Custom Sensor Case Study
Positek sensor steels another success for strand monitor
Monitoring the internal condition of a continuous steel casting plant is as awkward as it is essential unless you have a Strand Condition Monitor from Sarclad International.
This device is peppered with sensors and when passed through the plant between batches it gathers and relays accurate information on a number of parameters which affect steel quality. Any necessary adjustments or maintenance can thus be instantly identified and rectified with the minimum of down-time.
In order to achieve consistently top-quality steel blooms or slabs it is necessary to maintain the position, rotation and straightness of the guide rollers which determine the dimensions of the steel slabs. Also the correct functioning of water cooling sprays and the shape of the arc formed as the steel moves from vertical to horizontal orientation must be controlled in order to maintain the desired crystalline structure. Many of these parameters are monitored by the array of highly sensitive and accurate position sensors fitted to the Condition Monitor and the data is subsequently analysed so that the process can be adjusted.
Because no two continuous casting plants are the same, the Sarclad Strand Condition Monitor is custom-built for each customer and its electronic components must therefore be particularly flexible in their application. The wet and greasy conditions within the casting plant provide a harsh environment for sensitive instrumentation and, with the original position sensor, failure was a common problem during the early development phase of the machine. The problem was serious enough for Sarclad to find a new supplier. Over the past two years Sarclad International's design engineers have been working closely with the team from Positek to obtain a robust and sufficiently accurate position sensor for the monitor.
Describing their quest for the ideal position sensor, Jerzy Slowikowski, Sarclad's Instrumentation Engineer, says, "We needed a sensor with good linearity, and good stroke to body length, robust enough to be able to function accurately and reliably under hostile conditions. Because the monitors are so specialised and are custom-designed to suit each customer we needed components which could themselves be customised, backed by a company with a flexible approach to product development. We were able to work alongside Positek's designers who devised a sensor specifically for our needs which has proved reliable and accurate in operation."
Although the sensor is made only for Sarclad, it incorporates PIPS® technology developed by Positek and used in its other displacement sensor products. This PIPS® technology is based on inductive coils and uses a unique custom integrated circuit to produce an accurate analogue output signal. The complete interface circuit is built in to the sensor which simplifies the installation for Sarclad.
Because the sensors are contaminated with hot dirty water in operation and space was restricted, a particular challenge was to ensure that each one was fully waterproof even when subjected to thermal shocks. Special design techniques had to be employed to seal the sensor particularly where the cable enters the body. In production each sensor must pass a rigorous 24hr immersion test before it is delivered to Sarclad.
The sensor has a range of 25mm and produces an output of 0 to 5V from a 12V supply. The linearity achieved is 0.2% and each sensor is calibrated so that it produces a consistent output with position. Each sensor is put through a number of quality assurance tests to ensure compliance with the tough specification.
With the success of the first sensor, Positek was asked by Sarclad to develop a range of sensors for the latest developments in monitors. The latest monitors need to be able to operate in the new casting processes which produce thinner steel and therefore need thinner monitors with much more compact sensors. Positek was in a good position to provide a solution to this additional problem because the PIPS® technology is more compact than an LVDT sensor. Sensors with travels of 8mm and 15mm have been designed to suit these new applications.
Another essential requirement for Sarclad's instrumentation suppliers was the ability for small batch production; with every Strand Condition Monitor being a 'one-off' and with 12-24 Positek sensors per machine, this adds up to a manufacturing nightmare with which only the more personal and flexible companies such as Positek could cope.