OPTICAL ENCODER HEAD WITH IMPROVED LINEARITY ^Jernej Rozman, ^Anton Pleteršek ^IDSd.o.o., Ljubljana, Slovenia ^University of Ljubljana, Ljubljana, Slovenia Key words: encoder, optical scanner. Vernier scale, opto-sensors, interpolator. Abstract: We present a high precision optical scanner, combining averaged optical sensors array with appropriate 10 urn graduated scales on measurement - fix plate and Vernier sliced parallel scale on reading plate. The generated sine wave signals are at least 30dB less distorted by distributing and mismatching optical edges over a number of sine wave periods within a number of Vernier scaled periods. Reading plate which is positioned along optical array has a unit division smaller than those on a fixed scale - permit a far more precise positioned optically generated sine wave current. Position-like averaging of four generated signals was distributed over an opto-array and reading scale. Background of reduction as well as prototype is presented. Good matching was found between mathematically analyzed optical scanner and measurement, where comparable measurements were performed on optical head having redistributed and fixed opto-edges. Optični dekodirnik z izboljšano linearnostjo Kjučne besede: dekodirnik, optični pretvornik, Vernierjeva skala, optični senzor, interpolator Izvleček: V članku predstavljamo optični dekodirnik z visoko točnostjo pretvorbe. Sestoji se iz polja optičnih senzorjev, namenjenih povprečenju signala ter iz fiksne in čitalne letve z 10 um rastrom in z Vernierjevo porazdelitvijo. Zgradba sistema omogoča periodično prerazporeditev optičnih robov znotraj periode sinusnega signala in znotraj Vernierjeve periode ter s tem zmanjšanje popačenja proizvedenih sinusnih signalov za 30dB. Čitalna letev ima raster zmanjšan v primerjavi z rastrom merilne letve kar posledično pomeni veliko boljše pozicioniranje in razpršen vpliv uklonskih pojavov. Princip delovanja in prototipni optični dekodirnik so opisani v članku. Izdelana matematična analiza se dobro ujema z izsledki meritev. Poleg rezultatov meritev je podana primerjalna analiza sistemov z Vernierjevo porazdelitvijo - in sicer s konstantnim in s prerazporejenim rastrom. 1 Introduction Accurate measurement of displacement is of prime importance in any computer controlled machine (CNC). The need to manufacture "something" requires the ability to move "something" with a very high level of accuracy. And for this accurate position sensors are needed, also called position encoders /4/. Such encoders can be roughly split by the type of displacement they detect (rotary, linear) or by the quantity they detect (optical, magnetic) or by the type of output data they produce (incremental, absolute). This work deals with linear optical incremental position encoders. As this work deals with a very narrow field of research not much literature is available. Most information can be found in books about automatics and robotics /1, 2/. Our solution is also much differ from that in patents /10, 11/. 2 Optical encoder An optical encoder (Figure 1) is typically composed of a light source (LED), the main scale with a built-in optical grating with period PM and the optical scanner that is composed of an optical sensor and a reading scale with a built-in optical grating with period PR /3/. The main and read- ing scales are glass plates on which a thin layer of metal (chromium Cr) is deposited and then a regular pattern is etched into this layer. If periods PM and PR are the same then as the scanner moves along the main scale the patterns on the main and reading scale overlap to a different degree, depending on the momentary position of the scanner. In short, as the scanner moves along the main scale the amount of light from the light source (LED) is modulated. Therefore the electrical signal produced by the photo sensor is also modulated. In our case the photo sensor is a reverse polarized photodiode and the signal is in the form of electrical current. Real encoders use four such sensors placed apart L of the grating period (PM, PR) so that they produce four signals that are 90° shifted between each other. Figure 2 shows the four ideal quadrature signals +A, -A, +B, -B that are produced by the four sensors. The two pairs of signals are amplified with a differential amplifierto remove the large DC components and suppress even harmonics to produce the signals A and B. The signals in figure 2 are periodic; this corresponds to a movement of the scanner