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Design of incremental encoder interface involves designing a circuit to decode the positional and directional data embedded in the output signals of the encoder , as well as designing a counter circuit to count the number of pulses. This can be done either by using a custom IC or 5-10 discrete ICs. Four monostable circuits would be needed to implement the direction sensing unit. However, with the increase in number of components, system reliability is reduced. Also, systems are easily susceptible for interference by noise. This paper presents a way of implementing the above functions using software only. In addition, the paper provides a mechanism for efficient error correction, thus making it suitable for servomotor control designs, even under noisy conditions.
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Incremental Encoder Reader Circuit with Error Detection and Correction using FPGA
1Arindam Sanyal, 2Snehasish Das, 3Sonai Ray, 4P.Venkateswaran, 5S.K.Sanyal and 6R.Nandi
1,2,4,5,6Department of Electronics & Tele-Communication Engineering Jadavpur University, Kolkata – 700 032.
3Department of Computer Science & Engineering
Indian Institute of Technology Kharagpur Kharagpur – 721 302.
( email ids : [email protected], [email protected], [email protected], [email protected], [email protected] )
Abstract- Design of incremental encoder interface involves designing a circuit to decode the positional and directional data embedded in the output signals of the encoder , as well as designing a counter circuit to count the number of pulses. This can be done either by using a custom IC or 5-10 discrete ICs. Four monostable circuits would be needed to implement the direction sensing unit. However, with the increase in number of components, system reliability is reduced. Also, systems are easily susceptible for interference by noise. This paper presents a way of implementing the above functions using software only. In addition, the paper provides a mechanism for efficient error correction, thus making it suitable for servomotor control designs, even under noisy conditions. Keywords : Biphase , Control system , Encoder reader circuit , Error correction , Forward pulse train , FPGA .
I. INTRODUCTION
Whenever mechanical rotary motions have to be monitored, an encoder is the most important interface between the mechanical and the control unit. Encoders transform rotary movement into a sequence of electrical pulses. As only increments of rotation are detected by a single channel, a second signal, phase shifted by 90 degrees is also generated. This second signal, along with the first signal, enables the direction of rotation to be determined. The two signals from channel A and B are shown in Fig. (1) .
Fig. (1) Typical encoder waveform showing channel B being 900 offset from channel A An incremental biphase encoder is the most commonly used device in computer-controlled feedback system. The microcomputer actuates several final control elements based on the information obtained from the encoder. In addition to providing positional and directional information, the encoder interface circuit must also perform some special functions to maintain proper operations, such as missing pulse correction, error correction, reseting the position counter based on control signal from the microcomputer, etc. It requires 5-10 discrete ICs to implement all the above functions. Also certain types of combinational logic are error prone due to gate delays or threshold offsets and multivibrator imprecisions. This paper tries to do away with these errors as well as reducing drastically the amount of hardware needed. The scheme is explained in section II and the implementation details are given in section III.
channel A
channel B
3rd International Conference on Computers and Devices for Communication (CODEC-06)Institute of Radio Physics and Electronics, University of Calcutta, December 18-20, 2006.
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II. PROPOSED SCHEME
A. Edge Detection algorithm
The change of states of signals A and B as Fig. (1) can be represented in the form of the following state diagram:
Fig. (2a) State transition diagram of a biphase incremental encoder dashed lines indicate erroneous transition.
The above state diagram has been formed taking the value of channel A as the msb ( most significant bit ) and the value of channel B as the lsb ( least significant bit ). The state transition from 00→01→ 11 →10 is denoted by FPT( forward pulse train ) and the state transition from 00→10→11→01 is denoted by RPT( reverse pulse train ). The erroneous state transitions are shown by dotted lines. The above state diagram has been used to obtain the following state model : A counter is incremented at every forward transition and decremented at every reverse transition of the signals A and B. Thus the value of the counter at any instant gives the total number of edges detected.
Fig. (2b) State diagram of the encoder reader circuit
B. Error Handling algorithm
There may be errors introduced into the signals A and B due to noise. The effect of the noise is to change the input voltage level so that a 1 may appear as a 0 or vice versa. Any erroneous state transition can be found as shown in Fig. (2a). Depending on the direction of the previous state transitions, the program then takes the input signals to their correct states, for example in fig. 2b), if the current state is S1 and the input is 11 then the program will sense an error and send the next state to either S4 or S2 depending on the direction of the previous transitions. Also, the effect of the noise may be such that it does not trigger any invalid transition. This can happen if there are spikes between two cycles of signals A and B. The effect of a spike is to trigger an fpt and an rpt in succession, thus resulting in no change in the counter. Thus the algorithm can effectively correct any error due to noise.
III. IMPLEMENTATION
The program was simulated using ModelSim XE III 6.a. We give below two snapshots of our simulations in Fig.s (3a) and (3b). For the first waveform Fig. (3a), error free signals phase-shifted by 90 degrees applied to the inputs of the encoder reader circuit, and the count value is taken after 8 cycles. The counter rightly indicates the value as 32. For the second waveform Fig. (3b) a test bench was
S4 S2
S3
S1
00
01
11
01 00
11 10
00 10
11 01
10 00
11
10 01
3rd International Conference on Computers and Devices for Communication (CODEC-06)Institute of Radio Physics and Electronics, University of Calcutta, December 18-20, 2006.
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designed to simulate the effect of noise. The signals distorted by noise are applied to the inputs of the encoder reader circuit, and again the count value is taken after 8 cycles. Once again, the counter reads
32 after 8 cycles, thus indicating that the algorithm works even if the inputs are corrupted by noise.
Fig. (3a) Snapshot of the simulated waveform in the absence of noise
Fig. (3b) Snapshot of the simulated waveform in the presence of noise
3rd International Conference on Computers and Devices for Communication (CODEC-06)Institute of Radio Physics and Electronics, University of Calcutta, December 18-20, 2006.
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IV. CONCLUSION
An efficient error correcting mechanism for biphase encoder interface has been described. The implementation details and the simulation results have been given above. Using this implementation the system reliability can be increased thus making it a favourable tool for automatic control system.
REFERENCES
[1] E. S. Tez, “Interfacing bi-phase incremental encoders”, IEEE Trans. Ind. Electron, Vol. IE – 33, No. 3, Aug. 1986, pp. 337- 339 [2] Bernard Hebert, Michel Brule and Louis- A. Dessaint, “A high efficiency interface for a biphase incremental encoder with error detection” , IEEE Trans. Ind. Electron, Vol. 40, No. 1, Feb.1998, pp. 155 [3] J Bhasker A VHDL Synthesis Primer, Star Galaxy Publishing, Revised 2nd Edition [4] Xilinx Spartan 3 FPGA Trainer User Manual, Vi Microsystems Pvt Ltd, Chennai-600096
3rd International Conference on Computers and Devices for Communication (CODEC-06)Institute of Radio Physics and Electronics, University of Calcutta, December 18-20, 2006.
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