Design World – August 2019

Linear Motion

techno

analog signal, potentiometers can also be used.

Draw-wire sensors can measure distances

up to 50 km (50,000 m) and can operate with

wire extension and retraction speeds up to 10 m/

sec and acceleration rates up to 5 g. One of the

most important specifications when choosing a

draw-wire sensor is linearity, which refers to the

difference between the ideal output-position plot

(a straight line) and the sensor’s actual output-

position plot (See graph at right). Draw-wire

sensors typically range in linearity from ± 1%

to ± 0.05% of the measuring range, but designs

are available with linearity of ± 0.01% of the

measuring range.

A unique property of draw-wire sensors is

that they can measure linear distance even with

indirect measuring paths. When the measuring

path is not a straight line, low-friction, high-

precision pulleys, or guide rollers, can be used

to create diversions without jeopardizing the

measurement’s integrity.

Draw-wire sensors are simple, rugged

(available in IP67, IP68, and IP69K protection

classes), and low-cost. They’re a suitable option

for applications with intermittent measuring

requirements, but generally aren’t recommended

for applications with high or continuous duty

cycles. Typical applications include patient beds,

hydraulic and pneumatic actuators, lifting devices,

and mobile equipment such as forklifts and

construction machinery.

New draw-wire sensor offerings

A number of manufacturers provide a range of

draw-wire sensor options to accommodate a wide

variety of applications and uses. For example,

SIKO’s new SGH50 draw-wire sensor uses a

cable mechanism for absolute measurement of

hydraulic or telescopic cylinders. This sensor

110 August 2019 http://www.designworldonline.com DESIGN WORLD

is the latest generation of the SGH wire-

actuated encoder family from SIKO and now

makes it possible to measure distances up to

5 m. Suitable applications are agricultural

machinery, construction machines,

municipal vehicles, forestry machines,

piston accumulators and other industrial

applications. The main features include an

extension of measurement length, improved

durability, and compatibility with a wide

range of interface protocols.

The SGH50 can be integrated right

into hydraulic and telescopic cylinders. A

specially developed plastic is characterized

by an extension of fluid temperature

resistance from 85 to 105 °C, making for

an extremely durable design. It also meets

the specifications of extreme vibration

standards, which are used for testing

helicopter turbines, and can handle shocks

100 times the force of gravity. Unlike rod-

based sensors, its flexible cable completely

absorbs vibrations with the hydraulic

medium providing additional damping of

any vibration.

Designed for the highest requirements of

mobile hydraulics, the wire-actuated sensor

meets the prerequisite of an extremely long

service life. It’s been tested and designed for

the service life of a cylinder both electrically

and mechanically. In order to cope with the

harsh conditions of the mobile world, all of

the sensors integrated into the cylinder are

completely protected from external factors,

with a protection rating of IP69K and

making use of KV1H plug connections.

Analog interfaces, CANopen or SAE

J1939 provide communication between

the sensor and machine control system.

Applications in which maximum functional

safety is essential can be supplied with

redundant SGH50 versions (CAT3, PLd)

in accordance with EN13849. For safety

variants, SIKO also offers redundant or

analog interfaces, redundant CANopen,

redundant SAE J1939 or CANopen-safety.

Also from SIKO is the SG61 wire-

actuated encoder that can measure lengths

up to 6 m. These sensors feature a robust

aluminum housing, coupled with impact-

Ideal

Real-world

transducer

Voltage

Displacement

These tiny draw-wire

sensors from Micro-

Epsilon weigh just

8 g and are 19 x 19

mm. This MT series

of sensors feature

aluminum housings

and maximum

measuring range of

130 mm.

Linearity is a key

factor when choosing

a draw-wire sensor,

with some designs

achieving linearity

of ± 0.01% of the

measuring range.

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