Position sensing by draw wire.
The idea of 'draw wire' sensors' is not new, but often they are designed
for a one-off application solution. For this reason, results are often
variable, and usually lack precision.
One company which has been successfully producing draw wire sensors
since 1979 is ASM. Applications for the products are infinite, it says,
with examples being cited in lift control, airframe load testing, robot
operation, automation, mechanical handing systems, training testing,
crash testing and racing car development. One big advantage of draw
wire technology is its ability to monitor actual physical position, such
as in automotive lift applications, where exact car position is known,
regardless of rope stretch due to excess load. In addition, in this type
of application where the real-time position is constantly known, the
vehicle can be controlled to allow passengers a smooth, gentle stop.
Another application, in motor racing, uses a draw wire sensor mounted
to the suspension struts of a motor racing vehicle. At the various racing
tracks during testing, data is transmitted by telemetry back to the team
manager to provide information for optimum suspension damping for
that particular track.
ASM's draw wire sensors use a stainless steel cable winding onto a
precision machined drum, often via a tracking cable guide. Cable
tensioning is provided by a coil spring which gives constant, repeatable
pull-out and pull-in forces. The wire is wound onto the drum in a single
layer without overlap, enabling maximum accuracy and repeatability
with low hysteresis. Linearity of up to 0.01% or digital resolution to
0.01mm can be achieved.
A variety of outputs
The drum assembly is mounted on a shaft driving a high grade precision
potentiometer or encoder to give a variety of outputs suitable for most
real time electronic interfaces. Various models, ranging from 0-50mm
to 0-40,000mm are available and some models enable the user to
source a suitable encoder.
The Caterpillar truck racing team
use cable actuated position sensors
fitted to each suspension leg.
When taken to a new track, vital
infromation on suspension movement
is transmitted by telemetry and
analysis enables optimum suspension
setup for that track.
In general, the sensor cable can be run via pulley wheels, and a cable
extension can be provided to allow the sensor to be positioned remote
from the application. Harsh and corrosive environments can be catered
for and acceleration up to 95g is permissible.
Interfacing with PLCs and control systems is possible due to a large
range of outputs such as potentiometer, 0 to 10v, 4-20mA (2 or 3 wire),
ADSI, incremental encoder (1,5,10 or 25 pulses per mm), absolute
encoder parallel or synchronous serial interface (SSI), Profibus, CAN
or Interbus S.
Other options include the ability to increase the accuracy of incremental
encoder output using edge counting techniques, a choice of direct
readout on a LCD display, and a choice of RS232 data stream outputs.
Velocity output can be derived from the installation of a dc tachometer
driven by the cable drum assembly. This can be a direct drive in a
'velocity only' sensor or connected in parallel with a position sensing
device to give both parameters. The nominal 0 to 100v output can be
scaled or conditioned to give a velocity output range from 0 to 2mm/sec
up to 0 to 20m/sec.
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August 2001
