Motion Control Resources
Lika Electronic USA Posted 06/18/2019
Draw-wire encoders are a little-known type of measuring system. That’s why it is seldom considered when it comes to designing a new solution for an industrial application. But there are a number of good reasons why designers should appreciate its functional characteristics and discover that sometimes it can be an excellent answer to measuring needs:
- Measuring lengths from 2 m (3.28 ft), 5 m (16.4 ft), 10 m (32.8 ft), and more, up to 50 m (164 ft)
- Any type of encoder can be integrated: incremental, absolute, fieldbus, Ethernet, analogue (including potentiometer)
- Resolution down to 0.01 mm / 10 μm
- Speed up to 2 m per second / 78.7 inches per second
What is a draw-wire encoder?
A draw-wire encoder is a measuring system that’s basically made up of two parts integrated in the single device:
- a rotary encoder, it can be either optical or magnetic, with incremental, absolute, and analogue outputs (a potentiometer can be installed as well)
- a stainless-steel wire having different lengths (Lika’s range varies from 2 m up to 50 m / from 3.28 ft up to 164 ft). It is wound on a spring-wired drum and then pulled out and rewound during operation.
The stainless-steel wire is attached to the moving axis being measured. The encoder is coupled to the spring-wired drum, so as the cable moves back and forth, the encoder is rotated. In this way, the linear motion of the wire is translated into a rotary motion by the drum and then into a positional information by the encoder. Additionally, the wire’s flexibility enables curved travels through guiding rollers.
Why a draw-wire encoder?
Let’s consider that the encoder-drum assembly (the fixed part of the device and also the most delicate one) and the stainless-steel wire that is attached to the mechanism being measured (the moving and the most rugged part) can viewed in some way as two separate components. Thus, the encoder-drum assembly can be installed away from the wire and the axis being monitored operate. For this reason, the assembly can mount in a safe place, easily accessible, and adequately protected while the stainless-steel cable can be submitted to harsh environmental conditions at the same time - i.e. dirty, humidity, high temperatures, etc. Furthermore, by virtue of the small footprint of the cable, the operational space required by the wire can be very tight, even a few centimeters wide, since the encoder can be mounted in a different and larger location.
A further benefit is that the cable-pulling mechanism can be coupled to any encoder which preserves the same complete set of functions as the standard encoder. This provides excellent flexibility in terms of applications and environments. Lika’s range includes the following versions:
- with programmable incremental encoder and resolution down to 0.01 mm / 10 μm;
- with SSI and analogue absolute encoder and resolution down to 0.012 mm / 12 μm;
- with Ethernet and fieldbus absolute encoder: Profinet, EtherNet/IP, EtherCAT, POWERLINK, This allows the Ethernet technology to be available in the most uncomfortable conditions (the narrowest spaces, harsh industrial environment); resolution down to 0.024 mm / 24 μm;
- string pot.