Taking account of temperature in precision machining operations
11 August 2015
Deckel Maho Seebach compensates for temperature induced material expansions in its machining centres with the help of Murrelektronik Pt100 sensors.
Precise contour accuracy and surface quality in the micrometre range is what counts when producing lathed and milled parts. Any fluctuations in the product size pose real problems. Therefore, thermal influences caused by the environment have to be avoided in modern machining centres.
At Deckel Maho Seebach, the CNC controller makes up for temperature fluctuations with logarithmic compensation. Murrelektronik’s new MASI 68-Pt100 modules carry out this task in an economical and highly precise manner.
Metal 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.
In order to meet the most demanding requirements regarding precision, Deckel Maho Seebach compensates temperature induced material expansions in the machining centres. High precision Pt100 sensors (class A) are installed on spots in the machine that are crucial regarding temperature (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.
So far, 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 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. Distances of more than ten or twelve meters had to be covered, sometimes using C-tracks, which made this an expensive solution that required a lot of wiring work.
The electrical engineers from Deckel Maho Seebach were not happy with the long signal paths and correspondingly expensive, shielded sensor cables. When looking for an innovative solution, they worked closely together with Murrelektronik. The solution is a 4-channel MASI68-Pt100 module for decentralized and high precision data acquisition measurement.
Deckel Maho Seebach directly integrated Murrelektronik’s MASI68-Pt100 modules, the most compact in their class, into the existing AS-Interface bus system. They are installed in a decentralized position next to the test points. MASI68 modules feature a bus expansion, therefore, the bus cable can be daisy chained from one module to the other.
Analogue components that require a lot of wiring are no longer necessary in the cabinet, which saves a lot of space and costs, and it is no longer necessary to install the long, shielded cables. This means savings in time and money. 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 4 ports each in their machining centres.
This makes eight Pt100 inputs available. Deckel Maho 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) - which means those parts of the machine that are exposed to the most stress. Of course, more test points offer an even more exact basis for calculating the parameters for temperature compensation.
Keeping EMC interference as low as possible is a decisive factor to acquire 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 simple, easy and 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 are equipped with grounded metal ports and the modules themselves feature a functional earth connection where discharging grounding straps are attached. All these measures 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 engineers from Deckel Maho Seebach additionally 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 with sensors and to compensate for them correspondingly.
With regard to their mechanical design, Murrelektronik’s MASI68-Pt100 fieldbus modules are fully potted, which makes them resistant to shock and vibration, as well as being resistant to coolants and lubricants. This makes them suitable for applications in the demanding environment of machine tools.
MASI analogue modules are used in high precision systems of the DMU 40, 60, 80, 100 eVo series. Furthermore, they are used in the DMF series and in all highly innovative standard machinery.
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