Breakdown of grating based NiosII circuit design (1)

1 Overview

Currently, raster electronic subdivision is raising grating displacement sensor resolution of the main ways that can be divided into software subdivision and hardware subdivision.

Software subdivision while it is possible to achieve a higher number of subdivision, but due to the A/D conversion accuracy and conversion device time restrictions, to a certain extent affected the real-time measurement. Hardware subdivides the law generally used in the number of segments is not too high, and with the number of segments, the circuit will become more complex. This article uses a special interpolation chip (IC-NV) on front-end output orthogonal signal interpolation segmentation, use FPGA to interpolation of signal after the subdivision in the second subdivision; while leveraging the Component Editor in QuartusII tools designed the second fine resolution component to the component, speed measurement and control components, LCD and through Avalon bus and NiosII soft core processor, system integration and modularity.

2 moire and four times the frequency directly subdivision principle

Moire e breakdown is raising grating displacement sensor resolution of one of the main ways.

Moire is grating sensor work Foundation. Moire spacing approximation for grating blade spacing of 1/θ times (θ as the angle between the Deputy grating), and the direction of approximate and vertical line direction. When either grating along the perpendicular to the direction of reticle moves one line space, moire fringes in line direction moving streaks spacing, so you can detect the movement of the moire fringes to calculate the distance indicates the grating.

For the horizontal moire, in order to determine the direction of displacement in the direction of grating reversible counting and eroded DC level drift on measurement accuracy of a moire fringes in raster placed four optical transceiver components.

When striped sweep this turn four optical transceiver components, it will produce 4 road phase respectively 0 °, 90 °, 180 °, 270 ° signal through operational amplifiers differential amplification circuit to achieve four segments. However, the actual application to achieve four optical transceiver components spaced is very difficult. Currently, most grating displacement sensor using optical gate moire to achieve four segments, as shown in Figure 1.

Light gate grating Deputy of grating carved 4 crack-Windows, all Windows line and main grating consistently, and the adjacent two-one interval between Windows (n + 1/4) d.

Where d-blade spacing (here is 20 μ m), n is an integer. That way, when 0 ° window grid lines and master gratings full overlap, the lightest window, form the light band; 180 ° window grid lines and primary raster grid lines are obscured, the formation of dark band; 90 ° and 270 ° window grid line slot is half obscured, in half and half dark state. Therefore, when you move indicates the raster, the four Windows of light intensity by cyclical changes. In the window area where optical transceiver components for light to be detected, they can be the difference between π/2 turn 4 sine wave signals.

3 grating formation and realization of difference

3.1 system overall programme meter

System block diagram shown in Figure 2.

Photoelectric conversion output of 4-way differ by 90 ° for cosine current signal after 2 front differential amplifiers, converted into a voltage signal and eliminates the DC level, get 90 ° phase difference between orthogonal signal s I n c o s/θ θ. In order to eliminate the orthogonal signal noise signal in doping, design a source of second-order Butterworth lowpass filter. Filtering of signal interpolation ASIC IC-NV can give FPGA for second fine resolution, speed, and digital displays.

3.2 photoelectric conversion and preamplifier

Photodiode of photocurrent generally μ A level, and amplifier feedback resistor generally use M Ω level of resistance.

Therefore, the amplifier's input bias current of impact could not be ignored, to choose the input bias current of FET input operational amplifier. This selection of TI's 4-way TLC279CN LinCMOS amplifier. It has the input offset voltage is low and high input resistance, low noise, typical 25 ° anybody bias current for 60 pA, far less than the photodiode of photocurrent. Photodiodes can work in zero bias or reverse-biased manner. In the reverse bias, photodiodes can achieve high switching speed; but at the cost at the expense of linear and non-light conditions, there is still a very small current, is referred to as the "dark current." Zero bias circuit by dark current in a small, tiny illumination, you can keep the illuminance and output into a linear relationship.

Figure 3 using reverse and received a photodiode.

This way you can effectively undermine the DC calm even harmonics. Because the back-end chips in single-ended input interpolation on the input signal peak DC and has restrictions in normal phase input setting variable resistance adjustable output DC level to 2.5 V, at the same time by adjusting the feedback resistor to peak output voltage to 1 V.

3.3 low-pass filter design

Since the current raster movement speed to 120 m/min, maximum of 600 m/min and grating blade spacing to 20 μ m is the output of orthogonal signal frequency is not more than 500 kHz.

Thus, selected low-pass filter cutoff frequency for fc = 500 kHz, passband gain K = 1. The specific design of the circuit shown in Figure 4.

3.4 realization of difference circuit

IC-NV is IC-HAUS company monolithic A/D conversion chip, to be able to enter the s I n c o s/θ θ signal interpolation to output an incremental encoder signal quadrature.

IC-The internal structure of NV chips and peripheral circuits such as shown in Figure 5. Its internal integrates high speed comparators and Burr filter, to ensure signal integrity for high-speed conversion and; input/output pins with ESD protection, and with TTL, CMOS-compatible, the interface is simple and reliable.

[1] [2] [3]