Abstract
Among various nanometer-level displacement measurement methods, grating interferometry-based linear encoders are widely used due to their high robustness, relatively low cost, and compactness. One trend of grating encoders is multi-axis measurement capability for simultaneous precision positioning and small order error motion measurement. However, due to both lack of suitable hardware data processing platform and of a real-time displacement calculation system, meeting the requirements of real-time data processing while maintaining the nanometer order resolutions on all these axes is a challenge. To solve above-mentioned problem, in this paper we introduce a design and experimental validation of a field programmable gate array (FPGA)-cored real-time data processing platform for grating encoders. This platform includes the following functions. First, a front-end photodetector and I/V conversion analog circuit are used to realize basic analog signal filtering, while an eight-channel parallel, 16-bit precision, 200 kSPS maximum acquisition rate Analog-to-digital (ADC) is used to obtain digital signals that are easy to process. Then, an FPGA-based digital signal processing platform is implemented, which can calculate the displacement values corresponding to the phase subdivision signals in parallel and in real time at high speed. Finally, the displacement result is transferred by USB2.0 to the PC in real time through an Universal Asynchronous Receiver/Transmitter (UART) serial port to form a complete real-time displacement calculation system. The experimental results show that the system achieves real-time data processing and displacement result display while meeting the high accuracy of traditional offline data solution methods, which demonstrates the industrial potential and practicality of our absolute two-dimensional grating scale displacement measurement system.
Highlights
Grating encoders, a specific type of linear encoder, are key components for precision positioning in various industrial engineering applications
A four-step optical layout-based multi-axis displacement calculation algorithm and a multiple quadrant photo detector (QPD)-based multi-axis angle algorithm usually require simultaneous processing of around more than ten signals at high speed, which is challenging in terms of both hardware and programing
For solving this problem and to bridge the gap between lab prototype solutions and a product that can benefit most industrial applications, we investigate an alternative approach for real-time data processing
Summary
A specific type of linear encoder, are key components for precision positioning in various industrial engineering applications. A four-step optical layout-based multi-axis displacement calculation algorithm and a multiple quadrant photo detector (QPD)-based multi-axis angle algorithm usually require simultaneous processing of around more than ten signals at high speed, which is challenging in terms of both hardware and programing For solving this problem and to bridge the gap between lab prototype solutions and a product that can benefit most industrial applications, we investigate an alternative approach for real-time data processing. As mentioned above, the widely-used method in the above-mentioned multi-axis grating encoders is off-line data processing using MATLAB, where several channels of interference signals detected by photoelectric detectors are eventually converted to voltage signals that are easy to be analyzed This method limits the real-time processing capabilities of the measurement platform and hinders the practicality of real-time display of displacement measurement results [23,24]. 5, we discuss the results and compare them with traditional scale displacement calculation methods
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