and ferrous chips sitting on the tape should be avoided or shielded with a cover strip.
Side-by-side comparison
| Draw-wire sensor | Magnetic linear sensor | |
|---|---|---|
| Contact | Mechanical wire and drum | Non-contact, wear-free |
| Installation | Two fixing points, quick retrofit | Tape along full travel plus guided sensor head |
| Typical range | Up to several metres per sensor | Set by tape length |
| Resolution | Good for most positioning and monitoring | Down to the micrometre range |
| Best for | Hydraulic cylinders, lifts, presses, retrofits, mobile machines | Saws, woodworking and sheet-metal machines, gantries, precise long axes |
Which should you choose?
- Choose draw-wire when you need a quick retrofit, the moving part is hard to guide precisely, or the measurement is inside a hydraulic cylinder.
- Choose magnetic tape when you need high resolution, long travel, no mechanical wear, or a sensor head that can be guided closely along a fixed path.
- Choose an absolute version of either when the position must be known immediately after power-up.
For readouts, SIKO also offers matching position indicators and displays that show the position directly at the machine.
Get a recommendation
Rajdeep Automation supplies the complete SIKO linear measurement range across India. Tell us the measuring length, required resolution, output and environment through the enquiry form and our application engineers will suggest the right sensor and share the datasheet and price.