head with constant velocity along the main scale. One period of the signal corresponds to a movement of the scanner head equal to the grating period (PM, PR). The signals are not periodic in time but they are periodic in relation to the displacement along the main scale. The signals in figure 2 are also not pure sine/co- ph photosensor reading scale PR PM fS^WI fiViCTF y.'Wf^) tfViy/y::^,^ main scale glass plate f________I LED Fig. 1: Optical encoder. sine in reality they are distorted and contain harmonic components. -A +B The signals can be digitized directly so that the resolution of the encoder is determined by the grating period (PM, PR). Or interpolation can be used to increase resolution. \ : X K K A Fig. 3: Ideal signals. If we have available two quadrature (sine/cosine) signals then we can by scaling and adding or subtracting of the two signals produce a new signal shifted by an arbitrary amount as is illustrated by the simple circuit in figure 4 and the two trigonometric formulas (1) and (2). We can produce a number of such shifted signals and make XOR operations on them (figure 5) to produce a quadrature signal with four times higher frequency which results in a four times higher encoder resolution. sin(x +a)= cosa sinx + sina cosx (1) cos(x + a)=-sinasinx + cosacosx (2) R< sm(x)-- cös(x)-- R, R. ' - sm(x-ra) Fig. 4: Shifted signals. MJKXI —■! —i !in(si3=/Si—«. J......r" L X____J..................L ,n,.n.,.n.Ji„n,„.R„n..,n„n....n...n..,n IjmjlJTJTJTJlJlJTJ^ Fig. 2: Quadrature signals. 2.1 Interpolation As we have seen the encoder produces two 90° shifted (quadrature) signals A and B as can be seen in figure 3. The two quadrature signals enable us to detect the position of the scanner head at all time just by measuring the values of signals. Fig. 5: Interpolation by factor 4. One example of an integrated interpolation system is the IDS-EN400 /4/ which has a selectable interpolation fac-tor5, 10, 20, 25, 50, 100 and integrated signal conditioning circuitry. The main emphasis of this work is the improvement of the quality of the quadrature signals to enable higher interpolation factors and higher encoder resolution. With the industry standard grating period (PM, PR) of 20|jm, interpolation factors of 100 and more would enable resolutions in the nanometric range. 3 Improved encoder The old scanning head (figure 6) was composed of four photodiodes (+A, -A, +B, -B) that produced the quardra-ture signals and an additional photodiode (Rl) that generates the index signal which is used for absolute position encoding (for more information see /3/). Such a scanning head was basically described in Section 2. The spectral purity of such a scanning head is mainly determined by the diffraction and reflection of light by the two optical gratings in the main and reading scales. Without this the signals would have a triangular shape not sinusoidal. We have used several techniques to improve the signal quality as described below. 3.1 Vernier (Nonius) pattern In figure 7 we can see how two patterns with period P1 =1 and P2=0.9 produce a third pattern which has a much larger period P=9 (9 is the least common multiple of P1 and P2). This new Vernier pattern corresponds with the movement of the scanning head along the main scale therefore we can place four photodiodes in one period of the Vernier pattern for generating the quadrature signals as depicted in figure 7. Fig. 6: Old scanning head. n=9 □ □ ^OMMMiMMMMä. pi=i 1 +A ..... -A +B -B Fig, 7: Vernier (Nonius) pattern. The actual encoder uses a main scale with the grating period (pitch) of 20 M m and we chose a reading scale period of 19|jm to produce a Vernier pattern with a period of 380|jm. Inside this 380ijm we could place four photodiodes with maximum width of 95|jm but the semiconductor technology chosen limits the width to 83|jm. These dimensions are illustrated in figure 8. The two square shaped transmittances of the main and reading scales interact to create the Vernier pattern (380|jm) which corresponds to the intensity of the light that falls on the photodiodes. As the scanning head moves 20|jm the Vernier pattern moves 380|jm. This movement of the pattern is detected by the four photodiodes placed 95|jm apart to produce the 90° shifted signals. The "chopped" shaped of the Vernier pattern as seen in figure 8 is averaged (filtered) by the width of the photodiodes. iFan^miUance oi. inc iiuii'n scaic [ransmiuiinccof (he reading scale ^y^cfriiL-t ?.>attcrn photodiode