A hydraulic drill rig, a crane or a mobile lifting machine needs to know where its cylinder rod is during operation, because the control system acts on that position. A draw wire sensor can measure the stroke distance with a flexible steel cable wound on a spring-loaded drum. As the cylinder extends, the cable extends and turns the drum. A sensing element on the drum converts the rotational position into a proportional electrical signal. When designed with a suitable housing, a draw wire sensor provides a longer lifetime of operation when compared with a contact rotary potentiometer.
Why contact position sensors wear out on field equipment
Many mobile machines still track cylinder stroke with a rotary potentiometer. A potentiometer measures position by sliding a metal wiper along a resistive track. The resistance between the wiper and one end of the track changes with position, and the control system reads that resistance as a voltage output. Every cycle of the cylinder drags the wiper across the track. Each pass removes a small amount of the track material.
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In a clean, temperature-controlled environment, these sensors can last for years before needing replacement. However, on a drill rig, the same sensor is exposed to airborne dust, vibration and shock from drilling, and thermal fluctuation from cold overnight temperatures to midday heat. Dust works into the track and accelerates the abrasion of the sensing element. Therefore, when the same rig run hundreds of cycles per shift, the operating life of an exposed contact sensor shortens considerably from years to months.
The cost of replacing the sensor on a regular interval weighs more towards the cost of downtime rather than the cost of the sensor itself. When a position sensor fails between scheduled maintenance visits, the machine must be stopped until a technician can travel to site and change it. A worn track also causes a secondary problem before it fails. As the reading drifts, the control system acts on a position that is no longer true. On a drill rig, that means the cylinder may cycle past its set depth and this results in inconsistent hole depths. When positioned on a crane or a lifting platform, the machine believes the boom or the platform is somewhere it is not.
Therefore, the suitable position sensor should last the machine’s planned shutdown interval and not the other way around.
How a draw wire sensor measures cylinder stroke
Inside the sensor housing, a flexible steel cable is wound on a drum. A spring keeps the cable under tension. The free end of the cable attaches to the moving part, which on a hydraulic cylinder is usually the rod end or a bracket on the moving structure. As the rod extends, the cable pays out and turns the drum; subsequently, as the rod retracts, the spring rewinds the cable. The drum is coupled to a sensing element, either a multi-turn potentiometer, an incremental encoder or an absolute encoder, which job is to convert the number of drum turns into a resistance change or a count of increments.
By firstly converting the linear motion to rotation motion, it allows a long stroke to be measured by a compact sensor mounted at one end of the cylinder, with only the cable running along the stroke path. Our draw wire portfolio range offers a wide measuring ranges from 40 mm on the miniature models to 50 m on the long-distance encoder models.
A draw wire sensor with a potentiometric element still contains a wiper on a resistive track, so it is not a contact-free device. The only difference is where the contact point is. On an exposed rotary potentiometer, the wiper track is in contact with the object. Inside a draw wire sensor, the sensing element sits within a sealed housing, and on the outdoor industrial models the cable drum and the spring occupy separate compartments so dust or moisture entering one cannot reach the other. Where the application justifies it, an encoder element removes the sliding contact from the measuring element altogether.
What field conditions demand of the sensor
There are four distinct requirements that shape the specification of position sensor for mobile equipment.
- The sensor must be compact to allow mounting at the cylinder end cap inside the existing guarding, with the cable running along the stroke path.
- The sensor housing has to offer the suitable ingress protection rating (min of IP65) for protection against dust and water.
- Its electronic sensing element must also offer measurement reliability across the site temperature range, for example from frost overnight to a fully warmed hydraulic system at midday, to offer stable output signal.
- It must be able offer an output signal that can be accepted by the machine’s existing PLC, so the control logic does not need to change.
A 4 to 20 mA current loop is the most common interface into PLC and SCADA inputs on mobile and heavy equipment. It also carries a fault signal without any extra wiring. The zero position sits at 4 mA, not at 0 mA. If the loop current falls below 4 mA, the control system knows the cable or the sensor has failed, rather than reading a broken circuit as a valid zero-position measurement. Draw wire sensor can also offer voltage output, potentiometer output and, on the encoder models, incremental or absolute digital output as alternatives should the requirement calls for them.
Mounting configuration and cable routing
The cable connection decides much of the sensor’s service life. The cable should run in a straight line from the sensor outlet to the attachment point, with no potential contact such as rubbing on guarding, hoses or structure through the full extension and retraction of the cylinder. Any contact along that path abrades the cable coating and adds friction that the return spring must overcome. The mounting brackets should be installed to hold the sensor immobile, since vibration transmitted to the housing may show up as noise signal on the output. On mobile equipment, the attachment point also needs to allow for the small angular movement between the sensor and the moving part as the cylinder articulates, which is usually handled with a swivel or a clevis at the cable end.
Choosing a draw wire sensor for mobile and outdoor equipment
Which draw wire sensor suits a hydraulic cylinder in an outdoor environment?
For mobile machines, cranes and lifting equipment, the Micro Epsilon WPS-K100 draw wire sensor is suitable for this type of application. It offers a wide measuring range from 2300 mm to 5000 mm with a linearity of 0.2% FSO, operates from -40 °C to +85 °C and is protected to IP69K. The housing material is glass fibre-reinforced plastic with separate drum and spring compartments, and the cable is made from polyamide-coated stainless steel.
What if the stroke is longer, or the accuracy requirement is tighter?
Then, the WPS-MK120 draw wire sensor can be used as it covers a 3000 mm to 7500 mm measurement range with a linearity of 0.15%. It is also available with voltage, current or potentiometer output. On the other hand, users can also consider WDS-P115 draw wire sensor with a measuring range from 3000 mm to 15000 mm, offering both analog and digital versions, with a linearity of 0.01%, and it can also work with incremental or absolute encoder output to the analogue options. For strokes of 30 m to 50 m, the WDS-P200 can be used, and it offers output in incremental or absolute encoder.
When is a draw wire sensor not suitable?
If you need to measure short strokes with high accuracy in a controlled environment, LVDT is usually a better option. For measurement of small range of displacement on a metallic target that must be measured without contact suit an eddy current sensor. Laser triangulation sensor can be used for high speed non contact measurement, but it needs a clean optical path, so airborne dust and mist on a mine site can scatter the spot and disturb the reading. Choosing the right technology depends on many variables such as site environment, machine conditions, not just measurement range alone.
Can we assist with your displacement measurement application?
Selecting a draw wire sensor for a displacement measurement on hydraulic cylinder comes down to four things: measuring range, the environment the housing has to survive, the temperature range on site, and signal output. If a sensor needs to be replaced every few months, it usually failed on one of those four reasons, not on the product quality.
If you are working through those questions for a hydraulic cylinder, a crane or any long-stroke movement on mobile or outdoor equipment, have a chat with us about your application. Our applications engineers can help you define the project scope, select the suitable sensors and assist with system integration and technical support.
