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多尔衮身高Dipartimento di Elettronica, Università di Pavia
Via Ferrata 1, I-27100 Pavia, Italy
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Centro per la Ricerca Scientifica e Tecnologica, Istituto Trentino di Cultura
Via Sommarive 18, I-38055 Povo (TN), Italy
e-mail: JXLGR WRUHOOL#HOH XQLSY LW
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A rotary encoder system is an angular rotation/position transducer. It is made up of a glass disc with a suitable number of concentric traces, each having alternated opaque and transparent zones. All traces are illuminated by a LED from one side of the disc. On the opposite side, the light beam is modulated by the disc traces and, after passing through a collimating slit, is collected by a photodiode array (a photodiode is provided per each trace). From photo-generated current signals, the information corresponding to the angular position is extracted.
Two types of optical rotary encoders are available, namely absolute and incremental encoders [1]. An absolute encoder measures the angular position at any time, independently of the rotation speed of the disc; the number of traces is equal to the number of resolution bits. On the other hand, in an in
cremental encoder a pul is generated each time a transparent zone pass above the collimating slit and, therefore, each time the angle of rotation changes by a predetermined amount. Such an encoder needs to count puls to measure an angle, and has a fixed number of traces on the disc.一品白衫
Control systems for mechanical position require absolute encoders with resolution up to 16 bits, thus leading to large discs, difficult to reali with existing rigraphy techniques.
In this paper, we propo a method for increasing the resolution of a rotary encoder without actually increasing the number of traces on the disc.
The propod approach us an encoder system of mixed absolute/incremental type. It is made up of a disc with 13 traces, 11 carrying an absolute Gray code and 2 providing incremental information. Fig. 1 shows the four outmost traces of the disc (two absolute and two incremental traces).
祝母校Figure 1. Detail of the mixed absolute/incremental encoder disc
The output signal is extracted by 13 photodiodes, 13 readout channels and a dedicated digital signal processing unit (EPU16, 16 bit Encoder Processing Unit). The propod approach allows the angular position to be determined with 16 bits of accuracy in binary code. Moreover, our solution allows the ur to program the resolution over a certain range. Photodiodes, analogue readout channels and the digital processing unit are integrated onto the same chip.
In next ctions, we will describe the logic blocks that constitute the EPU16 unit, which performs the following main functions:
•Gray code to binary code conversion (from the absolute ction);•programmable resolution;
•increa of maximum resolution from 13 to 16 bits (from the incremental ction).
沙的组词EPU16 has been designed using VHDL [2].
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The digital processing unit must process two types of digital signals: tho coming from the absolute traces, and tho coming from the incremental ones. The signals must be handled parately, at least in the first processing stage. The absolute signal is an 11-bit word in Gray code and reprents
the 11 most significant bits of the output data. To provide an output binary-weighted code, the first processing step is the conversion from Gray code to binary code.
The incremental signal comes from the two outmost traces of the encoder disc. The two analogue signals corresponding to the two traces are sampled and converted by two 8-bit analogue-to-digital (A/D) converters with a sampling frequency of 110 kHz. Due to the rotation of the disc, the two analogue signals
can be approximated by sin([) and cos([) functions. The sinewave frequency depends on the rotation speed of the disc. The first stage of the incremental signal processing implements an interpolation algorithm that receives two samples of the sine function (with 4 bits of amplitude resolution) and provides the 5 least significant bits of the 16-bit output data.
朱国威The absolute and the incremental signals, adequately procesd, are merged into one 16-bit word by the cond processing stage, where the angular position is also calculated in ur-defined units of measure.
Fig. 2 shows the schematic diagram of EPU16, which has the following input signals:
• a 19 bit input bus. Bits from 0 to 10 come from the absolute traces, bits from 11 to 14 and from 15 to 18 are produced by A/D conversion of the two incremental signals;
• a clock signal (4 MHz);
• a ret signal;
• a load signal for programming the unit of measure for angles.
The 16-bit output bus carries the angular information measured by the nsor
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张黑女碑Figure 2. Schematic diagram of the digital processing unit EPU16
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The absolute encoder ction of the disc produces 11-bit words in Gray code.To merge the words together with the binary-weighted 5-bit words corresponding to the incremental ction, conversion into a binary code is necessary. The Gray2Bin11 cell converts a Gray word (J J J J Q Q −) into a binary-weighted word (E E E E Q Q −) in the following way:
−=⊕==+1
..., ,2 ,1 ,0for ,1Q L E J E J E L L L Q Q This conversion is implemented with EXOR gates [3].
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EPU16 determines the angular position with a resolution programmed during the load pha. This option allows the ur to lect how the angular information is provided, choosing between degrees or radians and lecting the desired resolution. The minimum resolution steps are 0.01° and 0.0001 rad,respectively. Programming the resolution is made possible through the input bus, under the control of the load signal.
手机处理器哪个最好The input cell Demux19 receives the signals from the external bus,discriminates between nsor and programming data, and parates the absolute from the incremental information.
Programming the scale factor is independent on both ret and clock lines.In particular, dynamic tting is possible, with a tting time of few nano-conds, which is negligible even with respect to the minimum period of the signal coming from the outmost trace (asynchronous operation).
The value of the scale factor is stored into the input cell (Demux19) and is fed to the output cell (Bin2Ang16) through the Scale bus. The latter cell merges the 11-bit absolute signal Abs11<10:0> with the 5-bit incremental signal Inc5<4:0>, thus producing a 16-bit binary-coded signal Bin<15:0>. Then it calculates the output data through the relationship:
16
2Scale Bin Outbus ⋅=In this way, the possible input word collisions (i.e., the events when two or more input words correspond to the same output word) are proportionally distributed along the whole circle.
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Cell Inc2Abs is the main cell of EPU16. Through an interpolation process, at every sampling period the two incremental signals coming from the two outmost traces of the encoder disc are procesd to obtain the 5 least significant bits of the output data.
The propod processing method consists in finding the time position of the two samples within the corresponding sinewave period 7, by comparing their amplitude with reference values stored into a ROM. The incremental signal frequency is exactly twice as large as the frequency of the signal corresponding
to the least significant bit (Abs<0>) of the absolute binary code, as is shown in Fig. 3. To achieve a correct result from the interpolation process, the allowed maximum frequency of the incremental signal is max inc,I = 2048 Hz, cor-responding to 60 rpm. This limitation comes from the sampling frequency ud in the A/D conversion. However, this does not result in performance degradation of the whole system, becau motion control requires a fine
To achieve a resolution increa of 5 bits through interpolation, 32 stored samples would be needed for each reference. However, it is possible to reduce the number of stored samples by suitably processing the input incremental values. In the interpolated word Inc5<4:0>, the 3 most significant bits Inc5<4:2> correspond to one eighth of a sinewave period. They are calculated directly using the following relationships:
≤>=><⊕=><=><Ch2
Ch1Ch2Ch1 2Inc5Ch2Ch1 3Inc5Ch2 4Inc5 if ,1 if ,0)
sign()sign()
sign(where Ch1 = cos([) (13-th trace) and Ch2 = sin([) (12-th trace).
After calculating Inc5<4:2>, we obtain the two least significant bits Inc5<1:0> by a comparison with four values of the sine and cosine references in the interval (0, π/4).
To obtain a more area-efficient storing, we combined the two reference signals into the function )sin(
)cos(1)ref([[[−−=. The function ref([) is periodical with period π/2. As shown in Fig. 4, it is very clo to linearity, and can be approximated by ,0[∈[.
