Ensuring reliable feedback from exposed linear encoders

13th June 2017
Posted By : Joe Bush

Exposed, incremental linear encoders are the feedback system of choice from the axes of CNC equipment for applications where very high positional accuracy is required, for example in semiconductor manufacturing and measurement, printed circuit board assembly machines, metrology equipment and high precision machine tools.

The problem is that the measuring standard (scale) of an exposed encoder is fastened directly to a mounting surface on the machine and can become coated with oil, coolant or dirt, as can the reticle that scans the scale. Signal stability and quality, and hence the integrity of information fed back to the control system, can be adversely affected.

The solution is to use HEIDENHAIN’s new application specific integrated circuit (ASIC), HSP 1.0, which provides a stable measuring signal that is unaffected by even severe levels of contamination. The intelligent circuitry compensates for fluctuations in signal amplitude due to interference, providing a low noise output that maintains interpolation accuracy of the machine axes.

HEIDENHAIN

The signal processing ASIC constantly monitors the scanning signal from a HEIDENHAIN linear encoder. If changes are detected, it compensates for the resultant deviations to establish the original signal quality so that interpolation deviation and position noise do not increase. The signal retains its ideal form and its amplitude stays nearly constant at 1 VPP. Even if the ASIC’s control limit were to be exceeded due to increasingly heavy contamination, it would not result in an abrupt failure of the signal, which would instead only slowly decrease in amplitude.

If it does, the HSP 1.0 readjusts it by increasing the LED current. The resulting increase in LED light intensity scarcely affects the noise level in the scanning signals, even when there is a high degree of signal stabilisation. This is in contrast to systems where the gain is made in the signal path, which increases the noise level.

Detailed accuracy data for better encoder selection
HEIDENHAIN has also introduced enhanced accuracy data for its exposed linear encoders to enable design engineers to refer to more detailed information when selecting the appropriate encoder for an application. More precise prognosis of achievable accuracy is the result.

In many applications, it is not the accuracy over the entire measuring range that is important, but rather the accuracy over a limited section of the scale. On PCB assembly machines, for example, it is the last few millimetres of the measuring path that determines the precision of the mounting process. For such applications, the user obtains substantially more relevant and precise information, enhancing machine performance.


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