Exposed Linear Encoders - DR. JOHANNES HEIDENHAIN GmbH - #9

Text version of the page

9 The sensor generates four nearly sinusoidal current signals (I0°, I90°, I180° and I270°), electrically phase-shifted to each other by 90°. These scanning signals do not at fi rst lie symmetrically about the zero line. For this reason the photovoltaic cells are connected in a push-pull circuit, producing two 90° phase-shifted output signals I1 and I2 in symmetry with respect to the zero line. In the X/Y representation on an oscilloscope the signals form a Lissajous fi gure. Ideal output signals appear as a concentric inner circle. Deviations in the circular form and position are caused by position error within one signal period (see Measuring Accuracy) and therefore go directly into the result of measurement. The size of the circle, which corresponds with the amplitude of the output signal, can vary within certain limits without infl uencing the measuring accuracy. Photoelectric scanning in accordance with the interferential scanning principle and single-fi eld scanning Scale Orders of diffraction –1 0 +1 Scale with DIADUR phase grating Grating period Scanning reticle: transparent phase grating Photovoltaic cells LED light source Condenser lens Interferential scanning principle The interferential scanning principle exploits the diffraction and interference of light on a fi ne graduation to produce signals used to measure displacement. A step grating is used as the measuring standard: refl ective lines 0.2 ìm high are applied to a fl at, refl ective surface. In front of that is the scanning reticle—a transparent phase grating with the same grating period as the scale. When a light wave passes through the scanning reticle, it is diffracted into three partial waves of the orders –1, 0, and +1, with approximately equal luminous intensity. The waves are diffracted by the scale such that most of the luminous intensity is found in the refl ected diffraction orders +1 and –1. These partial waves meet again at the phase grating of the scanning reticle where they are diffracted again and interfere. This produces essentially three waves that leave the scanning reticle at different angles. Photovoltaic cells convert this alternating light intensity into electrical signals. A relative motion of the scanning reticle to the scale causes the diffracted wave fronts to undergo a phase shift: when the grating moves by one period, the wave front of the fi rst order is displaced by one wavelength in the positive direction, and the wavelength of diffraction order –1 is displaced by one wavelength in the negative direction. Since the two waves interfere with each other when exiting the grating, the waves are shifted relative to each other by two wavelengths. This results in two signal periods from the relative motion of just one grating period. Interferential encoders function with grating periods of, for example, 8 ìm, 4 ìm and fi ner. Their scanning signals are largely free of harmonics and can be highly interpolated. These encoders are therefore especially suited for high resolution and high accuracy. Even so, their generous mounting tolerances permit installation in a wide range of applications. LIP and LIF linear encoders and the PP two-coordinate encoders operate according to the interferential scanning principle. XY representation of the output signals