MACHINE BUILDING, FRAMEWORKS & SAFETY FEATURE
WHEN TEMPERATURE makes a difference
With thermal influences capable of affecting the precision of lathed and milled parts, DECKEL MAHO Seebach has selected Murrelektronik’s fieldbus modules to compensate for temperature differences in its high-precision machining centres
W
hen producing lathed and milled parts, precise contour accuracy
and surface quality are essential – and this is especially the case for precision parts for industries ranging from medical engineering to automotive and the aerospace industry, where repeatability with small tolerances independent of thermal influences is required. However, many factors inside a
machining centre can have a negative effect on the precision of a workpiece – waste heat from the machining spindle, servo motors, ball-type linear drives, synchronous and torque motors, as well as temperature fluctuations of coolants and lubricants. In addition, thermal influences from the environment – such as fluctuations in ambient temperature, heating, air conditioning, insulation and so on can influence metal expansion.
THE SOLUTION To help overcome the problem, DECKEL MAHO Seebach has installed high precision Pt100 sensors (class A) on areas in its machining centres that are crucial regarding temperature: the hot spots. These sensors measure the local temperature and transfer the measured values to the control with Murrelektronik’s 4-channel AS-Interface Pt100 module via the ASInterface bus, where complex software evaluates this data. Based on the measured temperature deviations, the machine tools are then automatically corrected and repositioned. The values measured by the Pt100
sensors were directly transferred to the control cabinet with shielded sensor cables and then evaluated with remote I/O modules. Two Pt100 sensors were installed at the headstock to check the temperature – this is where a lot of power is needed because of the spindle drive, acceleration and deceleration, and the heat release is correspondingly high. This solution made it necessary to
lay shielded cables into the cabinet to protect the measured values from EMC interference. However, a distance of more than 12m had to be covered, sometimes
using C-tracks, which made this an expensive solution that required a lot of wiring work. In addition to this, engineers at DECKEL MAHO Seebach were not happy with the long signal paths and correspondingly expensive shielded sensor cables. To come up with an alternative, the engineers worked closely with Murrelektronik.
FIELDBUS TECHNOLOGY The solution is a 4-channel MASI68- Pt100 module for decentralized and high precision data acquisition measurement. The company integrated Murrelektronik’s compact MASI68-Pt100 fieldbus modules into the existing AS-Interface bus system – installed in a decentralized position next to the test points. The MASI68- Pt100 modules feature a bus expansion, meaning the bus cable can be daisy chained from one module to the other. They are also fully potted, which makes them resistant to shock and vibration, as well as resistant to coolants and lubricants. This makes them the ideal choice for applications in the demanding environment of machine tools. Of additional benefit, analogue
components that require a lot of wiring are no longer necessary in the cabinet, and there is no need to install long shielded cables. The Pt100 sensors are directly connected to the AS-Interface modules with short, shielded cables. The AS-Interface modules process the sensor signal at a resolution of 16 bits and transfer the high quality signal to the CNC controller. DECKEL MAHO usually uses two modules
with four ports each in their machining centres, resulting in eight Pt100 inputs available; and uses these inputs to evaluate significantly more Pt100 test points compared to the previous solution. Now, the test points are not only located at the headstock, but also at the NC rotary table as well as in the machine bed (in X and Y direction), meaning those parts of the machine that are exposed to the most stress. Of course, more test points offer an even more exact basis
The MAS168-Pt100 fieldbus modules used on the high-precision machining centres are potted, which makes them resistant to shock and vibration, as well as resistant to coolants and lubricants
for calculating the parameters for temperature compensation. For extreme EMC environmental
conditions, such as installations near synchronous and torque motors and motor cables, DECKEL MAHO Seebach uses special shielded round cables for the AS-Interface bus. Moulded and pre-wired for easy installation, these are designed according to AS-Interface specification. The MASI68-Pt100 modules are
galvanically decoupled inside from the AS-Interface supply voltage. All sensor connections are equipped with grounded metal ports and the modules themselves feature a functional earth connection where discharging grounding straps are attached. The result of this is significantly higher interference immunity towards EMC influences. In addition, the Pt100 sensors are
used in four-wire technology, making it possible to compensate for cable induced influences up to the measuring point. The maximum measuring current of just 1mA reduced the power loss and the intrinsic heating of modules and sensors to a minimum. All these actions protect the measurement values from corrupting influences caused by the application. In addition to this, the engineers also detected important test points, depending on the type of machining centre, to increase the precision of the milling parts. Now, significantly more test points can be integrated, which makes it possible to detect the temperature fluctuations of coolants and lubricants and to compensate for them accordingly.
Murrelektronik
www.murrelektronik.co.uk
DESIGN SOLUTIONS | JUNE 2016 15
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