Linear MotionWhen it comes to measuring position in linear systems, draw-wire
sensors are still a simple, durable, and low-cost option.
M
Miles Budimir • Senior Editor
Danielle Collins • Contributing Editor108 August 2019 http://www.designworldonline.com DESIGN WORLDMotion system designers have quite a few options for tracking
and measuring the position of a linear system. Some of the options include
magnetic and optical linear encoders that read changes in magnetism or
light, to linear variable differential transformers (LVDT) that operate on
the principle of mutual induction, to magnetostrictive sensors that rely on
torsional strain induced by magnetic fields.However, each of these technologies requires that a critical part of the
measuring system be attached to the object being tracked. What if the
environment, application, or installation space doesn’t permit attaching
critical measuring equipment — such as a read head, push rod or magnet — to
the target object? Another type of linear measuring device, known as a draw-
wire sensor, solves this problem.
Draw-wire sensors go by a few other names that include: draw wire
transducers, cable transducers, string potentiometers, string encoders, linear
position string pots, and yo-yo pots or yo-yo encoders (due to the way the
spring-wired drum pulls the wire back when it’s removed from the load).Sensors report down to
the wire
Draw-wire sensors are relatively simple
in construction, with four main parts: a
measuring wire, a drum (aka spool or reel), a
spring, and a position sensor.
The measuring wire is commonly
made of braided steel or stainless steel and
covered with nylon or polyamide, although
wires can be made of plastic and other
materials for harsh environments or special
application requirements. The passive wire
connects to the object being measured by a
clevis or loop, and the wire is mechanically
attached to and single-wrapped around aLinear Motion 8-19_Vs4.LL.indd 108 8/5/19 2:01 PM