Coping with temperature fluctuations

21 April 2016

Thermal influences caused by the environment have to be avoided in modern machining centres and, at DECKEL MAHO Seebach, the CNC controller makes up for temperature fluctuations with logarithmic compensation. Ralf-Dieter Kaulisch and Alexander Hornauer of Murrelektronik explain how the company's new MASI68-Pt100 modules carry out this task in an economic and precise manner

Precise contour accuracy and surface quality in the µm-range is what counts when producing lathed and milled parts. Expansion of workpieces and the machining centre itself is influenced by fluctuations in ambient temperature, heating, air conditioning, insulation and the workpiece's temperature, for example if it comes in from a cold warehouse.

Many factors inside a machining centre can have a negative effect on the precision of a workpiece. For example 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. However, precision parts for medical engineering, the automotive and aerospace industries and many more require repeatability and, at the same time, small tolerances independent of such thermal influences.

To meet the most demanding requirements, DECKEL MAHO Seebach compensates temperature induced material expansions in its machining centers. High precision Pt100 sensors (class A) are installed on spots in the machine that are crucial regarding temperature. 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 AS-Interface bus where complex software evaluates this data. Based on the measured temperature deviations, the machine tools are then automatically corrected and repositioned. 

Previously, 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. DECKEL MAHO Seebach used two Pt100 sensors installed at the headstock to check the temperature. This is where a lot of power is used, 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. Distances of more than 10 or 12m had to be covered, sometimes using C-tracks, which made it an expensive solution that required a lot of wiring work. 

When looking for a solution, DECKEL MAHO Seebach engineers worked closely with Murrelektronik. The solution is a 4-channel MASI68-Pt100 module for decentralised and high precision data acquisition measurement.

DECKEL MAHO Seebach integrated Murrelektronik's compact MASI68-Pt100 modules into the AS-Interface bus system. They are installed in a decentralised position next to the test points. MASI68 modules feature a bus expansion, therefore the bus cable can be daisy chained from one module to another. Analog components that require a lot of wiring are no longer necessary in the cabinet, which saves space and costs, and it is no longer necessary to install the long, shielded cables; 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 its machining centers. The company uses these inputs to evaluate more Pt100 test points than with its 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) – which means those parts of the machine that are exposed to the most stress. More test points offer a more exact basis for calculating the parameters for temperature compensation.

Keeping EMC interference as low as possible is necessary to achieve optimum measuring data. With extreme EMC environmental conditions, for example for installations near synchronous and torque motors and motor cables, DECKEL MAHO Seebach uses special and shielded round cables for the AS-Interface bus. These connectors are moulded and pre-wired and enable easy, error-free installation. Moreover, they are designed according to AS-Interface specification 3.0. MASI68-Pt100 modules are galvanically decoupled inside from the AS-Interface supply voltage. All sensor connections have grounded metal ports and the modules themselves feature a functional earth connection where discharging grounding straps are attached. Such measures are said to result in a significantly higher interference immunity towards EMC influences.

DECKEL MAHO uses Pt100 sensors in four-wire technology. This makes 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.

The MASI68-Pt100 fieldbus modules are fully potted, which makes them resistant to shock and vibration, as well as resistant to coolants and lubricants. This makes them suitable for applications in the demanding environment of machine tools.